What is Colour Gamut?
A display’s colour gamut tells us the different colours the display can or should display. We can think of gamut it two ways:
- As a two-dimensional triangle with x and y axis that hasthe most saturated primary colours on its outermost points: the most saturated blue, green and red. While the edges of the triangle hold the most saturated intermediate colours, which will be the three secondary colours (magenta, yellow and cyan) and the mix between those and the primary colours. The body of the triangle holds all the shades of those colours all the way to full white in the centre of the triangle.
- As a three-dimensional triangle that holds all the different brightness / luminance levels of those colours from 0% luminance to 100% luminance the display is able to achieve on its z axis. While this 3D model is also called the colour volume, the luminance value is part of the gamut, not a separate entity in and of itself.
Another way to demonstrate this would be by slicing up the colour volume chart above. What we would get is a cross-section in 2D Colour Space with our white point for the cross-section in the middle of the triangle, while the full greyscale and gamma going across the different light levels (or cross sections) of the colour volume.
HDTV uses a standard called BT.709 with its BT.709 gamut. This standard is thankfully a worldwide standard as opposed to the one we used for SDTV called BT.601 which had its NTSC gamut in North America while PAL in Europe with the rest of the World divided up between the two. (An extra complication was SECAM which I won’t go into, as most territories supporting SECAM also supported PAL.)
The BT.709 gamut is used for Blu Ray as well, and provides an extended amount of colours in addition to the BT.601 standard. However, the BT.2020 gamut used for UHDTV is a lot larger. So large in fact that 99% of displays cannot display it currently. This is why most content is actually mastered with a P3 gamut, which is also used for Digital Cinema applications. However, since there is no consumer P3 standard, the P3 gamut is encoded in the BT.2020 container.
Primary and Secondary Colours and CMS
Since displays create colours using an additive method (as opposed to subtractive like you learnt in art class and water colour), the primary colours that all colours are created from are red, green and blue. We can create all colours using these.
However, when it comes to calibration, we also need to define the secondary colours.
- Yellow is created from a combination of red and green
- Cyan is created from a combination of blue and green
- Magenta is created from a combination of red and blue
Each display has something called the CMS or Colour Management System. The CMS controls how the different shades of colours are created on the display by controlling this additive process. By calibrating the gamut using the CMS, we are able to ensure the display shows the colours as closely to the intended colours as possible.
Hue, Saturation, Luminance – Lights, Camera, Action!
When calibrating the gamut of a display, we normally need to calibrate 3 axis or aspect of colour. These are hue, saturation and luminance.
- Hue – in essence – rotates the colour on the CIE chart to change its colour: e.g. shifting red towards yellow or magenta or shifting green towards yellow or cyan.
- Saturation changes the colour’s intensity without changing its brightness. By increasing saturation, we are pushing the colour outward on the CIE chart. By decreasing saturation, we are pulling it in on the colour chart.
- Luminance is to do with a colour’s brightness. Each gamut standard specifies the relative brightness of the primary and secondary colours. By increasing colour luminance, the colour point is pushed towards the top of the 3D triangle. By decreasing the colour luminance, we are pushing it down towards black.
Calibrating the Gamut using the CMS
In its most basic form, the CMS can be controlled by a display’s
- Colour / Saturation Control
- Hue or Tint control
These controls move all colours at the same time, so there is very little room for correcting inaccuracies between colours. But if a display is fairly linear, or the colour errors are similar for all colours, this can help somewhat.
It can also be helpful if the multi-axis CMS (see below) does not have enough range to move the individual colours and for some reason the gamut is either rotated or too saturated across the board. So it can be used in combination with the multi-axis CMS.
A multi-axis (normally 6-axis) CMS gives us control over all the colours individually including its hue, saturation and luminance. This is very helpful as colours don’t usually have the same errors and this allows us a LOT more precision.
Do you like the above content? You can learn more about calibration using The Display Calibration Guide including a description of the procedures to use in tool-agnostic and HCFR-specific versions.