There is full black and full white. Between those two values, different shades of white are the greyscale. The luminance of those different shades of white are determined by the gamma.
We will cover SDR gamma in this article with a future article covering HDR’s EOTF (HDR10) and OOTF (HLG).
Gamma in SDR is a relative standard, which means that it accounts for our eye’s sensitivity relative to the ambient light in the room. So a display placed into a well-lit room will need a lower gamma (e.g. 2.0) than a display that is placed in a pitch black room (e.g. 2.4 or higher).
Additionally, displays with a higher black floor may need a lower gamma than high-contrast displays with a lower black floor. This is what the BT.1886 gamma standard is trying to solve.
The Power Law gamma standard (2.0, 2.2, 2.4, etc) is the original standard and is recommended as the first starting point when learning and calibrating gamma. BT.1886 is the newer standard, but may not work well with all displays (dependent on display controls) or all lighting conditions.
How is Gamma Calibrated?
Using Dedicated Gamma Controls
Some displays will have dedicated gamma controls that are configurable. This is helpful if there are no multi-point greyscale controls or they don’t respond as you expect them to.
Custom gamma controls can at times be fiddly as two adjacent gamma controls will interact – as they are normally parametric – so you may need to go back and forth a few times to get the gamma to line up correctly.
Using Multi-Point Greyscale
When a display is fairy linear, you can calibrate gamma using the multi-point greyscale controls. In fact, some displays give you only this option (such as some LG OLEDs).
Even if there are dedicated gamma controls, you may want to try to calibrate gamma using the greyscale controls first as it may allow you to have more precision, or get a better end result by using both greyscale and gamma controls.
The other benefit of using multi-point greyscale is that the original gamma choices on the display are still usable – instead of having calibrated the a custom gamma slot for one reference.
Using the Contrast Control
In some instances, if there are no multi-point greyscale or dedicated gamma controls. In such a case, the contrast control can be used – very judiciously – to correct smaller gamma issues in the high end.
The contrast control on a display sets the white level so large adjustments will reduce the display’s light output and contrast.
The contrast control is a lot more useful for calibrating the EOTF on some TVs where there are no EOTF controls beyond the basics. That is for another article.
Which Procedure to Choose?
All the above are covered in detail in The Display Calibration Guide including the following procedure decision tree and related procedures for both SDR and HDR including some advanced strategies and detailed HCFR steps.