Quantum Dot Color

4K ultra-high-definition (UHD) video is about more than just a 4x increase in the number of pixels of a regular 1080p TV. UHD video can also deliver an expanded color palette with more vibrant, deeper reds, blues, greens, and every color we can physically see.

Most liquid crystal display (LCD) televisions can adequately represent the color information in the traditional color standard (aka BT.709) used in DVD, Blu-ray, and broadcast programming. However, newer video formats, including Dolby Vision and HDR-10, can accurately represent a significantly increased color space like those defined in DCI P3, Pointer's gamut, and, realistically, complete BT.2020. This more-saturated pallet of colors can result in imagery that appears more like that experienced through our own eyes in nature.

Quantum dots to the rescue

LCD TV manufacturers experiment with a semiconductor technology known as a quantum dot (QD) to expand the color capabilities and improve the efficiency of their products. QDs are crystalline structures that convert incoming light energy into very pure and precise hues. 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 energetic wavelengths emitted by energy-efficient blue LEDs are an ideal fuel for green and red QD materials to generate 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) partially covering the device. The jar comprises 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 light source (backlight unit) and color filter. An LCD color filter splits each screen pixel into tiny red, green, and blue (RGB) windows (subpixels.) These subpixels are individually gated by a liquid crystal module (LCM) to create a wide range of colors we see. Blocking all subpixels produces black, and all subpixels illuminated will produce various flavors of white light.

The better optimized a BLU's light output is to the characteristics of a particular color filter that determines an LCD's ability to produce richly saturated hues. The white LEDs used in modern BLUs are often blue LEDs coated with a phosphor that fluoresces yellow, resulting in a cold (bluish) white light. This coolish-looking light also has limited spectral output in the green and especially red wavelengths.

LCD manufacturers are also expanding color output through blue LEDs coated with red and green phosphor materials. These advanced LEDs approach the color palette coverage of current QD-enhanced displays. However, inequal phosphor decay times can produce undesirable color-fringing artifacts in motion video.

For a wonderfully detailed look at the inner workings of an LCD, I encourage everyone to spend five minutes nerding-out with this excellent video hosted by Professor Bill Hammack.

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. Amazon's Kindle Fire HDX 7" was the first tablet display to incorporate QD materials. Nowadays, many companies including Hisense, Samsung, TCL, and Vizio offer premium televisions that feature quantum dot color enhancement.

Real-world example

The California-based company, Nanosys, is a leader in the development and manufacture of quantum dot materials in the display industry. They presented a pair of name brand 65-inch 4K UHD LCD TVs: one stock and the other equipped 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. In contrast, the bottom screen 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 through an HDMI amplifier. Differences in red and cyan were easiest to discern. Content properly 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 BT.709 optimized color filters can cover about 90% of the greatly expanded Rec. 2020 color space. Newly optimized color filters should bring BT.2020 display coverage into the 95%-96% range.

Non-toxic but tiny

Until recently, QD materials incorporated the toxic element cadmium. The latest generation of QD materials is cadmium-free. Still, this change has resulted in a significant reduction in crystal size to only a couple of dozen atoms across. Tolerances for crystal size at this scale is challenging at best, and the crystal's overall size affects its emitted color. Proposed solutions to the QD "crystal binning" issue include centrifuge-based separation and advances in the manufacturing process involving metal encapsulation.

The future

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. However, 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.) Given increasingly strict energy efficiency standards, an emissive QD display may become the consumer's best option for a bright and colorful high-resolution display.