Bandpass Filtering and LCD Testing
As promised here are some results of a test session with the LCD. I ordered a bandpass filter for 850 nm near-IR light for the PS3 camera a while ago at ebay. The filter blocks all light except in a small range around 850 nm wavelength.
I am trying to build a multi-touch screen with the DSI technique, explained here but instead of using a projector and some special projection surface I will be using the recently disassembled LCD.
So why not using a projector? The answer is quite simple: I don't own one 
But there are some other facts that lead me to try it with an LCD:
- Projectors need quite a lot of distance from the projection surface but I am not intending to build a multi-touch table.
- Even HD projectors which are quite expensive do have very limited resolution in relation to the size of the projection surface. So small text can hardly be read on such a table.
- I wanted to build something smaller so one person is able to reach the whole screen from one location.
There are basically two issues with the LCD approach:
- The camera is located behind the LC-panel thus has to track blobs through the panel which normally is covered by the backlight (with a white opaque layer) and some metal. This directly leads to the second problem.
- The backlight has to be replaced with one that is itself transparent.
My first approach was to use Plexiglass Endlighten for both backlight and IR illumination but I was not able to detect any blobs with the Plexiglass behind the LC-panel. So I bought another Plexiglass Endlighten which is located in front of the LC-panel for blob tracking purposes only.
I took some picture to test whether IR light is blocked on dark regions on the LCD image as visible light is. The results were quite surprising, the visibility of IR light is not affected by the picture shown by the LC-panel at all as can be seen below.
Display with backlight on.
Backlight and IR lighting on
Only IR lighting on.
Both backlight and IR lighting on. Bandpass filter on top of the camera.
My next step will be to build a prototype and experiment with different types of LEDs for a brighter backlight.