Optics - LCD backlight (prism light guide)

Hi Everyone!

I am doing a study on how a light guide works. I have been studying a sample for sometime and am still clueless on how it works.
Basically it is used behind a small LCD panel (for mobile or mp3 player) and the led is placed from the side. Thus light has to be reflected to the surface that is 90 degrees to the light propagation.

I would like to know

  1. Is Fresnel loss involved in the design, to achieve continuity. If I play around with only one prism, it should be quite easy to redirect a single ray to the surface. But the problem I don’t quite understand is how about the next few prisms? How do they get their light? (continuity problem)

  2. The prisms are only on the bottom surface (tapered side) of the light guide. Then what happens to the light rays that propagates at a height slightly above the prisms (i.e. doesn’t cross the prisms right from the start).

  3. Is there a simple ideology / testing methods to follow to determine the shape and sizes of prisms to be used.

I understand using softwares (LightTools, TracePro, etc) a very good light guide can be achieved. However my target this time round is just to get a basic design guideline to make an “acceptable” lightguide instead of a superb one.

Hope everyone can assist me with whatever input you may have.
THANKS a lot !


you don’t need prisms, just a good way to trap light in a confined area with only one way out. (think 5-sided cube) thinner volumes with interior reflective surfaces are best.