There can be a contrary situation as well when the display cannot distinguish between different light tones and they are all reproduced with the same brightness in the given range. This may occur when the contrast level is set too high (as you know, it is the contrast setting that controls the brightness of the white color). To give you an example, here is a graph taken from a display with the contrast set to maximum:
It is obvious from the picture above that about 40% of the red color is reproduced with the same – maximum – brightness.
Yet another parameter that can be measured is color temperature. It determines the tone of the onscreen image. The lower is the temperature, the warmer are the colors (you have to put up with this terminology, because a man perceives the spectrum of warmer objects as cold ones). Color temperature is measured in Kelvins (K) and equals the temperature of the absolute black body radiating the same spectrum. There are two most commonly used values. Printing and photography often involve 5500K temperature (this value was introduced by folks at Kodak under the name of “daylight”. There is a joke that in real world this value equals the color temperature of midday sunlight near the offices of this company). For computer-processed images (on displays), 6500K is used (for comparison: 6000K color temperature corresponds to bright sunlight in a clear sky, while a slightly clouded sky has 6500…7000K color temperature). Sometimes, but quite rarely, the color temperature is set to 9300K giving out a slight bluish hue (an analog from nature – it is the color temperature of a thin shadow on a bright day). Thus, the standard color temperature for PC displays equals 6500K and I am going to use this value later on. Furthermore, if the color rendition is defective, the color temperature as measured on white and on gray colors will differ. That is why I will always mention two temperatures: one for the pure white (255:255:255 in the RGB model) and the other for the 50% gray (128:128:128 RGB). The closer these two values are to each other, the better calibrated is the display.
The brightness of a display is not controlled by the “Brightness” button, despite the common delusion. This is true for the “Contrast” button, too. These are rather confusing terms, as they actually both adjust brightness: “Brightness” controls the level of the black color, while “Contrast” – the level of the white color. Contrast or rather contrast ratio is estimated as the ratio of the level of white to black. The sensitivity and dynamic range of the ColorVision Spyder sensors allow measuring the brightness of both: black (it is usually about 0.5-1.5 nit for LCD displays; 1 nit = 1 candela per square meter) and white colors (this may be up to 300nit). I will mention these two numbers for all the displays, and the contrast ratio will be then calculated from them. Moreover, as the brightness should be measured under a definite color temperature, all displays are calibrated to 6500K temperature before the tests.Brightness is the more interesting parameter to measure as many manufacturers include the matrix’s rather than the display’s parameters into the specifications. Meanwhile, the supported range of brightness and contrast values obviously depends on the electronics and firmware of the display, and the matrix only determines the maximum and minimum of this range. I guess it will be interesting to what extent the potential of the matrix is actually used. Moreover, there are two ways to control brightness in a display: by adjusting the matrix itself or by changing the backlight brightness. In the second case, the reduction of the backlight brightness may lead to a lower level of black than written in the matrix’ specs. That is, the contrast ratio will be higher than specified. Our tests are going to show you whether this is the case with some of the displays we use today.