Measurement Techniques, Vol. 49, No. 7, 2006

OPTOPHYSICAL MEASUREMENTS DENSITOMETER FOR MEASUREMENT OF THE OPTICAL DENSITY OF MOTION PICTURE AND PHOTOGRAPHIC FILM

A. I. Denisyuk, V. N. Kuz’min, S. E. Nikolaev, S. V. Safronov, K. A. Tomskii, and A. S. Troitskii

UDC 535.34

A specification of photometric problems is considered for the purpose of drawing up requirements for a densitometer for use in quality control of film products. One aspect of the requirements of these products is that it is necessary to measure the optical density of black/white as well as positive and negative color film. Key words: densitometer, zonal density, integral density, optical density, densitometry.

The lack of domestically produced devices that assure densitometric control of film materials has served as a basis for the development and organization of small-lot production of such devices. Densitometers (from the Latin, densitas, density and meter) are distinguished on the basis of the principle of measurement (direct reading and principle of comparison), type of light detector (human eye, photoelectric element, or photomultiplier), nature of data generated (nonrecording and automated recording instruments), and the magnitude of the measured field (densitometers and microdensitometers proper, also called microphotometers) [1–5]. The optical density D measured by the device is detetermined by the expression D = –logτ(λ), where τ(λ) is the transmission coefficient. The device, the outer appearance of which is represented in Fig. 1, consists of an illuminator and control block 1, clamping unit with photodetector 2, microscopic stage 3, and control panel 4. The optical circuit of the device (Fig. 2) includes a light source, lens system, rotating mirror, block with light filters, and diaphragm. A halogen bulb is used as the source of radiation. By means of a two-lens optical system, radiation is focused in the plane of the diaphragm, which is mounted within the microscopic stage, behind which is situated the sample which is to be measured (photographic film). Light which has passed through the sample scatters diffusely and is recorded by the photodetector; a silicon photodiode is used as the photodetector. Thus, the device implements the following geometry of measurement of the optical plane: illumination of a sample along a normal and recording of all the diffuse light behind the measured sample. The present densitometer is an integral-type device. The spectral sensitivity of photodetectors of visual, blue, green, and red light (Status A and Status M) requird by the Standard [1] is achieved by correction of the spectral sensitivity of the photoelectric diode through use of a combination of colored light filters. Measurements of the spectral sensitivity of the photodetector in combination with light filters were conducted on a precision apparatus using double monochromatization and TKA Scientific Production Association; e-mail: [email protected]. Translated from Izmeritel’naya Tekhnika, No. 7, pp. 39–41, July, 2006. Original article submitted March 30, 2006. 0543-1972/06/4907-0685 ©2006 Springer Science+Business Media, Inc.

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2

3

4

1

Fig. 1. Outer appearance of TKA-KM densitometer.

Radiation detector Photographic film

A Filters

Lens Light source Mirror

Fig. 2. Optical circuit of device.

Spectral sensitivity, relative units

Visual Status A Status M

Wavelength, nm

Fig. 3. Spectral characteristics of Status A and Status M filters and visual filter.

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TABLE 1. Characteristics of Colored Glass Used to Produce Status A and Status M Combination of colored glass Filter brand

thickness, mm

brand

thickness, mm

brand

thickness, mm

ZhS-20

3.0–4.0

SZS-21

1.4–1.6

–

–

blue

ZhS-12

1.0

SS-4

4.0–4.5

SZS-21

1.75

green

OS-11

1.0

SZS-22

6.0–7.0

–

–

red

KS-13

1.0

SZS-21

2.5–3.0

–

–

blue

ZhS-12

1.0

SS-4

2.5–3.0

SZS-21

1.75

green

OS-11

1.0

SZS-22

3.0–3.5

–

–

red

KS-14

3.0–4.0

SZS-21

2.5–3.0

–

–

Visual Status A:

Status M

equispectral spectrophotometry. The correction error for the spectral sensitivity of the photoelectric diode assuming a specified curve was less than 2%; the absolute error in the measurement of the optical density for different samples of photographic film was less than ±0.005 V for each combination of light filters with the photodetector. Different brands of colored glass and the thickness of each are presented in Table 1. The spectral curves of the selected light filters together with the photodetector for Status A and Status M are presented in Fig. 3. The device functions in the following way. A light flux from the radiation source is focused by means of the lens system on a limiting diaphragm, passes through the material which is being measured, and converges in the photodetector, which is turned on in a current generator mode. Analog-to-digital conversion, digital filtration, and normalization of the digital signal is realized by a microconverter, or integral “system on a crystal.” Through the use of a microconverter, it becomes possible to dispense with the need for additional development of the block that controls the quantizing motor. A sufficiently rapid 12-digit analog-to-digital converter and other built-in highly efficient analog peripheral devices, 8052 core, built-in program flash memory, and certain other useful functional components have made it possible to construct a compact, inexpensive device. As in many cases when microconverters are employed, the software includes complex code written in high-level C and execution time-critical fragments written in Assembler. The input circuits of the device must amplify a rather weak signal, function with different levels of signals that are received in the course of switching between filters with different transmission coefficient and measure signals over a broad range of rapidly varying optical densities (when conducting a series of measurements). Input of correcting profiles from a personal computer is enabled in the device, which provides some flexibility to the control system and makes the operator independent of the producer. In the course of measurements, different types of noise and interference may be superimposed on the signal, causing distortion in the working signal. When measuring densities greater than 3D, these electromotive forces may significantly exceed the useful signal. The principal sources of such distortions are as follows: • network interference with frequency of 50 Hz (or 60 Hz) and network voltage harmonics; • electromagnetic babble from other electronic devices; • high-frequency noise from surrounding devices. In order to assure precise and reliable operation of the device, it is necessary to take every measure possible to filter out or eliminate (minimize) the above types of interference. The use of analog filters may prove extremely expensive and difficult to implement. The use of a signal chain that includes an ADuC831 microconverter has turned out to be one possible method of simplifying the circuit which also solves the above problems. In the newly developed device, the analog-to-digital converter, filter, and computer module are replaced by a single integral microcircuit, making it possible to achieve flexi687

bility in the process of adjusting the parameters of the filter. A traditional circuit for converting current into voltage and subsequent amplification of the latter is used for the analog input, while a MAXIM ICL7611 multiplier with low consumption current is used as the amplifier. As a result, a prototype of a densitometer that supports measurement of the density of motion-picture and photographic film in accordance with the requirements of standards documents with metrological and technological support has been created. The device is distinguished by a high level of reliability and simplicity in operation and low price.

REFERENCES 1. 2. 3. 4. 5.

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GOST 9160-91, Method of General Sensitometric Testing of Multi-Layer Color Photographic Film [in Russian]. V. A. Zernov, Photographic Densitometry [in Russian], Iskusstvo, Moscow (1980). T. V. Korneeva, Explanatory Dictionary in Metrology, Measurement Technology, and Quality Control [in Russian], Russkii Yazyk, Moscow (1990). A. A. Belousov, A. I. Vinokur, and M. S. Vasin, Technology for Reproduction of Archival Photographic Film [in Russian], NIKFI, Moscow (2003). M. G. Kozlov and K. A. Tomskii, Technical Optics Measurements [in Russian], Izd-vo St. Petersburg Institute of Printing, St. Petersburg (2004).