CIE 1931 xy and CIE 1976 u′v′ gamut measurements are the most common way to report color, but they only describe color in two dimensions and can provide an incomplete picture of real-world performance.
On paper, these two projectors listed below appear nearly identical, covering 94.1% and 96.8% of BT.2020 in CIE 1931 xy. Yet their images look noticeably different. How can both be true?
| AWOL Vision LTV-3000 Pro | Valerion VisionMaster Max |
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| Tested CIE 1931 xy BT.2020 coverage: 94.10% | Tested CIE 1931 xy BT.2020 coverage: 95.97% |
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The reason is that traditional 2D gamut measurements only describe the chromaticity boundary of displays. They don't show how much color a display can produce at different lightness levels. Color volume provides a more complete and representative way to evaluate a projector's color-rendering capabilities.
What Is A Display's Color Gamut?
A display's color gamut is the range of colors it can reproduce within the range of visible colors, based on its primaries and gamma or EOTF. In reviews, we usually compare that range to a target used by real content. SDR content is commonly checked against BT.709, while HDR content is usually assessed against the DCI-P3 gamut and less commonly against BT.2020.
How We Used To Report Color: A Triangle In The CIE xy Or u'v' Chromaticity Chart
The most familiar way color gamut is presented online is via a CIE 1931 xy or a CIE 1976 u'v' chromaticity diagram. The triangle is made by connecting the display's measured red, green, and blue primaries, and the target gamut is drawn on the same chart as the large colorful triangle with targets around its edge.
These charts are useful because they're easy to understand at a glance. They tell us which hues and saturation levels a display can reach. However, the common xy or u'v' charts only use the x and y or u' and v' chromaticity coordinates. u'v' is an analog to xy that better represents differences in colors and more closely matches the human perception of color difference.
Neither CIE 1931 xy nor CIE 1976 u'v' account for luminance, which is the brightness part of the measurement, making them a flat map of color, only showing hue and saturation. This is an issue for two main reasons: two different colors, such as yellow and brown, can have the same chromaticity and only differ to us due to their luminance, and chromaticity in general has a hard time predicting the color-rendering abilities of displays that use non-RGB primaries to create color. Two projectors may have the same CIE xy coverage of a color gamut, but not appear as colorful in real content.
2D Color Gamut Is Broken
Leaving out brightness wasn't a real problem when most displays behaved in a similar way. CRT TVs had similar brightness outputs and used similar RGB phosphors to create colors, so the CIE xy chromaticity chart was an adequate way to compare color gamut between products. Those days are gone, and we now encounter issues like in the example of the AWOL Vision LTV-3000 Pro looking significantly less colorful than the Valerion VisionMaster Max, despite a very small 2% difference in CIE xy BT.2020 coverage.
Modern displays are different from CRTs. They have wildly different brightness outputs, and some don't create every color using only red, green, and blue light in the same simple way. Examples include RGBW OLED panels with a white subpixel and some projectors that use additional white, yellow, or cyan light paths or drive phases. These designs can improve brightness or efficiency, but they can also separate "white brightness" from "color brightness." This is the main reason we evaluate a projector's brightness both in terms of white light output and also color light output.
Color Area vs. Color Volume
The easiest way to think about this is in terms of area versus volume. The CIE xy or u'v' charts and their triangles are like measuring the footprint of a box on the floor. Color volume measures the whole box, including its height.
For displays, that height is brightness. Two displays can have a similar color "footprint" on the xy or u'v' charts, but one may be able to keep colors vivid at higher light levels while the other cannot. That is why color volume is a better way to describe modern HDR displays, laser projectors, and displays with extra non-RGB light sources.
What Is CIELAB?
CIELAB, also written as CIE L*a*b*, is a three-dimensional color space used in color measurement. It describes color using three values:
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L* for lightness.
- L* represents how light or dark a color appears relative to the reference white, L* = 100. For displays, it's essentially the perceived brightness of the color, not its measured luminance.
- a* for the green-to-red direction.
- b* for the blue-to-yellow direction.
CIELAB is useful here because numerical distances in the space are intended to roughly correspond to perceived color differences. It's not perfect, and no single color model perfectly predicts human vision in every situation. But for measuring display color capability, it's much more informative than a 2D chromaticity-only chart because it includes lightness, not just chromaticity.
What Are Gamut Rings?
A full CIELAB color volume is three-dimensional, which makes it hard to show in a simple review article. Gamut rings solve that communication problem by turning the 3D volume into a 2D chart.
The method slices the display's CIELAB color volume into lightness bands in increments of 10, from 10 to 100. Each slice is flattened into a ring. Darker colors are closer to the center, and lighter colors are farther out. The area of the rings corresponds to the CIELAB color gamut volume.
How To Read A Gamut Ring Intersection Plot
For reviews, the most useful version is the Gamut Ring Intersection Plot. It overlays the display's measured color volume against the target color volume. In the charts, the target is shown in gray, and the display's measured capability is shown in color. If gray sticks out beyond the colored rings, that part of the target is missing.
Colored rings show what the display can reproduce. The gray area shows the target gamut. If the colored rings cover the gray area, the display covers that part of the target. If gray sticks out, the display is missing those target colors. Inner rings represent darker colors; outer rings represent brighter colors.
In the case of the AWOL Vision LTV-3000, pictured below, we see that for the first three internal rings, L10 to L30, the projector essentially fully covers the BT.2020 color gamut. The L40 to L60 rings show coverage diminishing, particularly in the deep purples, reds, and greens. Looking at L80 to L100, the three outermost rings, we see that the projector struggles to produce very light green tones and isn't great at reds either.
Color Volume Results Match Real-World Performance
The goal of our reviews is to help our readers understand what they will experience with the products once they use them. A traditional xy or u'v' chart can still be useful for showing primary color locations and for comparing against older measurements. But as a main color-capability metric, it can hide important differences between modern displays. For example, the AWOL Vision LTV-3000 and Valerion VisionMaster Max both have very high coverage of the BT.2020 color gamut in CIE 1931 xy.
From our example at the top of this article, comparing a colorful scene side-by-side, we notice that bright, saturated colors are much more muted on the LTV-3000. This is indicated by the roughly 20% lower CIELAB BT.2020 color volume coverage of the LTV-3000 compared to the VisionMaster Max.
Looking at the Gamut Rings Intersection Plot for both projectors' volumetric coverage of BT.2020, it's easy to see that the LTV-3000 has deficiencies in bright, saturated colors, notably red and green, that the VisionMaster Max doesn't.
| AWOL Vision LTV-3000 Pro | Valerion VisionMaster Max |
|---|---|
| CIE 1931 xy BT.2020 coverage: 94.10% | CIE 1931 xy BT.2020 coverage: 96.80% |
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| CIELAB BT.2020 Color Volume: 72.78% | CIELAB BT.2020 Color Volume: 93.64% |
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CIELAB Color Volume is much more useful than CIE 1931 xy chromaticity because it shows whether a display can reproduce the target color range across different brightness levels. This is especially important for projectors and also for HDR displays such as TVs and monitors.
Limitations Of CIELAB Color Volume And Gamut Rings
CIELAB Color Volume and its 2D representation via the Gamut Rings Intersection Plot isn't a replacement for every color test. They show color capability and target coverage, but they don't automatically tell us whether the display is accurate out of the box. A display can have a large color volume and still need calibration.
They also don't replace other display measurements such as brightness, contrast, EOTF tracking, tone mapping, grayscale accuracy, and uniformity.