Converting DLP projectors for grayscale operation, part 1.

DLP projectors are interesting things. They are based on MEMS technology — in essence, a million or more tiny controllable mirrors, one for each pixel. But the mirrors are not colored. To get the final output color, the image is actually refreshed three times per frame, and a colored filter is spun over each sub-frame. It’s above the flicker fusion threshold, so normally you can’t see it with your eyes. But a camera with a sufficiently high shutterspeed can:

Red interframe
Green interframe
Blue interframe
Note the filtered light being reflected back into the lampbox.

There are many situations in which this kind of projection is undesirable, particularly temporal experiments where one would like to present a stimulus at some exact moment. The next two images illustrate the inter-frame refreshing of pixels. Remember, each pixel is a little mirror being turned on (toward the lens) and off (away from the lens) in a time sequence. To make a pixel darker, it is off longer. To make a pixel brighter, it is on longer.
It’s somewhat easier to see than to describe. Imagine a fixed number of events in a sequence. “1” represents on. “0” represents off.

Darker:
00000100

Brighter
:
11110101

Mid-Gray:
10101010

This is PWM — pulse width modulation. Because of the way PWM works, it is not possible to guarantee with suitable precision when a pixel might be “on” or “off”. Note the seemingly random patterns in the next two images, which were captured at the edges of the red/green and red/blue refresh intervals. The large blocky artifacts on the desktop are due to JPG compression, but the small artifacts (particularly in the toolbox) are from PWM.


It is actually possible to see the effects of color wheel operation simply by making a fast saccade across a projected image. If the projector is a DLP(DMD) device, you will be able to perceive rainbows at bright edges.

Now that I’ve explained some of the problems of DLP devices, the next post will explain a simple solution that will solve some of them.

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