Mathews table seems to have worked really nicely, so i wondered why more builders aren’t using the angled led method. Is it that the amount of light refracted is enough with the ally tube frame and the flat edge perspex?

http://digitalstratum.com/programming/ftir_build

because if you use strong leds you have a good optical performance anyway, so why bother with the time consuming less easy to construct angled led method?

Gotcha.thanks.

Odin

Just to echo gravano . . . I mounted the LEDs at ~55 degrees but the light boost wasn’t worth it, easier to just put in more LEDs.

I have a question about this. Wouldn’t having the angle allow closer points on the surface of the acrylic? Such as this comparison…

Angled:

Straight:

Wouldn’t having more points at the top of the acrylic create a stronger FTIR effect? Perhaps less lagging effect as well? Just a thought.

That looks interesting but also the angled one looks like it would create a less uniform light distribution.
If that matters i have no idea but thats what it looks like

Odin

I just threw that together real quick to show my point. I’m sure with some time and calculations you could make the uniformity just as good as the other pictured. I guess no one knows if having more points on the surface like that would make things more responsive (aka less lag) as well as result in a stronger blob.

Surely the responsivity is mostly a function of framerate and software processing speed?

i think the response mostly depends on the camera in the first place and on the software processing speed in the second place, my guess is that the angle of the leds is just about optical performance and not about response time

To clarify, my development board is very small, 8x10 inches so adding an LED makes more of an impact than it would on a bigger surface.  In theory, it makes sense to angle the LEDs but I don’t see bigger or brighter blobs when I angle at ~55 degrees which the applet suggests is optimal for my setup.

The design applet does not account for light reflecting off the edges, attenuation of refracted/transmitted rays, and can be very misleading. Also, it is not clear what dispersion profile the applet is using.

The more acute an angle of incidence, the greater the reflection and the less radiation actually making it into the panel. A major advantage of beveled surfaces is to minimize reflected light and maximize refracted/transmitted light into the panel.

I highly recommend getting a cheap red laser pointer, a small and thick scrap of acrylic, and cut/polish its edges at different angles, and play with it to see what is REALLY going on! (red is close enough to nir for the purposes of exploring edge reflection/refraction and waveguide propogation)

It took me about an hour to cut a 3” wide by 1.25” thick acrylic piece with a skill saw adjusted to 45 degrees, sand it with various, buff it, play with it, and discover how different it is from a polished 90 degree edge. Its no simulation, its real, and it nicely shows the problem of incident angle attenuation as well as waveguide ray tracing.