Articles: Monitors

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It is the so-called laser display that can produce the biggest color gamut for a system with three basic colors. In such a display, the basic colors are formed by three lasers (red, green and blue). A laser has a very narrow radiation spectrum, extremely monochromatic, so the coordinates of the corresponding basic colors lie exactly on the diagram’s border. It’s impossible to move them out beyond this border because points out there do not correspond to any light. And if the coordinates are shifted into the diagram, the area of the gamut triangle is reduced.

The picture shows that even a laser display cannot reproduce all the colors the human eye can see, although it is quite close to doing that. To enhance the gamut, more basic colors can be used (4, 5 or more) or a hypothetical system can be created that would be able to change the coordinates of the basic colors “on the fly”. But the former solution is too technically complex today whereas the latter is implausible.

Well, it’s too early yet for us to grieve about the drawbacks of laser displays. We don’t use them as yet, and today’s monitors have a far inferior gamut. In other words, in actual monitors, both CRT and LCD (except for some models I’ll discuss below), the spectrum of each of the basic colors is far from monochromatic. In the terms of the CIE diagram it means that the vertexes of the triangle are shifted from the border of the diagram towards its center, and the resulting area of the triangle is greatly reduced.

There are two triangles in the picture above, one for a laser display and another for the so-called sRGB color space. To put it short, the second triangle corresponds to the typical gamut of today’s LCD and CRT monitors. Not an encouraging picture, is it? I fear we won’t see a pure green color in near future.

Talking about LCD monitors, the reason for this is the poor spectrum of backlight lamps in LCD panels. Cold-cathode fluorescent lamps that are employed in them emit radiation in the ultraviolet range which is transformed into white color with the phosphors on the lamp’s walls.

In nature, it is various hot bodies that are the usual source of light for us. First of all, it is the Sun. The radiation spectrum of such a body is described by Planck’s law and the main thing is that it is continuous, with all wavelengths, and the radiation intensity at adjacent wavelengths differs but slightly.

A fluorescent lamp, just like any other gas-discharge light source, produces a linear spectrum with no radiation at some wavelengths and with a manifold difference in intensity between parts of the spectrum that are only a few nanometers apart. Our eye is insensitive to the type of the spectrum, so it perceives the Sun and the fluorescent lamp as radiating identical light. But it’s different in the PC monitor.

So, there are several fluorescent lamps standing behind the LCD matrix. They are shining through the matrix. On the other side, there is a grid of color filters – red, green and blue – that make up triads of sub-pixels. Each filter cuts a portion of spectrum, corresponding to its pass-band, out of the lamp’s light. This portion must be as narrow as possible to achieve the largest color gamut. Suppose there is a peak of 100 imaginary units at a wavelength of 620nm in the backlight spectrum. We use a red sub-pixel filter with a maximum of transmission at 620nm and seem to get the first vertex of the color gamut triangle, right on the border of the diagram. But it’s not that simple in reality.

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