Articles: Monitors

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So, a CRT monitor is much brighter than an LCD monitor at daylight when they are both turned off. If we now turn the monitors on, the LCD will get a bigger addition to the level of black than the CRT due to its lower specified contrast ratio, yet it will still remain darker than the CRT. But now if we “turn out” the daylight by drawing the blinds, we’ll get just the opposite – the CRT monitor will yield a deeper black!

Thus, a monitor’s real contrast depends on ambient lighting. The more intensive it is, the better for LCD monitors, which provide a high-contrast picture even under bright lighting whereas CRT monitors show then a faded picture. But CRT monitors have an advantage in darkness.

By the way, this partially explains why monitors with a glossy coating of the screen look so good, at least in the shop window. The ordinary matte coating disperses the light it receives in all directions while the glossy coating reflects it in one direction, like a mirror. So if the source of light isn’t behind your back, the matrix with a glossy coating will seem to have more contrast than a matte matrix. But if the light source is behind you, it’s all different: the matte screen is still dispersing the light more or less uniformly, while the glossy coating is reflecting it right into your eyes.

What I am talking now refers not only to CRT and LCD monitors, but to all other display technologies. For example, the forthcoming SED panels from Toshiba and Canon have a fantastic specified contrast ratio of 100000:1 (in other words, their black color is absolutely black in darkness), but are going to fade at daylight just as the CRT does. SED panels use the same phosphors that are shining when being bombarded with a cathode ray and there is a layer of black tone film in front of them. It is impossible to decrease the transparency of the film (to increase the contrast ratio) with CRTs due to the ray becoming unfocused, and it is going to be impossible with SED panels due to the reduction of the service life of the emitting cathodes at higher currents.

Recently there have appeared LCD monitors with high specified contrast, up to 3000:1, but using the same matrixes as are employed in the monitors with more traditional numbers in the specs. It’s because the so-called dynamic contrast ratio rather than the ordinary contrast ratio is specified.

The concept is in fact simple. Each movie has both bright and dark scenes. In both cases our eyes perceive the brightness of the whole picture at once. So if most of the screen is bright, the level of black in the small dark areas doesn’t matter, and vice versa. It is reasonable then to automatically adjust the backlight brightness depending on the onscreen image. The backlighting can be made less intensive in dark scenes, to make them even darker, and more intensive, up to the maximum, in bright scenes, to make them even brighter. This automatic adjustment is referred to as dynamic contrast ratio.

The official numbers for dynamic contrast are arrived at in the following manner: the level of white is measured at the maximum of backlight brightness and the level of black is measured at its minimum. So if the matrix has a specified contrast ratio of 1000:1 and the monitor’s electronics can automatically change the intensity of backlight brightness by 300%, the resulting dynamic contrast is 3000:1.

It should be made clear that dynamic contrast mode is suitable only for movies and, perhaps, for games, but gamers often prefer to increase brightness in dark scenes for an easier orientation in the surroundings rather than to lower it. For ordinary work this mode is not just useless, but often irritating.

Of course, the screen contrast – the ratio of white to black – is never higher than the monitor’s static specified contrast ratio at any given moment, but the level of black is not important for the eye in bright scenes and vice versa. That’s why the automatic brightness adjustment in movies is indeed helpful and creates an impression of a monitor with a greatly enhanced dynamic range.

The downside is that the brightness of the whole screen is changed at once. In scenes that contain both light and dark objects in equal measure, the monitor will just select some average brightness. Dynamic contrast doesn’t work well on dark scenes with a few small, but very bright objects (like a night street with lamp-posts) – the background is dark, and the monitor will lower brightness to a minimum, dimming the bright objects as a consequence. But as I said above, these drawbacks are negligible due to the peculiarities of our perception and are anyway less annoying than the insufficient contrast of ordinary monitors. I guess most users are going to like the new technology.

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