Articles: Monitors

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Contrast Ratio: Specified, Actual and Dynamic

The fact that a good CRT monitor provides better contrast than a good LCD monitor has long been regarded as an a priori truth that doesn’t need more proof. We can all see how bright a black background seems on LCD monitors in darkness. And I don’t want to deny this fact. It’s hard to deny what you can see with your own eyes even when you are sitting at a newest S-PVA matrix with a specified contrast of 1000:1.

The specified contrast is usually measured by the LCD matrix maker rather than by the manufacturer of the monitor. It is measured on a special testbed by applying a certain signal and at a certain level of backlight brightness. The contrast ratio is the ratio of white to black.

It may be more complicated in an LCD monitor because the level of black may be determined not only by the matrix characteristics, but sometimes by the monitor’s own settings, particularly in models where the brightness parameter is regulated by the matrix rather than by the backlight lamps. In this case, the monitor’s contrast ratio may turn to be lower than the matrix’s specified contrast ratio – if the monitor is not set up accurately. This effect can be illustrated by Sony’s monitors that allow to control brightness either with the matrix or with the lamps. If you increase the matrix brightness above 50% on them, the black color soon degenerates into gray.

There is an opinion that the specified contrast ratio can be improved by means of backlight brightness – that’s why many monitor manufacturers employ so bright lamps. But I want to tell you that this opinion is totally wrong. When the backlight brightness is increased, the level of both white and black grows up at the same rate, so their ratio, which is the contrast ratio, does not change.

Well, this all has been told a lot of times before, so let’s better move on to other questions.

Surely, the specified contrast ratio of modern LCD monitors is yet too low to match good CRT monitors – their screens are noticeably bright in darkness even if the image is all black. But we usually use our monitors not in darkness, but at daylight, sometimes very bright daylight. It’s clear that the real contrast ratio is going to differ from the specified one under such conditions because the monitor’s own light is added up with the external light it reflects.

Here is a photo of two monitors standing next to each other. One is a CRT monitor Samsung SyncMaster 950p+ and the other is an LCD monitor SyncMaster 215TW. Both are turned off and there is a normal ambient lighting, sunlight on a cloudy day. It can be easily seen that the screen of the CRT monitor is much brighter under than the screen of the LCD monitor. That’s exactly the opposite of what we see in darkness when the monitors are both turned on.

It’s easy to explain: the phosphor they use in cathode-ray tubes is light gray by itself. To make the screen darker, tone film is put on the glass. The phosphor’s own light goes through that film only once, but the external light has to pass it two times (on its way to the phosphor and then, bouncing back from it, to the outside, into the user’s eye). Thus, the intensity of the external light is weakened much more by the film.

CRT monitors can’t have an absolutely black screen. As the film becomes more opaque, the brightness of the phosphors must be increased since it is weakened by the film, too. And this brightness is limited to a rather moderate level with CRTs, because when the current of the cathode beam is increased, its focusing worsens, producing a fuzzy, unsharp image. That’s the reason why the maximum reasonable brightness of CRT monitors is not higher than 150 candelas per sq. meter.

With LCD monitors, the external light has almost nothing to get reflected from. LCD matrixes don’t have any phosphors, only layers of glass, polarizers and liquid crystals. A small portion of light is reflected from the external surface of the screen, but most of it penetrates the screen and is then lost forever. That’s why the screen of a turned-out LCD monitor looks almost perfectly black at daylight.

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