This is a simulation of the path of light from an LED entering an acrylic or polycarbonate sheet from the edge. You can adjust the LED position and properties, the edge angle, and the sheet material.
Evanescent field strength is a plot of the magnitude of the evanescent field as a function of distance outside the surface of the acrylic. The evanescent field is what illuminates the finger, so you want the field to be as large as possible at the contact distance. Unfortunately, I don't know how close your finger gets to acrylic when you touch it; I imagine it's a function of the surface roughness. I have seen plexiglas surface roughness reported as low as 5 nm in labs, but in the real world it seems to be somewhere in the 100 nm - 1000 nm range. If you have any better idea for this, please contact me.
The field strength is in arbitrary units, as a fraction of what it might be. This is for 880 nm IR light, although any near-IR will have approximately these units. For scale, 1000 nm = 1.0E-6 m = 0.001 mm. Field strength is reduced when (1) beams don't stay in the sheet and (2) beams reflect from the surface at a grazing angle, instead of near the limiting TIR angle.
Source code is here. It's not very heavily commented, but hopefully it's clear. If not, you can email me with questions at firstname.lastname@example.org.Author: Website