A properly set-up monitor reproduces the entire color range in a rather wide range of user-defined settings (for example, for the white color brightness from 50 to 150 candelas per sq. m), but some models can only work normally at the default settings. If you move the brightness or contrast controls just slightly off the ideal position, you lose either dark (when increasing the brightness) or light (when increasing the contrast) tones.
You can also notice that the curves of different colors do not coincide on the diagram – some go lower, so go higher… This leads to deviations of the image tonality (in other words – of the color temperature) from the desired, and as the difference between the curves of the basic colors may vary throughout the dynamic range, the deviation of the color temperature will also vary depending on whether we’re outputting light or dark tones.
This makes it impossible to correct the tonality precisely with the monitor’s standard settings – i.e. with the independent RGB controls – as having achieved the necessary color balance at one part of the range (for light grays, for example), you will find you’ve worsened this balance at another part of the range (dark grays). The only way out of this predicament is calibration of the monitor with a hardware calibrator that creates an ICC profile basing on all the peculiarities of the monitor’s color reproduction.
Well, you can try to create a profile manually, as special software for that purpose comes enclosed with many monitors, but manual calibration isn’t always a success. Sometimes the color reproduction of the monitor is originally so badly set up that even a hardware calibrator is useless.
I said “color temperature” above, so let’s consider this characteristic in more detail. The color temperature determines the tonality of the onscreen image. The lower the temperature – the warmer the colors are (that’s the way we’re all perceiving colors – the spectrum of hotter bodies is perceived as cold). The need of such temperature arises as the human eye doesn’t have any universal white color, which would always be white. The eye adjusts itself to a specific range depending on the environment.
You can see this in the following experiment: take a cell phone with white highlighting of the screen and put it on a sheet of white paper. Then take a look at it at normal daylight and at lamplight. In the first case, the phone screen would seem white or even slightly yellow; in the second case it will have a bluish tone, since the eye will base its “balance of white” on the sheet of paper and the color of the sheet of paper is determined by the spectrum of the light source. The spectrum of lamplight is bluish, of daylight – yellowish. In the same way the tonality of the white color on the screen of a monitor will vary slightly depending on the external lighting, only in a slighter degree, since the monitor’s screen is bigger than the phone’s and the eye is adjusting itself to the screen, too.
That’s why it is recommended to set a color temperature that would produce pure white color without any additional tones – under the given external lighting.