Understanding gamma

Display gamma is numerical expression that describes the relationship between signal input and light output of a display device. As you increase signal voltage, the traditional CRT display does not produce increases in light output in a linear way. The relationship between signal input and light output is non-linear. To correct for this, reciprocal non-linearity is applied at the mastering stage. This is typically referred to as camera gamma. The combination of these two opposite nonlinear luminance curves—camera gamma at the mastering end and display gamma at the device end—results in a linear system gamma of 1.0, which is what we want. However, when viewing material in a dim environment, it is generally thought desirable to have a system gamma that is slightly nonlinear: somewhere between 1.1 and 1.2 is most often quoted for a dim viewing environment.

Assuming a Rec. 709 camera encode gamma of 0.51, this means that display gamma should be in the 2.2-2.35 range. Thus, you should aim for a calibrated display gamma of between 2.2 and 2.35 (I aim for 2.25 in my calibrations), but what does this mean? For an idea of what different display gammas provide, see the chart below.

As you can see, the various gammas all begin and end with a one-to-one relationship between signal input and light output. Zero input produces zero output (actually, because of the display’s residual black level it is really just the minimum amount of light the display is capable of, not literally zero) and maximum input produces maximum output. This is as you would expect. However, the precise relationship between input and output as you gradually increase input is not linear, as explained above, and it varies depending on gamma.

A display with lower gamma increases its light output more quickly as you increase the signal input. If you look at the 10% input, you’ll see that a 2.8 gamma produces 1/5 the light output of 2.0 gamma. This is obviously an enormous difference. The difference becomes increasingly less significant as the level of input rises, so that at 80% input a display with a 2.8 gamma produces 9/10 of the output of a display with 2.0 gamma, which would be barely noticeable.

For this reason, the primary effect of system gamma on image quality will be with shadow detail and black levels. A gamma of 2.2-2.35 offers a nice compromise between these two opposing qualities. If you lower gamma below 2.2 you will achieve great shadow detail but your black levels will be noticeably elevated and contrast will suffer. If you raise gamma above 2.35 then you will create deep, dark blacks but with compromised shadow detail.

There are several points to keep in mind about gamma:

• The gamma of your display should be between 2.2 and 2.35.
• The gamma curve should be smooth and consistent. The curves presented in the chart above are idealized gammas. In the real world displays can create a gamma curve that varies considerably from this ideal.
• The gamma of red, green, and blue, should all be the same. If they are different, then you will have a problem with gray scale tracking.
• You can change system gamma by adjusting the contrast and, especially, the brightness control. If you lower the brightness control too much in an effort to maximize contrast, then you will get an excessively high gamma and crushed blacks with very little shadow detail.
• Because of gamma, 50% input voltage does not produce 50% of available light output. To get 50% of available output, you need a signal input of between 70-80%.