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Subject: [DPRG] LED ID
From: Mike McCarty jmccarty at ssd.usa.alcatel.com
Date: Tue Oct 17 09:59:29 CDT 2000

On Mon, 16 Oct 2000, gfe wrote:

> I've been deconstructing a defunt color copier and have a board with
18 yellow LEDs. They were situated below the image drum, but are group
in two sets of 9 rather than evenly across the drum. I was hoping they
might be some super bright led to use for line following but they won't
excite a phototransistor even if placed flush against it. They emit
yellow light. Any idea what these are?

Probably yellow LEDs. Phototransistors are usually made from Si, which
has very low sensitivity to that part of the spectrum. Red LEDs are much
better than yellow, and infrared are even better for this use.

There are two things confusing you, probably. One is that the eye has
its maximum sensitivity in the yellow to long green colors, making the
LEDs appear brighter to you than they "really" are, and the detector
has low sensitivity in this region, making them appear darker than they
"really" are to the detector.

Brightness is not the same as power output. The sensitivity of the
detector also figures in.


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