FYI: Have another thread running about my having to replace my pool light niche Pentair-Amerlite with a entire replacement of same (ie., the pool light assembly consisting of the housing, glass lens, gasket, circular face trim). This afforded me the opportunity to take the old light fully assembled and subject the lens to some "Kick Tests" , if you can call it that. Standing over the light assembly, I really gave the lens some hard kicks with tennis shoe on - and it didn't break or crack. I went to some pretty far extremses and was unable to break it.
The dome design of these lens and the composition and thickness of the glass lens combines to make them extermely durable IMO. My assumption is that the manufacturer anticipated that kids or adults could, in monkeying around or playing rough in the pool, subject the light to some pretty serious underwater abuse in the form of kicks, blows and other similar types of forceful contact with the lens.
I'm going back home later today and will try some other test - curiosity being an element of my nature. I am going to try to see if my blows can get it to crack or break. I'll post those results.
My preliminary opinion, however, is that these lens were designed to take some real abuse - i.e., that the manufacture's risk assumption analysis resulted in it exercising an over abundance of caution in designing these lens.