The second talk that Ken Werner was due to give was about quantum dots and their impact on the prospects for OLED TV. One of the claimed benefits of OLED TVs compared to LCD is a better gamut.
Werner used a talk given by Seth Coe-Sullivan, from quantum dot maker, QDVision at an SID event earlier this year when he questioned whether the claimed benefits of OLEDs were really true (Display Monitor Vol 21 No 8).
Quantum dots (QDs) are small particles of materials (not difficult to make according to Daniel den Engelsen of ABInfo at the event). They have the property that when optically excited, they emit light at lower frequencies, with the frequency dependent on the size of the particle. Used in a display, the QDs act like phosphors – when exposed to blue light, the QDs emit green or red light. The frequency of the light is very pure, without the waste of energy seen in, for example, white LEDs using YAG phosphor. Especially if the colour filters of an LCD are optimised for these frequencies, the emitted light gives very saturated colours and a wide gamut with good efficiency.
QDs in displays are typically either in tubes for use in edge-lit LCDs (QDVision) or films that replace the diffuser film in the LCD stack (Nanosys/3M). The white LEDs are replaced with blue ones. This makes the engineering simple, although filter optimisation is a different story. Sony has been shipping LCD TVs with QDs in its range since the model year 2013.
Going back to Coe-Sullivan’s list, LCD can compete in every area, in his view. (I think the problem for OLED will be if high brightness – up to 2,000cd/m² – HDR LCD displays start to appear – which they will. To go up in brightness by that amount will be very difficult indeed.)
Werner asked if consumers really care about colour? 3M did some tests to develop the DQS or Display Quality Score (Display Monitor Vol 21 No 22). This was a re-branding of its Perceptual Quality Metric (Display Monitor Vol 20 No 21) and is intended to give a single index of merit figure taking into account brightness, colour, contrast, resolution and display size. The formula to calculate it is based on 3M research and the idea is to allow a display maker to optimise the DQS by careful choice of those parameters.
Colour is the second factor for consumers after resolution, and boosted colour saturation can give viewers the impression of higher contrast (the Helmholz-Kohlrausch effect). When 3M tested an LCD (DQS 137) and an OLED (DQS 136) side by side with consumers, 75% preferred the OLED, which had lower brightness, but better contrast and a wider colour gamut than the LCD. When the firm boosted the contrast of the LCD to match the OLED (DQS 138), then 75% prefer the LCD, but when the gamut is also boosted (DQS 140), 85% to 90% prefer the LCD.
Werner said that QDs could be used to enhance LCDs to be good enough to threaten the development of OLED in the market and that, with LG & Samsung planning to adopt QD-based TVs in 2015, 2015 will be the “year of the QED”
Display Daily Comments
The talk included comments on the two different methods being used to put QDs into TVs and other systems, using tubes and films. In questions, it was pointed out that Pacific Lighting Technologies of Portland, Oregon, is putting QDs in with LEDs in the same packaging. Other companies have told us that this is very difficult because of the effects of the high heat from the LEDs on the QDs. (BR)