Updated on January 6, 2015: new product details and tech video link
4K ultra-high definition (UHD) video is about more than just an increased number of pixels compared to the HD formats we experience today. UHD video can also deliver an expanded color palette with richer, deeper reds, greens, and all of the colors in between.
However, before we can witness this UHD color revolution, liquid crystal display (LCD) televisions need a color boost to accurately represent a greatly increased color space over Rec. 709 (HDTV) - like those defined in DCI P3, Pointer's gamut, and Rec. 2020.
Quantum dots to the rescue
LCD TV manufacturers are experimenting with a semiconductor technology known as a quantum dot (QD) in order to expand the color capabilities and improve the efficiency of their products. QDs are crystalline structures that are engineered to emit a very pure and precisely colored light. The size of a QD crystal determines its color. Larger QD crystals produce a more reddish hue and smaller particles skew towards blue.
LEDs + QDs
QD crystals fluoresce when exposed to any light source up to the dot's emitted wavelength, and QD efficiencies improve slightly as the source (or "pump") wavelength shrinks. The relatively short wavelengths emitted by blue LEDs, in addition to being among the most energy efficient of the colors produced today, make them a good primary light to combine with the output of green and red QD materials to generate a white light with ideal spectral characteristics.
The above picture shows a disassembled backlight unit (BLU) with the light output of blue LEDs interacting with the glowing white diffuser sheet (containing green/red QD materials) that is partially covering the device. The jar contains green QD materials as noted by the color emitted closest to the light source.
Light mixing 101
The colorfulness of an LCD TV's picture is directly related to the interaction of its white light source (backlight unit) and color filter. An LCD color filter splits each screen pixel into tiny red, green, and blue (RGB) windows (subpixels) that are individually modulated by a liquid crystal module (LCM) into the wide range of colors we see as well as black (all subpixels off) and white (all subpixels on).
The better optimized a BLU's light output is to the characteristics of a particular color filter determines an LCD's ability to produce richly saturated hues. The 'white' LEDs used in modern BLUs are mostly blue LEDs coated with a phosphor that fluoresces yellow resulting in a cool (bluish) white light with limited spectral output in the green and especially red wavelengths.
LCD manufacturers are also expanding color output through the use of blue LEDs coated with red and green phosphor materials that approach the color palette coverage of current QD-enhanced displays.
QD LCDs now available
QD enhanced LCDs are not new. Sony introduced a trio of LCD televisions in 2013 that featured a QD enhanced BLU, and Amazon's Kindle Fire HDX 7" was the first tablet display to incorporate QD materials. More recently, the 4K UHD Asus Zenbook NX500 is another example of a color enhanced display using quantum dots.
LG recently announced a new LCD television with a BLU that uses white LEDs with an enhanced phosphor formula and specially tuned color filters to expand color output. At the same time, LG is also introducing a QD enhanced model with an even wider range of color output using current color filter designs.
Samsung has also introduced a new line of LCD televisions featuring quantum dot technology for enhanced color reproduction and efficiency.
Real world example
I recently visited the quantum dot technology experts at Nanosys in Milpitas, California to preview a pair of name brand 65 inch 4K UHD LCD TVs: one stock and the other with a modified BLU using blue LEDs plus a QD enhanced diffuser. Both LCDs used the original color filter.
In the slideshow above, the top display has the QD enhancement while the bottom display was left as-is out of the box (factory calibrated picture preset). The imagery used was sourced from the RAW output of a digital camera and distributed to the displays though an HDMI amplifier. I found the differences in red and cyan easiest to discern, and it highlighted how content that is authored and optimized for a wide color gamut display can appear pleasingly natural and not over-saturated or cartoon-y.
Nanosys claimed that current QD materials and Rec. 709 optimized color filters can cover about 90% of the greatly expanded Rec. 2020 color space. Newly optimized color filters should bring Rec. 2020 display coverage into the 95%-96% range.
Non-toxic but tiny
Until recently, QD materials were based around the toxic element cadmium. The latest generation of QD materials are cadmium-free, but this change has resulted in a significant reduction in crystal size to only a couple of dozen atoms wide. QD color precision becomes more difficult as an additional atom or two is now a much larger percentage of the crystal's overall size affecting its emitted color. Separating crystals by size at an atomic scale presents a significant challenge. Proposed solutions to the QD "crystal binning" issue include centrifuge-based separation as well as advances in the manufacturing process.
Currently, QD materials are stimulated by photons to compliment the light output of LEDs. Within the next few years, emissive displays using electron-stimulated QD materials may dominate the premium display market. The inorganic crystalline structure of QD materials has superb longevity although its surrounding chemistry has some oxygen sensitivity - hence the sandwiching between sheets of barrier film.
Quantum dots are already exceeding the brightness levels of OLED (at least in the lab), and given increasingly strict energy efficiency standards, an emissive QD display may end up becoming the consumer's best option for a bright and colorful high resolution display.
Read more about the best 2015 TVs and display technologies.