50 degrees is the half gain viewing angle left/right of a central position. Your drawing seems to indicate a 25 degree viewing angle, totally 50 degrees, but it is actually a total of a 100 degree viewing angle for half gain.
This is very typical of these types of screens. The non-Fresnel lens screens will have a wider half gain of the viewing angle, but will do worse with rejecting ambient light.
As is ALWAYS the case: These are band aid products to a bad room. What they do, they do well, but it is always (ALWAYS!) far better to control the light in the room. UST projectors with their matching ALR screens do a much better job with light rejection as well as delivering a good on screen image compared to long throw models. The steep projection angle is very helpful in this regard.
In a room with ambient light, at the end of the day, do whatever you possibly can to eliminate ambient light NEAR THE SCREEN. That last part is important because you can still have pretty good light in the room and maintain a great on screen image as long as you are doing what you can to keep light off the screen area.
See:
http://www.avintegrated.com/lighting.htmlThat's a 106" grey screen in a room using a fairly dim Panasonic PT-L300U projector from 20 years ago. The actual lighting is described on the page.
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