Not sure what you mean by "sperical fresnel"??? The Fresnel lenses I have seen are fairly flat. Older rear projection TV's have one right behind the linticular of the screen. As far as glass vs plastic... that depends on the glass and the plastic. Standard window glass has a bit of a green tint to it and somewhat reduces the infrared transmission. Salt windows are often used for good infrared transmission in analyitical systems, but are not compatable with moisture. Pull a Fresnel out of a big rear projection set and you will find you can get some pretty high heat, enough to light up a piece of pine in under a second. The amount of energy at the focal point is never more than the total energy striking the lens, just concentrated at the focal point.