What is Gamma?

You may have heard that gamma 2.2 is standard and wondered why. There is a power law relationship between the output luminance and the input voltage or digital value. Impact on luminance impacts the visual system, but the effects are not identical. Let’s take a look at Figure 2. The bottom row represents the intensity increasing from black to white in a linear fashion. The top row, visual encoding, represents the increased intensity from black to white in a low power fashion. Note that between 0.0 and 0.1, there is a big visual gap in linear intensity, with a smaller visual gap between 0.0 and 0.1 in visual encoding. And from 0.9 to 1.0 in linear intensity, the difference is subtle, whereas it is more obvious in visual encoding. Looking at the overall grays in visual encoding row, the perceived differences between each gray patch are almost identical. This phenomenon was also found in Ebner and Fairchild’s study in 1998, where they found using an exponent of 0.43 to convert linear intensity into lightness for neutrals can provide an optimal perceptual encoding of grays. The exponent of 0.43 is approximately 2.33, quite close to gamma 2.2. Hence, gamma value of 2.2 has become the golden standard of digital display for a proper calibration.

So how does gamma curve impact overall image quality or perception? Gamma 2.2 delivers a balanced or ‘neutral’ tone between highlights and shadows so you can distinguish the grays easily in between. Gamma 1.0 is an interesting curve to look at. This is a 45-degree straight line relationship between the input signal and output luminance. This is also a ‘by-pass’ scenario where no processing is done on the display end. So you will see a very bright and ‘flat’ image, where almost no contrast is present, such as Figure 3.

Gamma 1.8 was popular due to Mac OS. Gamma 1.8 curve produces slightly brighter images than gamma 2.2 curve so sometimes it is preferred. However, since Mac OSX 10.6, gamma 2.2 has become the standard gamma curve for Mac OS as well. An example of gamma 1.8 versus gamma 2.2 is illustrated in Figure 4.

Gamma 2.4 is widely used in movie and TV industries due to Rec. 709 standards. The slightly enhanced contrast brings out the saturation of the colors for better viewing experiences. Of note, overall brightness of images may be lowered. An example of gamma 2.4 versus gamma 2.2 is illustrated in Figure 5.

Gamma 2.6 has gained popularity because of the latest DCI-P3 standard. DCI-P3 is endorsed by most newer digital movie theaters. With a gamma 2.6 curve, the images definitely look darker but also very saturated. And this is the effect that is required by the director, hence, DCI-P3 required gamma 2.6 curve. An example of gamma 2.6 versus gamma 2.2 is illustrated in Figure 6.

If a gamma curve is not smooth, the transition from black to white is not in even increments, as shown in Figure 7. Grays are harder to distinguish and color and contrast suffer. It follows that image quality and viewing experience are negatively impacted.

Usually there are two ways to adjust gamma settings. The first way is straightforward--just look for the OSD (on screen display) on your monitor. A professional color management monitor, such as BenQ SW series monitor or PD series monitor, should have a Custom Mode in the Color Mode option. In Custom Mode, you can select different settings, such as color gamuts or gamma curves. Under the Gamma option, select factory calibrated gamma curves, ranging from 1.6 to 2.6 with 0.2 intervals.

Another way of adjusting gamma curve in SW series monitors is with BenQ proprietary calibration software, Palette Master Element. Connecting with an external calibrator, such as X-rite i1 Display Pro /Calibrite ColorChecker Display Pro, i1 Pro 2 or a Datacolor Spyder 5, you can simply select the gamma curve you would like in the Advanced mode. After calibrating, your gamma curve is ready for you to use in Calibration 1, 2, or 3. It's as simple as that. This is the preferred method to adjust your gamma curve if you have a BenQ SW series monitor.