LED Roll-Call Techniques

One solution that I hate is to relying upon moving a traversing column of detectors to scan the LEDs. You could run a special test pattern of the LEDs and thereby use less detectors; one detector would be arranged to detect several LEDs, the test pattern of which illuminates one at a time. This is what we call a Bozo-Solution.

I'll rarely share my favorite solutions but here's one:

Use a partially mirrored laminate on the inside surface of the transparent cover plate. Place zone detectors around the array that can pick up the diminished reflections from nearby LEDs. By illuminating LEDs in a timing pattern, the detection of the proper wavelength off the partially mirrored laminate into the appropriate detector confirms the proper working of that LED.

You could use special lensing channels of plastic to get the reflecting back paths, but laminated surfaces are much cheaper. Most companies that use this mechanical/optical solution don't even realize the partial reflectance technique when examining these products. :)

When do you test the LEDs? Why do it all at once? When you are flipping frames between the time you remove one image and before you post the next, illuminate one LED and check for reflectance. Since this takes only a split second, no one can detect it. Affectively you are running a LED test over a long stretch of time, over and over.

:)

-Jay C. Box