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Photocells
Fig. 2. Schematic drawing of projection-type optical system for controlled additive trichromatic illumination using a single lamp.
compared with three reference voltages, which are the analogs of the desired red, green and blue exposures. Any difference between photocell and reference voltage is amplified by one of three amplifiers and applied to the appropriate servomotor to reduce that difference to zero.
Figure 3 shows a Bell & Howell Model D Printer which has been modified to incorporate the optical system shown in Fig. 2. The outer half of the sprocket was removed to eliminate interference of the sprocket hub with the central light beam. The original light-control shutter mechanism was discarded. In its place a steel block was provided which carries bearings for a film-driven flange which supports the outer edge of the film in place of the original outer half of the sprocket.
The original cylindrical lamphouse has been replaced with a square box which houses the lamp and the optical system. The cylindrical housing on the front of this box houses one of the servomotors. The other two motors are on the rear side of the box. At the extreme right is the electronic complement consisting of the power supply below, above that the amplifier box, and on top a tape-controlled contactor. This contactor reads timing information
which is stored in the form of an array of holes punched in a strip of 1 6mm film.
Figure 4 is an interior view of the lamphouse showing the optical system. Two of the rotatable vanes can be seen to the left of the prisms. The third is seen in the foreground protruding from the hinged cover. The outer filmsupporting flange has been removed to show the plate which holds the beamsplitting mirror on its back side. The photocell enclosure has also been removed to show the three photocells beneath the sprocket enclosure.
The 16mm control tape is advanced, one frame at each scene change, by means of a solenoid to establish a new set of reference voltages. In the amplifier circuit of Fig. 5, the tape-controlled contactor controls a battery of fifteen relays, five for each color, which, in turn, control attenuators in the reference voltage circuits. These attenuators are calculated to provide attenuations of 0.4, 0.8, 1.6, 3.2 and 6.4 db, which are equivalent to exposure changes of 0.02, 0.04, 0.08, 0.16 and 0.32 log E. If several relays are energized, these attenuations add, so that a total exposure range of 0.62 log E in steps of 0.02 log E is provided for each color. The three knobs shown on the front of the amplifier box, Fig. 3, are
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November 1952 Journal of the SMPTE Vol.59