There is full black and full white. Between those two values, different shades of white are the greyscale. We normally express these as the % of white: e.g. 10%, 20%, etc
Greyscale is important as an incorrectly calibrated greyscale will introduce a colour tint to whites, but will also make it a lot harder to calibrate the the rest of the colours as the gamut and greyscale values do interact (more on gamut and CMS calibration in a follow-up article).
The correct shade of white at each % of the greyscale is expressed as the % of the primary colours: blue, green and red.
Greyscale is also called White Balance as each of the greyscale steps must be balanced to produce the correct shade of white.
How is Greyscale Calibrated?
Most displays will normally have a two-point greyscale control: one set to control the top of the range near white and one set to control the bottom of the range near black. The top controls are normally called Gains and the bottom controls are normally called Offset. However, there can be other names for them dependent on the manufacturer (see below table).
As the gain controls are touched, the greyscale changes the most at the top end but it also affects the rest of the scale. See below for an example with the green gain control increased. Please note the below diagram is theoretical and each display will behave a bit differently.
The opposite happens when we touch the offset controls.
We try and balance the offset and gain controls until the full greyscale is brought into balance at each % white point.
Some displays will have multi-point greyscale instead or in addition to the two-point one. Multi-point greyscale will control different points on the % scale for white. For example, a 10- point greyscale divides up the brightness range into 10 points from full black to full white.
How bright each of the steps are is controlled by the display’s EOTF (e.g. gamma) which is for a follow-up article. For now, what you need to be concerned with is the balance of colours at each step. Please see below for a theoretical example that demonstrates multi-point greyscale controls.
Which Procedure to Choose?
Multi-point greyscale controls are normally finer than two-point greyscale controls so they don’t normally have as much range. This is why you may need to use two-point controls before balancing multi-point greyscale.
However, there is an inherent issue with using offset controls that affects near-black gamma so should be avoided when possible. Thankfully, it is possible to simply use the gain controls then multi-point greyscale to balance greyscale normally, as long as the display is linear.
All these issues 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.