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- June 2004 -
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Gain is a term that receives a lot of attention when talking about projection screens. However, the way in which the term is typically used may lead to some confusion not only about what exactly "gain" is, but also about the inherent capabilities of projection screen materials to affect the projected image.
What is Gain?
Gain is typically thought of as a measure of the "brightness" of a projection screen, as compared to a block of magnesium carbonate, which serves as the industry's standard for a gain of 1.0. However, since a projection screen has no brightness of its own, gain can be seen more as a measure of the screen's reflectivity (front projection screens) or transmittance (rear screens). To measure a screen's gain, light is projected onto the viewing surface, and a light reading is taken by pointing a light meter at in the center of the screen, on-axis and perpendicular to the center. A number higher than 1.0 means the screen has a higher gain than the magnesium carbonate, or than standard matt white: in other words, it appears to reflect more light. A number lower than 1.0 means the screen has a lower gain than the magnesium carbonate, or than standard matt white. But what does that number mean, exactly?
Front Projection Screens
When light hits a front projection screen surface, that light is reflected back away from the surface. Light projected onto a matt white screen is reflected back in the shape of a semi-circle, the center of which is on-axis with the center of the screen. Measurements taken at any point along this semi-circle (think of it as half of a full, perfectly round balloon) will reveal the same concentration of reflected light. This is referred to as a 180° viewing cone. Matt white's 1.0 gain indicates the screen is reflecting back the same amount of light that is being projected onto the surface (see example in Fig. 1).
A number higher than 1.0 does not mean the screen is generating additional light. Rather, it means that the screen's characteristics (more reflective materials, for instance) are causing the semi-circle of reflected light to contract, while the overall amount of reflected light remains the same. This means that, when a light measurement is taken on-axis, at the center of the screen, there is a greater concentration of reflected light. However, as you move away from the center, the amount of reflected light decreases. In general, the higher the gain, the more contracted this "viewing cone" becomes, as our balloon of reflected light looks more and more like a hot air balloon with the air leaking out of it (see example in Fig. 2).
Rear Projection Screens
Gain is measured in much the same way for rear screens, except it is measured on the opposite side of the screen from the projector. In rear screens, gain is telling us how much light is being passed through the screen, as opposed to how much light is being reflected. In addition, because a rear screen doesn't transmit 100 percent of the projected light through the substrate and onto the viewing side, the "viewing cone"-even on a screen with a gain of 1.0--will not be uniform as in a matt white screen. A higher gain rear screen means more rays of light are passing through (because of thinner or less dense substrates) with no disturbance to their angle.
What It All Means
Screens that appear brighter on axis are concentrating the reflected light in a narrower area: the gain may be 2.5 on axis but 0.5 at 60° to the side. Screens with a lower gain are reflecting less light, and their "viewing cones" are similar to that of matt white. With today's brighter, high output projectors, gain is not as important as it once was because the screen no longer has to make up for a lack of projector brightness. Other factors-such as color contrast and accuracy, uniformity, reproduction of black and the ability to perform with more ambient light in the audience area-are equally important.
Draper Inc., located in Spiceland, Ind., is one of the world's largest manufacturers of projection screens, lifts and mounts for projectors and flat panel displays, window shades and basketball equipment. Established in 1902, Draper has offices and manufacturing facilities in Sweden, Hong Kong, the United Kingdom, California and Ohio, and ships products to dealers throughout the United States and more than 75 foreign countries. For more information, visit www.draperinc.com