What Does a >100% Rec.709 Color Gamut Mean?

In the field of colour reproduction, experts use the term colour gamut to describe the range of colours within the overall spectrum of visible colours (also known as a colour space) that an imaging device – in our case a projector – is able to reproduce; you can think of it as similar to a painter’s palette. As such, a projector with a wider colour gamut is by definition able to reproduce more colours in the visible spectrum.

One of the most common methods used to visually represent a colour gamut, is with a triangle drawn within the CIE colour space diagram, where each corner of the triangle represents the purest red, green, and blue that the device is able to reproduce.

But given that projectors are just one type of device within the larger imaging ecosystem – an ecosystem that includes other devices such as cameras, displays, and so on – defining their colour gamuts is not enough to describe their colour performance. This is because the manufacturers of each type of device in the ecosystem need to make sure that every colour in their devices’ color gamut are defined the same across the board, otherwise, when trying to reproduce the same image, each device would come up with different versions of each colour. That’s where the Rec. 709 colour gamut comes into play.

With the Rec. 709 standard in hand many brands have found that one of the most convenient ways to characterize their product’s colour gamut is as a percentage of the Rec. 709 colour gamut. While doing so seemingly helps the consumer quickly understand the degree to which a device can reproduce the Rec. 709 standard’s colours, this kind of description for a colour gamut can be easily misinterpreted, particularly when a brand advertises a colour gamut that exceeds the Rec. 709 colour gamut – such as 125% of Rec. 709. Because by advertising a colour gamut with 125% of Rec. 709, the implied – but incorrect – message is that this device’s colour gamut not only fully covers the Rec. 709 colour gamut but also includes more colours.

The truth is that the claim that a colour gamut 125% of Rec. 709 only means that the overall size of the respective product’s colour gamut is over 100% the size of the Rec. 709 colour gamut, it does not necessarily mean that said colour gamut covers 100% of Rec. 709 plus an additional 25%. There can be many instances where 125% of Rec. 709 colour gamut covers 90% or less of the Rec. 709 gamut, as seen in the image below.

The proper way of communicating how much of the Rec. 709 standard covered by a colour gamut instead is through colour coverage.

As mentioned above, a device with a colour gamut that exceeds the Rec. 709 standard does not necessarily mean that it is able to reproduce all the colours in the Rec. 709 standard. Colour coverage on the other hand better expresses a device’s capability to do so because it actually represents the percentage of overlap between the device’s color gamut and the Rec. 709 standard. So, a device with a colour coverage of 95% of Rec. 709 is guaranteed to be able to reproduce 95% of the Rec. 709 colour gamut accurately, while a product with over 100% of Rec. 709 might only be able to reproduce 90%.

Furthermore, colour gamuts that exceed the size of the Rec. 709 standard but feature a lower than 100% colour coverage will actually lead to issues with their colour accuracy, including colour deviation and oversaturation. This is due to the fact that because Rec. 709 is used as a standard to sync up colors across devices, any device whose colour gamut features large chunks outside of the Rec. 709 boundaries – as is the case with devices that have large colour gamuts but lower colour coverage – will invariably produce colours that deviate from the colours produced by another device that has adopted the Rec. 709 standard. Similarly, oversaturation will also occur when the areas of a large colour gamut that lie outside of the Rec. 709 boundaries skew towards a specific colour, most commonly red.