- HIKING TRAILS AND WILDFLOWERS by Keith and Barbro McCree -

Gamma Correction

Correcting for monitor non-linearity

In the cathode ray tubes that are used in many computer monitors, the brightness of the screen is not a linear function of the input voltage to the electron gun, but is proportional to the voltage raised to the power gamma, where gamma is about 2.5. This non-linearity causes the bright parts of a photographic image to be brighter than they should be, and the darker parts to be darker. In other words, the contrast is artificially increased by the monitor. The term 'gamma correction' is used to describe the correction of this error by applying an inverse power function.

In the television industry, a standardized gamma correction is applied in the camera, before the program is broadcast, so that every viewer sees a balanced image. No such standard exists in the computer industry. Most computer makers ignore the effect of monitor gamma, although sometimes a correction is built in to the video card.

Photographers who are using a computer that lacks built-in gamma correction will instinctively reduce the contrast with photo-editing software until their digital pictures look good on their machine. Unfortunately, this will make the pictures look 'washed out' on computers that already have gamma correction.

The Macintosh computer that I use has built-in lookup tables to correct for monitor gamma, but allows me to bypass the tables. I used these features to construct two images that show the effects of gamma correction (or the lack of it) on the appearance of digital photographs on different computers.

I adjusted the brightness and contrast of the first image to look good on my computer with hardware gamma correction switched on. To construct the second image, I switched off the hardware gamma correction, then I applied software gamma correction (using the 'Levels' slider adjustment in Adobe Photoshop LE) until the image looked about the same as the first image.

This image is optimized for viewing on a computer, such as the Macintosh, that has built-in gamma correction. When viewed with computers that don't have gamma correction, it will look too dark and have too much contrast. The frequency analysis shows that the image has an abnormal number of pixels (picture elements) of low brightness.

This image was software-corrected to a gamma of about 1.4. It will look 'washed out' on a computer with built-in gamma correction (because the correction has been applied twice), but should be acceptable on computers that don't have gamma correction. The analysis shows a more normal brightness distribution: most of the pixels are now near the center of the range.

If the first picture looks better on your screen, chances are that your computer has built-in gamma correction. You don't need to do anything. If the second picture looks better, your computer has not applied the proper correction for monitor error. You will have to adjust gamma manually.

Of course, how the pictures look will also depend on the quality of the monitor itself, and how you have adjusted the brightness and contrast controls.

Keith McCree, 1998

How to set your monitor controls for viewing digital photographs

Macintosh users:

The Monitors control panel allows you to select the number of colors shown on your monitor. You should select the largest number available, preferably millions of colors. If this level is not available, you may be able to reach it by installing additional video memory (VRAM).

The Monitors control panel also allows you to vary the gamma correction. On older systems, click the Options button to reveal your choices (you may have to hold down the Option key while clicking on the button). In newer systems, click the Color button to see the built-in color profiles. Click the Calibrate button to create a new profile ('Expert Mode' gives you the most choices). Save your new profile and select it. In general, you have the following choices :

• Mac Std Gamma applies an adequate amount of gamma correction. However, you will probably find that photographs will have a cold blue cast on your screen. Technically, this is because the color balance is set so that whites have an abnormally high color temperature, 9300 degrees Kelvin.
• Page-White Gamma applies the same gamma correction, and also shifts the color balance so that whites appear more normal. The color balance is adjusted to a color temperature of 6500 degrees Kelvin, which is the color of white paper in daylight.
• Uncorrected Gamma bypasses gamma correction altogether. This simulates viewing on a machine that does not have gamma correction. Try this option if the photograph you are viewing appears 'washed out'.

I recommend using Page-White Gamma, which applies both gamma correction and white color balance correction.

With the proper color correction, the white backgrounds of the diagrams on this page will match a sheet of white paper held up to the screen in daylight. If you have a digital camera, you can test your white color balance by taking a photograph of a sheet of white paper in daylight, then comparing the image on your screen with the white sheet.

Windows users should look for gamma adjustment as an 'Advanced' option in the Display settings.