Articles: Monitors
 

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Monitor Features: Behind the Scenes

In my previous reviews devoted to the features of 15” LCD monitors and 17” LCD monitors I mostly focused on one characteristic of an LCD display – response time. Other parameters were omitted altogether or measured subjectively. For example, take a parameter like “comfortable brightness”. This is a vague thing, which is not constant even for myself. It may depend on a number of factors like whether it is cloudy outside or the sun is shining through the windows.

However, there are a few characteristics of a display that can be measured with a certain precision: viewing angles, brightness, contrast ratio, color temperature and quality of color rendition. You may question the latter parameter, but color rendition can indeed be described in numbers. To do this (and to correct it if necessary) you need a device called “hardware color calibrator”. One calibrator like that, Pantone ColorVision Spyder with OptiCAL software, helped me throughout my tests.

ColorVision Spyder is a sensor attached to the screen (with CRT displays you can simply stick it to the glass with three suction cups, while with LCDs you hang it down the upper side of the display case, because the cups are too strong to use them with pressure-sensitive LCD matrixes). Eight photo-sensors are hidden behind the matte plate: seven of them are with color filters (to measure color rendition) and one without any filter (for measuring brightness).

The OptiCAL software, which is bundled with the calibrator allows creating a color profile for a display, and also checking out the main display properties like color curves, color temperature, black and white brightness and so on…Let’s explain the meaning of each of these terms first.

Color curves represent the dependence between the input signal (the color as produced by the graphics card) and the output signal (the screen brightness). The curve is described by the “gamma” number (output signal = input signal in the power of gamma; the maximum level – 255 in 8-bit-per-component representation – is taken for a reference unit equal to 1). The meaning of this parameter is in gamma compensation, which helps to increase precision of dark tones rendition. For example, the use of gamma 2.2 equals expanding the color representation to 10 bit per channel for dark tones. Of course, it is not for free – light tones are rendered worse (gamma 2.2 is equivalent to using 7-bit-per-channel encoding for them). In fact, gamma correction is performed twice. First, when the image is created (the resulting brightness will equal the gamma-root of the original brightness). Second, when the image is reproduced on the screen so that the user sees the original image.

 
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