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Gamma Correction
by Adam Powell 26 Jun 1997


Page 1

Everyone who makes images for the Web should know about gamma correction.

In a nutshell, gamma correction changes the overall brightness (and color saturation) of an image as it is displayed on a monitor. If a display is gamma corrected, the nonlinear relationship between pixel value (the number assigned to a particular color tone) and displayed intensity (the way it actually looks) has been adjusted for.

How's it work? Your computer tells your monitor to display colors on the screen by sending a particular voltage to a pixel that needs color information. Problem is, nearly all monitors multiply this voltage by a factor of 2.5. Since this voltage is usually between 0 and 1, when multiplied by a factor of 2.5 it usually equals less than 1, so monitors without the proper correction level tend to be too dark, with not enough contrast.

This is where the gamma correction comes in. The idea is to adjust the signal before it hits the monitor to make sure the proper color is displayed. But it's not just a simple boost because different colors and intensities call for different voltage increases (in fact, correction software boosts the voltage to a factor of 1/2.5). If the level of correction is off, or if the color isn't corrected at all, the image will look different on that monitor - hence the disparity between the way images look on the Web from monitor to monitor.

Macintoshes, designed in part for graphics manipulation, are built to recognize the gamma-correction factor in the monitor and adjust the output accordingly, starting from a gamma-correction factor of 1.8. PCs run on a default (2.5) and therefore tend to be quite a bit darker. To dispel a myth, it's not true that PCs don't have gamma correction. All platforms are set up to gamma correct monitors, but some are better at it than others, and PCs were not, until very recently, intended for the production of graphics.

It's worth noting that the amount of gamma correction applied to a signal changes not only the brightness of the image, but also the ratios of red to green to blue, so an old or poorly gamma-corrected monitor can really make your image look weird.

There's a wealth of good information available on the Web concerning gamma correction for those who want to know more. For many people, the most important thing to know is that gamma correction exists and is a factor in color display, and that any monitor-repair specialist will have the hardware and software necessary to adjust your monitor to accurately represent the pixel values you send to it.

Your local computer fix-it person can test your monitor and bring it back into line using gamma correction software. In some cases you can do this yourself. Once your monitor is true, you can design images for the Web with a median gamma correction in mind: 2.2 (the basic gamma setting on a Macintosh is 1.8, and the almost all PC monitors are set to 2.5). You can set this as an environmental global in Photoshop 3.0 by selecting Monitor Setup from the File/Preferences menu. Remember to change it back if you are outputting to print.

There are other issues involved in how your image looks on the Web, such as the browser-platform combination it is viewed on. And there's nothing you can do about folks viewing your slick images on their own poorly adjusted monitors. Test your entire site on as many monitors as you can, and make sure the monitor you design with is all tuned up.


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