First off GREAT forum for much information on MT stuff.

I havent yet started to design and build an MT setup yet but after reading through countless posts and web info, one of the questions I have is about the plexi-glass and how the LED’s are applied to get the most IR light into the plexi.

thought #1

#1 if you mount the LED’s directly into the side wall of the plexi, w/(polished holes), would this not intitially put all the LED IR into the glass.

#2 to keep as much IR in the glass itself, blocking it’s escape through the side walls ( of the glass itself ) only seems priority.

#3 polishing any edges allows IR in but also out, and in fact reflecting any IR at the edge back into the glass would be more efficient. possibly using a reflective material against all exposed glass edges. I’m not talking about a chrome u-channel that holds the wiring for the LED’s in place , I mean material directly on the glass surface at the edge.

#4 if some “plexi-glass” type material blocks the passage of IR more than others then is it worth trying a laminated glass surface with plexi on the bottom and lets say a thin layer of “poly-carbonate” on top to at least reduce the IR loss through the flat surface even more and at the same time blocking more surrounding outside IR light ? Even better, how about 1 way plexi-mirror laminated facing down on the regular plexi-glass?

#5 if IR loss is inevitable then why not pack the edges with as many LED’s as possible… there cheap. eg; if overall IR loss is ... say 30 % then double your LED count to over compensate the loss?

These are just a few thoughts I have. I like to perfect the basics as much as possible before slapping a project together to reduce the amount of trial and error issues.

P.S.  lol I am a newbie of MT , so I prefer to get as much info as possible.  Also I expect to post a tons of questions as I learn more. I hope this community won’t mind, or mind if I post my perspectives on the normal methods of MT.

.........have a good one............

All you really need is about 1 LED per inch mounted in aluminum channel on all sides of the acrylic sheet. Leave them outside the acrylic and polish the edges. Mounting at an angle, or, beveling the edge helps a great deal with reflection angles. You want that angle. It will help with even illumination. Take a look at this simulator:
http://www.outgribe.com/jason/ftir/ftir.html

Here’s the LED rail on the setup I built:

The minimal IR light loss in a FTIR setup isn’t a big deal. It’s mostly trapped light and that’s why the blob shows up when you touch the acrylic… not glass BTW. Glass doesn’t work.
As far as IR blocking goes, the projection screen material usually helps quite a bit. I’ve been experimenting with houshold IR blocking window film. It works to some extent. I use it during the daylight hours when ambient sunlight produced IR is making it’s way into my work room.

A lot has been tried and documented here, and if you do a lot of reading before you start (as you are doing) you’ll save lots of time.

The led simulator is great. It really lets you play with diff. factors to get the most IR into the acrylic.
I image a tighter pattern is best seeing that the pattern does widen up at a distance. It appears that an IR LED with a wider beam width degree would create a more even and tighter pattern at the right angle of tilt.
To bad the sim. doesn’t allow for the LED to go into the acrylic, I’d like to see the refraction.

Keep in mind the simulator doesn’t show the light refracted upon entrance into the acrylic as at certain angles, the light is bounced off the side even before it enters the acrylic. Placing the LEDs directly inside the acrylic basically removes the whole lensing system of the LED which isn’t necessarily a good thing.

I’m really interested in this issue. I can’t seem to find any testing that shows how the IR would travel if its already in the acrylic.
I know there are different saturation levels when the IR LED ( outside the acrylic ) , is either put on an angle or the acrylic is beveled, also a wider beam width could play a major factor.
These little details like this , tallied up could make for an even better MT unit.
If anyone has a link to a test or simular I would love to get it.
THX JAY.