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

as those cited below, may be expected to absorb as much as 99.8% of the available light, in terms of system response. In comparison, then, with a densitometer designed to read only black-and-white densities, the sensitivity of a densitometer for color materials must be higher by a factor of as much as 500. Description Figure 1 shows the Type 31 A densitometer with its separately housed amplifier unit. The units are connected by two 5ft cables, one carrying signal leads and the other containing power and control connections. The separated construction allows the amplifier to be placed in an out-of-the-way location where it need be reached only for servicing. All operator's controls on the densitometer proper are mounted in a superstructure above the sample plane, since it is sometimes convenient to have the instrument flush-mounted in a cut-out table top. In the center of the superstructure is the indicating meter, which has an approximately uniform density scale graduated in 0.02 divisions from 0.00 to 1.10. Four density ranges — 0 to 1, 1 to 2, 2 to 3 and 3 to 4 — are covered by the one meter scale and by the fourposition range switch at the lower left of the panel. Overlap between successive ranges is provided by the extra 0.1 density range at the end of the meter scale. At the lower right of the panel is the color switch for selection among eight different color filters. The filters are normally used in sets of three — red, green and blue — but provision of eight filter positions allows quick selection of additional filters, and also facilitates the use of the instrument for abridged spectrophotometry. Beside the indicating meter are four zero-adjustment knobs, mounted edgewise behind panel slots, for setting the meter to zero density with the sample removed. A separate zero adjustment is connected to the circuit for each of four positions of the color switch; this arrangement makes it possible to read a sample through four filters in succession without removing the sample to reset the zero with each filter. The remaining four filter positions utilize the same set of zero adjustments. From the superstructure extends a hinged reading arm containing the measuring phototube. The arm has sufficient span for reading the center of a sample 20 in. wide. The weight of the arm is counterbalanced by a spring at the hinge; an additional toggle-action spring holds the arm in the lowered position for reading, but causes it to remain up when lifted for inspection or insertion of a sample. On the top surface of the arm, near the back, is a graduated knob for adjustment of a stop that limits the downward travel of the arm. This control brings the window of the measuring phototube into contact with the surface of the sample when the arm is lowered, as required for accurate measurement of diffusing samples, but prevents the weight of the arm from exerting pressure. The sample may then, if desired, be moved about without raising the reading arm ; moreover the position of the phototube remains unchanged when the sample is removed for zeroing. The latter condition is a necessity when thick samples, such as photographic plates, are to be read. Figure 1 shows the densitometer equipped with an accessory strip-reading attachment comprising a strip holder and a spring-driven indexing mechanism. This equipment accelerates the routine measurement of standard 16mm and 35mm sensitometric strips having up to 21 steps. The strip to be read is first slid endwise into the holder (which can be disengaged from the indexing mechanism and handled separately) and pinned in position by release of the film clamp at the lefthand end. The holder is then engaged with the indexing mechanism and moved to the right against a stop. This action winds the spring and brings 698 June 1953 Journal of the SMPTE Vol. 60