Color volume measurements showing 3D charts with volume L coordinates on two displays.
As 4K Ultra HD televisions supporting high dynamic range (HDR) and a wider color gamut have evolved over the last several years, an additional criteria for measuring a set’s performance – called color volume – has increasingly entered the TV performance lexicon.
Suddenly the discussion is centered on color accuracy and stability over the full range of brightness, and not just a television’s ability to produce 1,000 nits of peak luminance and cover a DCI-P3 2D color gamut. This is because color volume can be expressed in a three-dimensional space that adds a brightness dimension to a 2D color gamut chart. Previously, the familiar 2D color gamut measured the range of colors that a display can reproduce at a single brightness level, typically 75 or 80 percent of the peak luminance.
The International Committee for Display Metrology (ICDM) has named this new method of evaluating TV performance VCRC (volume-color reproduction capability).
Color volume is the way to quantify the color capability of a display by looking at color coordinates plus the actual brightness or luminance levels. Back in the days of CRT TVs and first-generation flat-panel TVs this wasn’t too much of an issue because all of them were virtually the same luminance levels. As display technologies have progressed, we can now see very different luminance and color capabilities as well as much higher levels of overall brightness.
“HDR really unleashed the whole notion of color volume,” said Patrick Griffis, VP office of the CTO for Dolby Labs. “After 50 years in the world of NTSC, we have just started looking at that little horseshoe [CIE 1931 color space] diagram and we forgot about how bright stuff was, because we all knew it was 100 nits. With HDR, suddenly, we’ve taken the lid off and are able to reproduce the full volume of colors at brightness levels we could never do before. I kind of think of it as unleashing the potential of what’s possible. I tell people that every time you open up your eyes you see HDR. It’s just that we’ve never been able to reproduce it before.”
Various workflows are now being perfected to let TV calibrators and product reviewers accurately measure this aspect.
Meanwhile, color volume measurement is being used this year by manufacturers to pitch the various benefits and limitations of premium 4K Ultra HDTVs, particularly in comparing the benefits between today’s two primary flat-panel TV technologies – LED back- or edge-lit LCD TVs and organic light-emitting diode (OLED) TVs.
Read more of our discussion on color volume after the jump:
Typically, LED LCD TVs can generate higher levels of brightness and peak luminance while reproducing colors very accurately and with greater visible detail in very bright scenes. They sometimes can have issues with their ability to produce the deepest black levels, which can make some dark areas of an image take on grayish tones instead of true black, impacting low-end contrast and colors.
OLED TVs are self-emissive, meaning they generate their own light source without the need for separate backlighting. These excel at their ability to handle dark elements, black level and color accuracy at low levels of light.
Television performance measurement dates back to 1931 when the International Commission on Illumination (CIE) developed the CIE 1931 color space charting the relationship between physical pure colors in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. This included various technical values including the transposition of wave forms into coordinate values plotted on a chart. This can give us a 2D graphical representation of what a display is doing with color and what its maximum color reproduction capabilities are.
In 1976 a slight change was made to this with the addition of the MacAdams ellipsis used to more accurately match colors to the capability of the human eye. This was felt necessary because, for example, the CIE 1931 color chart covered a wide area of green in which the human eye could not fully perceive all of the varying differences in shading.
Brightness variations were there and measurable but the eye was not capable of perceiving the differences between some of them. Also in 1976, the CIE Color Lab was introduced using luminance in its coordinate systems. It was determined to be necessary because instances existed where certain shades of color in the 1931 two-coordinate system were perceived as noticeably different to the human eye, yet measured exactly the same.
If you measure the gamut for the Digital Cinema Initiative (DCI) P3 color space recommendation on two different displays they may both measure at 100 percent of that color space, but after adding a luminance value – or L –they appear very different. One display might be able to get to 100 percent DCI-P3 when the other only gets to 56.65 percent, after the luminance portion of the equation is added.
To illustrate this in the context of the claimed superior benefits of their own quantum dot-based 4K Ultra HD LCD TVs, Samsung engineers like to compare the 2016 Samsung flagship KS9800 full-array LED LCD TV against one of LG’s 2016 WRGB-based OLED TVs. In Samsung’s comparison, the 2D x, y coordinates of each display look very similar — 97 percent vs. 96 percent. But when luminance is added in the CIE Lab Color Space (or L*a*b* coordinate space) the KS9800 covers 86 percent vs. 67 percent of the full 3D-value of the DCI-P3 color space.
The addition or reduction of light can have dramatic effect on the value of perceivable color.
Going forward, now that television manufacturers are able to produce content with high luminance and wide color gamut values, color volume in supporting displays becomes very important. Content today can be mastered with background elements at brightness levels exceeding 1,000 nits. Engineers with Dolby Labs, the proponent of the Dolby Vision end-to-end HDR system, explain that on the mastering side, the state of the art today is about 4,000 nits of peak white, and the Dolby Vision mastering standard used by a number of major studios allows for going all the way up to 10,000 nits, which is the “aspirational goal.”
Griffis explained “the Dolby Vision specification also specifies 12-bit mastering because it provides the precision needed to go from zero to 10,000, and any step along that range is below the level of perception, so the creators will know that no matter what the content is they’ll never get any potential artifact. This is very important to understand, because it’s color volume not HDR or color gamut that is the key thing.”
New displays such as quantum dot LED LCD TVs and certain phosphor-coated LED approaches are now attaining some of these very high measurements and LG is using various techniques like RGBW, where a white subpixel is added to red, green and blue subpixles, to boost brightness in its OLED displays.
Samsung asserts that OLED sets using RGBW technology like LG’s, desaturate colors — red in particular — as brightness is boosted closer and closer to 1,000 nit levels. This washes out the reds in the color volume making a form of pink. Samsung argues that the brighter the OLED RGBW display gets the more it loses its ability to hold on to detail and color information. Samsung executives contend that manufacturers of OLED with RGBW purposely sacrificed color saturation (and accuracy) in the color volume to get desired brightness levels.
Dolby’s Griffis, whose Dolby Vision system supports both OLED and LCD technologies, acknowledged that RGBW as used in LG’s OLED TVs “does boost the white but it also brings out some other colors. Many specular highlights are often in that white domain. You’re not going to get a really, really red specular, for example. Technically it’s in the white area. But [RGBW is] a good engineering approach in order to get higher brightness on speculars.”
Specular highlights are highly bright areas of a picture where reflected light is intensified by rounded or curved edges on reflective surfaces, like the rounded hood of a shiny red sports car projecting sunbeams on a cloudless day.
Timo Kunkel, staff researcher applied vision science group, imaging R&D for Dolby, explained that Dolby Vision, which is supported on LG OLED TVs among others, can compensate for some of this desaturation in OLED RGBW displays.
“Dolby Vision renders into the color volume the best it can taking into account what’s in the content — based on if it would desaturate or if it would make the color darker — to adjust it to the actual scene. So, it can do whatever fits the best. It’s not that we just generally dim or we generally desaturate. With RGBW it is not an issue of reduction of color but the fact that we can do more than you can do with similar RGB technology and go basically whiter. If you would switch [the fourth white subpixel] off you would just get a dimmer white. So RGB pixels alone are not as effective.”
In the meantime, how well a television is able to map color volume and luminance elements to fit a display says a lot about the quality of a TV. Systems with dynamic metadata capability, like Dolby Vision or new HDR10+ developed by Samsung as an extension on the HDR10 static system, will be able to map the wider and brighter HDR information on a scene-to-scene or even frame-to-frame basis to exactly what the TV is capable of doing. Using dynamic mapping, the system will know what the content is doing and use that to guide the mapping of the display.
“The challenge now as we’ve gotten increasingly larger color volume is are we able to get to the practical limits of the human visual system? Because, after all, that is what we are designing for, ultimately,” Giffis said. “Whatever the image is from the production side, whatever story they are trying to tell inside that color volume, our challenge as engineers is to make sure we faithfully reproduce it whether it’s in legacy SDR/Rec.709/100 nits CRTs to the new world of HDR OLED TVs with wider color gamut and brighter peak whites and LCDs. Make no mistake and have no doubt about it, the nits race is on and we are going to get closer and closer to that aspirational goal of 10,000 nits because we can see the difference and light sells.”
By Greg Tarr
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