Light Sources for Optical Reader
By Robbie Rhodes
Mark, brother John looked into your suggestion of using a laser diode "line of light" generator as the illumination for the music roll in an optical roll reader (digest 960104).
John found a catalog from Edmund Scientific Company several months ago which also listed the little light source. It doesn't produce a scanned (moving) beam of light; rather, the laser beam is spread-out to a line by a simple lens. The complete hand-held barcode reader that used the light source most likely had a galvanometer-driven mirror which scanned the reflected light into the photodetector.
To get the best efficiency from the a light source some sort of condensing lens system is needed, such as in the overhead projector, which uses a Fresnel lens at the object (transparency) plane as the last element of its condenser lens system. The rays of the light source ideally converge at the iris stop in the projection lens. In a good quality 35mm enlarger or slide projector you can see this: put a piece of paper where the center of projection lens normally sits, and you can see the image of the lamp filament focused there.
You could easily use an overhead projector for the light source, if the granularity of the Fresnel rings doesn't cause trouble. Here's a diagram in "ascii-art". (View with mono-space font.)
========= tracker bar with photo-detectors
_________ music roll
| | | | | light rays are close to parallel here
wwwwwwww 2nd condenser lens (Fresnel lens)
\ /
\ /
( ) 1st condenser lens
\/
o light source (filament)
Returning to the subject of the "Scanned Light Beam":
The point-of-sale scanner uses a powerful red laser beam because the total optical system is so variable -- the salesclerk is waving the barcode image at an indefinite distance and attitude. I think it's just amazing that the system works as well as it does!
However, it's terribly expensive, and that's why facsimile "fax" machines don't use a scanned light beam. It's much, much cheaper to flood the text paper with light, and "photograph" it with a CCD array.
That's why we're studying fax machines and computer image scanners: to adapt the technology for reading music rolls.
-- Robbie Rhodes |
(Message sent Sun 7 Jan 1996, 23:51:22 GMT, from time zone GMT-0800.) |
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