further investigation via a message i posted on facebook

i found this answer

Bar code readers read the ratios between the black and white parts of the lines.

The laser scans across the lines and is reflected back to a sensor. When it’s passing over the white part, more of the photons are reflected back than when it’s passing over the black part. The scanner measures the time it spends in each part.

But the scanner can’t control how far the sensor is held from the scanned item, nor can i control the angle at which the path of the laser passes over the lines. Both of these change the timing—holding the sensor farther away means the “end” of the laser’s beam is moving faster, shortening the time it spends in any section, and when the beam passes diagonally over the lines it has more line to cross, increasing the time it spends in any section. Both of these effects happen in equal amounts to the white and the black parts, though, so the ratio of the times is invariant.

The scanner needs both parts—light and dark—to work. It doesn’t “read” just one part, it uses both


of course i googled it ... lost of info available if you want to look further

Posted By: ghostof barry butler, Apr 3, 08:52:18