As others have stated, the screen burn you see in the projection monitor is just CRT burn. Projection monitors (and television sets) are basically just three separate CRT tubes, one for each color. They're small, about five inches, and mounted behind lenses. There is also a cooling fluid in there too, because these CRT's are run very, very bright and get very hot. They have to be, in order to create enough light to fill a massive projected screen.
They burn even worse than normal direct view CRT's, because the little tubes are run so bright. In fact, home video game consoles used to always come with warnings in the manual, saying that they should not be used on a projection television, and for exactly this reason.
The TV set in my parent's living room is a 42" Sony rear projection. It has the "USA" network logo burned into the tubes from too many Law and Order marathons. Seriously. It's faint, but you can see it when the screen is white, like during a commercial.
Even in normal use, projection tubes burn evenly and get dimmer. If you take apart a junked projection TV, you can see a browned rectangle in all the tubes where the image was, and clean phosphor along the very edges. Back when TV sets were worth fixing, replacement projection tubes were a pretty common requirement, after several years of heavy use. In some sets you have to change the cooling fluid now and again too - it gets yellowed from the heat.
-Ian