The viewing angle (or the 'viewing cone') is the angle from the perpendicular from which the screen is best viewed. People within the viewing angle will enjoy the best picture quality while viewers outside of the viewing angle will experience a change in brightness, and possibly color shifting.
Hot spotting is when the center of the image will be brighter than the edges. Again this is not a problem with low gain screens, but can become a problem for screens with a higher gain. Hot spotting is caused by insufficient light diffusion.
Color shift is only a problem for CRT projectors. Since CRT projectors have three light sources and only the Green one is in the middle, the Red and the Blue are off to the sides. When you move to the left of the viewing cone the image will turn redder and if you move to the right the image turns blue. Screens with a low gain tend to not have this problem, while screens with a high gain will exhibit color shifting.
Projection Screen Gain
Gain is a measure of brightness as compared to a block of magnesium carbonate, which is the industry's standard for gain of 1.0. The higher the gain number the more reflective the screen, therefore the brighter the image appears to the viewers.
- How to add brightness to the image on the screen?
One thing to keep in mind is that a screen cannot add brightness to your projector; the only way a screen can show a perceived brightness advantage is to focus the light it reflects back to the viewers. This light has to come from somewhere and it comes from the viewing angle. The higher the gain of a screen the brighter it will be when you are directly within the viewing angle and the dimmer it will become the farther you get from the viewing angle.