Light pipes, light guides, optical waveguides, etc

Theory

There are two major groups of light pipes: Those that use total internal reflection (TIR), and those that use a reflective coating.

Applications

• Fiber optics
• LED light pipes
• Light-guide panels, used for high-end lighting, or as a fancy backdrop.
• Edge-lit numerical display
• Other Wikipedia articles:
  • (more generally) Optical waveguides
  • Light tube

Edge-lit acrylic sign

(Background info:   Edge-lit acrylic functions according to the same principles as light pipes and .)

Sanding one of the faces roughens it, causing light to be emitted from that area.

Things you can do to prevent light from coming out in a specific area:

• Edges can be polished, using some combination of sanding/buffing/flame. [2] [3] These edges reflect the light back inside.
• Cover edges or faces with reflective material, such as aluminum foil tape (for ductwork) or silver/chrome vinyl film.
• Painting an area prevents light from being visible in that area. (but it doesn't reflect that light back inwards)

Smaller details

• Covering all sides of an LED with reflective tape can reduce "hotspots" somewhat.
• General advice on edge-lit acrylic signs: [1] [2] [3]
• It can help to get cell cast acrylic, instead of extruded. (TODO: Why? Is that needed specifically when doing laser etching?)
• HackADay articles on 'light guide panels' and acrylic signs: [1] [2] [3] [4] (more)
• You can stack acrylic layers to get interesting effects. [1] [2] [3]
• Black acrylic can cause interesting effects. [1] [2] [3] [4]