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Would a stripped (backlight-removed) LCD pass IR light through? Answered

I'm looking to create a desktop touchscreen interface.  I've seen the LCD projectors, but I was thinking that instead of creating a projector, just put a stripped LCD similar to an LCD projector:
http://www.engadget.com/2006/10/17/how-to-build-your-own-hd-projector-part-1/

underneath an IR display like this:
https://www.instructables.com/id/Interactive-Multitouch-Display/

Beneath it all, I would put an IR converted webcam & a lamp.  The goal is to make it as compact as possible under the desk. 

This design hinges upon the IR light from the touch screen passing through the stripped LCD.  

Thoughts?





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Treknology (author)2010-03-28

 Start playing with the IR capable camera first. You will note that the IR-pass filter is completely opaque to the human eye, therefore you can indulge in a great deal of experimentation to see what materials pass IR light, and you will be surprised.

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Re-design (author)2010-03-04

I don't know why it wouldn't but I don't know what the screen is made out of.  I don't see any benefit in them making the screen block IR though.

My question is "how are you going to read the touch location and translate that into coordinates?".

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bullschmidt (author)Re-design2010-03-04

Just check the multitouch display instructible :-)

https://www.instructables.com/id/Interactive-Multitouch-Display/

The reason I ask this question is because, AFAIK, LCD's work by blocking the backlight in controlled manner, right? 


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Re-design (author)bullschmidt2010-03-04

Right.  But the blocking is done on the visible light.  You'll have to test to see if the ir goes thru the glass.  You can test that by shooting a tv remote thru the screen and see if it appears to weaken the beam controlling the tv.

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Re-design (author)Re-design2010-03-04

Now I know more about what you are doing.  It might work, it all depends on the ir going thru the lcd screen.

YOu need to test it when the screen is clear and when the pixels are turned on.  When turned on they may well block ir.

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bullschmidt (author)Re-design2010-03-04

Yeah - thats what I figured.. I was hoping to know before I tore up a LCD.  I'm also looking to find out if it will be the same from one LCD to another (so I can test on a cheapo one).

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Re-design (author)bullschmidt2010-03-04

Well, you could carefully take off the cover of one and check it without taking out the back lights.

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