Update: Projection Screens
It's easy to see hot spotting in high gain screens. In most rear-screen projector sets, if you are up close to the screen, you'll see a significant hot spot, usually a wedge of bright light. As you back away from the screen, you'll see the bright part of the picture getting larger. As you continue to move back, the bright spot will eventually fill the screen. The point at which the screen appears to be equally bright over the entire surface of the image might be said to be the focal point of the screen. Unfortunately, the distance away from the screen is too far back to see any of the detail in a high-definition image.
To take advantage of the resolution in an HD image, you have to sit rather close to it, probably about 1.4 times the width of the image for a true 1080p source. In the days of 1.5 focal length lenses on CRT projectors, we determined gain was 1.3. Using screens of that era we don't see enough of a difference in the image from projectors at a 2.2 focal length to revise our numbers. We've stayed with the 1.3 gain number as being the highest you should go in screen gain for an HD image.
Then came gray screens with gains of 0.8 to 1.1. Theoretically, if you don't see hot spots in a 1.3 gain screen you shouldn't see any hot spotting in something under a 1.3 gain. But that isn't what you see. There is significant hot spotting in these 'low gain' screens. So what happened? What happened is that many gray screens have really high gains. If you equate screen gain to light falling off from center to edge, these screens have a really high gain even though the number given for their gain is around 1.0. How is this possible?