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Condenser Lens
Rotate ble Objective SemiVane Lens Mirror
Printer Gate
Diffuser Photocell
Chopper
Fig. 1. Schematic drawing of a projection-type optical system with servocontrol of intensity at printer gate.
system designed to meet these requirements is described below.
A simple projection-type optical system with a high-aperture condenser lens and a high-wattage lamp is illustrated schematically in Fig. 1 . The condenser lens forms an image of the lamp filament in the objective lens, and the objective lens, in turn, forms an image of the uniformly illuminated condenser lens in the printer aperture. One method of controlling the intensity of illumination at the printer aperture without affecting uniformity is to change the aperture of the objective lens by means of an iris diaphragm or other mechanical masking means, such as the rotatable vane placed near the lens (Fig. 1).
In general, however, the intensity will not change linearly with changes in position of the iris or the vane, because of the nonuniform structure of the filament image. The necessity for a calibrated relation between intensity and vane position can be obviated by means of a servosystem in which the intensity is measured photoelectrically and adjusted automatically and continuously to the correct value (Fig. 1). This is done by comparing the voltage developed by the photocell with a reference voltage, shown schematically as the output of the potentiometer. Any difference between these voltages is amplified and fed into the servomotor which rotates
the vane in a direction to reduce the error. The reference voltage set up by the potentiometer is thus the analog of the desired exposure, and scene-toscene changes in exposure can be made simply by readjusting this reference voltage.
An additive system of color exposure requires three simultaneous exposures, red, green and blue, whose intensities are controlled individually and preferably independently. In Fig. 2 is shown an optical system in which three beams are derived from adjacent segments of a common condenser lens. Mirrors reflect light from the upper and lower segments of the condenser lens into prisms which direct these beams onto the printer aperture at angles of 15° to the central beam. The sizes of the prisms are chosen to compensate for the difference in path length between the outer and central beams; in this way, identical objective lenses can be used in the three beams. Red, green and blue filters at the objective lenses substantially restrict the exposure of each beam to one of the three color primaries. A beam-splitting mirror reflects a small fraction of the filtered light onto an opal glass which acts as an integrator. Beneath the opal glass are three photocells with red, green and blue filters over them. The output voltages of the three photocells are
J. G. Streiffert: Control for Color Printing
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