Abstract: A light source for providing controllable illumination having the red, green, and blue components in selected intensity proportion, uses a lamp/filter assembly which projects light through a diffusing plate into a mixing chamber from the output end of which mixed light is emitted for illuminating photographic paper through a negative. The chamber is rectilinear, and the red and green lamps project toward opposite corners while the blue lamp projects along the axis of the chamber. A light pipe which has an input end facing the output end of the chamber extends into the chamber and extracts light which is processed in red, green, and blue channels having temperature stabilized phototransducer circuitry and control circuitry wherein signals corresponding to the intensity of the color components are processed in the channels and are modified by control signals which can be produced in the photo printer control computer.

Abstract: An r.m.s. voltage regulating circuit for maintaining substantially constant the r.m.s. output voltage of an A.C. source. The circuit preferably includes a triac and a timing circuit for controlling the non-conduction angle thereof, preferably at the beginning of each half cycle. A function generator compensates for the non-linearity in the relationship between the r.m.s. value of the output voltage and the amplitude of the A.C. voltage source.