i was using FTIR at first, then i removed the acrylic to wipe it clean or something, and realized it wasnt necessary, so i kept it off. (touching lcd glass feels a lot nicer than cheap acrylic, i just use the cloth that came with my ipod touch to wipe the screen and my ipod)
but this method works well if there are LED’s along all four sides of screen, because there is nothing now to trap the light, so the further away from the source, the more the IR light disperses. Meaning on my prototype, the bottom edge of the screen gives more faint blobs than the ones at the top. FTIR is more cost effective because you need only 1/4 of the LED’s.
but like i said, the FTIR or DI part is the easy part. if you still cant get distinguishable blobs, take a look at the layering of your LCD monitor.
‘cheaper’ LCD monitors will have fewer layers and generally work better. for example, my samsung LCD monitor had 4 layers; a diffuser, a fresnel, a reflective polarizer, and another diffuser. Whereas, the LG monitor im using, only has 2 layers, a diffuser and a fresnel. So if your LCD layers has 4 layers or something, remove the reflective polarizer and the extra diffuser.
the flourescent lights give off IR, but because its reflected inside the acrylic block and the dots, it multiples, so line up IR blocking film along those edges which will cut down some of the stray IR light coming through.