When Samsung unveiled its QLED TVs at CES (which you can read about here), the company placed a lot of emphasis on the concept of color volume and how QLED achieves a much larger color volume than OLED. This emphasis was maintained in Samsung’s booth and Innovation Room, a meeting room in the Las Vegas Convention Center where various demos were set up for journalists and other invited guests.
Up to now, most manufacturers and calibrators have specified and measured the range of colors that a display can reproduce in terms of a color gamut, which is the familiar triangle within the CIE diagram. However, that diagram is 2-dimensional; it represents the color gamut at one brightness or luminance level, typically 75% of the display’s peak luminance.
But what happens as the image approaches the display’s maximum and minimum luminance? The colors become less saturated and converge on white or black. To represent this behavior, a third dimension—luminance—must be added to the color-gamut diagram. The result is known as a color volume, a 3-dimensional shape that depicts how the display reproduces colors at different luminance levels.
A 3-dimensional color volume represents the range of colors that a display can reproduce throughout its luminance range from black to white. The 2D color gamut is a slice through this volume at a particular luminance level.
In the world of standard dynamic range (SDR), a 2D color gamut sufficed because calibrated displays behaved in a similar manner—peak luminance was 100 nits and colors were generally well saturated at 75% of that luminance. But in the world of high dynamic range (HDR), things are different. For example, peak luminance is 5 to 10 times higher—in the case of Samsung’s QLED TVs, 15 to 20 times higher—and displays in the future are likely to go even higher than that. Also, the peak luminance of the display used to master the content can be quite different than the peak luminance of a consumer display, and consumer displays can be quite different from one another.
To accommodate these differences, a process called tone mapping is used to adjust the dynamic range of the content to fit gracefully within the capabilities of each display. Also, the color volume of each display becomes very important. If you measure only 2D color gamut, you don’t know how colors will look at different luminance levels. Two displays can have the same 2D color gamut measured at a given luminance level, but they might have very different color volumes. On one display, colors might look fairly saturated near its peak luminance, while on other display, they might look very desaturated. There’s always a tradeoff between color saturation and luminance, but the exact characteristics of that tradeoff depend on the display’s color volume and tone-mapping algorithm.
In this example of real-world measurements of two different displays, they exhibit nearly identical color gamuts but very different color volumes. The wireframes seen in this graphic represent 100% of the DCI color volume. The ICDM (International Committee for Display Measurement) established an evaluation method for color volume called Vcrc (Volume Color Reproduction Capability) based on measuring luminance (L*) and two color axes, red-green (a*) and blue-yellow (b*), also known as CIELAB.
Among the color-volume demos presented by Samsung at CES, I found one to be particularly illuminating. A QLED and OLED TV were set up next to each other and calibrated, displaying the same content. A colorimeter was used to measure certain colors on each display, and the measurement appeared as a dot on a separate screen showing the color volume of each one.
As you can see in this photo, the points measured on the QLED (blue dots) are within its own color volume on the left, but they fall outside the color volume of the OLED on the right. Also in this photo below the screen, you can see the color volumes as re-created by a 3D printer, which is very cool. The colors are not correct, since they are the result of reflected light rather than emitted light, but holding them in your hands is a very effective illustration of the concept.
The best news of all is that SpectraCal is working on adding color-volume measurements and representation in its CalMan calibration software. The program will also feature an auto-calibration routine for 2017 Samsung TVs in both SDR and HDR, which will make calibrating them much easier than it’s been up to now, especially in HDR mode. For more on this, click here.