Edge-lit acrylic

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

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]