On the one hand, our eye and its complexity : it is capable of perceiving thousands of subtle colours. On the other hand, our computers, today overpowering, but only knowing the bits. How do they manage to talk about the same thing ?
Colors in the digital age
How can you manage to code with 0s and 1s since a computer can only do that (but it does it very very fast and repeatedly if necessary, wow!), thousands or millions of subtle colors ?
Colors and computer science
Researchers have succeeded in digitally modelling the complexity of colors as seen by a human being. They had to start from the basic functioning of computers, i.e. 0 and 1 (called a bit) and the human eye which sees in RGB on 200 distinct shades per color for the most efficient of them and again, never in the three primary colors. To do this, they had to invent models based on these 0 and 1s to display, for example, a particular colour on a screen. These mathematical models are the bit and the byte (one byte is equal to 8 bits).
It is said that the signal is encoded in 24 bits - 3 × 8 bits - so on three bytes since one byte = 8 bits. Each signal is therefore a succession of eight 0 or 1 and this three or four times, for each RGB or CMYK color. An RGB signal will therefore be written as 255, 112, 44 for example (Red fully lit, green at 112 and blue at 44 is relatively dark).
Numbers and colours
Now that we know how to represent a color digitally, let's send a given RGB signal, always the same - 255, 112, 44 - to several different monitors, such as on the walls of home appliance stores' monitors. What's going on? What's going on? None of them have the same color ! Indeed, the RGB filters placed in front of each pixel of a monitor will be different in each monitor but even more so in different monitor models, of different brands. The RGB color model therefore does not allow the same color to be displayed directly on several different devices. As we saw in the introduction, it was therefore necessary to invent a color model independent of human eyes and their slight differences as well as of the different peripherals, i. e. absolute: the L*a*b* model. The color space L*a*b* was invented in 1976 by the CIE, always it. To a color L*a*b* corresponds only one color therefore only one wavelength. So for the same red pixel on each screen to which we have sent the same RGB signal, a slightly different L*a*b* color will correspond.
Very important ! A given digital signal - therefore a given RGB triplet, in our example - 255, 112, 44 - can therefore correspond to several different colored sensations for a person with a "normal" view if it is sent to several displays, for example. It is fundamental in color management to always have this notion in mind and therefore in what follows.
An RGB signal does not "really" represent a color - even if it is based on the human eye model - but a digital data (a color definition) which, when sent to a given device, is translated into a given color (an L*a*b* color) and depends on the components that were used to manufacture it. The RGB model is therefore practical in its operation because it is based on the functioning of the eye, but it is not absolute in any way, unlike the L*a*b*.
On the other hand, if I want to display the same color on several monitors, I will have to send them a different RGB signal! If I want to display a medium neutral grey (normally 100, 100, 100) on different monitors, I will have to send them different RGB signals so rather x, y, z for one, x', y', z' for another rather than x, x, x as one might expect. Since it is almost impossible to know what value to send to a given screen to display the "right color", it is necessary to calibrate it with a calibration sensor and create, for each monitor, a small file containing this precious information: an ICC profile. The ICC profile is therefore the basis for color management.
How then can I imagine a single color perceived by a standard eye for a digital pair if, for a given screen, I have to send it an RGB signal xyz and not xxx so that it displays a neutral color? It is once again to the ICE's credit that it has answered this question. This is what we will see with color spaces and colorimetric models on the next page. But before that, I would like to make one more important point, since it is the notion of the gamma of the eye or a screen - The gamma of the eye, of a screen - 4 / 10