Journal of the Society of Motion Picture and Television Engineers (1950-1954)

Figure 3 is a view of the densitometer with the top raised for access to the optical system. At the left is a brace mechanism that counterbalances the weight of the hinged top and locks the top in either a half-open or a fully open position. Optical System The optical system of the densitometer is shown, approximately to scale, in Fig. 4. For clarity, a part of the system is shown as if turned 90° from its actual position; otherwise, the figure is a top view. The sample is illuminated from below by light that has passed successively through lens A, a rotating light chopper (modulator), a heat-absorbing glass, a color filter, a neutral filter used in shifting the meter range, lens B, and the defining aperture in contact with the sample. Since the readings are to conform to the American Standard for diffuse density, all of the light emerging from the sample must be collected, or at least equally weighed, in the receiving system. For this purpose, advantage is taken of the structure of the miniature, end-on 1P42 phototube. The sensitive surface of this tube is coated on the inside of a glass window that covers one end of the tube. The window, being in close contact with the upper surface of the sample, admits not only the specularly transmitted light but nearly all the scattered light as well. This method of diffuse collection avoids the losses that occur with other arrangements, such as an integrating sphere or a diffusing plate, and therefore gives a higher sensitivity. The phototubes used are stringently selected for high sensitivity, low drift, and normal spectral response. The light source is a blower-cooled 100-w projection lamp operated at line voltage. A spherical mirror behind the lamp forms a filament image that fills the spaces between the filament coils and nearly doubles the available light. Lenses A and B together form an optical relay; that is, lens A images the lamp filament on lens B, which in turn images the aperture of lens A on the under surface of the sample. Since the circular aperture of lens A is nearly uniformly illuminated by the lamp, its image at the sample, is a nearly uniform circular spot. The spot is slightly larger than the 5-mm window of the measuring phototube. The aperture of lens B is stopped down until all the rays strike the sample within 10° of normal incidence. A wider cone of illumination would give more intensity at the sample and therefore higher sensitivity, at the expense of impaired conformance with American Standard diffuse density. A second 1P42 phototube is employed in a comparison system which removes the effects of line-voltage variation and amplifier drift, in a manner to be described subsequently. This phototube receives an amount of light that is independent of the sample density but that varies with the intensity of the lamp. A filament image is formed at the comparison phototube by lenses C and D and two small mirrors. The illumination in the image can be adjusted by means of an iris diaphragm at lens D ; this control has the effect of a wide-range zero adjustment. The light chopper, shown in Fig. 5, is made of photographic film and is driven by an 1800-rpm synchronous motor. The three opaque areas at the outside of the chopper interrupt the measuring beam at a frequency of 90 cycles/sec. At a smaller radius is a ring containing 160 opaque areas, which interrupt the comparison beam at 4800 cycles/sec. The resulting 90 and 4800-cycle electrical signals from the phototubes are combined and amplified as a single composite signal. It is worth noting that, in a system using chopped light and an a-c amplifier, phototube dark current is ordinarily of little consequence since its direct component is not amplified. The eight color filters are mounted in Kodak Adapter Rings which clip onto a 700 June 1953 Journal of the SMPTE Vol. 60