Update: Projection Screens
High & Low Gain
The primary reason for high-gain screens has been to compensate for poor viewing environments. Other screens have been throwing in significant compromises to image quality, so the image will still be far less that it could be. Basically, if you don't control the environment you won't get a good image. If you do control the environment then you can make good use of a screen that will deliver the best image quality we've seen.
Prior to the introduction of gray screens, the gain has been fairly easy to understand. A gain of 1.0 suggested light was being reflected equally in every direction. Anything higher than 1.0 suggested the surface of the screen was reflecting light forward towards the audience. There was a significant fall off in the amount of light that could be seen at the left, right, top or bottom. About 14 years ago, we determined that a gain above 1.3 caused hot spots in the images at the proper viewing distance for high-definition images.
Some screen manufacturers were quick to point out that the visibility of these hot spots was partially dependent on how far away the projector was from the screen. They are right in that the 1.3 value was determined using CRT projectors using rather short throws. The projector was placed at about 1.3 to 1.5 times the width of the screen back away from the screen. The actual number, 1.3 to 1.5, was projector lens dependent. Some had a 'short' throw, 1.3, while others were able to provide a more uniform picture by placing the projector slightly further back from the screen, 1.5 times the width of the screen.
Along came lamp based projection technology and lenses that would allow the throw distance to be much longer. The Samsung projector we're using can be as far away as 2.2 times the width of the screen. In theory, the screen gain could be higher and you still wouldn't see hot spots. A higher-gain screen would provide audience members seated directly in front of the screen with more light than they would see from a low-gain screen. So what could be bad about that?