We continue our study of the features of 19” liquid-crystal displays. The methodology of our tests has not changed a jot. We still measure the response time characteristic and use a Pantone ColorVision Spyder calibrator for measuring the brightness and contrast ratio of the tested displays and drawing their color curves. For more information on the testing methodology see our articles called Closer Look at 19” Monitors Features: Pixel Response Time and More! and Closer Look at 15” LCD Monitors Features: Pixel Response Time.
Before proceeding to the tests, I would like to say a few words about color rendition. Why does it matter for an LCD at all?
In fact, color rendition is determined by the correlation between the input signal (as shaped by the graphics card, in our case) and the output signal (the brightness of a pixel). This dependence is described by the formula “Output = Input in the power of gamma”. We automatically, without any correction, arrive to this correlation if we use a CRT display since this is a property of the electron gun itself. There is only interfering factor. The gamma equals 2.5 for CRT displays, while the industry standard, sRGB, specifies the gamma to be 2.2. This divergence is easily eliminated by the display electronics. Thus, we can see straight and neat color curves in CRT displays:
Note that these graphs have been taken on a cheap 17” CRT display, Samsung SyncMaster 750s rather than on a professional color-calibrated display. The only noticeable drawback we can see here is that the curves for different colors may go a little higher or lower than the theoretical curve. For example, in the above case, the green curve goes a little above the red and blue ones. It means the screen colors will have a slight greenish tint. This is easily fixed by adjusting the RGB settings of the display or the graphics card – just shift the green color slider a little to the left and the green curve will go down to coincide with the other two. Thus, if you don’t demand anything exceptional from the color rendition of your CRT display, you can just set up the white balance to feel quite happy.
It’s all much more complicated with those liquid crystals. LCDs don’t have an “inborn” power dependence of the pixel’s light-transmission coefficient (it determines the luminosity) on the input signal. So, it is the display electronics that should produce a well-shaped power curve from the non-linear and irregular characteristic of the liquid crystal cell. The result is often disappointing. You can view the color curve graphs for most of the tested displays to see that the ratio of the three basic colors varies along with the level of the input signal. For example, the blue curve may go much higher than the red and green ones in the middle of the range. Then they may all coincide at the end of the range, among the light tones, while the red curve may dominate the dark tones. The visual outcome is quite predictable: the dark tones are a little reddish, the gray color is a little bluish, while white is all right. You can do nothing about such RGB settings. For example, if you lower the level of blue you get a normal gray color (the middle of the range), but blue will also go down along the entire range, making the dark tones more reddish and the white color – yellowish (too much of red and green). These color defects can be easily perceived by the eye, even if you are just viewing your family photos, not expecting any professional quality of the color reproduction.