Display Color Gamuts: NTSC to Rec.2020
Modern color gamuts are one of the most misunderstood aspects of display metrology. Here, we will examine the usefulness of the still widely referenced 1953 NTSC gamut and also compare accurately colorized gamuts in the 1931 and 1976 CIE color spaces.
by Raymond M. Soneira
THE color gamut defines the palette of available colors that a display can produce – so it is the most important defining visual characteristic of any display. While color gamuts have changed over the years, in the past, virtually all displays needed just a single gamut to produce all of the content that a user wanted to see. But with the recent development of several wider-color-gamut standards for producing new content, including DCI-P3 for 4K UHD TVs and digital cinema, all future TVs, monitors, smartphones, tablets, and laptops will need to support at least two color gamuts.
So, there is a big learning curve for consumers, reviewers, content producers, and even manufacturers with regard to the proper use of the new color gamuts. In this article, we will examine and compare some of the most important display color gamuts that have been appearing in consumer products over the last 60-plus years, from the earliest NTSC gamut up through the latest DCI-P3 and Rec.2020 gamuts. In short, gamuts have been evolving and getting progressively larger.
Display Color Gamuts and Standards
Over the years, there have been an incredibly wide range of color gamuts that have been implemented for displays. Many are simply based on the particular native primary colors conveniently available at the time at low cost for different display technologies such as CRT, plasma, LCD, OLED, LED, quantum dots, phosphors, lasers, etc. Many applications just need any suitable range of colors to satisfy a user’s needs. However, essentially all imaging-based applications need a specific well-defined color gamut in order to accurately reproduce the colors in the image content. Over the years, this has given rise to many different standard color gamuts for image content, and they have generally been based on what the current displays at the time could produce. So both the displays and content have evolved together over time, and many different gamuts have been defined, but they have not all been created equal.
What makes a color gamut important and a true standard is the existence of lots of content created specifically for that gamut: manufacturers then need to include that standard in their products. So, it is the content and content producers that define a true color-gamut standard – the displays then need to deliver it as accurately as possible on-screen. Every display needs to adapt its native color gamut for the content that it has to show. This is implemented using color management, which we discuss below.
While people primarily think of color gamuts in terms of their outermost saturated colors, most image content is generally found in the interior regions of the gamut, so it is particularly important that all of the interior less-saturated colors within the gamut be accurately reproduced.
And if you are not sure of the set of colors that the different gamuts actually produce, we will show you accurately colorized versions of the two most important standard gamuts being used today so you can evaluate them visually. In case you think you have already seen colorized gamuts before, the colors shown in essentially all published color gamuts are fictitious and wildly incorrect. We have accurately calculated them here.
Color Gamuts and Ambient Light
One very important point that applies to all displays is the color gamut that you actually see on-screen is reduced by any existing ambient light falling on the screen. Since very few users watch their displays in absolute darkness (0 lux), the color gamut that is actually seen is noticeably less than 100%. We examined this very important effect and its solution in an online DisplayMate article in 2014.1
NTSC Color Gamut
The first official color gamut standard for displays was the NTSC color gamut, which made its debut in 1953 for the beginning of U.S. color-television broadcasting. However, the NTSC primary colors were too saturated and could not be made bright enough for use in the consumer (CRT) TVs of that era, so the NTSC color gamut was never actually implemented for volume commercial production of color TVs. As a result, the NTSC gamut was never an actual standard color gamut, and there is essentially no consumer content based on the true NTSC color gamut. This is amusing (and annoying) because now, more than 60 years later, many manufacturers and reviewers are still quoting and referring to the NTSC gamut as if it were some sort of state-of-the-art standard, while in fact it has been obsolete and colorimetrically disjointed from most other standard gamuts for an incredibly long time.
Manufacturers of high-tech display products should be embarrassed for publishing their specifications in terms of NTSC, an obsolete technology more than 60 years old. So please everyone – let’s stop referring to the very outdated NTSC and instead move on to the actual color gamuts that are being used in today’s displays.
The Real Analog-TV and Standard-Definition-TV Color Gamuts
Instead of the official NTSC gamut colors, the practical phosphor colors that were actually used in early color TVs were developed by the Conrac Corporation. The result became the SMPTE-C color-gamut standard. TV production studios used Conrac color monitors to produce their broadcast TV content, so it was the Conrac color gamut rather than the NTSC gamut that was the real color-television standard gamut. The SMPTE-C gamut is not that different from today’s sRGB/Rec.709 gamut, which is 13% larger than that of SMPTE-C. Many later gamut standards were based on SMPTE-C, including up to Rec.601 for Digital Standard-Definition TV. We are now going to skip over lots of history and get to the display color gamuts that are in use today.
sRGB/Rec.709 Color Gamut
For over 10 years, the main color gamut that has been used for producing virtually all current consumer content for digital cameras, HDTVs, the Internet, and computers, including photos, videos, and movies, is a dual standard called sRGB/Rec.709. If you want to see accurate colors for this content on just about any consumer product, then the display needs to match the sRGB/Rec.709 standard color gamut – not larger and not smaller. If your display does not faithfully render the content in this gamut, the colors will appear wrong and also be either too saturated or under-saturated.
There are still widely held beliefs by many reviewers and consumers that viewing content on a display with a larger color gamut is actually better, but it is definitely worse because the display will produce colors that were not actually present in the original content, creating an oversaturated or generally incorrect rendering. Below, in Fig. 1–4, we will show both visually and quantitatively what the sRGB/Rec.709 color gamut looks like in both the 1976 and 1931 CIE diagrams.
Accurately Matching the Color-Gamut Standard
For reasons similar to what occurred long ago with the NTSC gamut, up until recently a reasonable fraction of all displays could not produce 100% of the sRGB/Rec.709 color gamut. This was especially true for mobile displays, which in many cases provided less than 70% of the sRGB/Rec.709 gamut due to luminance and efficiency issues similar to those that had plagued the NTSC gamut. As a result, their on-screen images appeared somewhat bland and under-saturated. But today, most good-quality products have displays that produce close to 100% of the sRGB/Rec.709 color gamut.
Similar issues also apply to the newest and largest color gamuts, DCI-P3 and Rec.2020, which we examine in detail below. 4K UHD TVs only need to provide 90% of the DCI-P3 color-gamut standard to receive a 4K UHD Alliance certification, and the currently available Rec.2020 displays typically only provide 90% of the Rec.2020 color-gamut standard. It has always taken some time for displays to fully and properly implement the latest color-gamut standards. However, this delay introduces color errors that reduce the absolute color accuracy of the displayed content, which we discuss below.
Adobe RGB Color Gamut
Most high-end digital cameras have an option to use the standard Adobe RGB color gamut, which is 17% larger than the standard sRGB/Rec.709 color gamut that is used in consumer cameras. The Adobe RGB gamut is also used in many other advanced and professional imaging applications. For consumers, Samsung’s Galaxy smartphone and Galaxy tablet OLED displays accurately produce the Adobe RGB gamut as covered in DisplayMate’s Mobile Display Technology Shoot-Out article series.2
DCI-P3 Color Gamut
The newest standard color gamut that has significant content is DCI-P3, which is 26% larger than the sRGB/Rec.709 gamut. It is being used in 4K UHD TVs and in digital cinema for the movie industry, so while the amount of existing DCI-P3 content is still relatively small compared to that for sRGB/Rec.709, it is now starting to grow rapidly. DCI-P3 is also being adopted in many other new displays and applications that need to provide a wider color gamut with a wider range of more saturated colors. DisplayMate recently tested the new Apple iPad Pro 9.7, which has a very accurate native 100% DCI-P3 gamut and also produces a very accurate 100% sRGB/Rec.709 gamut by using color management.3
Rec.2020 Color Gamut
The next-generation standard color gamut will be the impressively large Rec.2020 standard, shown in Fig. 1. In fact, it is 72% larger than the sRGB/Rec.709 and 37% larger than the DCI-P3. The color gamut is extremely wide and the color saturation extremely high. However, there is almost no current existing content for Rec.2020. And there are very few existing displays that come close to providing Rec.2020, which requires quantum dots for LCDs. Of course, continuing progress is being made in extending the color gamuts for both LCD and OLED panels, so Rec.2020 will become an important new standard gamut within the next several years.
Comparing the Standard Color Gamuts
Figure 1 shows the color gamuts for most of the standards that we have been discussing. They are all plotted on a CIE 1976 uniform chromaticity (color) diagram that quantitatively evaluates color in a perceptually uniform manner for human color vision with (u’, v’) color coordinates. All of the color regions and visual differences among colors remain consistent throughout the entire 1976 CIE color space, so it provides an excellent and accurate method for specifying, manufacturing, marketing, comparing, measuring, and calibrating displays.
Note that the older 1931 CIE diagrams with (x,y) color coordinates that are published by many manufacturers and reviewers are very non-uniform and distorted, so they are effectively meaningless for quantitatively evaluating color gamuts and their color accuracy. The color gamuts shown in Fig. 1 would appear very differently in the 1931 CIE diagram. We will examine this in detail for the sRGB/Rec.709 gamut below.
In all of the CIE diagram figures, the outermost white curve is the limit of human color vision – the horseshoe is the pure spectral colors and the diagonal is the line of purples connecting red and blue at the extreme ends of human color vision. Green is between red and blue in the spectrum and is on the extreme left in the CIE diagrams.
A given display can only reproduce the colors that lie inside of the triangle formed by its three primary colors, which are always based on red, green, and blue, following the eye’s own spectral color response. The wider the color gamut, the greater the range of colors that can be produced. Some displays have more than three primary colors. In such cases the color gamut is then defined by a polygon. Sharp’s Quattron, for example, includes a fourth yellow (non-standard) primary color that actually improves the display’s brightness and efficiency more than enlarging the gamut as can be seen in Fig. 1.
When content is being produced, colors that are outside of the content’s color gamut move automatically to the closest available color and no longer exist and cannot be recovered later by using a wider color gamut. So, the highly saturated colors outside of the color gamut are still reproduced but with lower color saturation.
The standard color of white for almost all current color-gamut standards is called D65, which is the color of outdoor natural daylight at noon with a color temperature close to 6500K and is marked in the figures as a white circle near the middle. To deliver accurate image colors, a display must match the same color gamut and also the same color of white that was used to create the content. Unfortunately, many displays accurately reproduce the color gamut, but then use an inaccurate (typically too blue) white point, which then introduces color-accuracy errors throughout the entire inner regions of the gamut.
Fig. 1: Shown are the standard color gamuts plotted on a CIE 1976 uniform chromaticity diagram.
Color-Gamut Size Comparisons in Terms of Area
A common metric for comparing the relative sizes of the color gamuts is by using their relative areas within the Uniform 1976 CIE diagram. The relative gamut sizes that are calculated from the non-uniform 1931 CIE diagram are significantly different and are compared in a later section below.
• The Adobe RGB color gamut is 17% larger than that for sRGB/Rec.709. • The DCI-P3 color gamut is 26% larger than that for sRGB/Rec.709. • The Rec.2020 color gamut is 72% larger than that for sRGB/Rec.709 and 37% larger than that for DCI-P3.
And for those of you still interested in NTSC-gamut statistics: The NTSC color gamut is 98% of the Adobe RGB color gamut. So while they are both very close in gamut area and size, note how very different their triangular gamut shapes and color regions are in Fig. 1, proving that the still current practice of using NTSC for gamut specifications, and comparisons has little colorimetric meaning or useful quantitative value for the current gamuts and displays (and is doubly incorrect when combined with the non-uniform 1931 CIE color space).
Accurately Colorized sRGB/Rec.709 Color Gamut
Figures 2 and 3 show an accurately colorized sRGB/Rec.709 color gamut. For displays this can only be done for a single color gamut at a time. The colors in the figure have been accurately calculated to show the real colors within the sRGB/Rec.709 gamut – the colors shown in most published color gamuts are fictitious and wildly incorrect. Also included are 41 reference colors that we use for measuring absolute color accuracy throughout the entire gamut, which is discussed below.
Note that printed versions of the colorized gamuts depend on the particular inks being used and also their spectral absorption of the particular ambient light you are viewing them in, so they cannot be as accurate as when viewed on an emissive display, and they also generally provide smaller gamuts than most displays. (See sidebar above.)
Note that every color within the gamut is shown at its maximum brightness (luminance). White is the brightest color near the middle because it is the sum of the peak red, green, and blue primary colors. The secondary colors of cyan, magenta, and yellow radiate from the white point as ridges because they are the sums of two primary colors.
One particularly interesting result seen in Fig. 2 is how relatively small the green region of the sRGB/Rec.709 color gamut is in the accurate 1976 CIE uniform color space. However, the green region appears considerably larger in the distorted and non-uniform 1931 CIE chromaticity diagram, as shown in Fig. 3. The newer color gamuts – Adobe RGB, DCI-P3, and Rec.2020 – all significantly enlarge the green region of their color space within the uniform 1976 CIE diagram.
Absolute Color Accuracy and Just Noticeable Color Differences (JNCD)
One very important issue is the accuracy of each display’s color gamuts, and the absolute color accuracy for all of the colors within the entire color gamut. One vital reason for accurately colorizing and rendering each color gamut in the 1976 CIE uniform color space is that the display’s color accuracy and color calibration can be accurately analyzed uniformly, and then the true color errors uniformly minimized for all of the colors within the color gamut. The errors are expressed in terms of Just Noticeable Color Differences (JNCD), which correspond to fixed linear distances within the CIE diagram. Figure 2 shows distances corresponding to 1 JNCD and 3 JNCD, with 1 JNCD = 0.0040 in the (u’,v’) 1976 color space.
For each display we test, we measure the absolute color accuracy of 41 reference colors, which are shown for sRGB/Rec.709 in Fig. 2. See this color accuracy analysis for both the DCI-P3 and sRGB/Rec.709 color gamuts in the DisplayMate article on the Apple iPad Pro 9.7, which includes a more detailed discussion of JNCD.4 In the Absolute Color-Accuracy Shoot-Out online article, we show the colors for a wide range of facial skin tones and fruits and vegetables so that you can get a good idea of where these important colors fall within the 1976 CIE diagram.5
Fig. 2: An accurately colorized sRGB/Rec.709 color gamut appears with reference colors.
Accurately Colorized 1931 CIE Diagram for the sRGB/Rec.709 Color Gamut
The best way to demonstrate the large differences between the 1976 uniform and the older 1931 non-uniform CIE diagrams is to show an accurately colorized sRGB/Rec.709 color gamut for both side-by-side in Fig. 3. Note that for comparison, both of the color triangles have been scaled to have the same geometric area in the figures.
Note how differently the colors are distrib-uted within each color space. The obsolete but still widely used 1931 CIE diagram has a very non-uniform color space that significantly expands the green region and significantly compresses the blue region, providing a very distorted representation of human color perception. Specifying and analyzing displays in terms of the very non-uniform and distorted 1931 CIE color space introduces significant performance, calibration, and color-accuracy errors. Many manufacturers also specify their guaranteed display color accuracy in terms of the non-uniform (x,y) 1931 CIE coordinates, which results in large variations and differences in color accuracy throughout the color space.
The 1976 CIE diagram transforms and corrects the distortions in the original 1931 version to produce a uniform color space that accurately renders human color perception and color accuracy. It’s about time that manufacturers and reviewers abandon the obsolete 1931 CIE color space for all of the above reasons!
The color-gamut size comparisons that are calculated and specified by many manufacturers using the 1931 CIE diagram are also inaccurate and misleading. For example, in the non-uniform 1931 CIE color space, the Adobe RGB color gamut is 35% larger than that for sRGB/Rec.709, more than double the accurate 17% value listed above from the 1976 CIE uniform color space. And in the 1931 CIE color space, the DCI-P3 color gamut is 36% larger than that for sRGB/Rec.709, a 38% size exaggeration compared to the accurate 1976 CIE value of 26% larger. Manufacturers should be embarrassed for specifying their products in terms of the obsolete and very misleading non-uniform 1931 color space.
Fig. 3: Accurately colorized comparisons of the 1976 Uniform and 1931 Non-Uniform CIE color spaces. For the comparison, both color triangles have been scaled to have the same geometric area in the figures.
Accurately Colorized DCI-P3 Color Gamut
Figure 4 shows an accurately colorized DCI-P3 color gamut with an inscribed sRGB/ Rec.709 gamut in order to demonstrate the differences between the two gamuts. DCI-P3 is being used in 4K UHD TVs and in digital cinema for the movie industry, so while the amount of existing DCI-P3 content is still relatively small compared to that for sRGB/Rec.709, it is starting to grow rapidly.
Note how much larger the green region in the DCI-P3 color space is in comparison to sRGB/Rec.709. The extreme reds have also been significantly expanded. Based on the measurements in DisplayMate’s Absolute Color Accuracy Shoot-Out, most fruits and vegetables are found in the most saturated red-to-orange-to-yellow-to-green regions of the color space (so they visually attract animals who will eat them and spread their seeds), and the most highly saturated colors are also heavily utilized in a great deal of human-generated content in order to get people’s visual attention. Therefore, the enlarged red-to-green sliver in the DCI-P3 color space is actually very important.
Fig. 4: An accurately colorized DCI-P3 color gamut is shown with an inscribed sRGB/Rec.709 gamut.
Color Management for Multiple Color Gamuts
When a display needs to support one or more additional color gamuts such as sRGB/ Rec.709, which is smaller than its native color gamut, it can accomplish this with digital color management performed by the firmware, CPU, or GPU for the display. The digital RGB values for each pixel in an image being displayed are first mathematically transformed so they colorimetrically move to the appropriate lower saturation colors closer to the white point. The available color gamuts can either be selected manually by the user or automatically switched if the content being displayed has an internal tag that identifies its native color gamut, and that tag is recognized by the display’s operating system or firmware. The Apple iPad Pro 9.7 implements color management that automatically switches between the DCI-P3 and sRGB/Rec.709 gamuts.
Another, more advanced color-management approach is for the content to include a detailed specification for the colorimetry of the content, and then it is up to the display to implement it as accurately as possible using its native color gamut.
Out With the Old, in With the New
This overview from the earliest NTSC gamut to the latest DCI-P3 and Rec.2020 gamuts demonstrates the importance of eliminating the widespread use of the obsolete 1953 NTSC gamut and the obsolete 1931 CIE diagram in the display industry.
The 1953 NTSC gamut was never actually used for production displays and is colorimetrically different from current standard gamuts, so it is misleading to use it as a reference gamut. The 1976 CIE diagram transforms and corrects the large distortions in the original 1931 diagram to produce a uniform color space that accurately renders human color perception and color accuracy.
Switching to current display technology and colorimetry standards is tremendously overdue. It is essential not only for properly specifying, measuring, manufacturing, and accurately calibrating displays, but also for comparing and marketing them to both product manufacturers and consumers.
The display industry and manufacturers of high-tech products should be embarrassed for using obsolete 63-85 year old standards and colorimetry! At Display Week 2017, I am really hoping not to see any obsolete NTSC gamut specifications and obsolete 1931 CIE diagrams.
References
1http://www.displaymate.com/Display_Technology_2014.htm#Ambient_Light
2http://www.displaymate.com/mobile.html
3http://www.displaymate.com/iPad_Pro9_ShootOut_1.htm
4http://www.displaymate.com/Colors_35.html
5http://www.displaymate.com/Color_Accuracy_ShootOut_1.htm •
Raymond M. Soneira is the founder and President of DisplayMate Technologies Corporation. This article is based on original content from www.displaymate.com. He can be reached at rmsoneira@displaymate.com.