Solving the OLED Burn-in Problem?

We, wise industry observers that we are, had believed until recently that LG had solved the OLED burn-in problem, at least as far as TV applications were concerned.

However, careful and continuing long-term testing by as well as extensive anecdotal evidence, have shown the problem is very much with us.

At LG’s press conference CES 2019, Tim Alessi announced the companies first 8K OLED TV, which includes “pixel dimming control technology.” At the risk of taking an unjustified logical leap, I assume Alessi’s comment is a veiled admission that burn-in exists and that LG is trying to do something about it. But it does seem like a somewhat cobbled-together approach. If you have carefully designed the electro-optical transfer function of your set to faithfully reproduce the media creator’s intention, why would you then want to further dim any of the pixels unless you had a problem and could think of no other way to fix it?

But is there a more integrated way of handling the differential aging and burn-in issues? Years ago, the Canadian company Ignis developed a system to checking for changes in the switching threshold of OLED pixel switches and compensating with appropriate changes to the drive voltage on a pixel by pixel basis. I have heard the Ignis technology was licensed by LG Display. (Can anybody confirm that?) (Ken – a search on Ignis on the very handy Display Daily website shows that we published an announcement on this several years ago! – Man. Ed. Ignis Signs Patent License With LGD We also confirmed that the technology was being used.)

However, Ignis was compensating only for changes in the pixel switches, not for changes in the OLED emitters themselves. In his latest patent, granted in September 2016, Nongqiang (Nick) Fan — an alumnus of UC Berkeley, Motorola, and Microsft — addresses both problems. In addition to what appears to be a novel dynamic approach for setting the pixel switch’s threshold voltage, Fan includes within each pixel switch a photodiode that senses the output of the OLED emitter. The diode’s photo-induced current is used to change the bias on a transistor in the pixel switch, thus controlling the drive current to the OLED. It’s a neat solution that should not add much cost.

CAPTION: In Fan’s dynamic compensation pixel switch, some light from the OLED emitter illuminates the photodiode D1, whose photo-induced current changes the bias on transistor T1. This circuit can compensate for OLED emitter aging, including burn-in. (Figure: Nongqiang Fan)

Fan has been working on methods for dealing with pixel non-uniformity since 1996, has been awarded a number of patents on the subject, and has sold some of his IP. He knows this field. On the other hand, this combined dynamic compensation approach — convincing as it is — has not yet been reduced to practice. Perhaps LG Display would like to give it a try. Ken Werner

Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications, including mobile devices, automotive, and television. He consults for attorneys, investment analysts, and companies re-positioning themselves within the display industry or using displays in their products. He is the 2017 recipient of the Society for Information Display’s Lewis and Beatrice Winner Award. You can reach him at [email protected].