Hello everyone,

I am new to FTIR and Multi-Touch LCDs, but I have been reading up on the topic.  I know a very good amount about electronics and electrical engineering, so I have no trouble wiring the LEDs up.  My main problemn is I do not know how many to use.  I have been reading around the forums, and most people seem to favor the SFH-485’s.  Is a 40 degree angle really enough?  How LEDs many would you recommend for use on a 22 1/2 by 17 inch display?  How thick acrylic would you recommend?

Thanks a ton,

Alex

Alex,

The more the LEDs the BETTER. I keep spacings less than 2” (and go much closer for larger panels). Decide first on what your power supply voltage is and the number of LEDs in a series. The total number should of course be a multiple if this, then determine their physical positions.

Have the tips of the LEDs touch the edges of the panel. You can play with wider dispersion angles by placing KY jelly between the LED and panel edge (this reduces the strength of the lens by replacing the air’s low index of refraction with the jelly’s higher index). While wider angles helps evenly distribute the radiation, it also results in more radiation placed beyond the critical angle (the bright un-usable “halo” around the edges of the panel). If you put the LEDs close enough, you will not need a wider dispersion angle.

For small panels, I would go with 5mm or 1/4”. Thin panels “bounce” the light around more and this helps with distribution for small panels (not for large panels were attenuation becomes a greater problem).  5mm or 1/4” polycarbonate is strong enough and readily available in many hardware stores.