Update: Projection Screens
Higher-gain screens introduce hot spots into the picture. The visibility of those hot spots is partially determined by the viewing distance, as well as the projector distance. The projector being further away from the screen can reduce that visibility, but it won't make hot spots go away.
Up to this point screen gain has been measured based on the reflectivity of a magnesium carbonate surface. If light from the screen is higher than that reflected from a magnesium carbonate surface, it is said to have gain. Obviously, if the screen were to actually have true gain, it would have to be a light amplifier. In reality, it gets its gain by reflecting light directly back at the viewer instead of reflecting it equally in every direction, as does the magnesium carbonate surface.
We measure gain by shining a light in the direction of the screen and measuring what comes back in the direction of the light source. We can go further and measure the amount of light that comes off the screen at various angles. Ideally, with a screen with a gain of 1.0, the light should be equal at all angles from head on to the screen to almost all the way out to the edge of the screen. Light should also be equal in the horizontal and vertical directions away from the center.
What you should also see, depending on the capability of the projector, is a uniform field of light over the entire surface of the screen with a gain of 1.0. In describing a screen with gain, we see this uniformity compromised. There are hot spots and they move, depending on your viewing position. Someone on the left side of the screen will see it as being brighter than the right side. Someone on the right side will see it as being brighter than the left side.