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A video display cathode ray tube (CRT) makes light for generating the image by bombarding a screen with a scanning electron beam. The screen is coated with suitable chemicals called phosphors which generate either red, green or blue light. The actual phosphors used are not those needed to create the maximum range of colours, since these are not available from any known chemicals. Compromises are also made for consumer displays to generate brighter pictures. The best phosphors available are used to define a reference ‘Grade One’ monitor and these can then used to reverse engineer an ideal response from the camera red, green and blue processing channels to get the best colour reproduction. Unfortunately the required response includes regions of negative output which are impossible with real world sensors. Instead, a simulation of negative response is created by adding a proportion of reverse polarity signal from one channel to another. There is thus a ‘matrix’ whereby each of the three channels is fed to each of the other two, creating 6 paths in total.
The system is nothing less than an engineered bodge, but the result is a very significant improvement in colour accuracy, particularly for pastel and flesh tones. Note however, that it is very difficult to make adjustments to the default Colour Matrix values without the requisite test charts, instrumentation and considerable experience!