Like on TV – The reason for the complicated procedure

You are here: Nature Science Photography – Lightness and color – The perception of lightness and color

Now, one may rightly ask why evolution has produced the rather complicated system of the opponent color mechanism. There are two good reasons for this. Let’s start with the simple part of the explanation and go back in evolutionary history to the time when living beings were not yet color capable. Their visual perception was limited to distinguishing brightness values and to accomplish this, the signals from the photoreceptors were summed up, just as we still do in the brightness channel today. With the advent of the receptors responsible for color perception, evolution then did not take the actual logical path of developing one red, one green and one blue channel each, but simply extended the existing system in the most effective way by two further axes: those for the difference between red and green or blue and yellow. This has the additional advantage of reducing the amount of information to be transmitted, because instead of transmitting the data for black, white, red, green and blue (5 channels) separately, 3 channels are sufficient with the difference values.

The development of modern television confirms that this is the most efficient method of information processing. There, too, at the beginning (in the camera) and at the end of the process (in the television set), one signal each is used for red, green and blue. However, during the broadcast of the video signal, the system also utilizes a brightness signal and two color difference signals. The reason for this can be found in the technical development and also has something to do with efficiency. After RCA introduced the first television system in 1935, the regulatory authority realized that it had to divide the usable range of the electromagnetic spectrum among the companies interested in the new technology. At that time, of course, there was only the technology for the black and white picture and, although 3.7 Mhz was sufficient for the transmission of this signal, each station was granted a generous 6 Mhz. By 1940, the CBS television company had finished developing the first color television system, which also separated the signals for red, green and blue in the broadcast phase. There were two fundamental problems with this. First, it required three separate 3.7 Mhz bands, and second, it excluded users of the previous B/W equipment from receiving the new color signals. After years of litigation to enforce its own standard, the Federal Communications Commission refused to grant CBS the additional bandwidth it needed, leading to the system’s withdrawal from the market after only a few months of operation. In the meantime, other companies led by RCA had developed a B/W compatible color system by summing the red, green and blue signals into one brightness signal and simultaneously splitting them into two difference signals (red-minus-brightness and blue-minus-brightness). A third green-minus-brightness channel was unnecessary because it was sufficient to subtract the sum of the individual channels from their total to determine the value of the third primary color. The brightness signal needs 4.2 Mhz; the two difference signals 1.5 Mhz and 0.5 Mhz respectively, but by slightly overlapping the channels, the engineers could ensure that the available 6 Mhz was not exceeded. In 1953, this system became the standard and continues to shape the operation of color television today.

The second part of the explanation deals with the fact that the stimulus response of the photoreceptors alone does not provide reliable information about the color of the light that activates them. Here’s what happens. The three types of cone receptors responsible for color vision have wide-ranging sensitivity curves. So, for example, if an M cone responsible for the medium-wavelength green region of the spectrum is hit by, say, 100 photons of wavelength 580 nm (yellow) and responds with a specific reaction, its response to a light of that wavelength that is twice as bright would also be twice as strong. But it would be even greater if it were stimulated with 100 photons at 520 nm (green), because its sensitivity is highest at this wavelength. The stimulus response therefore only provides information about the brightness of the light but not about its chromaticity. The visual system circumvents this indeterminacy by balancing the signals from the receptors in the opponent color channels. If we revisit the 2nd example above and flood part of the retina with green light of 520 nm wavelength, the M cones in this area will give a stronger response than the L cones (the S cones are not activated by light of this wavelength) because they are most sensitive to this wavelength. Now, as the brightness doubles, the signal strength of the two receptors also doubles, but relatively speaking, the signal from the M cones is still greater than that from the L cones. As a result, information about chromaticity is contained in the ratio between the stimulus responses of the three receptor types, which remains constant even with changing brightness. And this relation is determined by the opponent color mechanism.

Next Third processing stage – Adding a spatial aspect for color

Main Lightness and Color

Previous Second processing stage – Conversion of signals into opposite color channels

If you found this post useful and want to support the continuation of my writing without intrusive advertising, please consider supporting. Your assistance goes towards helping make the content on this website even better. If you’d like to make a one-time ‘tip’ and buy me a coffee, I have a Ko-Fi page. Your support means a lot. Thank you!