Three years ago, just when the TV makers appeared to have settled on LCD TVs with LED backlighting as the dominant technology, a completely new technology — OLED — appeared. That year saw the introduction of OLED models by two major companies, LG and Samsung. One delivered it in the form of a slim and sexy panel, the other with strange styling involving a weird outer frame.
We heard from insiders, however, that the yield on large OLED panels was shockingly low — around 10%, so that nine panels were being rejected for every one that worked, which explained why the initial prices were so high. The next year, Samsung (along with some other companies) dropped its use of the technology for TVs, just as LG.Display worked out how to deliver it more cheaply through an improvement in its fabrication processes that allowed far higher yields.
The following year the Samsung returned with its a new technology — SUHD. Samsung’s new range was marked by Ultra High Definition panels, and something from a company called Nanosys called ‘Quantum Dot’ technology, which was said to improve the colours and efficiency of the TV panel.
So what are these Quantum Dots? Little glowy things in the pixels of the TV? Or just marketing hype, like most products claiming magical ‘nano’ or ‘quantum’ properties?
I was inclined to go with marketing hype as the explanation. After all, everything electronic works on quantum principles, seeing as how the basis upon which transistors work is at the quantum level. Quantum refers to the fact that units of energy don’t scale smoothly to zero, but at extremely low levels must be at one of several discrete levels. But there’s a whole bunch of other stuff, such as wave/particle duality, uncertainty, and so on.
This year, though, Samsung had someone at its 2016 AV product launch who was able to actually describe what a ‘quantum dot’ is. Professor David Reilly is an experimental physicist who, among many other things, leads the Quantum Nanoscience Laboratory at Sydney University. He explained what was going on with this technology.
Many of us (myself included) may have thought that to the extent that there was anything especially quantum about it, the action was happening in the TV pixels themselves. Wrong. The quantum dots are used for the light source.
Here’s how it works. Instead of using standard LED edge-lighting or backlighting for a relatively conventional LCD panel, this technology uses LEDs producing light at the blue and ultraviolet end of the spectrum. Bundles of two specifically engineered quantum dots are then excited by the UV to produce red and green light. And thus you have these mixed together to produce what looks like a white light, but is actually rather different in its quality.
The human eye perceives colour by using three different kinds of cone cells, each sensitive to a relatively narrow range of wavelengths. These ranges overlap, but sensitivity is at its highest in the centre of each range. One type of cell being hit in its sensitive band is interpreted as blue. The other cells’ triggering is interpreted as red and green. The sensitive areas for the red and green cones is quite close together on the spectrum, while that for blue is rather further away.
That is, of course, why all display technologies use red, green and blue pixels (we’ll ignore one company’s strange flirtation with yellow). The important thing for this discussion is that, with two provisos, it doesn’t really matter where in the sensitive band that wavelength of the perceived light sits. If it is in the blue band, it is perceived as blue, and only one shade of blue. Other subjective shades result from the output of that cone being mixed with the output from the red and green cones.
The two provisos? First, if the range of wavelengths of the light is too broad, then it might inappropriately also trigger the red and green cones. Second, wavelengths that fall outside the point of highest sensitivity for a particular colour are relatively inefficient. Higher intensities are required for the same cone response.
So? Professor Reilly explained that quantum dots are nano-sized crystallite — tiny crystals — that fluoresce when stimulated by UV/blue light. Because their (nano)size determines the wavelengths of the light they produce, they can be very precisely engineered for well-defined wavelengths. So they are tuned to hit the wavelengths to which the cones are most sensitive, increasing efficiency and reducing spillover into the sensitive ranges of the other cones.
At the front of the TV there is still a regular LCD panel with RGB coloured filters, operating in the same old way. But the light being filtered isn’t normal broadband white light with a large proportion of its energy falling in places in the spectrum to which the eye’s colour receptors are rather insensitive.
Instead it is composed of three narrow bands of wavelengths, well targeted for maximum efficiency and accurate control of colour.