What are Quantum Dots and how could they help your next TV?

March 8th, 2013

David Schulberg the 3rd

Let’s take a quick look at the nanotechnology that could be in your next television.

(Credit: Sarah Tew/CNET)

At the Consumer Electronics Show (CES) in January, Sony announced several LCD TVs with “Triluminos”, a new backlighting method that they promise offered a “rich, authentic colour and excellent red and green reproduction.” Digging deeper, it turns out that Triluminos includes an optical component produced by QD Vision called “Colour IQ” which uses quantum dots to help create light.

What are quantum dots?

Quantum dots are a “semiconductor nanocrystal technology”. If you don’t remember your high school physics, avail yourself of the Wiki page.

If you don’t know your valence bands from your conduction bands, you can think of a quantum dot as this: tiny pieces of matter with unique properties, including the ability to emit light at very specific wavelengths. Sort of like microscopic pieces of glitter that glow green, red or blue, depending on their size.

Red, green and blue spectra for red, green and blue quantum dots.
(Credit: QD Vision)

Specific wavelengths of light are good. We need specific wavelengths of light — the more specific the better. All televisions create an image by combining the three additive primary colours: red, green and blue (RGB). Sharp adds yellow, a secondary colour, but this isn’t in any content and is created by the TV. Mixing RGB in varying amounts gives us all the colours possible in our current TV system.

All LCDs create these colours with filters. Plasma displays create them with phosphors that glow in the required colour (similar to the way CRT tube TVs worked). OLED, depending on the company, is one or the other: LG’s method creates a “white” OLED then adds colour filters. Samsung’s method has specific red, green and blue OLED sub-pixels.

So where do quantum dots come in? Sony has a method.

Sony’s X900 and W900 lines

Three of Sony’s 2013 TVs will use quantum dots in their backlighting in the guise of QD Vision’s Color IQ tech (the 65X900, 55X900 and 55W900). A traditional LED LCD uses blue LEDs coated with a yellow phosphor to create “white” light. While reasonably efficient compared to other technologies (ie, CCFL LCDs and plasmas), this still creates a lot of “wasted” energy. Orange, for example, doesn’t make it past the colour filters on the front of the TV (instead, red and green are combined to create orange).

Triluminos uses blue LEDs, but instead of coating them with a yellow phosphor, the blue light from the LEDs passes through the Color IQ optical element containing red and green quantum dots. So the blue LEDs have two functions: create blue light, but also energize red- and green-emitting quantum dots, so they in turn can create red and green light. About two-thirds of the light created by the blue LEDs is used to excite the QDs. Cool, right?

This diagram is a top-down view of one side of two edge-lit LCDs (the front is “up” in this case, the back is “down”). The upper image shows a traditional “white” LED (blue, with yellow phosphor). The lower image is the method used in Sony’s Triluminos: a blue LED that passes through red and green quantum dots. This RGB light bounces off the light guide, and out through the liquid crystal and other layers just like a regular LCD TV.
(Credit: QD Vision)

Oh, and if the “Triluminos” name sounds familiar, Sony has used it before. The 2013 version described here is referring to an edge-lit LED technology, not the RGB full-array LED backlighting from 2008.

How dots can help TV colour

Sony claims its new backlight technology allows for a wider colour gamut compared to LCD TVs using “white” LEDs, as in more potential colours. Since all modern TVs are fully capable of reproducing every colour in all current HDTV content, this is a bit of marketing hyperbole.

However, the benefits of this could go beyond cool, futuristic tech and “wow, neato”-based marketing. When I’ve reviewed LED-lit projectors, I’ve found that the colour possible from RGB LEDs looks more realistic than the same Rec 709-calibrated colours created by colour filters (DLP) or dichromatic mirrors (LCD/LCOS) as lit by UHP lamps. One TV engineer I asked about this phenomenon replied “LEDs are like painting with purer paint”.

Our own David Katzmaier often remarks in his reviews on the bluish cast seen on some conventional LED-based TVs compared to, say, plasma sets. “It’s usually most prevalent in dark areas, but I sometimes see a slight bluish ‘coldness’ in brighter material and skin tones, too. In some cases, I see it despite seemingly excellent colour measurements from my instruments.”

So it’s possible that even with the same measured colour points, quantum dot-enhanced displays could produce more realistic colour. Will they? Will the colour mixing required to create Rec 709 from wildly oversaturated colour points cause other issues? What effect will the colour filters, which are still necessary on LCDs, have on this “purer” light? These are questions we can’t answer until we see the X900 series and any future TVs with quantum dots.

This whole column, and not one “quantum leap”

Atomic Force Microscopy (AFM) image of sparse QDs (white) on a semiconductor background (black). Individual QDs, as well as close-packed small groups can be resolved.
(Credit: MIT)

The current generation of quantum dot technology requires a primary light source like the blue LEDs in Sony’s Triluminos. This won’t always necessarily be the case, as it will be possible to excite the quantum dots directly. This could be a full QD backlight, but it could be more. How about a direct-emissive display like OLED, but instead of Organic Light-Emitting Diodes, it’s sub-pixels filled with red, green or blue quantum dots. QD Vision calls this a “QLED”, and it could have similar performance characteristics as OLED (like a truly infinite contrast ratio). Will it be easier to produce better colour or have even lower power consumption? At this point, we have no idea. Given the production difficulties OLED has had, just the fact that there’s something on the horizon that could offer potentially similar performance is exciting.

Bottom line

Unlike many of the new technologies on display at CES every year, quantum dots are real and are potentially very cool. For now, they reside only in a few high-end LCDs, but like OLED, they could hint at what a display of the future might be. We shall see.

Via CNET.com

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