What is Gamma?

Many of you may wonder why a standard gamma curve is defined as gamma 2.2. The main reason that a power-law relationship exists between the output luminance and the input voltage or digital value is because the visual system does not operate in a linear way. Let’s take a look at Figure 2. At the bottom row, linear intensity represents the intensity increase from black to white in a linear fashion. At the top row, visual encoding represents the intensity increase from black to white in a power-law fashion. Please note that between 0.0 and 0.1, there is a big visual gap in linear intensity whereas it is much less apparent between 0.0 and 0.1 in visual encoding. And from 0.9 to 1.0 in linear intensity, the difference is not perceivable, whereas it is perceivable in visual encoding. Looking at the overall grays in the 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 used an exponent of 0.43 to convert linear intensity into lightness for neutrals to provide an optimal perceptual encoding of grays. And the exponent of 0.43 is approximately 2.33 and is quite close to gamma 2.2. Hence, the gamma value of 2.2 has become the golden standard of digital displays for proper calibration.

So how does the gamma curve impact overall image quality or perception? Gamma 2.2 delivers a balanced or ‘neutral’ tone between highlights and shadows, and 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 in Figure 3.

Gamma 1.8 was very popular due to Mac OS. The gamma 1.8 curve produces slightly brighter images than gamma 2.2 so sometimes it is more preferred in some cases. However, since Mac OS X 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 the movie and TV industries due to the Rec. 709 standard. The slightly enhanced contrast brings out the saturation of colors and stimulates viewer perception and preference. However, the 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 started to gain in popularity because of the latest DCI-P3 standard. DCI-P3 is endorsed by many newer digital cinema theaters. With a gamma 2.6 curve, images definitely look darker but very saturated. And as this is the effect required by directors, DCI-P3 needs a gamma 2.6 curve. An example of gamma 2.6 versus gamma 2.2 is illustrated in Figure 6.

Nevertheless, what will happen if a gamma curve is not smooth? If a gamma curve is not smooth; that means the transition from black to white is not strictly incremental, as shown in Figure 7. Then the visual differences in grayscale will look not perceptually different, and also with coloration (not neutral gray). And when this phenomenon applies to a grayscale image, the image will lose detail and result in a different image than it is supposed to be.

Since we now have some basic ideas of how gamma curves impact image quality and how important it is to have a good gamma curve, how do we adjust the gamma curve? Usually there are two ways of doing that. The first way is straightforward, you can just look for the OSD on you monitor. Usually on a professional color management monitor, such as BenQ SW series monitors or PD series monitors, there is a ‘Custom Mode’ in the ‘Color Mode’ option. In ‘Custom Mode’, you are can select different settings such as color gamut or gamma curve. Under the ‘Gamma’ option, you can select factory calibrated gamma curves, ranging from 1.6 to 2.6 with 0.2 intervals.

Another way of adjusting the gamma curve on SW series monitors is to utilize BenQ proprietary calibration software, Palette Master Element. Connecting with an external calibrator, such as X-rite i1 Display Pro, i1 Pro 2, or a Datacolor Spyder 5, you can simply select the gamma curve you would like in Advanced Mode. Following the instructions on the screen, after completing the calibration, you can be sure your gamma curve is ready for you to use in Calibration 1, 2, or 3. It is as simple as that. This is the preferred method to adjust your gamma curve if you have one of our SW series monitors.