X-bit labs

X-bit’s Guide: Contemporary LCD Monitor Parameters and Characteristics (page 7)

Category: Other

by Oleg Artamonov

[ 10/26/2004 | 02:58 PM ]

Brightness and Contrast

Strictly speaking, it’s not correct to talk about brightness and contrast of an LCD monitor, because the manufacturers often just copy the specification of the matrix the monitor is based on. However, while the monitor’s electronics doesn’t affect such parameters as response time and viewing angles, it is not so with brightness and contrast.

Let’s first clarify the meanings of the terms: brightness is the luminosity of white color (i.e. the matrix receives the maximum signal) in the center of the screen, and contrast (or contrast ratio) is the luminosity ratio of white to black, also in the center of the screen.

The contrast-related problems are natural for LCD matrices due to the very principle of their operation. Unlike with the absolute majority of electronic display devices (CRT, electroluminescent and LED displays, OLED and so on), the LCD matrix is a passive rather than active element. It can’t radiate light; it can only modulate the light that is passing through it. There’s a backlight unit behind the LCD matrix, while the matrix only controls its own opacity, weakening the light to a certain extent. It achieves this by turning the polarization plane: the liquid crystals are placed between two co-directional polarizers.

So if the light hasn’t changed its polarization plane between them, it goes through the second polarizer without any losses. But if the polarization plane has been turned by the liquid crystals, the second polarizer will stop the light stream and the corresponding cell will look black. But due to various reasons (the polarizers are not perfect, and the crystals cannot be perfectly positioned, too) it is impossible to stop all the light, so some portion of the backlight will pass through the matrix, highlighting slightly the black color of the monitor.

As I mentioned above, these measurements are performed by the matrix rather than monitor manufacturer, on a special platform where the matrix is attached to a test-signal source and the backlight lamps are powered with an electric current of a definite value. Thus, they arrive at some etalon values. In a real monitor, however, the electronics adds to the results, too. The electronics differ from the laboratory’s signal generator and is also adjusted by the user to some extent who can control brightness, contrast, color temperature and other parameters of the device.

Thus, the real parameters of a monitor often don’t comply with its specs. For example, if the monitor’s electronics “highlight” black color a little (of course, it is an obvious defect, but quite a widespread one among some inexpensive models), the real contrast ratio proves much lower than specified.

Moreover, a contrast ratio of 500..700:1 as declared by many matrix manufactures for their products is still far from perfect, notwithstanding the seemingly high number. In fact, the monitor with such a contrast ratio cannot provide the really deep black color – black is going to look dark-gray in a dimly lit environment. If the real contrast ratio is only 200..300:1, it is easy to notice the backlight lamps to shine through the black color.