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CIE 1931 xy vs CIE 1976 u'v': why gamut numbers differ

By TechCompare · Updated · How we check

sRGB covers 73.7% of DCI-P3 in CIE 1931 xy and 79.6% in CIE 1976 u'v'. Neither number is wrong. Check which chart a review used before you compare two screens.

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CIE 1976 u'v' chart with DCI-P3, Adobe RGB and sRGBThe curved outline holds every color people can see, with wavelengths in nanometers along its edge. Each standard is a triangle of its red, green and blue primaries. A larger triangle holds more saturated colors.
  • sRGB, the solid line
  • DCI-P3 (Display P3): Wide color monitors, phones and HDR movies. Display P3 is the same triangle with a D65 white.
  • Adobe RGB: Photo editing and print. It reaches further into green and cyan than DCI-P3.
sRGB against each reference, in CIE 1976 u'v'
ReferenceCoverageArea
DCI-P379.6%79.6%
Adobe RGB85.7%85.7%

Coverage is the part of the reference inside the triangle, so it stops at 100%. Area is the triangle's size next to the reference, and it can pass 100%. Spec sheets don't always say which one they print.

Open this chart in the Color Gamut Visualizer. Check any standard, enter a monitor's primaries or spec sheet numbers, and switch between CIE 1931 xy and u'v'.
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A gamut chart is a map of color, and like any map it stretches some places. The CIE 1931 xy chart stretches greens. The CIE 1976 u'v' chart was made to space colors more evenly, so the same triangles change shape on it. This page draws them in u'v'. Switch the chart in the tool to see both.

Topic
CIE 1931 vs CIE 1976
Two charts
On the chart
sRGB, DCI-P3, Adobe RGB
CIE 1976 u'v'

How this is calculated

The xy chart came from color matching tests in the late 1920s, and the CIE published it in 1931. It's good for saying exactly which color a primary is, but distances on it don't match what people see. A step in green is much harder to notice than the same step in blue.

In 1976 the CIE added the u'v' chart, a rescaled version that's closer to even. Greens shrink, and blues and reds get more room. Coverage and area change with it.

That's why you'll read that DCI-P3 is about 26% larger than sRGB in one place and about 36% in another. The first number is from u'v' and the second from xy.

The move between the charts keeps straight lines straight, so a triangle that holds all of a standard in one chart holds all of it in the other. Only the numbers under 100% change. Spec sheets usually give one number without naming the chart, so two numbers from different sources may not compare.

Verdict

Compare coverage numbers only when they come from the same chart. If a review gives both, xy is the older one that most spec sheets mean, and u'v' is closer to what you'd see. Either is fine for picking between two screens, as long as both use it.

More Color Gamut Visualizer scenarios

sRGB vs DCI-P3
DCI-P3 holds all of sRGB and reaches further into red and green. Here's how big the gap is, and when it matters.
View details
DCI-P3 vs Adobe RGB
Two wide ranges of about the same size, pointed different ways. P3 reaches further into red, Adobe RGB into green and cyan.
View details
sRGB vs Adobe RGB
Adobe RGB keeps sRGB's red and blue and moves green far out. Here's what that adds for photos and print.
View details

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Same numbers, different light on each screen.
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Another way to map color, for picking.
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Frequently asked questions

Which is more accurate, xy or u'v'?
u'v' is closer to how people see differences, so many color scientists prefer it for coverage. Neither is perfect, and both leave out brightness.
Does the chart change whether a monitor covers 100% of sRGB?
No. The change from xy to u'v' keeps straight lines straight, so a triangle that holds all of sRGB on one chart holds it on the other. Only partial coverage numbers change.
Why does the green part of the chart look so big?
The xy chart gives greens far more room than the eye needs to tell them apart. The u'v' chart squeezes them back, which is why the triangles look different there.

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