How do bar code scanners read a bar code from a cell phone screen?

[mrspeedybob]

A quick google search found that there are 2 kinds of bar code scanners. One uses ambient light and a CCD device, these obviously would have no problem with a bar code on a screen since it is essentially just taking a picture.

The other kind sweeps a laser over the bar code and reads the reflection. From what I have read these scanners are more particular about the angle and distance to the screen when reading a bar code from a cell phone screen, but I can't figure out how it could read the screen at all. The image I see on a cell phone screen is made by the screen emitting light, not reflecting it. The light reflected from any given spot on the screen is, as far as I know, unaffected by what is displayed on the screen (a problem clearly demonstrated every time you have to tilt your screen to avoid glare from a bright light). Since the laser type scanner relies on reflected light, how is it able to "see" what is on the screen?

 

[dschlink]

Even though the surface of a the screen on a cellphone can reflect a fair amount of light, a LCD works by blocking the backlight. This produces more than enough contrast for the high intensity beam of a laser scanner to read by reflection.