Suppose two monitors have the same matrix with a maximum brightness of 250 candelas per sq. m (and, accordingly, the same nb coefficients), but we want to have a screen brightness of 100 candelas per sq. m. In this case, the monitor with the backlight-controlled brightness reduces the value L in 2.5 times compared to the maximum, but L remains the same in the monitor that controls the brightness through the matrix. Clearly, the level of black (Lb) is going to be 2.5 times lower in the monitor that controls the brightness with the backlight lamps.
Besides that, as I mentioned in the previous section, controlling brightness with the help of the matrix has a negative affect on the response time characteristic. These effects can be clearly seen in Sony’s monitors that allow controlling the screen brightness either with the matrix (the “Brightness” parameter in the screen menu) or with the backlight lamps (the “Backlight” parameter).
But what screen brightness should you choose? It all depends on the task and the external lighting. The screen brightness should be from 70 to 130 candelas per sq. meter for working with text, while 200 and more candelas per sq. meter may be comfortable for games and movies. In contrast to LCD monitors, CRT ones typically have an operational brightness of 90..100 candelas per sq. m (models produced in the last two years feature extra brightness modes, made available by the technologies for high-precision focusing of the ray, but they are still suitable only for movies and games; LCD panels, with their ideal image sharpness at any brightness, have long surpassed them), but the contrast ratio of a good CRT monitor easily exceeds 1000:1, an unattainable peak for the majority of LCD monitors.
Irregular backlighting may also be seen with LCD monitors, mostly with low-contrast matrices. This is often perceived as light or dark stripes and spots (light spots may sometimes correspond to the locations of the backlight lamps), or as light stripes near the edge of the matrix – they occur if the matrix is too pinched in the case during the assembly process (I mean the metallic case of the unit, rather than the plastic bezel of the monitor, which serves decorative purposes only).
So, the conclusion to this section actually repeats what I said at the end of the preceding sections: we can compare two monitors on matrices of the same type by their specified contrast ratios (the higher specified contrast ratio will most probably result in a higher real contrast ratio), but we can’t compare monitors on matrices of different types. Then, we also cannot say anything about the absolute (not relative, as in terms of “better-worse”) contrast ratio basing only on the numbers provided by the monitor manufacturer.