Automatic white balance control circuit

Abstract

A fully automatic white balance control circuit which is sensitive to changes in the ambient or incident illumination as well as to changes in the object being viewed includes a main photo-sensor having a video output from which three primary color signals are derived, three external or incident light sensing elements having spectral characteristics the same as those of the main photo-sensor for the three primary colors, three capacitors for converting output currents from the three external light sensing elements to respective voltages, a voltage comparator for comparing a voltage across at least the one capacitor corresponding to green light with a reference voltage, and three sample-and-hold circuits for sampling and holding voltages from the three capacitors, respectively. Charging of the three capacitors by output currents from the three external light sensing elements, respectively, is started at the commencement of each cycle, for example, each vertical period, and, when the voltage comparator produces a compared output in response to the voltage across the one capacitor having become equal to the reference voltage, the charging is stopped and the voltages then present on the three capacitors are sampled and held by the three sample-and-hold circuits to thereby provide white balance control signals by which the level ratios of the three primary color signals derived from the main photo-sensor are controlled to be 1:1:1 thereby to adjust the white balance.