Actually Lexan is a type of polycarbonate, and both are plastics.
The easiest lens material to take care of is plain glass or Pyrex/Borofloat. Uncoated there is nothing to worry about damaging. You can also follow up with Rain-X to help it repel water and dirt. Just use water, or Windex to clean. The coated lenses like the UCL have anti-reflectivecoatings to worry about. Follow the manufacturers directions to avoid damage. Glass lenses can break, but normally won't from just dropping unless the lens hits a rock or other hard object. Of course it depends on how the glass is tempered. Obviously there is no chance of the lens being melted. Lexan/polycarbonate lenses should only be cleaned with water, or a soap and water. Any type of chemicals have the potential to cause damage to the lens, some will just weaken the lens, others will cause it to become couldy or streaked. On the higher end (and hotter) lights, you have the chance of melting the lens if your light is on for too long a time period, especially if it is in an enclosed space such as a pocket or carrying pouch. For any plastic lens you also have the problem of the lens scratching easily. The better ones will have an anti scratch coating, but deeper scratches can occur (scrape it against a nail or the like), and will eventually wear off. There are some plastic polishing and scratch removing products that will remove the fine surface scratches. Also most clear coat safe, scratch removers for car paint will work fine. For the cleaning of any type of plastic (to include Lexan) I like to use a product called Plexus. It's actually made for cleaning the lenses in paintball goggles, and because of that it's safe for use on polycarbonate without fear of damage. If a lens on a pair of those breaks, a person can lose an eye, so they want to avoid being sued. I use this on pretty much everything I have that is clear plastic, my paintball and SWD goggles, sunglasses, light lenses, cell phone and Game Boy screens, etc. It also leaves a slick, anti-static surface to repel dust.
