August 29, 2011
| by Greg Robinson
When shopping for a front projection screen, there are several factors you need to consider before making a purchase. How big should the screen be? Does the material need to be perforated to allow speaker output to pass through unmolested? Should the fabric be white or silver? How reflective does it need to be? Each of these questions is worthy of an article all by itself, but today we’re just going to focus on that last one.
Screen gain is the measurement of how much light is reflected by the projection surface as compared to the reflectivity of a matte white board coated with magnesium carbonate, a pure white mineral found in nature. A screen gain of “1.0” means that the screen reflects the same amount of light as does the white board. Similarly, a screen with a gain of “0.9” reflects 90% as much as the white board whereas a screen gain of “2.0” reflects twice as much light.
Is Brighter Better?
When first considering the concept of screen gain, your initial reaction might be to promptly seek out a high gain screen. After all, brighter is better, right? Like everything else in this world, it depends. If your venue is a conference room and you’re giving a Powerpoint presentation to a room full of people with the lights on, yes - brighter is definitely better. However, in a darkened home theater, you’d be surprised at how little light you really need.
In a darkened theater, your primary goal is a screen image that’s bright enough to deliver sufficient contrast without being so bright that it results in eye strain. If you’ve ever gone to the movies and the scene cuts suddenly from a dark interior to a bright outdoor shot, you know how overwhelming this can be. Ideally your image brightness, measured in foot-Lamberts, should fall somewhere between 10-20fL. And like driving from the east coast to the west coast, there’s more than one way to get there.
If your goal is to achieve a brighter or darker screen image, there are several angles from which you can attack the issue. For starters, you can change the light output of your projector - or try a different projector entirely. You can also adjust the size of your screen; like an adjustable flash light, a front projector lets you choose between a narrow cone of focused bright light (i.e. a small screen) or a wide cone of diffused, dimmer light (i.e. a large screen) with a multitude of options in between thanks to the beauty of a zoom-equipped lens. Last but not least, screen brightness can be modified by using a screen material with a higher or lower gain factor.
High and Low
The thing to remember with screen gain is that a screen’s gain factor is directly related to its viewing angle. Since gain is measured at the screen’s brightest point - front and center, perpendicular to the screen - high gain screens are designed to maximize brightness for those seated in that same vantage point. This is fine for a small conference room, where everyone is huddled around a central table, but it’s going to mean an unsatisfying viewing experience if you’re watching a movie from an off-axis theater chair.
As its name implies, “Peak Gain” is the gain rating at the brightest position - front and center. A telling measurement is a screen’s “Half Gain Viewing Angle” - the angle at which screen brightness drops 50% relative to its Peak Gain. Generally speaking, high gain screens have narrow Half Gain Viewing Angles whereas with a low gain screen that angle can be considerably wider.
When you consider high gain screens’ narrow viewing angles and the fact that they are also more conducive to color shift and hotspotting (where portions of the image appear brighter than others), it’s hard to make the case for using a high gain screen in a dedicated home theater environment.
Although your mileage will certainly vary depending on your projector, desired screen size, and ambient light level, a screen gain between 1.0-1.3 is arguably the safest bet if you’re seeking to outfit a typical home theater. And what exactly is a “typical” home theater these days? That sounds like yet another article. Stay tuned.
Check out the slideshow of screens to fit nearly every home theater.
Greg Robinson is a freelance technology writer whose work has appeared in several national publications. When he's not evaluating Blu-ray Discs or calibrating televisions, you can usually find him thumping volleyballs at his local gym in rural northeast Connecticut.