Some people try to justify the LCD monitor manufacturers saying that an extra-high contrast ratio strains the eyes. This is not true at all. You just can’t have a “too low” level of black. Ideally, black should be not just low, but equal to zero – this would mean that the monitor is reproducing the pure black color, without any tricks. Of course, the specified contrast ratio would be very high in this case (the screen surface is not an ideal black body and will reflect external light, but I’m speaking about the specification now, which is measured without any external lighting).
There’s one more myth: some say that the manufacturers are increasing the specified contrast ratio of their matrices by increasing the brightness of white color, keeping the intensity of black color on the same level. Thus, the specified contrast ratio grows up while the effective one remains the same since the user works with a contrast ratio that’s comfortable for him/her rather than at the maximum possible. It’s clear from the operational principle of LCD matrices that their brightness can only be increased by intensifying the backlight.
Given L is the backlight intensity, the level of white color is then Lw=L*nw, where nw is the pass-through coefficient of the open pixel (it is slightly below 1, since some portion of the backlight is still lost on its passing through the crystals and polarizers). The level of black is then calculated as follows: Lb=L*nb, where nb is the pass-though coefficient of the closed pixel (it’s slightly above zero). The contrast ratio C is thus described by the formula: C= L*nw/ L*nb=nw/nb. The pass-through coefficients of the closed and open pixels depend on the characteristics of the matrix only, but not on the backlight intensity, so the specified contrast ratio of the matrix is in no way dependent on the backlight, but is only determined by the matrix’s own properties. Thus, the manufacturer cannot better the specs of a matrix by increasing its brightness – this common opinion is not grounded at all.
Sometimes it is argued that the external lighting should also be taken into account. For example, a normal daylight in the room does contribute to the level of black. In this case, the “visual” contrast ratio is Cvis=(L*nw+Lext)/(L*nb+Lext), where Lext is the external lighting. As follows from the formula, higher L values lead to higher Cvis values. But I want to repeat once again that I’m talking about the specified contrast ratio of the matrix as measured by the manufacturer without any external lighting.
Besides the fact that the contrast ratio of the matrix is measured on a special test platform, rather than in the finished monitor (i.e. without counting in the specifics of the electronics of this monitor), the user is also free to tweak the brightness and contrast settings, thus affecting various parameters of the image. The way the settings affect the image depends on the realization of the controls in the particular monitor model.