JPS6199825A - Fourier transform multiwavelength photometer - Google Patents

Abstract

PURPOSE:To obtain a final result in a short time by employing discrete Fourier transform and performing integration at every plural spectral points every time data on one point in an interference pattern is obtained. CONSTITUTION:The interference pattern 15 measured by an interferometer 10 is inputted to an A/D converter 20 in time series to obtain a digital value 42, which is inputted to a digital multiplier 40. The value of specific wavelength lambda1 stored in a wavelength storage device 60, on the other hand, is stored in a memory 61 and inputted to a cosine function generator 30, which inputs a phase corresponding to the optical path difference (x) of the interference pattern f(x) and the cosine function value 43 of frequency in inverse proportion to the wavelength lambda1 to a multiplier 40. The multiplier 40 multiplies the digital value 42 and function value 43 to obtain the product 44. A memory 51, on the other hand, is stored with spectral intensity S(lambda1) corresponding to the wavelength lambda1 and this value and product 44 are added together and stored in the memory 51 again. Thus, real-time processing is performed to obtain the final result in a short time.

Description

[Detailed Description of the Invention] [Quality of the Invention] Compared to a dispersive spectrophotometer, a Fourier transform spectrophotometer has the following advantages:
It is well known that since it is a simultaneous spectroscopy method that does not use a slit optical system, it has a high utilization rate of light energy and is advantageous for weak light analysis and high-speed spectroscopy. On the other hand, the Fourier transform spectroscopy method calculates a useful spectrum for analysis by Fourier transforming the interference pattern obtained from the interference means.
T) requires a very long calculation time and has traditionally been replaced by Fast Fourier Transform (FFT).

Now, if both the number of interferogram data points and the number of spectrum points are N, the number of multiplications in DFT is N2 times, and in FFT, even considering complex number operations, it is 2 N ], og2N times, and the effect is larger with N. It was a natural tendency for FFT to be generally used because the time required for multiplication occupies most of the total calculation time.

However, for quantitative measurements using spectrometry, it is sufficient to obtain the absorbance (logarithm of transmittance) or the relative value of emission intensity at a specific wavelength; for some types of qualitative analysis measurements,
Data for all spectral points within the spectroscopic range is not required. In particular, it is known that in quantitative analysis, it is generally sufficient to have two points of spectral data for one analysis item, and even for a mixed sample, five points are sufficient.

Now, assuming that the number of analysis items is 10, the number of measured spectrum points is 20, and the number of data points of the input interferogram is 4096 points, the number of multiplications in DFT is T1 = 4096 x 2
0=8 =2-4096-12=9.6 XIO' (
2), which is slightly larger than DFT.

[Purpose of the invention]

An object of the present invention is to provide a Fourier transform multi-wavelength photometer that eliminates the waste of the prior art and is equipped with specific wavelength Fourier transform means particularly suitable for quantitative analysis.

[Summary of the invention]

The present invention is characterized by employing a discrete Fourier transform (DFT) to calculate a specific spectral intensity in decimal numbers, and performing integration for each of multiple spectral points each time data for one point of an interferogram is obtained. This means performing so-called real-time processing.

The second feature of the present invention is that either a mechanical scanning method or an electronic scanning method can be used as a means for measuring the interferogram.
This means that there is no essential difference between them.

[Embodiments of the invention]

This will be explained below according to the drawings.

FIG. 1 is a basic configuration diagram of the present invention, in which an interferogram 15 measured by an interferometer 10 is transmitted to an A-n converter 20 in time series.
The signal is sequentially inputted into a digital quantity 42 and inputted into a digital multiplier 40. On the other hand, wavelength memory! The value of the specific wavelength λ1 stored in the polygon is stored in the memory 61, inputted to the cosine function generator 30, and the value of the interferogram f (x
) and a cosine function value 43 of a frequency inversely proportional to the wavelength λ are input to the multiplier 40 and multiplied by the interferogram data 42, resulting in a product 44.

On the other hand, the memory 51 stores the spectral intensity S for the wavelength λ.
(λ,), the already stored value and the product 44 are added and stored in the X memory 51 again. In other words, the calculation performed here is: ω, = 2πΔX/λ1 (3) S, (λt) = S--- (λ1) + f(n- Ax)・eos(n・ωt) (4) However, S, (λ*) = f (o)/2 (
5). Further, Ax is the extraction interval of the interferogram, and according to the sampling theorem, it must be equal to or less than 1/2 of the shortest wavelength for analysis. Furthermore, all S(λ□) are cleared to zero prior to calculations (5) and (4).

In equation (3), ω is an angular frequency uniquely determined by the wavelength λ, and if it is calculated in advance before a series of calculations, the cosine calculation can also be performed quickly.

The cosine function generator 30 is, for example, a storage device having a function table corresponding to angles in the first quadrant, and has n·ω.

By reading out the value stored at the address corresponding to , the calculation time for calculating the cosine function by giving the angle can be greatly reduced. Multiplier 40, integration storage device 51.
The 58-wavelength storage devices 61 and 68 can be substituted for one arithmetic storage device, but their internal functions are the same as those shown in FIG.

Figure 2 shows a plane mirror scanning type Michelson interferometer.
The analysis light emitted from the light source 1 becomes a parallel beam in the optical system 2,
The light enters the semi-transparent mirror 3. The light beam transmitted through the semi-transparent mirror 3 is reflected by the movable plane mirror 4, and then reflected from the semi-transparent mirror 3, and then transferred to the fixed plane mirror 5.
It is combined again with the light beam reflected by the light beam, and is focused on the detector 7 by the focusing optical system 6.

When the movable plane mirror 4 moves in the direction of the arrow 9, the optical path difference between the two beams changes, and the interference pattern f as a function of the optical path difference
(x) is obtained as signal 15.

FIG. 3 shows the same Michelson interferometer as in FIG. 2, but a fixed plane mirror 5 is arranged at an angle of a small angle α with respect to a fixed plane mirror 4, and a photoelectric detector placed on the condensing surface of the condensing optical system 6 is arranged. Equiatomic fringes are projected onto a detector array (photoarray) 8. When the photodetector array is electronically scanned, an interferogram signal 15 is output at regular intervals.

FIG. 4 shows an example using a pi-lens type interferometer, in which light from a light source 1 is focused on a sample cell 25 by a lens 2, and transmitted light is incident on two lenses 31 and 32.

The lights collected by the lenses 31 and 32 overlap to form interference fringes in space, and are incident on the photoelectric detector array 8 placed on the interference surface.

FIG. 5 is an example of an interferogram 70, in which the amounts of optical path difference 0 and maximum optical path difference are measured and converted into data.

FIG. 6 is an example of a spectrum 80, in which a specific wavelength λ□
The spectral intensities of two points between ~λ and 81° and 82 are shown.

If the characteristic absorption of the analyte is at the wavelength λ, and there is no absorption at the wavelength λ, then the ratio of the two spectral intensities 81 and 82, i.e., a=S(λ4)/S(λ3) (6)
It is well known that this corresponds to the concentration of the analyte present.

〔Effect of the invention〕

As described above, the present invention provides a Fourier transform type spectrophotometer suitable for quantitative analysis of predetermined substances, and has the following effects.

1. Has all the advantages of Fourier spectroscopy.

2. Only the spectrum point corresponding to the specified arbitrary wavelength is calculated, so it can be used immediately for quantitative analysis.

3. If there are 20 or fewer spectral points, the final result can be obtained in a shorter time than the fast Fourier transform method.

4. Since a real-time method is adopted and there is no need to temporarily store the interferogram, the data storage memory has a sufficient walking capacity corresponding to the number of wavelengths.

5. Since fast Fourier transform is not performed, the number of digits to be executed may be small; for example, 16 bits is sufficient.

6. A plurality of wavelengths can be arbitrarily specified and easily changed as necessary.

7. By calculating the spectrum at a specific wavelength that is not absorbed by any of the substances to be measured and using the intensity ratio with other absorption spectra, we compensated for the influence of light source intensity fluctuations and scattering by the sample itself. Relative photometry can be obtained.

In addition to these effective features, the Fourier transform spectroscopy method naturally provides the high brightness of the optical system and the high energy efficiency associated with simultaneous photometry, making it suitable for weak light analysis and ultra-trace analysis. Therefore, its application range is wide.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of the present invention, FIG. 2 is a diagram showing an embodiment of an interferometer used in the present invention, FIG. 3 is a diagram similarly showing another embodiment, and FIG. Another example is shown in FIG. 5, which shows an interference pattern, and FIG. 6, which shows a spectrum. 10... Interferometer, 20... A-D converter, 30...
- Cosine function generator, 40... Multiplier, 50... Arithmetic storage device, Random... Storage device.

Claims (1)

[Claims] 1. Means for obtaining an interference pattern including an optical path difference of 0 up to a constant optical path difference, means for A-D converting the interference pattern for every constant minute optical path difference, and cosine functions corresponding to a plurality of given wavelengths. and means for integrating the product of the output of the A-D converting means and the value of the cosine function for each given wavelength, and the number of points is several tens compared to the number of data points of the interferogram. A Fourier transform multi-wavelength photometer that is characterized by obtaining spectral points from a fraction of a fraction to a fraction of a hundredth.