Gain is a property of a projection screen, and is one of the specifications quoted by projection screen manufacturer. The number that is typically measured is called the peak gain at zero degrees viewing axis, and represents the gain value for a viewer seated along a line perpendicular to the screen's viewing surface. The gain value represents the ratio of brightness of the screen relative to a set standard (in this case, a sheet of magnesium carbonate). Screens with a higher brightness than this standard are rated with a gain higher than 1.0, while screens with lower brightness are rated from 0.0 to 1.0.
Half Gain Angle
The half gain angle is the area of the surface where the projected image produces the fullest spectrum view. This area is roughly in the shape of a cone with its point toward the center of the screen. Higher gain reflective screens produce narrower viewing cones, while matte surfaces produce a broader viewing cone. Half gain angle is 1/2 of the viewing cone. Multiply the half gain angle by two to get the total viewing cone.
Signal-to-noise (SNR)
SNR is the ratio between the signal (intended information) and noise (background, undesirable information). A high SNR means that the image can be clearly separated from the background, a low SNR means the signal and noise ratio are comparable and thus the image (signal) is harder to discern from the background(noise)
Screens having less brightness uniformity usually present a concentration of light in the middle of the surface; this is known as the hot spot. Often an effect of high gain/high reflectivity or when a surface has poor light diffusion properties.
The moiré effect refers to the lines you may see on your projector screen when you are trying to watch movie in cinema. On projectors, it often has to do with the pixellated image from the projector being interfered with by the weave pattern of the projector screen.
Moiré occurs when the natural patterns on your projector screen align with the pixel patterns of what is being projected, merging to form lines that run across your screen. This can occur when the projection screen has an uneven patterning.