At the last Maker Faire I had a bunch of interactive gizmos for people to play with, but one question I was asked more than any other was "How did you make those displays?" On the Rock Paper Scissors Playing Glove one small display showed three different images and text while on the Secret Knock Detecting Gumball Machine the light just seemed to float in the clear plastic without an obvious source. The answer is edge lighting.
Edge lighting exploits a fun property of light called internal reflection where light can get trapped inside a transparent material. It's the same concept that makes both a diamond sparkle and optical fibers work. But we’re going to use it to make light come out of a sheet of plastic.
Simply put we shine a light in the edge* of a clear piece of plastic and etch it where we want the light to come out. With this simple, cheap method you can use a single LED to display words or images. Because it's etched into a transparent surface the images can seem to float in mid air or you can layer several of them to make a single display with more than one image or color.
Keep reading to see how easy and quick it is to get this effect.
*And thus the name "Edge Lighting"
Step 1: Credits And Theory
First I need to give credit where it's due. This idea is not new or original. At very least it was used in calculators in the 60's. I was turned on to it by the always creative Evil Mad Scientist Labs and their great Christmas cards. They also did the research and found great materials which make the whole process easier
When light passes between two different materials (like plastic and air) at a very high angle it will get reflected back rather than passing through. You can see this in a glass of water. Look at it from above and you can see right through it. Looking at it almost edge on and it will be very reflective. This means that if we shine a light into the edge of clear plastic the light will bounce around inside until it finds a way out. By etching and/or carving the surface of the plastic we control where it escapes.