Reflective vs. Transmissive
This is an old thread, but in case anybody is strolling through this and still wondering....
You have to understand how an LCD, like a phone, works. LCD screens use a bright white light behind the screen. This light is then passed through a linear polarizer that ensures all the light is polarized horizontally.
The front layer of the screen (just behind the glass) is another linear polarizer, except vertical.
If you just pass light through a horizontal, then vertical polarizer, none of it will make it through. The liquid crystal element turns the polarization a quarter of the way around when a pixel is turned on.
Now hypothetically, if you had the scanner at just the right angle, those pixels that were turned on could transmit light all the way to the backplane, which would be slightly reflective and transmit the laser light all the way back. But it's not a mirror back there: the reflection of the glass at the front is probably going to be brighter.
This is why people are universally saying that laser barcode readers don't seem to work on LCD screens. A camera is different, because it sees transmitted light just like our eye. A barcode relies on reflected light to bounce back off the white part of the paper.
It's basically identical to the reason why you can't just turn your laptop display all the way down to zero and use sunlight. If you struggle hard you might slightly make out the image, but not enough to make it useful.
All the new electronic ink displays, like the Kindle, are a different story. They have no backlighting or polarization scheme, and rely on small, electrically charged capsules of ink sitting inside an oily medium. There are black capsules that absorb light and white capsules that reflect light, and they switch using an electric field. These reflect light just like paper does.
This is why you'd have no difficulty scanning barcodes off a Kindle using a laser. (And yes, you could use a camera too: if your eyes can see it, so can a camera scanner.)
