Display calibration

	http://people.csail.mit.edu/jaffer/Gamma
	Display calibration

This page is primarily about Display Gamma. But display gamma is not the only parameter affecting displays; adjusting it will yield scant improvement of poorly configured displays.

Display geometry can have a large effect on visual fidelity. If the pixels are spread (by insufficient video bandwidth, for instance), then the pixel intensities can be skewed. This is because display technologies intrinsically have non-linear transfer functions.

Gross maladjustments are easy to detect. This astigmatism chart allows you to determine whether pixels are being spread more in one direction than another. If the horizontal and vertical lines in the chart do not appear to have equal density (black vs. black; white vs. white), then your display will show artifacts sufficient to make optimal gamma settings dependent on the content and orientation of images. Click on Astigmatism for more information and solutions to this problem.

Astigmatism
The horizontal and vertical lines should be of equal brightness

In A Standard Default Color Space for the Internet - sRGB, the authors examined the breadth of then current (1996) practice for viewing of photographic images via computers. Finding that CRT displays (then in overwhelming use) exhibited gamma exponents of approximately 2.2, they wisely chose to make 2.2 be the default gamma expected of viewing equipment -- thus assuring instant and widespread conformance with sRGB.

Display Gamma
Gamma chart - Copyright 2000 Aubrey Jaffer

This chart enables human eyes to determine a monitor's gamma at each of 9 distinct intensities. The gamma matches where there is vanishing intensity difference between the solid gray rectangles and the stippled horizontal strips. If most of the gray rectangles are stippled, then your video card is not displaying the 85 grays in this image; and its readings will be skewed towards lower numbers.

The black-level (brightness) control on monitors moves the dark gammas. Adjust it so that the gamma is uniform across the chart (horizontal). On some monitors, the other control (contrast) will not affect the gamma. But with this chart, you can judge for yourself.

If your monitor's gamma is not 2.2, it may be possible for your computer to compensate.

On (Unix) Systems running XFree86 version 4.0, setting xgamma to 2.2 divided by the display's measured gamma, compensates. Thus, if you measure a gamma of 2.0, then the command:
```
xgamma -gamma 1.1
```
should correct it; the gamma chart should then read 2.2.
On Windows 95, 98, and 2k, Adobe PhotoShop has a utility for calibration.
photo.net has directions for changing gamma on other types of computers.

Once you have calibrated your monitor to a (flat) uniform gamma of 2.2, you should find images from recent digital cameras look realistic. The pictures in The Canoe of Damocles were taken with MagicImage 500. A doily drawn using my anti-aliased graphics primitives constitutes a different rigorous test of display fidelity.

I am a guest and not a member of the MIT Computer Science and Artificial Intelligence Laboratory. My actions and comments do not reflect in any way on MIT.
	agj @ alum.mit.edu	Go Figure!