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OBJECTIVE LENS
FILM PLANE
INTEGRATING BAR \°J RE.CELL Fig. 1. Optical schematic of the integrating-type color densitometer.
the photosensitive surface of the photoelectric cell in the red and visual setting of the densitometer and (b) eliminating undesirable near-infrared light reaching the photoelectric cell passed by all blue and green filters.
Either of two apertures can be used for making density measurements. One is a round one, illuminating a circle of about -^ in. in diameter at the film plane; the other is rectangular, illuminating a 0.015 by 0.125 in. area. The round aperture is meant for general work and the rectangular one for the measurement of variable-area sound track.
A fully achromatic corrected objective lens focuses the aperture onto the film to be measured. The color and visual filters and a front-surfaced mirror are located between the aperture and the objective lens.
Four filters are mounted in a wheel. A neutral density is mounted with each filter to bring the photoelectric cell output to the same value. This minimizes zero adjustment when going from one color band to another. An integrating bar collects the light passing
through the film and delivers it to the photoelectric cell.
In checking all available commercial photoelectric cells, we found only two that lend themselves readily to the measurement of density, namely the RCA 1P42 and 929, or the equivalent in other makes.
We found that the 1P42 had an undesirable lag and color fatigue in the blue and green bands with the amount of light necessary to measure to a color density of 4.0. This can be partially overcome by raising the anode voltage above the ionization potential (18 v).
Raising the anode voltage sufficiently to overcome the fatigue problem raises two others: (1) If any trace of gas is left in the photoelectric cell, the cell will become nonlinear in the low density range when the greatest amount of light is present. (2) It will raise the dark current which means more noise, reducing the small margin between signal to noise in tffe high density range.
The 929 photoelectric cell has a greater output for a given light input, and does not seem to suffer from the above-mentioned shortcomings of the
Frank P. Herrnfeld: Color Densitometer
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