Absortion loss in light guide

Question

Dear Sir/Madam,I am analyzing a system which is containig a LED and a rectangular light guide of material “PMMA” with Silver coating on the side faces with 6.99mm lenght. LED is having power of 29 lumens and light guide is touching the LED as a result approximately all power is coupling to the light guide.But at the output of light guide I am getting only around 22 lumen. But light guide is having only 6.99mm length, so it is not digestable to have only 22 lumen at output. While ray tracing I am enabling “use polarization” and “ignore error”. Is this power at output is fare enough or I am doing something wrong, can you help me out?I am attaching the zemax file of  my design.Thanks.

Ray · Accepted Answer

You have multiple reasons for not all the light exiting the light-pipe

As David wrote, silver is not 100% reflective. If you remove the coating and use “object default”, thus TIR, you will get around 25 lm instead of 22 lm. This tells you which amount of light (~3 lm) is loss to absorption by silver.
Fresnel reflection from the entrance and exit faces of the light-guide will also count. If you change your first detector “Front Only” from 1 to 0, you can evaluate this amount of light. You get around 32.5 to 33lm (silver vs TIR): the 29lm incident from the LED and the rest is the reflected light: 3.5 to 4lm.

So your lightguide loses 3.5lm to Fresnel reflection and 3.5lm to Silver absorption, of which ~0.5lm is loss on the way back after Fresnel reflection at the exit.

David · Answer

I am not able to download the attachment. Perhaps a forum issue. However, it is possible the silver coating is not 100% reflective. In this case, many bounces would result in significant loss. To investigate, you could set the source to deliver one ray and save a trace to a ray database. Then view it with the ray database viewer. You could also use Path Analysis if your version supports it.

Absortion loss in light guide

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