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Original Article www.jmp.org.in

Portable and wide-range solid-state transmission

densitometer for quality control in film radiography
Javier Morales Aramburo, Sigifredo Solano Gonzalez, Jorge Toledo Toledo
Physics Department, National University of Colombia, Calle 59A No 63-20 - Núcleo El Volador, Medellín – Antioquia,
Colombia, Medellín-Colombia

Received on: 23.11.09 Review completed on: 02.02.10 Accepted on: 22.02.10 DOI: 10.4103/0971-6203.62135

ABSTRACT
In biology, materials science, radiography quality control or film dosimetry in radiotherapy, a transmission densitometer is
useful for measurements of optical density. The design proposed here is oriented to quality control in radiographic films. The
instrument described here utilizes low-cost solid-state devices and is easy to construct. The use of 1-watt white light-emitting
diode in this densitometer enables low power consumption and a cold light source. Moreover, the instrument does not need a
reference light, which results in decreasing the number of parts and reducing the overall size of the apparatus.

Key words: Densitometer, junction temperature, light-emitting diode, optical density

Introduction (photodiode) to measure the light intensity transmitted

to the film. The acquisition and signal conditioning are
Organizations like the International Atomic Energy achieved by implementing a circuit with the well-known
Agency (IAEA) and the International Radiation Protection integrated logarithmic amplifier LOG102. A microcontroller
Association (IRPA) supported by World Health Organization performs the calculation of optical density and its display in
(WHO) establish quality assurance programs in diagnostic a low–power consumption liquid crystal display (LCD). The
radiology and radiotherapy, within which densitometry is an calibration is achieved from two variable voltage dividers;
important part of quality control of image-forming systems the values of these voltages are used like an offset to adjust
with X-rays and dosimetry. In densitometry the degree of the values of optical density displayed to the values of a
the film blackening is measured by determining the optical calibration density strip tablet with the National Institute of
density (OD) by a densitometer. OD is defined as the ratio Standards and Technology (NIST) traceability.
of light incident on the film to the light transmitted.
⎛I ⎞ Materials and Methods
OD = log10 ⎜ 0 ⎟ (1)
⎝ Ii ⎠ Signal acquisition
The range of OD measured by the instrument is between The heart of the instrument is the logarithmic amplifier
0.15 and 4.0 with an uncertainty of ±0.02 OD. Even other LOG102 [Figure 2]. This amplifier is able to measure the
instruments have been constructed for this purpose, but 5 decades of the current (i2) that produces the photodiode
the technology is old[1] or they do not cover a wide range of with optical densities from 0 to 4. The light detector
optical density.[2] In other fields, a similar technique has been comprises a photodiode with spectral response in the visible
used with good results as those presented here.[3] An overview range connected to the logarithmic amplifier. This design
of the system framework of the transmission densitometer takes advantage of the linear (light intensity/diode current)
is shown in Figure 1. The instrument consists of a light relationship obtainable in the short circuit mode of operation
source, viz., light-emitting diode (LED), a tiny aperture of the photodiode. Moreover, this mode of operation gives
through which the light is directed; and a light detector the short response time required for fast film scanning.

Address for correspondence: Current and voltage sources

Prof. Javier Morales Aramburo The logarithmic amplifier gives a voltage response
National University of Colombia, Calle 59A No 63-20 - Nucleo El proportional to the ratio of the current i1 to the i2., where
Volador, Medellín Antioquia, P.O. Box: 3840, Medellín, Colombia. i1 is a current set to 500 μ A with a 3-terminal adjustable
E-mail: jmorales@unal.edu.co current source. This current adjusts the voltage range of the

Journal
Journalof
ofMedical
MedicalPhysics,
Physics,Vol.
Vol.35,
35,No.
No.3,
3,151-3
2010
152 Aramburo et al.: Transmission densitometer for quality control

Figure 1: System framework of the transmission densitometer Figure 2: Current source LM234Z and the Log102

logarithmic amplifier to a maximum value in the case of value from the equation (2), where VLOG102 is the voltage
the 4.00 OD suitable for A/D conversion; and in the case of from the logarithmic amplifier.
measurement without film, a minimum voltage that will be
displayed like a 0.00 OD in the LCD. The instrument takes OD = −α + β × VLOG102 (2)
power directly from four AA batteries, which produce 6 volts,
4.5 volts for the microcontroller; and the other voltages Mechanical and thermal noise
required for the operational amplifiers and a negative An important source of noise found in the measurement
voltage supply are obtained from a complementary metal- is the mechanical structure. The photodiode and the LED
oxide-semiconductor (CMOS) switched-capacitor voltage should be aligned, and only up and down movement is
converter (TL7660). The current source for the LED was allowed. The distance between LED and photodiode should
constructed with a linear voltage regulator; it is inexpensive, always be 1.7 mm. Light from the LED should pass through
and due to the feedback and an internal voltage reference of a pinhole of radius 1 mm, and the cup of the LED should
1.25 V, it keeps a constant current in the LED. be removed because the nearest distance between LED and
photodiode is required by the use of little forward current.
Calibration and measurements The thermal noise comes from the increment of the junction
With all the blocks working together, the next step is the temperature in the LED diode junction by the forward
calibration. The reference voltage for the A/D conversion current, causing relative photometric output changes,[4] as
is obtained from a 2.5-volt precision voltage reference well as loss of calibration. The current used to ensure the
integrated circuit. From its output are connected two accuracy of the measurements is only 25 mA, and the time
variable voltage dividers. With high input impedance by which the LED is turned on is about 4 seconds, to avoid
problems of accuracy due to the increment in the junction
voltage followers, the two voltages from the two voltage
temperature. The white LED used is a neutral white LED.
dividers are carried to the analog-to-digital converter of the
microcontroller. The logarithmic amplifier gives a curve of Results and Discussions
response in the range of the OD values measured, which
may be adjusted to the calibrated values of the density The calibration standard for optical density values was an
strip in each step by a linear relation. The constants of AGFA - STRUCTURIX–calibrated step tablet with 14 steps,
this simple linear relation (2) are set by means of variable identification number 6414163, and NIST traceability.
voltage dividers. In the calibration of the instrument, one The uncertainty of this strip tablet is ±0.006 OD, and the
of the potentiometers is used to adjust an offset voltage, measurements made with this instrument in each step of
represented by α, and the other is used to set the constant the tablet give a value with a maximum deviation of ±0.02
β. Finally the microcontroller obtains the optical density OD with respect to the value in the calibration tablet

Journal of Medical Physics, Vol. 35, No. 3, 2010

 Aramburo et al.: Transmission densitometer for quality control 153

Table 1: Measurements made with the for optical density measurement of radiographic films was
densitometer in each step of the tablet developed. The calibration system is straightforward and
OD measured with OD values in the Error = xmeasured − xtrue provides stability in measurements because of the circuit
the instrument calibrated step used to maintain the values of the constants of calibration
tablet with a precision voltage reference integrated circuit.
0.00 0.000 0.000 Excellent repeatability was obtained, and the uncertainty in
0.13 0.146 0.016 the measurements was found to be similar to that obtained
0.30 0.296 -0.004 by using commercial densitometers (±0.02 OD). This type
0.59 0.588 -0.002 of densitometer can be used easily in many radio-diagnostic
0.91 0.903 -0.007
and radiotherapy centers.
1.21 1.200 -0.010
1.50 1.500 0.000
1.82 1.810 -0.010
Acknowledgment
2.10 2.090 -0.010
2.40 2.390 -0.010
The authors would like to thank the Dirección de
2.69 2.680 -0.010
Investigación Medellín (DIME) of the National University
2.98 2.980 0.000 of Colombia for their economic support.
3.31 3.300 -0.010
3.58 3.590 0.010 References
3.86 3.880 0.020
4.16 4.180 0.020 1. Brown WN Jr, Birtley WB. A densitometer which records directly in
units of emulsion exposure. Rev Sci Instrum 1951;22:67-72.
2. Karkiewicz LM, Panitz JA, Fowler GL. Low drift optical densitometer.
[Table 1]. The repeatability of the measurements was Rev Sci Instrum 1980;51:1267-8.
achieved by solving the problems related to the different 3. Fu-Ming Tzu, Jung-Hua Chou. Optical density measurement of TFT-
sources of noise. The instrument provides accuracy in LCD by PMT coupled monochrome LED. IEE Electronic Materials
measurements with respect to the values of the calibration and Packaging, 2008. EMAP 2008. International Conference on
Publication Date:22-24-Oct 2008. p. 65 - 8.
standard; and in cases when a deviation was observed, the
4. Gabor M, Gergely N, Zoltan S. A Novel Procedure and Device to
difference with respect to the calibration standard was not Allow Comprehensive Characterization of Power LEDs over a Wide
higher than ±0.02 OD. Range of Temperature. IEE THERMINIC 24-26 Sept 2008 p. 89-92.

Conclusions
Source of Support: National University of Colombia, Conflict of
Interest: None declared.
A simple method using a white-light LED and photodiode

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Journal of Medical Physics, Vol. 35, No. 3, 2010