X-bit labs

Closer Look at 19” Monitors Features: Pixel Response Time and More! (page 2)

Category: Other

by Oleg Artamonov

[ 10/06/2003 | 07:18 AM ]

Originally, in the times when Apple Macintosh was the basic graphics workstation, the standard gamma number was 1.8 (1.72, to be more exact). But IBM-compatible PCs were gaining their ground and they had gamma = 2.5 (the gamma of the crystal-ray tube itself varies in a range from 2.45 to 2.55). Of course, this diversion was not satisfying. If an image is intended for a 1.8-gamma display, it will look too bright on a 2.5-gamma one and vice versa. As a result, Microsoft and Hewlett-Packard, later joined by Pantone and Corel, developed the sRGB standard (“A Standard Default Color Space for the Internet”). This standard established the gamma to be 2.2. Below you can see three variants of one and the same image, but compensated for different platforms: the first one is linear, the second has a higher gamma value, and the third has the highest gamma. It is clear that a higher gamma leads to crisper dark tones, but light ones begin to blur into one single background. Of course, if we look at these images on displays corrected for the corresponding gamma, we would see three pictures of the same brightness, but with different precision of dark and light tones rendition.

Linear

Gamma 1.8

Gamma 2.2

Thus, we now have two widely accepted modes: gamma 1.8 for the Mac and gamma 2.2 for the PC. I will stick to gamma 2.2 in this review, since only two displays of the tested bunch allow choosing some other value. The figure below is an example of measuring the color curve for the blue component at 2.2 gamma and 6500K color temperature: the measured curve is black, and the estimated curve is blue. The X-axis is the input signal level, while the Y-axis – the output one.

I chose this curve on purpose, so that it could be easier to reveal possible color rendition errors. There are two of them in this example. First, the measured curve goes below the estimated one everywhere, save for the light tones (in the right part of the graph). It means that we cannot achieve an adequate color rendition by simply increasing the brightness of the blue color in the display settings, as the entire curve would go up. That is, giving enough blue to dark tones, we get surplus of blue in light tones. To cure this situation, display color profiles are used that can correct color rendition more flexibly. Second, and this is a more serious drawback, the display with the current settings does not distinguish between the darkest tones of blue in the first third of the range: no matter how intense the input signal is, the screen brightness equals zero. This is usually cured by increasing the display brightness, although you may encounter various pitfalls. For example, brightness increase may cause growing intensity of the black color, so that the black color will look more like dark-gray pretty soon.