Edge-lit acrylic

Sanding one of the faces roughens it, causing light to be emitted on that side.

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

• Polish those edges, using some combination of sanding/buffing/flame. [2] [3]
• Cover those edges or faces with reflective material, such as silver/chrome vinyl film.
• Paint that area.

Smaller details

• 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)