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Getting rid of the burn-in

Because the VFDs already had seen a fair amount of use with mostly static text being displayed on them, there was a fair amount of burn-in: the phosphors in the individual pixels was degraded, in some cases so badly that you could read the text that usually was shown in those positions:

Unfortunately, there's no easy way to get rid of the burn-in: as far as I know, there's no way to rejuvenate the degraded phosphors. With the display driver being able to output grayscale data, however, what we can do is apply a correction factor to the data being sent to every pixel to get an even display. This means dimming the display a bit to the maximum brightness the most degraded pixel can send out: Say, we have two pixels we want to turn entirely on. One is degraded to output just 50% of its nominal light output, so we indeed turn it entirely on. The other one is not degraded and sends out the full amount of light. If we set its intensity to only 50%, the two pixels will have a similar amount of light output and the fact that one of them is severely degraded is effectively negated.

To do this, we need a map of how badly deteriorated each pixel is. Ofcourse, I could hand-tweak every pixels intensity, but for sixteen thousand pixels, I found it more prudent to automate the process. I ended up quickly hacking something together that turned on all the pixels in the display and using my DSLR to take a photo of it that was as straight and sharp as I could:

I then postprocessed the image in The Gimp to do some perspective correction and to crop the image to only the active part containing the pixels. A quick PHP program then tried to find each pixel to calculate its average intensity: it would guess the estimated position of a pixel, then finetune that estimate by looking for the pixels borders. It outputs the intensity value found for each pixel, as well as a calibration image that shows where it has detected a pixel (in red) and the edges (blue pixels):

The end result is an image where the burned-in image is almost entirely gone. Unfortunately, it's pretty hard to use a camera to show the difference; the camera seems to grab on to the intensity differences that are still there and amplify them, making the de-burned-in image a lot noisier than it really looks. What is visible is that the de-burning-in process gets rid of the big bright and dark surfaces while it introduces some high-frequency noise that is way less visible from a distance.

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