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Color Reproduction: Color Temperature

As I mentioned in the previous section, the term white color is subjective and depends on external conditions. Now I’m going to dwell upon this topic for longer.

So, there is actually no white color that could be regarded as an etalon. We could take a flat spectrum (with equal intensity of each wavelength in the optical range) as such, but there’s one problem. This color would look a cold, bluish color to the human eye in most cases. It wouldn’t be white.

As a matter of fact, our brain regulates the white balance, just like you do with your camera, depending on ambient lighting. An incandescent lamp looks just a little yellowish in the evening at home, although the same lamp will look totally yellow if you turn it on in a slight shadow on a sunny day. It’s because our brain adjusts its white balance for predominant lighting, which is different in these two cases.

The desired white color is commonly described with the term color temperature. It is the temperature an ideal blackbody must have to radiate a light of the corresponding color. For example, the surface of the Sun is about 6000K hot. And the color temperature of daylight on a sunny day is indeed defined to be 6000K. The filament of an incandescent lamp is about 2700K hot and the color temperature of its light is 2700K, too. Funnily enough, the higher an object’s temperature is, the colder its light seems to us because blue tones become predominant.

The notion of color temperature becomes somewhat irrelevant for line-spectrum sources like the above-mentioned CCF lamps, because their radiation cannot be compared with the continuous spectrum of an ideal blackbody. So when it comes to such light sources, we have to consider how their spectrum is perceived by our eye, and the instruments for measuring the color temperature must perceive the color just like the eye does.

You can set up the color temperature of your monitor from its menu. You’ll find some three or four preset values in there (much more in certain models) along with the option of adjusting the levels of the basic RGB colors. The latter option isn’t convenient in comparison with CRT monitors in which it was the temperature rather than the RGB levels that you could adjust, but it is a kind of a standard option for LCD monitors, except for some expensive models. The goal of color temperature setup on a monitor is obvious. It is the ambient lighting that serves as the sample for setting up the white balance, so the monitor must be set up in such a way that its white color looked like white, without a bluish or reddish tone.

The most disappointing thing is that the color temperature on many monitors varies greatly between different levels of gray. It’s clear that gray differs from white with brightness only, so we can as well be talking about a gray balance rather than a white balance, and it would be even more correct. And the balance for different levels of gray proves to be different on many monitors.

Above you can see a photograph of the screen of an ASUS PG191 monitor with four gray squares of different brightness. To be exact, there are three versions of the photograph put together. In the first of them, the gray balance is based on the rightmost (fourth) square, in the second – on the third square, in the last one – on the second square. It’s wrong to say that one version is correct and the others are not. They are all incorrect because the monitor’s color temperature shouldn’t depend on what level of gray you measure it by. This is not so in this case. This situation can only be corrected with a hardware calibrator – not with the monitor’s settings.

That’s why I publish a table with measured color temperatures of four different levels of gray in my monitor reviews. If the temperatures differ by much, the onscreen image will have a tincture of different colors like in the picture above.

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