Every screen material on the market has a so-called gain factor. This is perhaps the most important specification for any screen. Some screens are "low gain" and some are "high gain." Most white home theater screens are low gain, and carry ratings in the range of 1.0 to 1.3 gain. Some of today's gray ambient-light-rejecting screens are rated as low as 0.6 gain. High gain screens go up to 4.0 or beyond. But what do these numbers really mean in terms of your viewing experience?
Gain is a measurement of the reflectivity of any screen or projection surface. The gain number represents a ratio of the light that is reflected from the screen as compared to the light reflected from a standard white (magnesium oxide) reference board. Therefore, a screen with a gain of 1.0 will reflect the same amount of light as that from a white board. A screen rated at 1.5 gain will reflect 50% more light as that from a white board, whereas a gray screen with an 0.8 rating will reflect 80% of the light from a white board.
It might be easier to understand the nature of gain by thinking of it in terms of a trade-off between brightness and the size of the optimal viewing window. Technically speaking, gain is measured from the vantage point where the screen is at its brightest, which is directly in front and perpendicular to the screen—that is, right at the center "sweet spot." The measurement taken at this point is known as Peak Gain at Zero Degrees Viewing Axis. (Now, there's a mouthful!) If you move to the side and view the screen at an angle, the brightness of the projected image starts to drop off. The angle at which the gain reading drops to 50% of the peak value is known as the Half Gain Viewing Angle. That is to say, a person viewing the screen from this angle will see an image half as bright as the person seated at the center position.
Low gain screens have wider Half Gain Viewing Angles than do high gain screens. That is because the low gain screen diffuses light more evenly over a wider angle of view. A high gain screen is constructed to reflect more of the projector's light energy back toward the centerline of the projection path, and less light energy to the oblique angles of view. Thus brightness falls off more rapidly as you move away from the zero degree viewing axis, and the Half Gain Viewing Angle is relatively narrow.
Is a High Gain Screen a Good Thing?
It is easy, and wrong, to jump to the conclusion that a high gain screen must be preferable to a low gain screen. After all, what could be wrong with getting more light coming off the screen than the projector is putting in? There are situations where high gain makes sense, but there are some real potential downsides to higher gain in a home theater environment.
First, as just noted, it starts with that trade-off between brightness and viewing angle. A 1.0 gain screen diffuses light evenly in all directions. Thus seating can be placed in a wide viewing angle relative to the screen and all seats will afford a similar viewing experience regardless of the angle of view. With a high gain screen the brightness of the image increases to those seated in the center, and diminishes for those seated at the outside. For viewers sitting well off center, the relative brightness of various portions of the image can shift quite dramatically. So a high gain screen may put limitations on the number of optimum viewing seats you can have in your theater.
Second, a high gain screen does not typically reflect red, green, and blue equally. So it can generate color shifts in the image that are noticeable as you move around the screen viewing it from different angles. Once again, the image looks different to each viewer depending on where they are seated.
Third, any screen with a gain higher than 1.0 has at least some degree of hotspotting. That is, when viewing the screen from a center position, the middle portion of the image will be brighter than the edges. On screens under 1.3 gain or so this is not very noticeable, but as gain increases beyond 1.3 it can become a real distraction.
High gain screens do have a definite place in the world. In conference rooms and classrooms where you want some lights on and most of the seats can be positioned within the screen's narrow cone of reflectance, high gain screens can be quite effective in boosting image brightness. However, the videophile looking for the optimum image quality in a home theater environment will usually want to opt for a low gain screen.