Barcode Scanner Bar
Posted by admin on Friday, June 19, 2009 · Leave a Comment
Physics problem: Bar code scanners - how can they work???
The scanner 'reads' a bar-code by firing a laser at the bar code and (I assume) assessing how the beams are reflected back into the scanner. (?) That would make sense - there MUST be information heading back into the scanner for it to read the bar code and Iassume that this is the reflection. It's like throwing a ball against a wall and catching it.
My problem is that you can line up a hand held Bar Code Scanner at almost a completely flat angle to the barcode and it will still read it. How ? Any reflected beams would go no where near the scanner but scoot off into the distance. Ie you throw the ball, but hit the wall 20 metres to your right. The ball flies off, no where near you.
This works on barcodes on smooth surfaces such as plastic, from some distance.
Its been annoyoing me all morning. Thanks.
Reflection from most surfaces is diffuse and not specular. That means light comes off the surface in all directions. You can see this easily by looking at the surface - if you can see the line the laser illuminates then it is reflecting diffusely, because specular relfection would just leave you to see an image of what is being reflected (like in a mirror) not an illuminated image of the surface.
Specular reflection is very rare in nature.
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