In the book "SAS Survival Handbook" by John Wiseman they make a reference on page 29 (just in case anyone has the book and would like to see what I read) to a "beta light." He describes it as a small light emitting crystal about the size of a small coin that is fairly expensive but almost everlasting. It's used to read maps in the dark, and provide small amounts of light. I have been trying to figure out what the heck he is talking about for about a month now. I can't find anything on the internet about it (even the mighty google has failed me) and no one I know has any idea either. I wish he wrote more about this light crystal because it seems interesting to me. I figure that if I was going to find an answer to this question anywhere it would be here. I appreciate the help.