GB1291601A - Improvements in or relating to colour television cameras - Google Patents

Abstract

1291601 Television BRITISH BROADCASTING CORP 6 Jan 1970 [6 Jan 1969] 750/69 Heading H4F A sensing device comprising at least two photoelectric receptors with different spectral responses is exposed to the same light as illuminates a scene viewed by a colour television camera, the outputs of the receptors being compared to provide an error signal or signals which control variable gain amplifiers in the colour channels of the camera or associated apparatus so as to maintain a given white balance despite variations in the spectrum of the incident light. The error signal may also be used to control a variable gain amplifier in one receptor channel, so that the signal is a measure of the amplification required to match the outputs of the channels, Fig. 2 (not shown). The effect of variations in the intensity of the incident light on the error signal is reduced by providing AGC amplifiers in each receptor channel, which are either all controlled by a single signal derived from one or a plurality of receptor outputs, or from a further (broad spectrum) receptor, or are individually controlled by different combinations of such outputs Figs. 3-5 (not shown). Each error signal may be provided by comparision of a single receptor output with a broad spectrum signal obtained from a combination of the recepttor outputs, or from the further receptor, such signal also controlling an AGC amplifier in its own channel unless the receptor outputs are taken after AGC has been applied in their own channels, Fig. 6 (not shown), and Fig. 7. Using red, blue and green receptors only, Fig. 8 (not shown), the variable gain amplifiers in the green camera and receptor channels may be omitted since the relative adjustment in green intensity may be expected to be small compared with red and blue. If black body radiation is assumed, Fig. 9 (not shown), the outputs of two receptors (red and blue) may be compared and error signals derived for each camera channel. In Fig. 11 (not shown) the AGC amplifiers are controlled by a signal derived by comparing the output of a further receptor with a fixed reference signal. To limit the light intensity reaching the receptors a shutter or neutral density arrangement may be provided, Fig. 10 (not shown), and this may also replace the AGC amplifiers e.g. by linking with the camera exposure controls.