How to find the gamma of your display

To view images (colour or monochrome) you must have the gamma on your monitor set correctly. If the gamma is incorrect, then images will either look too dark or light.

Take a look at this alignment image... Please wait whilst the image downloads fully (13Kb)...


Now view this chart without gamma correction, ie. without adjusting your monitor. Find the square on the right which appears the same brightness as the strip on the left. The number inside that box is the gamma correction factor for your monitor.

To make adjustments, your monitor should have been powered up for at least one hour and the ambient light should be normal (medium brightness). The background colour on this page has been specially choosen so not to distract from the alignment chart.

The theoretical gamma correction for your monitor should be the 1.0 box. Adjust your monitor's contrast and brightness controls so that the 1.0 box displays the best brightness match with the left hand strip. Your monitor should now be setup correctly.

However in practice you may find that your monitor now seems too bright or dull to which you are use to, so you may wish to adjust again !!! My own monitor which is a Sony 17", Seems to have a gamma of 2.5. If I align for 1.0 images look far too dark. Monitor Gamma changes from different manufacturers and can also change as the CRT(tube) ages.

You may find that one of the photo or graphic applications on your PC has a gamma correction facility hidden away somewhere. If you find one, use it. It should be more accurate that the above image.


Gamma Correction - The Technical Stuff
When images are created, a renderer generally expects that the file will be displayed in a photometrically linear fashion. That is, value 200 in the file should produce a pixel that gives off twice as much light as a pixel with value 100. Alas, this is far from the case for cathode ray tubes (CRT's). The relationship between voltage and brightness of the phosphors is more closely approximated by: I = V^gamma where I = intensity of the pixel and V = voltage on the signal. The term 'gamma' refers to the nonlinearity of the transfer function. Gamma correction refers to altering the pixel values (or the lookup table) by the inverse of this function, i.e.: I' = I^(1/gamma) This corrects for the nonlinearity in your display, so that you will view the file in the way that the renderer intended. If you use the wrong gamma correction, your image will appear too dark (among other problems). It is important to remember that gamma is a property of your display device. It may be different for each monitor that you use. It so happens that the newer SGI monitors have a gamma of about 2.2. Strangely enough, the default hardware lookup gamma correction is 1.7, which is not correct! So to display high quality images properly, you need to tell your display software to do the gamma correction for you. For example, the popular "xv" program takes a command line argument "-gamma", followed by the gamma of your display.