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Color Reproduction: Color Gamut and LED Backlighting

About two years ago I wrote in my articled called Parameters of Modern LCD Monitors that the color gamut parameter was actually insignificant for monitors just because all monitors had the same gamut. Fortunately, things have changed for the better in this area since. Monitors with an enhanced gamut have emerged in shops.

So, what is the color gamut?

As a matter of fact, the human eye sees light in a wavelength range of 380-700nm, from violet to red. There are four kinds of light-sensitive detectors in the eye: one type of rods and three types of cones. The rods have excellent sensitivity but make no difference between wavelengths. They perceive the whole range at once and endow us with black-and-white vision. The cones have worse sensitivity (and do not work in twilight), but give us color vision under proper lighting. Each type of cones is sensitive to a certain wavelength range. If our eye receives a ray of monochromatic light with a wavelength of 400nm, only one type of cones will react – the one that is responsible for blue. The other two types, red and green, won’t notice anything. Thus, the different types of cones play the same role as the RGB filters standing in front of a digital camera’s sensor.

It may seem that our color vision can be easily described with three numbers, each for a signal from a certain type of cones, but that’s not so. Experiments back at the beginning of the last century showed that our eyes and brain process information in a less straightforward manner, and if color perception is described with three coordinates (red, green and blue), it turns out that the eye can easily perceive colors which have a negative red value in this system! Thus, it is impossible to fully describe the human vision with the RGB model.

The experiments eventually led to the creation of a system that encompassed all the range of colors our eyes can perceive. Its graphical representation is called a CIE diagram and it is shown in the picture above. All the colors perceived by the eye are within the colored area. The borderline of this area is made up of pure, monochromatic colors. The interior corresponds to non-monochromic colors, up to white. It is marked with a white dot. As a matter of fact, white color is a subjective notion for the eye as we can perceive different colors as white depending on the conditions. The white dot in the CIE diagram is the so-called flat spectrum dot with coordinates of x=y=1/3. Under ordinary conditions, this color looks very cold, bluish.

Any color perceived by the eye can be identified with two coordinates (x and y) on the CIE diagram. But what’s the most exciting thing is that we can create any color by mixing up monochromatic colors in some proportion because our eye is totally indifferent to what spectrum the light has. It’s only important how each type of the detectors, rods and cones, has been excited by that light.

If the human vision were successfully described with the RGB model, it would suffice to take three sources (red, green and blue) and mix them in necessary proportions to emulate any color the eye could see. But as I said above, we can see more colors than the RGB model can describe, so there is another question: having three sources of different colors, what other colors can we make by mixing the sources?

The answer is simple: if you mark points with the coordinates of the basic colors in the CIE diagram, everything you can get by mixing them up is within the triangle you can draw by connecting the points. This triangle is referred to as a color gamut.

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