Gain and Diffusion In Action

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Gray vs. White

The question of screen color is one of the most common, especially since gray models are often called "high-definition" screens. White screens are the standard in the local movie theater, so why should you use gray at home? The answer lies in the technology (or lack thereof) in the early digital projectors, which suffered from very low contrast ratios. Gray screens help address that problem by giving the projected image the illusion of higher contrast.

So the next logical question is: Are gray screens obsolete with today's high-contrast projectors? Surprisingly, no. Gray screens are useful in any environment where you don't have total control over the light. Most projection screens today are being installed in multi-use settings such as media rooms, which often have some ambient light as well as reflected brightness from light-colored furnishings. These reflective surfaces cause stray light to shine back onto the screen, reducing the picture quality. A gray screen can help offset the impact of these reflections. In a true dedicated home theater where the light is fully controlled and the room contains dark furnishings and low-reflection materials, however, the combination of a high-contrast projector and a white screen is the way to go.

Gain and Diffusion

Gain is a measure of how well a screen material reflects light, while diffusion is a measure of the light-scattering across a viewing area. Gain is determined against a white (magnesium oxide) board reference, and it's measured from the point where a screen is the brightest, which is directly in front of and perpendicular to its surface. A screen that reflects back 80% of the light reflected from the white reference board gets a rating of 0.8 gain. If a screen reflects 50% more light than the board, its gain is 1.5.

Gain is usually inversely proportional to the screen's diffusion angle. In other words, the higher the gain, the narrower your viewing area will be. When light hits a screen, it scatters in many directions. A viewer at the left or right edge of the screen will see an image that's about half as bright as the one seen by anyone sitting in front of the center of the screen. This is the edge of the screen's "viewing angle." If a screen has a 120° viewing angle, you can move 60° in either direction from the center and still have an image half as bright as the one seen at the center.

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