CA1310508C - Opto-electronic method for spectral analysis - Google Patents
Opto-electronic method for spectral analysisInfo
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- CA1310508C CA1310508C CA000546631A CA546631A CA1310508C CA 1310508 C CA1310508 C CA 1310508C CA 000546631 A CA000546631 A CA 000546631A CA 546631 A CA546631 A CA 546631A CA 1310508 C CA1310508 C CA 1310508C
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Abstract
OPTO-ELECTRONIC METHOD
FOR SPECTRAL ANALYSIS
ABSTRACT
An opto-electronic method for rapid, accurate spectral analysis of the reflectivity or transmissivity of samples is disclosed. A concave, holographic diffraction grating is oscillated at high speed to provide a rapid scanning of monochromatic light through a spectrum of wavelengths, The grating drive system is an electrically driven mechanical oscillator which utilizes the back EMF of the oscillator motor to maintain oscillation at the desired amplitude and frequency. A passive optical shutter mounted to the grating alternatively blocks the light entering and exiting the monochrometer as the grating is oscillated.
The resultant dark period is utilized in the method to provide a reference offset value and to control cooling of the detectors. A phase-locked loop circuit provides sample commands at precisely determined intervals to correctly correlate the spectral data with the output of the monochronometer. Source and exit optics are employed to optimally shape the light passing through the system. A detection head allows measurement of both light transmission or reflectance with only slight modification of the system. An optical fiber diverts light from the beam at or near the sample. This light is electronically detected and the electronic detection signal is used to adjust the gain of the sample signal so as to compensate for atmospheric bands, temporal variations, and system response. This provides a true double (dual) beam operation.
FOR SPECTRAL ANALYSIS
ABSTRACT
An opto-electronic method for rapid, accurate spectral analysis of the reflectivity or transmissivity of samples is disclosed. A concave, holographic diffraction grating is oscillated at high speed to provide a rapid scanning of monochromatic light through a spectrum of wavelengths, The grating drive system is an electrically driven mechanical oscillator which utilizes the back EMF of the oscillator motor to maintain oscillation at the desired amplitude and frequency. A passive optical shutter mounted to the grating alternatively blocks the light entering and exiting the monochrometer as the grating is oscillated.
The resultant dark period is utilized in the method to provide a reference offset value and to control cooling of the detectors. A phase-locked loop circuit provides sample commands at precisely determined intervals to correctly correlate the spectral data with the output of the monochronometer. Source and exit optics are employed to optimally shape the light passing through the system. A detection head allows measurement of both light transmission or reflectance with only slight modification of the system. An optical fiber diverts light from the beam at or near the sample. This light is electronically detected and the electronic detection signal is used to adjust the gain of the sample signal so as to compensate for atmospheric bands, temporal variations, and system response. This provides a true double (dual) beam operation.
Description
()PTO-EI.ECTRONI ~ l~lk'l ~ r ~`~)R SE~E(''I'RAL. ~NAL.Y~; L~;
Bacl~ground_o~ the Invention The present invention relai:es generally to methods f or spec t ra 1 I y meclsurillg arl(l ana Iy~: ing opti.ca I
properti.es of salllples. Sucl-l rnethods are presentlY used i n i n d u s t r .i a I a n d a g r i c u I t u r a L a p p 1. i c a t i o n L o r co:lori.rnetry and for qlJanti-tati.vely analyzing the const-ituents of samples. AdditionaI applicatiorls are being developed in the fi.eld of rnedi.c~ine in which samples are spectrally analyzed for diagnostic purpnses.
Examp.les of agricultural applications presently in use are implerrlellted by means of instrllmen~ts whi.ch accurately deterrnine the oll. proteill and water content irl grain or soybeans. The trad:itional analytical laboratory techniques~ such as the K jelda}ll method for measuring protein, are extremely accurate but requi.re the servi.ces of a ski lled chem:ist. The lesults, furtherrrlore, are not immediately or readi.ly availah]e.
Buyers of agricu:l.tural products have dernons-trated an increasing interest -in accIlrate and rapid determinatiorls of the moisture, protein and oil percerlt.lges ot the various produces purchased. 'I'lIe wlleat export market, for example, has seen the wi.despread ~introductioll of selling on the basis of guaranteed protein content.
This competitive pressure has increased -the requirement of the commodity handler, f rom the country eleva-tor to the export terminal, to sort rapidly and accurately grains and other products by their content, where applicable .
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131û50~
Additional agricultural and food applications include measurement of constituents in dairy products, cereal, beverages, fruits, meats, etc.
In the industrial market, this type of measurement is successfully applied to the following area:
¦ the textile industry for measuring lubrication on yarn and ¦ for fiber finish in nylon, polyester, cotton and others;
¦ tobacco industry for measuring the percent of tar and ¦ nicotine;
¦ paper industry for the chemical analysis of paper including ¦ coatings, thickness and moisture;
¦ plastic industry for measurment of tapes and film ¦ thickness;
¦ gasohol and petroleum industry for composition ¦ determination;
¦ cosmetics and perfume industry for measurement of oils and ¦ other ingredients.
¦ In the pharmaceutical industry, this instrument is applied ¦ for measurement and identification nondestructively of drug composition.
The need for versatile, yet low cost, advanced equipment, which combines and improves upon recent scientific findings in the field of nondestructive testing of products has greatly increased. For maximum usefulness of commodity handlers, such an instrument must not place high demands on the skillfulness of the operator or reguire a specialized knowledge of the scientific basis for the end result.
O
~ 131050~ 1 Rec~nl- developments h~ve provided me-thods and inst-~uments which are able to satisfy some of the above requirements of c~mmodity handlers. The optical analyzer disclosed by Isaac J. Landa in U.S. Patent No. 4,285,596 entitled "Holographic Diffraction Grating System for Rapid Scan Spectral Analysis" provides an optical system for rapid, accurate spectral analysis of the reflectivity'and/or transmissivity of samples. A concave holo-graphic diffraction grating oscillated at high speed is uti-lized to provide a rapid scanning of monochromatic liyht through a spectrum of wavelengths. The grating is positively driven at a very high speed (typically, ten scans per second) by a unique cam drive structure comprising identically shaped conjugate cams. The rapid scan by the grating enables reduc-tion of noise error by averaging over a large number of cycles.
The rapid scan also reduces measurement time, and thus prevents sample heating by excessive exposure of light energy. A filter wheel having dark segments for drift correction is rotated in the optical path in synchronism with the grating. Source optics is employed to shape optimally the light source for the particular application. The system optics further includes an arrangement of lenses, including cylindrical lenses, to obtain the best light source shape which results in maximum light throughput. Fiber optics are also employed and arranged to meet the optimum requirements of the,system for light collec-tion and transmission through portions of the optical system.
~ related instrument is disclosed by Isaac J. Landa in U.S.
Patent ~o. 4,264,205, entitled "Rapid Scan Spectral Analysis ~ 3 1 050~
System Utllizing Higher Order Spectra] Re~lections of IIologr.lpIlic Dift'ract~ion ~;ratings'~, wh~ch is related to the previously mentioned l,anda patent. The disclose(I
optica] systern is similar to that shown in the previoux patent application~ but includes a Eilter wheel divided inIrJ Iwo arcuate segments separated by opaque segmenIs arrange(I approxirrIatelv I~O apart. One arcuate segment of the wheeI trarIslr~ ,s onIy E:irst order ligllt. The other arcuate segment transmits only second order light.
Separate photodetectors are employed dur:irIg -inErared anal)~sis of samples for detecting first order and second wavelengt:h transmiss-iorIs, and an eIectronic deco-ler app.Iratlls is uiiIized t'or SW:it(`llillg l)etWr`ell (I(?t(`(`t.(lrS.
The analyzers disclosed in the two l,anda patents suffer from a number of disadvantages. First SllCh optical analyzers are limite(I in the accuracy of t:heit rneasurement by the particuIar Irive mecIlanisrrI empIoyed for oscillating the dift:rac-tion grating. SpeciEically.
the complex can, drive mechan:ism emplo~ed to provide a linear spectral scan is re]atively costly and inaccurate. 'I'he cam drive mechanisrn is needetI to control the var:iation in the ve'Iocity of thl` grat:i.rlg during each scan in order to obtain the desired linear spectral scan. The complex cam drive mechanism introduces error in the analysis because of the very tight tolerances required of the camming surfaces.
Another disadvantage is that the filter wheel employed for blocking the light to provide a dark offset value requires careful synchronization with the oscillating of the grating to !
~ 3 l ~51~;
ensure that the light is blocked al the appropriate time.
This increases Lhe likelihood of error as well as increasing the number of parts in the ar-aly~er.
Still another disadvantage o-f -the former methods of allalysis is thdt lhe trarlsmissivity arld retlec-tivity of sdmples is measured by different detec-tors. 'I'his approach introduces arl additiondl varidble into the analysis ot -the samples which necessarily compounds errors. 'I'he requirement of -two se-ts of de-tectors also increases cos-ts.
Summary of the Invention 'l'he present invention is a method for optically and analy~lng a sample comprising the steps of: a) providing rnonochromatic light at varying instantaneous waveleng-ths by: (i) repeatedly rotating d concave diffraction grating mounted to a drive shaft which is connected to a motor back and forth; (ii) illuminating the sarnple through a collima-ted optical path which includes said diffraction grating; (iii) repeatedly blocking said light pd-th said blocking step being synchroni~ed with reference to back EMF of said motor (iv) whereby Bragg reflection by said diffraction gra-ting along the axes of said collimated ligh-t path provides light which is substantlally monochromatic at an instan-taneous wavelength determined by the instantaneous position of said grating; and b) sensing optical characteris-tics of the sample at various wavelengths provided by said monochromating step a).
In a narrower aspect, s-tep (a)(iii), above, comprises the substeps ol: (d ) sensing the operation of said torque motor -to provide a back EMF signal, and (b) blocking said optical train in accordance wi.th said back EMF signal.
More narrowly still, the s--ep of sensing optical characteristics ol' the sample ot various wavelengths comprises -the substeps of: (i) sensing the operatiun of said torque motor to provide a back EMF signal, (ii) synchroni~ing said sensillg step in accordance with said back EME` signal, and (iii) detecting properties of the sample at plural instantaneous wavelengths determined in accordance wlth said synchroni~ing substep (ii).
The present invention electronically provides : 15 sample commands at precisely deLerrnined intervals in order to cor:relate correctly -the spectral data with -the output of the monochronometer. This is carried out by a phase-locked loop circuit:which receives pulses from an lnexpensive shaft encoder. I'he phase-locked loop t.racks the sinusoidal motion of the grating, corrects for the non-uniform relationship of the g:ra-ting equa-tion of wavelength to inciden-t angle, and provides resolution enhancement of the shaf-t encoder.
Finally, -the present invention provides a method which allows the same detectors to be used for measure-~ 3t ~Q8 ment o~ the transrni.ssivitY or reflectivity o~ samp:les with only mino~ modification. The detectors are advant.lgeollslY placed in a specially desi.gned detector head which irlcludes a removable rtlirror asseml)ly. in the transmiss-ion mode the sample is placed before the detector head. and the detector head :is directed toward tlle inc:idelltal light. I.ight p.lssirlg thrtJIlgh the sample is reflected off the mirror assembly in -the detector head and into the detectors. In the reflectallce mode the mirror assembly is removed. the detector hea(l is rotated 180 so that the detectors ~ace away fror,l the light source. The entire detector head is placed before the sarnple. Light from the monochronometer passes through an aperture in the detector head (previ.ously occupied by the rnirror assembly) and is reflected off the sample and back to the detectors.
The method may be irnplemente(l with a double beam instrument as fol]ows. An optical fiber d:iverts light from the beam at or near the samp:le. This light is electronically detected and the electronic detection signal adjusts the gain of the samp:le s:ignal so as to compensate for atmospheric bands temporal variationsl and system response. This provides a true double (dual) beam operation.
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BRI~F` D~RIPTION OF THE DRAWIN~S
Further objects and advantages of the present i.rlventioll will become apparent by referellce to the following detailed descripti.on ol' the aT)paratus for carrying out the consi.(lered in CC111 j Ull(:tiOll w,ith the accompanying drawings. in wllich:
Fi.~ure l -is a perspecti.ve v:iew o~ the opt:i.cal cornponents and layout o~ the apparatus for implementing the present invention;
F'igure 2, comprisin~ Fi~ures 2A and 2B, i,llustrates the grating drive system o~ the preserlt invention;
Figure 3 is a ci,rcuit di.agram sh o w :i ng the electronics utilized to drive the motor of the gra~ting drive systern;
Figure 4. comprisillg Figures 4A and 4B, is a representation or the scan angle oE the grating and the events that occur as the grating is oscillated;
Figure 5 is a block dia~ranl nf the phase-locked loop circu:it for providing sàmple pulses ~to the detector electronics ot` the apparatus;
~. 131U5~
Figure 6 is a circuit diagram of the data acquisition elec--tronic~ of the apparatus;
Figure 7, comprising Figures 7A, 7B, and 7C, is an illus-l lrat:ion oE the detector stage of the apparatus; and S Figure 8 is a circuit diagram of the detector electronics oE the appara-tus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Briefly, and referring first to Figure 1, the present method ls implemented using an electro-optlcal sys tem and pro-vides a rapid, accurate spectral analysis of the reflectivity and/or transmissivity of samples 126. A light dlspersing ele-ment 116, preferably a concave~ holographic diffraction grat-ing, is sinusoidally osclllated at a preselected high speed to provlde a rapid scanni4g of monochromatic light through a selected spectrum of wavelengths. The present invention in-cludes a simple and elegant drive mechanism 200 (See Figures 2A
and 2B) for sinusoidally oscillating grating 116. The sinusoi-dal oscillation of grating 116 produces a scan of monochromatic light which is expanded at both ends and compressed in the mid-dle of the frequency spectrum with respect to time. The grat-ing 116 also exhibits a non-linearity in the monochromatic light output, since the grating equation of monochromatic wave-length ~ to incident angle of light focused on grating 116 is non-uniform. This non-linearity results in the spectrum creat-ed during the scan being compressed at one end and expanded at the other by a relatively small amount as compared to the ~-t~8 expansion and compression produced by the sinsusoidal oscillation. The present invention electronically compensates for both of these non-linearities hy prnvid-ing sarnple comrnallds of the detector(si anillng Olltp~lt(S) at preciselv dete r min e d intervals o~ time.
rhiS allows the present invention to produce digital output data correlated to the spectral scan o~
rnonochromatic light. The pres~ent invention -~urther provides an oftset signal produced by means of a pass-ive optical shutter. This offset signal is used (I) to provide an offset value for de-tectors 712 (see F'igure 8) ; and (2) to control -temperature compensation of detectors712 to insure that they operate in a desired temperature range. The present invention also can include a single detector assembly 106 which allows measurement of reflectivity and transmissivity without a change of detector(s) ~12.
A representative illustration of the optical aspects of the method of the present invention is shown in F'igure 1, Broadly, the oE)t:ical aspects o~ the present invention cornprise a source optics stage 100, a monochrometer stage 102, an exit optics stage 104, and a detector stage 106. Light passing through the system generally follows the path indicated by the dot, dash line in F'igure 1.
Source optics stage 100 provides a properly focussed broadband beam Oe light to monochrometer stage 102. A broadband light source 108 emits light energy used by the present invention. Any type of suitable light source may be used which emits broadband light in the desired spectral range, which range is over the wavelength region that corresponds to ] () 1~ 131050~ ~
¦ ultraviolet (UV), through visible, through near infrared (NIR) ¦ and up to infrared (IR~. In the preferred embodiment, a ¦ tungsten halogen lamp for source 108 is used for measurement in ¦ the near infrared range. On the other hand, for measurement in ¦ the ultraviolet range, a xenon light source might be used, for ¦ example. In the preferred embodiment, an indicator lamp tnot shown) is lit as an indication that light source 108 is functioning properly.
l A spherical lens 110 collects light emitted from light ¦ source 108 and images such light on an entrance slit 112 of monochrometer stage 102. A field lens 114, adjacent to en-trance slit 112, focusses light incident from lens 110 to assure proper filling of dispersing element 116.
l Optionally, a stationary filter 118 is located adjacent to 15 ¦ field lens 114 to eliminate undesired wavelength orders from light source 108. Obviously, stationary filter 118 is not necessary with a light source 108 which emits light only in the desired spectral range, or with a detector 112 which detects light energy only in the desired spectral range. Stationary filter 118 may consist of a high pass, a low pass, or a band pass filter as required by the particular application. More-over, the exact location of filter 11~ is not critical. For instance, filter 118 could be located in exit optics stage 104.
An optional ellipsoidal mirror segment 120 is located behind light source 108 and serves to maximize light throughput by redirecting light into entrance slit 112. Mirror segment 120 is positioned with respect to the filament of disclosed light source 108 so as to double the amount of energy entering the monochrometer stage 102.
' -11- ' The monochrometer stage 102 is now described wi-th reference to Figure 1. Monochrometer stage 102 functlo11s to separate the broadband light provided by llght source 108 into its spectral components.
Monochrorneter stage 102 is comprised of entrance slit 112, dispersing ele1nent ]16, a passive optica] shul:ter 117, and an exit slit 120.
Dispersing element 116 is filled with the light entering monochrometer stage 102 throllgh entrance slit 112. Dispersing element 11~ separates incident light into its spectral components. Therefore, when dispersing elemer1t 116 is sent into oscillatory motion, a spectral scan of monnchroïnatic 1ight is pro(luced at exit slit 120.
lS Dispersing e1ernen~ 116 is a concave holo~raph:ic diffraction grating. In a co11cave holographic grating, the lines oE the grating are formed by a holographic technique. Concave holograph:ic gratings are presently available on the market and are marketed by Instruments, S.A. of Metuchen, New .Jersey. The use of a concave gra-ting is preferable to other light dispersing elements such as plane grating systems becallse it involvex the use of a single optical component as opposed to three or more optical components required in plane 8rating systems. As a result, the alignment in calibration procedures are simplified and the cost of precision mechanical mounts are reduced. The use of a concave holographic grating makes possible the design of a system with low ~ numbers, for example as low as F/l in some cases. High performance plane grating systems are usually limited F'/4 and higher. The lower optical F
numbers makes it possible to . f~Q5~
pass a greater amount of light energy through the optical system thereby making it possible for the present invention to analyze darker samples than possible by conventional optical analyzers. In addition, holographic gratings are free of ghosts and have a lower stray light level in comparison to rule gratings. Another advantage of the holographic grating used in the system of the present invention is that it can be made with a very high groove density which enables high resolution while maintaining high light energy throughput. Yet another advantage of the holographic grating is that it is corrected for astigmatic aberration and spherical and coma aberration are also reduced.
It should be understood, however, that any suitable optical dispersing element, such as a plane grating or a prism (not shown), could be used for dispersing element 116 in the present invention.
~ight shutter 117 is provided in order to obtain an offset signal for zeroing the output of the detectors 712. In the preferred embodiment, light shutter 117 consists of a passive synchronous light chopper constructed of a light aluminum foil, and attached ts concave holographic grating 116 by four rigid wires, designated by reference number 122. It should be understood that light shutter 117 can be constructed of any suitable material and can employ any suitable mounting structure.
The width of entrance slit 112 and the width ~f exit slit lZ0 are adjustable to enable selection of an optimal spectral bandwidth for the particular application. The entrance and I ` ( 1 3 1 050~
exit slit planes are also adjustable to permit the use of a wide range of dispersing components.
Holographic diffraction grating 116 is made to oscillate at a very high speed by means of a grating drive system 200. As S shown in Figure 2A, grating drive system 200 is an electrically driven mechanical oscillator comprising a shaft 202 (having an upper end 202a and a lower end 202b), a precision bearing assembly motor mount 204, a 2-pole brushless motor 206~ and a flat spiral return force spring 208 (having an inner end 208a and an outer end 208b)o As shown in Figure 2B, the lower end of 202b of shaft 202 is secured to the inner end 208a of spring 208. The outer end 208b of spring 208 is securely fastened to a stationary piece 203 fixedly attached to mount 204. It should be understood that the configuration of spring 208 can vary and can include configurations such as the spiral spring, the axial helix spring, the torsion bar, the single-leaf spring, as well as non-linear sprin~s or multiple sprin~
arrangements.
Referring again to Figure 2A, grating 116 is mounted on the upper end 202a of shaft 202. The mass of grating 116, and the force constant of spring 208, together produce a natural har-monic oscillator having a sinusoidal motion. Once set into motion, the drive system 200 would theoretically oscillate for-ever, but for the friction of the bearings in motor mount 204.
A small drive voltage, described below with reference to Figure 3A, is applied to motor ~06 to produce controlled torque which ac~s to compensate for the friction in the oscillator system 200. Since oscillator system 200 requires no direct mechanical . ~ ~310S0~
drive, it has an intrinsically smooth performance with very little vibration, making it ideal for use as a precision scan-ner. Additionally, the simplicity of drive system 200, and its relatively small number of parts, makes it highly reliable.
The sinusoidal motion of grating drive system 200 produces a corresponding sinusoidal spectral scan of monochromatic light with respect to time. Advantageously, the system electronics shown in Figure 5 (described below) accurately compensate for this non-linearity.. In conventional systems, such system elec-tronics is not present, necessitating a grating drive system 200 which produces a linear scan. Such a drive system, as described in U.S. Patent No. 4,285,596 to Landa requires a com-plex cam drive mechanism, which is costly and inaccurate rela-tive to the simple and reliable drive system 200 of the present invention.
The oscillating system 200 of Figure 2A and 2B further in-cludes a shaft encoder 210. Shaft encoder 210 provides three outputs: l and 2 (designated by reference numerals 212 and 214, respectively), and an index pulse 216. These signals are used by the system electronics of Figure 5 for producing an output to sample the detectors 312 at the appropr.iate intervals of time corresponding to discreet wavelength increments of monochromatic light, as will be described in greater detail below.
Referring now to Figure 3, the electronics to drive torque motor 206 is a feedback circuit which supplies just enough energy in the form of a drive signal 312 to maintain oscilla-tion of grating 116 at the desired amplitude (i.e., a scan -15- ~
`. , ~310508 angle 4~2~. As is well known, electric motors produce a back EMF ~electromagnetic flux) which is proportional to the rota-tional velocity of the motor.
The circuit of Figure 3 makes use of a back EMF signal 312 generated by torque motor 206. Note that the back EMF is mask-ed by the drive voltage 312, which has a much greater ampli-tude. The circuit uses a balanced bridge arrangement to extract the back EMF signal 310. In the circuit, a metering , resistor 302 is connected in series with the motor coil 304. A
¦ p~tentiometer 306 is adjusted so that the drive voltage on line 314 is bal~nced out and only the pure sinusoidal back EMF sig-nal 310 appears across the inputs ~16a and 316b of an opera-tional amplifier 308.
~oth amplitude Ithe scan angle 402) and frequency ~the num-ber of scans in a defined time period~ of oscillating system 200 are easily and precisely adjustedO The amplitude is con-trolled by electronically limiting the current level of drive signal 314 by employing a drive amplifier 310 in the feedback loop of the motor control electronics of Figure 3.
Referring now to Figure 4A, in the preferred embodiment, the amplitude of oscillation of grating 116 is set to limit scan angle 402 to approximately 42 . As shown in Figure 4B, the central 25 portion ~designated by reference number 404) covers the spectral range of interest, leaving approximately 7 1/2 overrun (reference numbers 406a and 406b) at either end.
The overrun regions 406a and 406b are advantageously used for storing a dark offset value described below. It should be understood that the minimum scan angle 402 is determined by the angle needed for complete blockage by the light shutter 117.
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The frequency of the oscillation of grating 116 can be varied by adjusting the force of spring 208 in Figure 2. The freguency of oscillation preferably is in the range of 5 to 500 l cycles per minute, which is 10 scans to 1000 scans per minute.
¦ A preferred frequency is 300 cycles per minute, or 10 scans per second. Because the holographic grating 116 is oscillated at a very high speed, the present invention eliminates noise by electronically averaging output data over a lar~e number of scans. In the preferred embodiment, a lamp ~not shown) is pro-vided which flashes once per scan as an indication that oscil-lating system 200 is functioning properly.
In order to measure spectral reflectivity or transmissivity in discrete wavelength increments, the output of dPtectorls) 712 must be sampled at precisely determined intervals. This is necessary to correlate correctly the spectral data with the output of monochrometer 1020 The absolute position of grating 116 must be known throughout the scan in order to produce the desired correlation. Moreover, repeatability of angular posi-tion is critical. In order to obtain one nanometer tlO 9 meter) wavelen~th resolution of output light for the concave holographic diffraction grating of the preferred embodiment, the 25 spectral region 404 must be divided into 1200 sample points. In other words, for the particular grating 116 employed in the present invention, the output of detector(s~
812 must be sampled 1200 times per scan to achieve an output of one data measurement for each wavelength increment of one nanometer. This is the equivalent of 48 sample points per degree of scan of spectral region 404.
.~ ~ ~ 131D~
Conven~ional shaft encoders can provide the 48 sample p~ints per degree resolution, but are quite expensive and pr~-duce outputs only at uniform increments of incide~t angle.
Further, due to the non-u~iform relationship of the yrating e~uati~n of wavelength A to incident angle ~, the spectrum created during the scan is compressed ~t one end and expanded at the other. Therefore, the circuit of Figure 5 i~ used to enable spectra of a unifor~ wavelenyth resolution to be pro-duced, without the need of a~ éxpensive shaft encoder.
Referring to Figure S D a phase-locked loop circuit 502 is shown which provides three functions. Firstly, phase-locked loop circuit 502 tracks the sinusoidal motion of oscillating grating 116 to provide detector sample commands at the correct intervals of time (note that since the motion of grating 116 i5 constantly changing, these time intervals are constantly changing~0 Second, phase-locked 1QP circuit 502 adjusts these intervals by a small amount to compensate for the slight non-linearity of the grating 116 output to incident angle of light input. Third, phase-locked loop circuit 502 provides an en-hancement of wavelength resolution by producing a number of output pulses ~at correctly spaced intervals) for each output pulse from shaft encoder 210. This allows the use of an encoder for encoder 210 which produces significantly fewer out-put pulses than would otherwise be required for the desired one nanometer resolution. The output of phase-locked loop circuit 502 is thus a series of correctly timed commands for sampling the analog output of detectors 812 of detector stage 106.
~ . ` ` 1 3 1 050~
The three reference signals 212, 214, and 216 ,from shaft encoder 210 provide a frequency reference to phase locked-loop circuit 502. Two of the reerence signals, l (214~ and 2 (216~, which are produced at equal increments of scan, indicate the absolute angle of grating 116. The l (214) and 2 (216) signals are square waves, separated in phase by 90. The phase relationship of these two signals l (214~ and 2 ~216) is indicative of the direction of scan of grating 116.
The third reference signal, index pulse 218, i5 produced by shaft encoder 210 at the start of each scan. Index pulse 218 is used to initialize a position counter SD4 (8 bit~ by loading in the contents of a preset switch 506. In this manner, the lower limit ~i.e., the start) of the spectral scan i~ established.
The l ~214) and 2 (216) signals are fed to an up-down logic unit 508, which determines the direction of scan from the phase relationship of the pulses. Based on the direction of scan, up-down logic unit 508 either feeds ~P clock pulses on a line 526 or DOWN clock pulses on a line 528 to position counter 504. Up-down logic 508 has the special property of identifying the direction of rotation unambiguously to avoid count error.
An UP/DOWN signal 524 from up-down logic 508 is also forwarded to a utilization device such as a computer 614 ~Figure 6, not shown) f~r proper analysis of the data from detector(s) 712.
Position counter 504 addresses a read only memory (ROM) 510 (8 x 8 bits), which stores the adjustment information for cor-recting the non-linear relation between angular position 0 and wavelength . The data addressed in ROM 510 is fed to a binary rate multiplier 512, which functions as an 8 bit programmable divider, As grating 116 rotates across the spectral region 404, binary rate multiplier 512, in conjunction with data from ROM ~10~ slightly alters the frequency of the output on a line 530 of the phase-locked loop circuit to adjust for the non-linearity of grating 116. The resultant signal on a line 532 is then fed to a divide-by-M counter 514 to provide enhancement of resolution. In order to track the sinusoidal motion of grating 116, the frequency of the output from divide-by-M counter 514 on line 534 is compared with the frequency of the l (214~ sig-nal by a phase comparator 51S. The resultant output on line 536 is fed to a voltage-controlled oscillator (VCO) 518, whose out put on a line 530 is thereby forced to a value so that the fre-quency of the two signals ~l ~214) signal and the signal on line 534) compared by phase comparator 516 i9 e~ual. Since the output of VCO 518 is great~r in frequency than the sample rate that the system requires, the output signal on line 530 is fed to a divide by N counter 520 to produce the resultant sample signal 522.
As is discussed below, detector stage 106 includes detec-tors 712 which each produce an analog voltage indicative of the amount of light incident on its detector surface. A data acquisition system, designed generally by reference numeral 600, shown in Figure 6, produces a digital word representative of the detector 712 analog output each time a sample command on a line 522 is received from the circuit of Figure 5~
Data acquisition system 600, is comprised o~ a sample-and-hold circuit 604 and a 16-bit analog-to-digital ~A/D) converter 602. Sample and hold circuit 604 receives an adjusted detector _~o_ ~
output signal on line 816 ~from Figure 8) and the sample signal ¦ on line 522 (from Figure 5). Thus, sample-and-hold circuit 604 captures the analog output of detectors 812 on each sample com-l mand ~rom line 5~2. The held analog signal is then processed by 5¦ A/D convertor 602 to convert the held analog value to a digital binary word, for example, 16 bits in length. Sixteen bit reso-lution (one p~rt in 65,536) is employed in the preferred em-bodiment to provide a dynamic range of measurement of more than four orders of magnitude without resbrting to range switching, 10log amplifiers, or other means of adjusting the gain to suit the signal amplitude.
Data acquisition occurs only over spectral region 404. For each sample signal present on liDe 522, a 16-hit word is generated and sent to computer 614 via line drivers 615. A
15transmission of 1,200 readings, for example/ comprises one spectrum scan.
As discussed above, a light shutter 117 is attached to grating 116. As grating 116 oscillates, light shutter 117 enters the light beam and alternately blocks the light from 20reaching or leaving grating 116. During the resultant "dark~
periods, a signal is produced by the detector stage 106. This signal is used as the system reference to zero the detectors, and is further used to control the temperature stability of the detectors. Obviously, any suitable shutter could be applied in 25the system. For example, a filter wheel synchroniied with the oscillatory motion of grating 115 could be used to block the light onc er s=an.
1 13lo5o8 Just prior to the start of the scan of spectral region 404, when shutter 118 is blocking the light path, a single reading of dark offset is taken. The sample signal for this dark sample is derived fr~m the change in direction of motor 206, and is indicated by a signal on a line 606.
Da~k current compensation is accomplished by latching the dark offset reading in a 12-bit digital latch 608. Note that only the twelve least significant bits are necessary, since the d~rk offset value will always be much lower in amplitude than the spectral scan data. The twelve latchPd bits of dark refer-e~ce data are converted to an analog voltage by a 12-bit digi tal-to-analog (D/A) converter 610. This analog voltage offset value, present on a line 612r is applied to the circuit of Figure 8 to zero the measurements of the spectrum transmission i~ the subsequent scan. Additionally, this analog voltage i~
used to control thermal stabilization of detectors 812.
Referring again to Figure 1, exit stage 104 functions to focus the monochromatic light provided at the output of exit slit 120 onto the sample 126. The light exiting monochrometer 102 may be focussed directly onto sample 126 by spherical lens 122, or may optionally be directed to a fiber bundle 124, which carries the light to the location of sample 126.
Detector stage 106 functions to collect the li9ht passing through sample 126 in the transmissive mode, or reflected off sample 126 in the reflective mode. Detector stage 106, shown in greater detail in Figures 7A, 7B and 7C, comprises a solid detection head 702 and the sample 126 to be measured. Detec-tion head 7~02 can cvmprise any suitable structure for col-lecting light from sample 126. In a preferred embodiment, I ( ~31050~
detection head 702 includes a pair of angled mirrors 706 mounted on a removable triangular unit 708, an aperture 710, and four detectors 712, shown in greater detail in the front view of detector head 702 in Figure 7C.
As shown in Figure 7A, detection head 702 is configured for measurement in the transmissive mode. However, as illustrated in Figure 7B, detection head 702 may also be used in the reflective mode. For measurement of reflecti~n, detection head 702 is rotated 180, and sample 126 is placed on the opposite side of ~etection head 702. Mirror unit 708 is removed so that light passes through aperture 710 and is reflected off sample 126 onto detectors 712. The beam on sample 126 can have a line shape, which is primarily advantageous for samples in motion, or it can have a circular shape for stationary samples. Detec-tors 712 are selectad to match the spectral range being mea-sured, and may consist of, for example, silicon, lead sulfite or lead selenide detectors, or photomultiplier tubes.
~e~erring to Figure 7c, it should be noted that detectors 712 are positioned parallel to the long axis of the exit slit 120. This allows detectors 712 to be brought closer to the light beam axis, which results in collection of more refl~cted light with no interference of specular light. This arrangement is superior to the conventional approach where the detectors are e~ually spaced in a circular array about the light beam axis because it takes into account the variation in beam divergence in the horizontal and vertical directions.
Figure 8 illustrates the detector system electronics. In the preferred embodiment, detectors 712 are operated in the _ ~3 _ `` . 1 31 05~
photoconductive mode. Therefore, a bias is required. A current mirror bridge, designated generally by reference numeral 802, a modification of the Howland constant current-source, supplies the bias to detectors 712. Current mirror bridge 802 acts as a constant current sink, which draws an equal amount of current through the detectors 712 and a balance resistor 804.
Advantageously, the use of current mirror bridge B02 requires only a single power supply for ~iasing detector~s) 712: Note that in a cvnventional bridge configuration, a posi-tive and a negative power supply is required. Furthermore, since the balance arm of the current mirror bridge (i.e., balance resistor 804) is connected to the 5ame voltag~ source (designated 806) as detectors 712, any variation in v~ltage source 806 is cancelled out of the signal produced. Finally, the constant current sink of current mirror bridge 802 appears to detector 712 as a very low impedence. The signal applied to the non-invertin7 input of the operational amplifier of a stage 814 is collected as a voltage developed across resistor Rs or, alternatively, is provided by a current-to-voltage converter.
The detector system electronics also includes detector coolers 808 for maintaining detectors 712 at a constant tem-perature. Thermal stabilization is controlled by the thermal properties of detectors 712 themselves by applying the dark reference slgnal 612 ac~uired by thè data system (Figure 6) as an error signal to an amplifier 810 driving coolers 808~ A
reference signal 612 in the negative direction indicates a ~too warm~ condition, which causes ampllfier 810 to deliver a larger current to coolers 808, resulting in cooling o$ detectors 712.
., Il - 24 -) 1310508 Conversely, a p~sitive dark re~erence offset signal 612 results in less cooling, allowing detectors 712 to warm to the desired temperature.
Dark reference offset signal 612 is also inverted by inver-ter 812 and added to the output from detectors 712 via summing circuit 814 to form the adjusted detector output signal on line 816.
The present invention optionally can provide true double ~dual) beam operatio~. Referring to Figure 7B, an optical fiber optic 770 has a f irst end disposed in aperture 710 of detection head 702 t~ divert a small amount of light from the beam passing through aperture 710 at Dr near the sample 126.
This light is propagated by optic f~ber 770 to its second end, which i~ optically coupled to a double beam electronics stage 772, which converts the propagated light to an equivalent electronic signal used to pr~duce an ele~tronic detection signal. The electronic detection signal is effectively used to adjust the gain of the sample signal 552 so as to compensate for atmospheric bands, temporal variations, and system response. This provides a true double (dual) beam operation.
Alternately, optical fiber 770 ~an be moved to occupy the position of an optical fiber 770a, shown also in Figure 7~.
Optical fiber 770a is disposed so as to pass through the middle of sample 126, so that the first end of fiber 770a is at sample 126 for receiving the light beam passing through aperature 710 and hitting the sample 126. It should be understood that the ~irst end of fiber 770 or 7a can be moved to other positions , . " , 131050~
adjacent sample 126 which allow it to receive a portion of the light at or adjacent the sample 126.
Bacl~ground_o~ the Invention The present invention relai:es generally to methods f or spec t ra 1 I y meclsurillg arl(l ana Iy~: ing opti.ca I
properti.es of salllples. Sucl-l rnethods are presentlY used i n i n d u s t r .i a I a n d a g r i c u I t u r a L a p p 1. i c a t i o n L o r co:lori.rnetry and for qlJanti-tati.vely analyzing the const-ituents of samples. AdditionaI applicatiorls are being developed in the fi.eld of rnedi.c~ine in which samples are spectrally analyzed for diagnostic purpnses.
Examp.les of agricultural applications presently in use are implerrlellted by means of instrllmen~ts whi.ch accurately deterrnine the oll. proteill and water content irl grain or soybeans. The trad:itional analytical laboratory techniques~ such as the K jelda}ll method for measuring protein, are extremely accurate but requi.re the servi.ces of a ski lled chem:ist. The lesults, furtherrrlore, are not immediately or readi.ly availah]e.
Buyers of agricu:l.tural products have dernons-trated an increasing interest -in accIlrate and rapid determinatiorls of the moisture, protein and oil percerlt.lges ot the various produces purchased. 'I'lIe wlleat export market, for example, has seen the wi.despread ~introductioll of selling on the basis of guaranteed protein content.
This competitive pressure has increased -the requirement of the commodity handler, f rom the country eleva-tor to the export terminal, to sort rapidly and accurately grains and other products by their content, where applicable .
,~
131û50~
Additional agricultural and food applications include measurement of constituents in dairy products, cereal, beverages, fruits, meats, etc.
In the industrial market, this type of measurement is successfully applied to the following area:
¦ the textile industry for measuring lubrication on yarn and ¦ for fiber finish in nylon, polyester, cotton and others;
¦ tobacco industry for measuring the percent of tar and ¦ nicotine;
¦ paper industry for the chemical analysis of paper including ¦ coatings, thickness and moisture;
¦ plastic industry for measurment of tapes and film ¦ thickness;
¦ gasohol and petroleum industry for composition ¦ determination;
¦ cosmetics and perfume industry for measurement of oils and ¦ other ingredients.
¦ In the pharmaceutical industry, this instrument is applied ¦ for measurement and identification nondestructively of drug composition.
The need for versatile, yet low cost, advanced equipment, which combines and improves upon recent scientific findings in the field of nondestructive testing of products has greatly increased. For maximum usefulness of commodity handlers, such an instrument must not place high demands on the skillfulness of the operator or reguire a specialized knowledge of the scientific basis for the end result.
O
~ 131050~ 1 Rec~nl- developments h~ve provided me-thods and inst-~uments which are able to satisfy some of the above requirements of c~mmodity handlers. The optical analyzer disclosed by Isaac J. Landa in U.S. Patent No. 4,285,596 entitled "Holographic Diffraction Grating System for Rapid Scan Spectral Analysis" provides an optical system for rapid, accurate spectral analysis of the reflectivity'and/or transmissivity of samples. A concave holo-graphic diffraction grating oscillated at high speed is uti-lized to provide a rapid scanning of monochromatic liyht through a spectrum of wavelengths. The grating is positively driven at a very high speed (typically, ten scans per second) by a unique cam drive structure comprising identically shaped conjugate cams. The rapid scan by the grating enables reduc-tion of noise error by averaging over a large number of cycles.
The rapid scan also reduces measurement time, and thus prevents sample heating by excessive exposure of light energy. A filter wheel having dark segments for drift correction is rotated in the optical path in synchronism with the grating. Source optics is employed to shape optimally the light source for the particular application. The system optics further includes an arrangement of lenses, including cylindrical lenses, to obtain the best light source shape which results in maximum light throughput. Fiber optics are also employed and arranged to meet the optimum requirements of the,system for light collec-tion and transmission through portions of the optical system.
~ related instrument is disclosed by Isaac J. Landa in U.S.
Patent ~o. 4,264,205, entitled "Rapid Scan Spectral Analysis ~ 3 1 050~
System Utllizing Higher Order Spectra] Re~lections of IIologr.lpIlic Dift'ract~ion ~;ratings'~, wh~ch is related to the previously mentioned l,anda patent. The disclose(I
optica] systern is similar to that shown in the previoux patent application~ but includes a Eilter wheel divided inIrJ Iwo arcuate segments separated by opaque segmenIs arrange(I approxirrIatelv I~O apart. One arcuate segment of the wheeI trarIslr~ ,s onIy E:irst order ligllt. The other arcuate segment transmits only second order light.
Separate photodetectors are employed dur:irIg -inErared anal)~sis of samples for detecting first order and second wavelengt:h transmiss-iorIs, and an eIectronic deco-ler app.Iratlls is uiiIized t'or SW:it(`llillg l)etWr`ell (I(?t(`(`t.(lrS.
The analyzers disclosed in the two l,anda patents suffer from a number of disadvantages. First SllCh optical analyzers are limite(I in the accuracy of t:heit rneasurement by the particuIar Irive mecIlanisrrI empIoyed for oscillating the dift:rac-tion grating. SpeciEically.
the complex can, drive mechan:ism emplo~ed to provide a linear spectral scan is re]atively costly and inaccurate. 'I'he cam drive mechanisrn is needetI to control the var:iation in the ve'Iocity of thl` grat:i.rlg during each scan in order to obtain the desired linear spectral scan. The complex cam drive mechanism introduces error in the analysis because of the very tight tolerances required of the camming surfaces.
Another disadvantage is that the filter wheel employed for blocking the light to provide a dark offset value requires careful synchronization with the oscillating of the grating to !
~ 3 l ~51~;
ensure that the light is blocked al the appropriate time.
This increases Lhe likelihood of error as well as increasing the number of parts in the ar-aly~er.
Still another disadvantage o-f -the former methods of allalysis is thdt lhe trarlsmissivity arld retlec-tivity of sdmples is measured by different detec-tors. 'I'his approach introduces arl additiondl varidble into the analysis ot -the samples which necessarily compounds errors. 'I'he requirement of -two se-ts of de-tectors also increases cos-ts.
Summary of the Invention 'l'he present invention is a method for optically and analy~lng a sample comprising the steps of: a) providing rnonochromatic light at varying instantaneous waveleng-ths by: (i) repeatedly rotating d concave diffraction grating mounted to a drive shaft which is connected to a motor back and forth; (ii) illuminating the sarnple through a collima-ted optical path which includes said diffraction grating; (iii) repeatedly blocking said light pd-th said blocking step being synchroni~ed with reference to back EMF of said motor (iv) whereby Bragg reflection by said diffraction gra-ting along the axes of said collimated ligh-t path provides light which is substantlally monochromatic at an instan-taneous wavelength determined by the instantaneous position of said grating; and b) sensing optical characteris-tics of the sample at various wavelengths provided by said monochromating step a).
In a narrower aspect, s-tep (a)(iii), above, comprises the substeps ol: (d ) sensing the operation of said torque motor -to provide a back EMF signal, and (b) blocking said optical train in accordance wi.th said back EMF signal.
More narrowly still, the s--ep of sensing optical characteristics ol' the sample ot various wavelengths comprises -the substeps of: (i) sensing the operatiun of said torque motor to provide a back EMF signal, (ii) synchroni~ing said sensillg step in accordance with said back EME` signal, and (iii) detecting properties of the sample at plural instantaneous wavelengths determined in accordance wlth said synchroni~ing substep (ii).
The present invention electronically provides : 15 sample commands at precisely deLerrnined intervals in order to cor:relate correctly -the spectral data with -the output of the monochronometer. This is carried out by a phase-locked loop circuit:which receives pulses from an lnexpensive shaft encoder. I'he phase-locked loop t.racks the sinusoidal motion of the grating, corrects for the non-uniform relationship of the g:ra-ting equa-tion of wavelength to inciden-t angle, and provides resolution enhancement of the shaf-t encoder.
Finally, -the present invention provides a method which allows the same detectors to be used for measure-~ 3t ~Q8 ment o~ the transrni.ssivitY or reflectivity o~ samp:les with only mino~ modification. The detectors are advant.lgeollslY placed in a specially desi.gned detector head which irlcludes a removable rtlirror asseml)ly. in the transmiss-ion mode the sample is placed before the detector head. and the detector head :is directed toward tlle inc:idelltal light. I.ight p.lssirlg thrtJIlgh the sample is reflected off the mirror assembly in -the detector head and into the detectors. In the reflectallce mode the mirror assembly is removed. the detector hea(l is rotated 180 so that the detectors ~ace away fror,l the light source. The entire detector head is placed before the sarnple. Light from the monochronometer passes through an aperture in the detector head (previ.ously occupied by the rnirror assembly) and is reflected off the sample and back to the detectors.
The method may be irnplemente(l with a double beam instrument as fol]ows. An optical fiber d:iverts light from the beam at or near the samp:le. This light is electronically detected and the electronic detection signal adjusts the gain of the samp:le s:ignal so as to compensate for atmospheric bands temporal variationsl and system response. This provides a true double (dual) beam operation.
l3~0sa~
BRI~F` D~RIPTION OF THE DRAWIN~S
Further objects and advantages of the present i.rlventioll will become apparent by referellce to the following detailed descripti.on ol' the aT)paratus for carrying out the consi.(lered in CC111 j Ull(:tiOll w,ith the accompanying drawings. in wllich:
Fi.~ure l -is a perspecti.ve v:iew o~ the opt:i.cal cornponents and layout o~ the apparatus for implementing the present invention;
F'igure 2, comprisin~ Fi~ures 2A and 2B, i,llustrates the grating drive system o~ the preserlt invention;
Figure 3 is a ci,rcuit di.agram sh o w :i ng the electronics utilized to drive the motor of the gra~ting drive systern;
Figure 4. comprisillg Figures 4A and 4B, is a representation or the scan angle oE the grating and the events that occur as the grating is oscillated;
Figure 5 is a block dia~ranl nf the phase-locked loop circu:it for providing sàmple pulses ~to the detector electronics ot` the apparatus;
~. 131U5~
Figure 6 is a circuit diagram of the data acquisition elec--tronic~ of the apparatus;
Figure 7, comprising Figures 7A, 7B, and 7C, is an illus-l lrat:ion oE the detector stage of the apparatus; and S Figure 8 is a circuit diagram of the detector electronics oE the appara-tus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Briefly, and referring first to Figure 1, the present method ls implemented using an electro-optlcal sys tem and pro-vides a rapid, accurate spectral analysis of the reflectivity and/or transmissivity of samples 126. A light dlspersing ele-ment 116, preferably a concave~ holographic diffraction grat-ing, is sinusoidally osclllated at a preselected high speed to provlde a rapid scanni4g of monochromatic light through a selected spectrum of wavelengths. The present invention in-cludes a simple and elegant drive mechanism 200 (See Figures 2A
and 2B) for sinusoidally oscillating grating 116. The sinusoi-dal oscillation of grating 116 produces a scan of monochromatic light which is expanded at both ends and compressed in the mid-dle of the frequency spectrum with respect to time. The grat-ing 116 also exhibits a non-linearity in the monochromatic light output, since the grating equation of monochromatic wave-length ~ to incident angle of light focused on grating 116 is non-uniform. This non-linearity results in the spectrum creat-ed during the scan being compressed at one end and expanded at the other by a relatively small amount as compared to the ~-t~8 expansion and compression produced by the sinsusoidal oscillation. The present invention electronically compensates for both of these non-linearities hy prnvid-ing sarnple comrnallds of the detector(si anillng Olltp~lt(S) at preciselv dete r min e d intervals o~ time.
rhiS allows the present invention to produce digital output data correlated to the spectral scan o~
rnonochromatic light. The pres~ent invention -~urther provides an oftset signal produced by means of a pass-ive optical shutter. This offset signal is used (I) to provide an offset value for de-tectors 712 (see F'igure 8) ; and (2) to control -temperature compensation of detectors712 to insure that they operate in a desired temperature range. The present invention also can include a single detector assembly 106 which allows measurement of reflectivity and transmissivity without a change of detector(s) ~12.
A representative illustration of the optical aspects of the method of the present invention is shown in F'igure 1, Broadly, the oE)t:ical aspects o~ the present invention cornprise a source optics stage 100, a monochrometer stage 102, an exit optics stage 104, and a detector stage 106. Light passing through the system generally follows the path indicated by the dot, dash line in F'igure 1.
Source optics stage 100 provides a properly focussed broadband beam Oe light to monochrometer stage 102. A broadband light source 108 emits light energy used by the present invention. Any type of suitable light source may be used which emits broadband light in the desired spectral range, which range is over the wavelength region that corresponds to ] () 1~ 131050~ ~
¦ ultraviolet (UV), through visible, through near infrared (NIR) ¦ and up to infrared (IR~. In the preferred embodiment, a ¦ tungsten halogen lamp for source 108 is used for measurement in ¦ the near infrared range. On the other hand, for measurement in ¦ the ultraviolet range, a xenon light source might be used, for ¦ example. In the preferred embodiment, an indicator lamp tnot shown) is lit as an indication that light source 108 is functioning properly.
l A spherical lens 110 collects light emitted from light ¦ source 108 and images such light on an entrance slit 112 of monochrometer stage 102. A field lens 114, adjacent to en-trance slit 112, focusses light incident from lens 110 to assure proper filling of dispersing element 116.
l Optionally, a stationary filter 118 is located adjacent to 15 ¦ field lens 114 to eliminate undesired wavelength orders from light source 108. Obviously, stationary filter 118 is not necessary with a light source 108 which emits light only in the desired spectral range, or with a detector 112 which detects light energy only in the desired spectral range. Stationary filter 118 may consist of a high pass, a low pass, or a band pass filter as required by the particular application. More-over, the exact location of filter 11~ is not critical. For instance, filter 118 could be located in exit optics stage 104.
An optional ellipsoidal mirror segment 120 is located behind light source 108 and serves to maximize light throughput by redirecting light into entrance slit 112. Mirror segment 120 is positioned with respect to the filament of disclosed light source 108 so as to double the amount of energy entering the monochrometer stage 102.
' -11- ' The monochrometer stage 102 is now described wi-th reference to Figure 1. Monochrometer stage 102 functlo11s to separate the broadband light provided by llght source 108 into its spectral components.
Monochrorneter stage 102 is comprised of entrance slit 112, dispersing ele1nent ]16, a passive optica] shul:ter 117, and an exit slit 120.
Dispersing element 116 is filled with the light entering monochrometer stage 102 throllgh entrance slit 112. Dispersing element 11~ separates incident light into its spectral components. Therefore, when dispersing elemer1t 116 is sent into oscillatory motion, a spectral scan of monnchroïnatic 1ight is pro(luced at exit slit 120.
lS Dispersing e1ernen~ 116 is a concave holo~raph:ic diffraction grating. In a co11cave holographic grating, the lines oE the grating are formed by a holographic technique. Concave holograph:ic gratings are presently available on the market and are marketed by Instruments, S.A. of Metuchen, New .Jersey. The use of a concave gra-ting is preferable to other light dispersing elements such as plane grating systems becallse it involvex the use of a single optical component as opposed to three or more optical components required in plane 8rating systems. As a result, the alignment in calibration procedures are simplified and the cost of precision mechanical mounts are reduced. The use of a concave holographic grating makes possible the design of a system with low ~ numbers, for example as low as F/l in some cases. High performance plane grating systems are usually limited F'/4 and higher. The lower optical F
numbers makes it possible to . f~Q5~
pass a greater amount of light energy through the optical system thereby making it possible for the present invention to analyze darker samples than possible by conventional optical analyzers. In addition, holographic gratings are free of ghosts and have a lower stray light level in comparison to rule gratings. Another advantage of the holographic grating used in the system of the present invention is that it can be made with a very high groove density which enables high resolution while maintaining high light energy throughput. Yet another advantage of the holographic grating is that it is corrected for astigmatic aberration and spherical and coma aberration are also reduced.
It should be understood, however, that any suitable optical dispersing element, such as a plane grating or a prism (not shown), could be used for dispersing element 116 in the present invention.
~ight shutter 117 is provided in order to obtain an offset signal for zeroing the output of the detectors 712. In the preferred embodiment, light shutter 117 consists of a passive synchronous light chopper constructed of a light aluminum foil, and attached ts concave holographic grating 116 by four rigid wires, designated by reference number 122. It should be understood that light shutter 117 can be constructed of any suitable material and can employ any suitable mounting structure.
The width of entrance slit 112 and the width ~f exit slit lZ0 are adjustable to enable selection of an optimal spectral bandwidth for the particular application. The entrance and I ` ( 1 3 1 050~
exit slit planes are also adjustable to permit the use of a wide range of dispersing components.
Holographic diffraction grating 116 is made to oscillate at a very high speed by means of a grating drive system 200. As S shown in Figure 2A, grating drive system 200 is an electrically driven mechanical oscillator comprising a shaft 202 (having an upper end 202a and a lower end 202b), a precision bearing assembly motor mount 204, a 2-pole brushless motor 206~ and a flat spiral return force spring 208 (having an inner end 208a and an outer end 208b)o As shown in Figure 2B, the lower end of 202b of shaft 202 is secured to the inner end 208a of spring 208. The outer end 208b of spring 208 is securely fastened to a stationary piece 203 fixedly attached to mount 204. It should be understood that the configuration of spring 208 can vary and can include configurations such as the spiral spring, the axial helix spring, the torsion bar, the single-leaf spring, as well as non-linear sprin~s or multiple sprin~
arrangements.
Referring again to Figure 2A, grating 116 is mounted on the upper end 202a of shaft 202. The mass of grating 116, and the force constant of spring 208, together produce a natural har-monic oscillator having a sinusoidal motion. Once set into motion, the drive system 200 would theoretically oscillate for-ever, but for the friction of the bearings in motor mount 204.
A small drive voltage, described below with reference to Figure 3A, is applied to motor ~06 to produce controlled torque which ac~s to compensate for the friction in the oscillator system 200. Since oscillator system 200 requires no direct mechanical . ~ ~310S0~
drive, it has an intrinsically smooth performance with very little vibration, making it ideal for use as a precision scan-ner. Additionally, the simplicity of drive system 200, and its relatively small number of parts, makes it highly reliable.
The sinusoidal motion of grating drive system 200 produces a corresponding sinusoidal spectral scan of monochromatic light with respect to time. Advantageously, the system electronics shown in Figure 5 (described below) accurately compensate for this non-linearity.. In conventional systems, such system elec-tronics is not present, necessitating a grating drive system 200 which produces a linear scan. Such a drive system, as described in U.S. Patent No. 4,285,596 to Landa requires a com-plex cam drive mechanism, which is costly and inaccurate rela-tive to the simple and reliable drive system 200 of the present invention.
The oscillating system 200 of Figure 2A and 2B further in-cludes a shaft encoder 210. Shaft encoder 210 provides three outputs: l and 2 (designated by reference numerals 212 and 214, respectively), and an index pulse 216. These signals are used by the system electronics of Figure 5 for producing an output to sample the detectors 312 at the appropr.iate intervals of time corresponding to discreet wavelength increments of monochromatic light, as will be described in greater detail below.
Referring now to Figure 3, the electronics to drive torque motor 206 is a feedback circuit which supplies just enough energy in the form of a drive signal 312 to maintain oscilla-tion of grating 116 at the desired amplitude (i.e., a scan -15- ~
`. , ~310508 angle 4~2~. As is well known, electric motors produce a back EMF ~electromagnetic flux) which is proportional to the rota-tional velocity of the motor.
The circuit of Figure 3 makes use of a back EMF signal 312 generated by torque motor 206. Note that the back EMF is mask-ed by the drive voltage 312, which has a much greater ampli-tude. The circuit uses a balanced bridge arrangement to extract the back EMF signal 310. In the circuit, a metering , resistor 302 is connected in series with the motor coil 304. A
¦ p~tentiometer 306 is adjusted so that the drive voltage on line 314 is bal~nced out and only the pure sinusoidal back EMF sig-nal 310 appears across the inputs ~16a and 316b of an opera-tional amplifier 308.
~oth amplitude Ithe scan angle 402) and frequency ~the num-ber of scans in a defined time period~ of oscillating system 200 are easily and precisely adjustedO The amplitude is con-trolled by electronically limiting the current level of drive signal 314 by employing a drive amplifier 310 in the feedback loop of the motor control electronics of Figure 3.
Referring now to Figure 4A, in the preferred embodiment, the amplitude of oscillation of grating 116 is set to limit scan angle 402 to approximately 42 . As shown in Figure 4B, the central 25 portion ~designated by reference number 404) covers the spectral range of interest, leaving approximately 7 1/2 overrun (reference numbers 406a and 406b) at either end.
The overrun regions 406a and 406b are advantageously used for storing a dark offset value described below. It should be understood that the minimum scan angle 402 is determined by the angle needed for complete blockage by the light shutter 117.
. ~ osn~
The frequency of the oscillation of grating 116 can be varied by adjusting the force of spring 208 in Figure 2. The freguency of oscillation preferably is in the range of 5 to 500 l cycles per minute, which is 10 scans to 1000 scans per minute.
¦ A preferred frequency is 300 cycles per minute, or 10 scans per second. Because the holographic grating 116 is oscillated at a very high speed, the present invention eliminates noise by electronically averaging output data over a lar~e number of scans. In the preferred embodiment, a lamp ~not shown) is pro-vided which flashes once per scan as an indication that oscil-lating system 200 is functioning properly.
In order to measure spectral reflectivity or transmissivity in discrete wavelength increments, the output of dPtectorls) 712 must be sampled at precisely determined intervals. This is necessary to correlate correctly the spectral data with the output of monochrometer 1020 The absolute position of grating 116 must be known throughout the scan in order to produce the desired correlation. Moreover, repeatability of angular posi-tion is critical. In order to obtain one nanometer tlO 9 meter) wavelen~th resolution of output light for the concave holographic diffraction grating of the preferred embodiment, the 25 spectral region 404 must be divided into 1200 sample points. In other words, for the particular grating 116 employed in the present invention, the output of detector(s~
812 must be sampled 1200 times per scan to achieve an output of one data measurement for each wavelength increment of one nanometer. This is the equivalent of 48 sample points per degree of scan of spectral region 404.
.~ ~ ~ 131D~
Conven~ional shaft encoders can provide the 48 sample p~ints per degree resolution, but are quite expensive and pr~-duce outputs only at uniform increments of incide~t angle.
Further, due to the non-u~iform relationship of the yrating e~uati~n of wavelength A to incident angle ~, the spectrum created during the scan is compressed ~t one end and expanded at the other. Therefore, the circuit of Figure 5 i~ used to enable spectra of a unifor~ wavelenyth resolution to be pro-duced, without the need of a~ éxpensive shaft encoder.
Referring to Figure S D a phase-locked loop circuit 502 is shown which provides three functions. Firstly, phase-locked loop circuit 502 tracks the sinusoidal motion of oscillating grating 116 to provide detector sample commands at the correct intervals of time (note that since the motion of grating 116 i5 constantly changing, these time intervals are constantly changing~0 Second, phase-locked 1QP circuit 502 adjusts these intervals by a small amount to compensate for the slight non-linearity of the grating 116 output to incident angle of light input. Third, phase-locked loop circuit 502 provides an en-hancement of wavelength resolution by producing a number of output pulses ~at correctly spaced intervals) for each output pulse from shaft encoder 210. This allows the use of an encoder for encoder 210 which produces significantly fewer out-put pulses than would otherwise be required for the desired one nanometer resolution. The output of phase-locked loop circuit 502 is thus a series of correctly timed commands for sampling the analog output of detectors 812 of detector stage 106.
~ . ` ` 1 3 1 050~
The three reference signals 212, 214, and 216 ,from shaft encoder 210 provide a frequency reference to phase locked-loop circuit 502. Two of the reerence signals, l (214~ and 2 (216~, which are produced at equal increments of scan, indicate the absolute angle of grating 116. The l (214) and 2 (216) signals are square waves, separated in phase by 90. The phase relationship of these two signals l (214~ and 2 ~216) is indicative of the direction of scan of grating 116.
The third reference signal, index pulse 218, i5 produced by shaft encoder 210 at the start of each scan. Index pulse 218 is used to initialize a position counter SD4 (8 bit~ by loading in the contents of a preset switch 506. In this manner, the lower limit ~i.e., the start) of the spectral scan i~ established.
The l ~214) and 2 (216) signals are fed to an up-down logic unit 508, which determines the direction of scan from the phase relationship of the pulses. Based on the direction of scan, up-down logic unit 508 either feeds ~P clock pulses on a line 526 or DOWN clock pulses on a line 528 to position counter 504. Up-down logic 508 has the special property of identifying the direction of rotation unambiguously to avoid count error.
An UP/DOWN signal 524 from up-down logic 508 is also forwarded to a utilization device such as a computer 614 ~Figure 6, not shown) f~r proper analysis of the data from detector(s) 712.
Position counter 504 addresses a read only memory (ROM) 510 (8 x 8 bits), which stores the adjustment information for cor-recting the non-linear relation between angular position 0 and wavelength . The data addressed in ROM 510 is fed to a binary rate multiplier 512, which functions as an 8 bit programmable divider, As grating 116 rotates across the spectral region 404, binary rate multiplier 512, in conjunction with data from ROM ~10~ slightly alters the frequency of the output on a line 530 of the phase-locked loop circuit to adjust for the non-linearity of grating 116. The resultant signal on a line 532 is then fed to a divide-by-M counter 514 to provide enhancement of resolution. In order to track the sinusoidal motion of grating 116, the frequency of the output from divide-by-M counter 514 on line 534 is compared with the frequency of the l (214~ sig-nal by a phase comparator 51S. The resultant output on line 536 is fed to a voltage-controlled oscillator (VCO) 518, whose out put on a line 530 is thereby forced to a value so that the fre-quency of the two signals ~l ~214) signal and the signal on line 534) compared by phase comparator 516 i9 e~ual. Since the output of VCO 518 is great~r in frequency than the sample rate that the system requires, the output signal on line 530 is fed to a divide by N counter 520 to produce the resultant sample signal 522.
As is discussed below, detector stage 106 includes detec-tors 712 which each produce an analog voltage indicative of the amount of light incident on its detector surface. A data acquisition system, designed generally by reference numeral 600, shown in Figure 6, produces a digital word representative of the detector 712 analog output each time a sample command on a line 522 is received from the circuit of Figure 5~
Data acquisition system 600, is comprised o~ a sample-and-hold circuit 604 and a 16-bit analog-to-digital ~A/D) converter 602. Sample and hold circuit 604 receives an adjusted detector _~o_ ~
output signal on line 816 ~from Figure 8) and the sample signal ¦ on line 522 (from Figure 5). Thus, sample-and-hold circuit 604 captures the analog output of detectors 812 on each sample com-l mand ~rom line 5~2. The held analog signal is then processed by 5¦ A/D convertor 602 to convert the held analog value to a digital binary word, for example, 16 bits in length. Sixteen bit reso-lution (one p~rt in 65,536) is employed in the preferred em-bodiment to provide a dynamic range of measurement of more than four orders of magnitude without resbrting to range switching, 10log amplifiers, or other means of adjusting the gain to suit the signal amplitude.
Data acquisition occurs only over spectral region 404. For each sample signal present on liDe 522, a 16-hit word is generated and sent to computer 614 via line drivers 615. A
15transmission of 1,200 readings, for example/ comprises one spectrum scan.
As discussed above, a light shutter 117 is attached to grating 116. As grating 116 oscillates, light shutter 117 enters the light beam and alternately blocks the light from 20reaching or leaving grating 116. During the resultant "dark~
periods, a signal is produced by the detector stage 106. This signal is used as the system reference to zero the detectors, and is further used to control the temperature stability of the detectors. Obviously, any suitable shutter could be applied in 25the system. For example, a filter wheel synchroniied with the oscillatory motion of grating 115 could be used to block the light onc er s=an.
1 13lo5o8 Just prior to the start of the scan of spectral region 404, when shutter 118 is blocking the light path, a single reading of dark offset is taken. The sample signal for this dark sample is derived fr~m the change in direction of motor 206, and is indicated by a signal on a line 606.
Da~k current compensation is accomplished by latching the dark offset reading in a 12-bit digital latch 608. Note that only the twelve least significant bits are necessary, since the d~rk offset value will always be much lower in amplitude than the spectral scan data. The twelve latchPd bits of dark refer-e~ce data are converted to an analog voltage by a 12-bit digi tal-to-analog (D/A) converter 610. This analog voltage offset value, present on a line 612r is applied to the circuit of Figure 8 to zero the measurements of the spectrum transmission i~ the subsequent scan. Additionally, this analog voltage i~
used to control thermal stabilization of detectors 812.
Referring again to Figure 1, exit stage 104 functions to focus the monochromatic light provided at the output of exit slit 120 onto the sample 126. The light exiting monochrometer 102 may be focussed directly onto sample 126 by spherical lens 122, or may optionally be directed to a fiber bundle 124, which carries the light to the location of sample 126.
Detector stage 106 functions to collect the li9ht passing through sample 126 in the transmissive mode, or reflected off sample 126 in the reflective mode. Detector stage 106, shown in greater detail in Figures 7A, 7B and 7C, comprises a solid detection head 702 and the sample 126 to be measured. Detec-tion head 7~02 can cvmprise any suitable structure for col-lecting light from sample 126. In a preferred embodiment, I ( ~31050~
detection head 702 includes a pair of angled mirrors 706 mounted on a removable triangular unit 708, an aperture 710, and four detectors 712, shown in greater detail in the front view of detector head 702 in Figure 7C.
As shown in Figure 7A, detection head 702 is configured for measurement in the transmissive mode. However, as illustrated in Figure 7B, detection head 702 may also be used in the reflective mode. For measurement of reflecti~n, detection head 702 is rotated 180, and sample 126 is placed on the opposite side of ~etection head 702. Mirror unit 708 is removed so that light passes through aperture 710 and is reflected off sample 126 onto detectors 712. The beam on sample 126 can have a line shape, which is primarily advantageous for samples in motion, or it can have a circular shape for stationary samples. Detec-tors 712 are selectad to match the spectral range being mea-sured, and may consist of, for example, silicon, lead sulfite or lead selenide detectors, or photomultiplier tubes.
~e~erring to Figure 7c, it should be noted that detectors 712 are positioned parallel to the long axis of the exit slit 120. This allows detectors 712 to be brought closer to the light beam axis, which results in collection of more refl~cted light with no interference of specular light. This arrangement is superior to the conventional approach where the detectors are e~ually spaced in a circular array about the light beam axis because it takes into account the variation in beam divergence in the horizontal and vertical directions.
Figure 8 illustrates the detector system electronics. In the preferred embodiment, detectors 712 are operated in the _ ~3 _ `` . 1 31 05~
photoconductive mode. Therefore, a bias is required. A current mirror bridge, designated generally by reference numeral 802, a modification of the Howland constant current-source, supplies the bias to detectors 712. Current mirror bridge 802 acts as a constant current sink, which draws an equal amount of current through the detectors 712 and a balance resistor 804.
Advantageously, the use of current mirror bridge B02 requires only a single power supply for ~iasing detector~s) 712: Note that in a cvnventional bridge configuration, a posi-tive and a negative power supply is required. Furthermore, since the balance arm of the current mirror bridge (i.e., balance resistor 804) is connected to the 5ame voltag~ source (designated 806) as detectors 712, any variation in v~ltage source 806 is cancelled out of the signal produced. Finally, the constant current sink of current mirror bridge 802 appears to detector 712 as a very low impedence. The signal applied to the non-invertin7 input of the operational amplifier of a stage 814 is collected as a voltage developed across resistor Rs or, alternatively, is provided by a current-to-voltage converter.
The detector system electronics also includes detector coolers 808 for maintaining detectors 712 at a constant tem-perature. Thermal stabilization is controlled by the thermal properties of detectors 712 themselves by applying the dark reference slgnal 612 ac~uired by thè data system (Figure 6) as an error signal to an amplifier 810 driving coolers 808~ A
reference signal 612 in the negative direction indicates a ~too warm~ condition, which causes ampllfier 810 to deliver a larger current to coolers 808, resulting in cooling o$ detectors 712.
., Il - 24 -) 1310508 Conversely, a p~sitive dark re~erence offset signal 612 results in less cooling, allowing detectors 712 to warm to the desired temperature.
Dark reference offset signal 612 is also inverted by inver-ter 812 and added to the output from detectors 712 via summing circuit 814 to form the adjusted detector output signal on line 816.
The present invention optionally can provide true double ~dual) beam operatio~. Referring to Figure 7B, an optical fiber optic 770 has a f irst end disposed in aperture 710 of detection head 702 t~ divert a small amount of light from the beam passing through aperture 710 at Dr near the sample 126.
This light is propagated by optic f~ber 770 to its second end, which i~ optically coupled to a double beam electronics stage 772, which converts the propagated light to an equivalent electronic signal used to pr~duce an ele~tronic detection signal. The electronic detection signal is effectively used to adjust the gain of the sample signal 552 so as to compensate for atmospheric bands, temporal variations, and system response. This provides a true double (dual) beam operation.
Alternately, optical fiber 770 ~an be moved to occupy the position of an optical fiber 770a, shown also in Figure 7~.
Optical fiber 770a is disposed so as to pass through the middle of sample 126, so that the first end of fiber 770a is at sample 126 for receiving the light beam passing through aperature 710 and hitting the sample 126. It should be understood that the ~irst end of fiber 770 or 7a can be moved to other positions , . " , 131050~
adjacent sample 126 which allow it to receive a portion of the light at or adjacent the sample 126.
Claims (14)
1.A method for optically analyzing a sample, comprising the steps of:
a) providing monochromatic light, at varying instantaneous wavelengths, by:
(i) repeatedly rotating a concave diffraction grating, mounted to a drive shaft which is connected to a motor, back and forth;
(ii) illuminating the sample, through a collimated optical path which includes said diffraction grating;
(iii) repeatedly blocking said light path, said blocking step being synchronized with reference to back EMF of said motor;
(iv) whereiby Bragg reflection by said diffraction grating along the axes of said collimated light path provides light which is substantially monochromatic, at an instantaneous wavelength determined by the instantaneous position of said grating; and (b) sensing optical charcteristics of the sample at various wavelengths provided by said monochromating step a).
a) providing monochromatic light, at varying instantaneous wavelengths, by:
(i) repeatedly rotating a concave diffraction grating, mounted to a drive shaft which is connected to a motor, back and forth;
(ii) illuminating the sample, through a collimated optical path which includes said diffraction grating;
(iii) repeatedly blocking said light path, said blocking step being synchronized with reference to back EMF of said motor;
(iv) whereiby Bragg reflection by said diffraction grating along the axes of said collimated light path provides light which is substantially monochromatic, at an instantaneous wavelength determined by the instantaneous position of said grating; and (b) sensing optical charcteristics of the sample at various wavelengths provided by said monochromating step a).
2. The method of claim 1, wherein said optical path comprises a passive optical filter.
3. The method of claim 1, wherein said blocking step a) (iii) comprises the substeps of (a) sensing the operation of said torque motor to provide a back EMF signal, and (b) blocking said optical train in accordance with said back EMF signal.
4. The method of Claim 3 wherein said blocking step a) (iii) comprises opening and closing a passive shutter.
5. The method of Claim 3, wherein said blocking step synchronized using a phase-locked loop.
6. The method of Claim 3, wherein said sensing step b) comprises the substeps of (i) sensing the operation of said torque motor to provide a back EMF signal, and (ii) synchronizing said sensing step in accordance with said back EMF signal.
7. The method of Claim 3, wherein said sensing step b) comprises the substeps of (iii) detecting properties of the sample at plural instantaneous wavelengths determined in accordance with said synchronizing substep (ii).
8. The method of Claim 1, wherein said sensing step b) comprises the substeps of (i) sensing the operation of said torque motor to provide a back EMF signal, and (ii) synchronizing said sensing step in accordance with said back EMF signal.
9. The method of Claim 8, wherein said sensing step is synchronized using a phase-locked loop.
10. The method of Claim 8, wherein said sensing step b) comprises the substeps of (i) synchronizing said sensing step with said blocking step, and (ii) differentially detecting wavelength-dependent properties of the sample by comparing optical measurements of said sample at instants when said optical train is blocked with optical measurements of said sample at other contemporaneous instants when said optical train is not blocked.
11. The method of Claim 2, wherein said sensing step b) comprises the substeps of (i) sensing the operation of said torque motor to provide a back EMF signal, (ii) synchronizing said sensing step in accordance with said back EMF signal, and (iii) detecting properties of the sample at plural instantaneous wavelengths determined in accordance with said synchronizing substep (ii).
12. The method of Claim 8, wherein said sensing step b) comprises the substeps of (i) sensing the operation of said torque motor to provide a back EMF signal, (ii) synchronizing said sensing step in accordance with said back EMF signal, and (iii) detecting properties of the sample at plural instantaneous wavelengths determined in accordance with said synchronizing substep (ii).
13. The method of Claim 1, wherein said blocking step a) (iii) comprises opening and closing a passive shutter.
14. The method of Claim 1, wherein said blocking step is synchronized using a phase-locked loop.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000546631A CA1310508C (en) | 1987-09-10 | 1987-09-10 | Opto-electronic method for spectral analysis |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000546631A CA1310508C (en) | 1987-09-10 | 1987-09-10 | Opto-electronic method for spectral analysis |
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| Publication Number | Publication Date |
|---|---|
| CA1310508C true CA1310508C (en) | 1992-11-24 |
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| Application Number | Title | Priority Date | Filing Date |
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| CA000546631A Expired - Lifetime CA1310508C (en) | 1987-09-10 | 1987-09-10 | Opto-electronic method for spectral analysis |
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| CA (1) | CA1310508C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116222984A (en) * | 2023-05-09 | 2023-06-06 | 上海隐冠半导体技术有限公司 | Grating ruler reflectivity measuring device |
-
1987
- 1987-09-10 CA CA000546631A patent/CA1310508C/en not_active Expired - Lifetime
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116222984A (en) * | 2023-05-09 | 2023-06-06 | 上海隐冠半导体技术有限公司 | Grating ruler reflectivity measuring device |
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