Gamma is a correction to the contrast of images and displays, performed by either software or hardware, and designed to correct for the fact that the intensity displayed on a monitor is not linearly related to the input voltage. With gamma values higher than 1.0 the pixel intensity becomes non-linear, such that you can see more dark shades and less bright variations. This relationship is:
Displayed_Intensity = Pixel_Value to the power of Gamma ("to the power of" is often shown as "^").
Gamma correction consists of applying the inverse of this equation to the image by computing
Macintosh monitors have a default gamma of 1.8 (you can use your broadband connection to take advantage of the various links and downloads offered on this page. You may change default gamma with System Preferences: Displays-Color-Callibrate...), PCs with Windows should have 2.2 (to correct it use Control Panel-Adobe Gamma...), and Silicon Graphics workstation monitors are set to gamma of 2.4. Note that Photoshop has its own settings that can be changed within View-Proof Setup menu, so that what you see in the Photoshop environment may look different outside this application.
Most systems are set up for gamma value to be 2.2. I recommend setting this up that way even on Macintoshes, so everybody on internet looks at the given image the same way. If it's too high, pictures created on other systems may be too washed out, if it's too low the monitor's contrast seems too high with light areas being too bright. Also, ghosting of anaglyphs is exaggerated with uncalibrated monitors. To calibrate your monitor first look at this image:
If both sides of the image above have the same brightness intensity, and you see that it consists of horizontal and vertical lines, click on the chart thumbnail below to get the test chart in FULL resolution. Important note: when you open the above links, you must view the charts in 100% magnification. Otherwise, it won't work at all. If the brightness of both sides is different, you are using an older type of monitor with analog input where the signal bandwidth is limited, so that you need use this chart. Otherwise, click on the chart thumbnail below.
Open it with an image editor or by a web browser, but you must view it at 100% magnification. Decrease the window size to see only 9 spots (without the text to prevent confusion), and scroll the image down so that you see darker circles than the background on the top right and lighter on the bottom left. Then, the circle in the middle (should be the best blended with the background) will show your monitor gamma. Squinting your eyes may help finding it. Widen the window to read the value. Adjust contrast of your monitor to get the gamma value of 2.2 (read the fine print below). Now your monitor is standarized and you'll see minimum ghosts. More on gamma you may find on answers.com and AbleStable pages.
