For many years now, we have been talking about color gamut (or color gamut) as the measure of the colors that a television can reproduce. But recently there has been more and more talk about color volume of a TV. What is it and how does it compare to the range of colors we’ve been using up to now?
Just repeat: color range
When we talk about color range, we are actually talking about the triangle on the chromaticity diagram that is formed by the base red, green and blue of a display or standard. We can form all colors that fall within that triangle with those three basic colors.
The above figure shows the color gamut of the Rec.709 standard used for all HDTV content, Blu-ray, and streaming video. The white point (in this case D65, the daylight standard) is central in the triangle. This diagram is two-dimensional, and it represents the color range at one specific level of luminance (or, more simply, brightness). We make the measurements for all our televisions at 75% of the peak luminance of the TV.
What about other luminance levels?
Anyone who wants to talk about color and take all factors into account should actually expand this 2D diagram with a third axis (L: for luminance) to a 3D representation. And contrary to what you might hope, the color range is not the same at every luminance level. For example, very saturated colors (those are the colors closest to the points of the triangle, and therefore the most intense) are very difficult to reproduce at very low luminances. And since white is created by mixing red, green and blue (simply put, by adding them up), no base color can be reproduced as brightly as pure white. In fact, red, green and blue do not have the same maximum brightness. If we have a maximum white of say 1000 nits, then the brightest blue is only 72 nits, for red this is 213 nits and for green 715 nits. You can clearly see these differences in the representation of the color volume below. On the left you see the traditional way in which we indicate color range in the xy Chromaticity diagram, on the right the Y (luminance) axis has been added and you see the 3D xyY color volume.
Moreover, two screens with the same 2D color range (measured as we usually do at 75% peak luminance) can perform very differently as we approach zero luminance (black) or maximum luminance (peak white). And the situation gets worse if we have to take HDR into account. For our regular SDR content, we could assume that it was mastered at a fixed maximum of 100 nits luminance. But with the rise of HDR, that’s no longer the case. Some televisions can go up to 800 nits, others up to 1,500 nits and more. Such televisions will therefore have a very different color volume (more on that below).
The simple presentation below shows what happens when you switch from the color range Rec709 to Rec2020 (the larger basic triangle) and at the same time take into account a higher maximum brightness. The larger volume contains many more color tones.
The color volume
The concept of color volume is therefore relatively simple in itself: it is the 3D volume based on a certain color range, and a maximum brightness. The screen can then display all colors within that volume. So we will probably indicate it with those two data: for example (Rec.2020 / 1000 nits) or (P3 / 1000 nits).
A possible standard for how we will measure this in the future is VCRC (Volume Color Reproduction Capability), a method recognized by the ICDM (International Committee for Display Measurement). The method was, not unexpectedly, co-designed by Samsung, which this year is heavily investing in color volume to highlight the possibilities of its TVs. Spectracal, the manufacturer of our measurement software (Calman), has already announced that the color volume will enable measurements. We will then be able to clearly indicate the difference between two devices that have a similar color range, but a different color volume.
The impact of greater color volume
All well and good, but what kind of differences will you see on a television with a larger color volume? A larger color volume is especially important for HDR display. After all, HDR is all about displaying very intense brightness, and we don’t just want to be white. That’s good for a sparkle on a chrome bumper, or the waves of the sea, but there are also a lot of things we perceive both very brightly and very colorfully. Think, for example, of a clear blue sky or the orange-yellow glow of a volcanic eruption.
The photos below show the effect of a larger color volume. The first image shows what the volcanic eruption looks like on a classic television. The orange-yellow color is within the color range, but is severely limited by the low maximum brightness. The result is an intense yellow color that appears much too dull.
The same volcanic eruption in a larger color volume shows much more spectacle. The orange has not changed its hue per se, but thanks to the greater color volume, it can be much brighter. In addition, more shades are visible in the image.
ou can see another example below. On the left you see a test image on Color volume of a TV:that almost completely fills P3 / 1000 nits. The same image on the right, but on a television with a smaller color volume.
Color volume of a TV: Conclusion
By adding a third axis for luminance to the traditional color gamut, we get a better picture of the color performance of a television over the full brightness range. We call the resulting 3D diagram a color volume. Since we are in a ‘brightness race’, manufacturers are trying to produce higher and higher peak brightnesses, Color volume of a TV clearly becomes important. After all, in addition to a high peak brightness for white, you also want intense colors. Larger color volumes have the advantage that they can show intense, deeply saturated colors with a very high brightness.