Color Volume: Why it Matters

color volume

The history of visual art is a constant progression across different media, but always with the same arc. Our ancestors drew simplistic monochromatic images on cave walls that—over time—evolved into paintings, printing, photography, cinema, and television. Each of these methods of reproducing imagery had its monochrome phase before color entered the picture. Today, we take the ability to capture, create, and consume colorful visual content for granted.

When it comes to video displays, the combination of high contrast and rich, accurate color allows a display to render scenes with sufficient fidelity to make them look realistic. Yes, there is also a role for resolution in this formula, but once you get to UHD/4K, the benefit of additional detail starts to drop relative to other image quality parameters.

The last couple of years have brought tremendous advances to consumer displays, with OLED leading the charge when it comes to contrast ratios, and LED-lit LCDs making great strides when it comes to peak brightness and color rendition. UHD/4K TVs already out-resolve what the human eye can see from normal viewing distances, and OLEDs offer unprecedented native contrast ratios, thanks to their ability to display true black by turning individual pixels completely off.

With standard dynamic-range (SDR) content such as Blu-ray, OLED’s strengths are immediately apparent. Wide viewing angles and ultra-deep blacks regularly garner near-universal praise, and deservedly so. But with the advent of HDR and improvements in LCD-TV backlights using quantum dots, several LED-lit LCD TVs now excel at achieving extreme peak-luminance levels as well as a wide color gamut (WCG).

Improvements in LED-lit LCD TVs allow them to offer incredible image quality with properly mastered HDR video. Indeed, during numerous demos over the past year, I’ve compared HDR LCDs and OLEDs side by side. On every occasion, I saw that the better-looking image depended on the content.

In these comparisons, OLEDs excel with scenery that includes lots of deep shadows and blacks—watching a clip from a movie like Gravity or Star Wars: The Force Awakens is a thrill. But, if you switch to something brighter and more colorful—sports, nature, animation, or video games, for example—the tables are turned.

When viewing HDR scenes full of vibrant colors, LED-lit LCDs that feature a wide color gamut and high peak luminance consistently looked as good as or better then OLED. In side-by-side comparisons of a Samsung 65″ KS9800 versus a 65″ LG G6 OLED, I’ve consistently seen how the OLED stumbles when asked to render bright, vivid colors. The reason for this is not simply that these LCDs render brighter specular highlights, it’s because they can reproduce a wider color gamut at high brightness levels.

In other words, they have greater color volume.

You might be thinking, “OLEDs cover over 90% of the DCI/P3 gamut, same as quantum-dot LED-lit LCDs.” That might well be true at a certain luminance level, but while OLED can cover a wide color range at lower illumination levels, once the image gets bright enough, the percentage of DCI/P3 covered by the TV drops significantly. As noted earlier, this effect is clearly visible in side-by-side comparisons with quantum dot-based LED-lit LCDs.

So, how is color volume defined? Technically, it is the 3D representation of the maximum color-reproduction capabilities of a given display throughout the entire range of brightness levels, resulting in a 3-dimensional shape or volume as seen below. By contrast, the much more familiar color gamut is the range of colors that a display can reproduce at a single brightness level, typically 75 or 80% of the peak luminance. This is represented in the familiar CIE diagram with a triangle defined by the red, green, and blue points at that brightness. The ICDM (International Committee for Display Metrology) has named this new method of evaluating TV performance VCRC (volume-color reproduction capability).

DCI P3 Example with Color Volume
On the left is the DCI/P3 gamut expressed in a 3D color volume diagram. On the right is a 2D color gamut chart showing BT.709, DCI/P3, and BT.2020 gamuts.

Color geeks are surely familiar with the 3D lookup tables (3D LUTs) that are used to make precise color corrections across a wide variety of hues and at multiple brightness levels. Measuring color volume works in much the same way, and you can express it visually using the L*a*b* coordinate space from CIE, as seen in the following diagram.

color colume chartThe TV represented in the graph on the left can reproduce DCI P3, even at high luminance levels. The TV on the right cannot.

In the latest OLED TVs, each subpixel has a slight concave depression called a micro-cavity, which is used to create constructive interference that narrows the bandwidth of the light emitted by each subpixel. This results in purer, more saturated primary colors that can express a wider color gamut with on-axis viewing.

However, an interesting side effect of these micro-cavities is an off-axis color shift, which is most visible in the yellows and cyans. The overall loss of image quality is different than what’s commonly seen with VA (vertically aligned) LCD panels, which tend to lose contrast and saturation as you move to the side.

on and off axisThese images show LCD and OLED both on-axis and off-axis. Photos by Mark Henninger.

Going forward, we’re bound to see a greater focus on the HDR capabilities of TVs. After all, the content is already there—Amazon, Netflix, and Vudu already offer a significant amount of streaming HDR movies and shows. To that end, you’ll see more focus on color volume, since it brings together the features that define HDR for consumers (higher contrast, greater peak luminance levels, and ability to render a wide color gamut over a large range of luminance levels).

The days when emissive displays like plasma and OLED were unquestionably superior to LCD-based transmissive displays are gone. Today, finding the best TV for the job requires weighing various factors, including cost, image size, and viewing habits. For example, if you love watching movies in total darkness, OLEDs are likely to remain very appealing. But what if you are not ultra-rich and yet desire something larger than a 65″ picture? LED-lit LCD suddenly looks a lot more attractive.

color volume
The quantum dot LED-LCD (on the left) has greater color volume than the OLED. Note the added vibrancy in the yellows.

Of course, it’s exciting to watch Star Trek Beyond on a 55″ or even 65″ OLED while sitting in a totally dark basement or dedicated AV room. But it’s equally thrilling to watch a bright, colorful movie like X-Men: Days of Future Past on Ultra HD Blu-ray, or an episode of an Amazon or Netflix series in HDR, on a 75″ or larger HDR-capable LCD while chilling in the living room, perhaps in the evening with the lights dimmed—but also perhaps on a sunny Sunday afternoon.

You can get a 77″ OLED (specifically, the LG 77G6) for $20,000 MSRP, but the price gap between that and a top-tier 78″ quantum dot-based LED-lit LCD like Samsung’s UN78KS9800 ($10,000 MSRP) is quite dramatic. More than ever, choosing the right TV for a given application depends on weighing factors like screen size, viewing environment, and cost.

2017 is shaping up to be a very exciting year for AV enthusiasts. There is genuine competition between TV makers and real progress in achieving picture quality that truly conveys what the artist intended. Rendering 100% of the DCI/P3 gamut (currently the standard for commercial cinema) at HDR luminance levels appears to be the next great milestone for consumer displays to blow past—it will mean viewers will see the same colors the director did. From there, the race is on to 4000 nits and BT.2020 color.

It’ll be interesting to see what advances and compromises are made to achieve this, in both LED-lit LCD and OLED. In this brave new world—where your TV could very well beat your local cinema when it comes to overall picture quality—color volume is going to be a significant part of the equation.