A Government Services Administration official makes a presentation using a projector and screen. Photo by J.J. Smith
Because the August Buyer’s Guide is spotlighting projectors, it makes sense to provide some tips on projector screens including insight on areas that should be considered when acquiring a screen such as “gain,” “brightness” and size.
An “unity-gain screen,” which is a screen that dispurses light evenly in every direction, would be considered an “ideal screen,” said Wendy Long, vice president of martketing for Da-Lite Screen who made the presentation “Projection Screen Technology Basics” at InfoComm 2011. “Unity-gain means a gain of one,” she added.
Gain is simply the measurement of brightness over the reflectivity from the front of a projection screen, Long said. Measuring a screen’s gain needs to be done in a completely darkened room in which a pure white light is projected onto the screen. Using calibrated optical measuring equipment and a standard white, magnesium oxide board, measure the brightness of the light source, and then measure the brightness that reflects off both the board and the screen. The gain number represents a ratio of the light that is reflected from the screen as compared to the light reflected from the board. For example, a screen with a gain of 1.0 will reflect the same amount of light as that from a white board. A screen with a gain rating of 1.5 reflects 50 percent more light as that from a white board, whereas a screen with a gain rating of 0.8 reflects 80 percent of the light that the white board reflects.
A screen’s “brightness” can be measured using a simple calulation based on the lumens produced by the projector and the area of the screen, Long said. The calulation is used to determine how many foot lamberts a user wants for an application, she added.
Commercial applications generally need from 30 to 50 foot lamberts. If a projector produces 1,500 lumens, calulate the area of the screen. If a screen is 6 feet by 8 feet, the area of the screen is 48 square feet. Then divide 1,500 by 48, which equals 40.625 foot lamberts, which is right in the middle for most commercial applications.
There is also a calculation for sizing a screen to a room for commercial applications, Long said. “These are industry standards that will work for most applications,” she said, adding they “provide a really good starting point.”
When determining the size of a screen, the traditional approach is the “one-sixth rule,” she said. That is, the screen’s height should equal 1/6 the distance from the position that is furthest away from the screen, or the back row. For example, if the furthest seat from the screen is 30 feet, the height of the screen should be the result of 30 divided by six, or five feet. “For most environments the one-sixth rule is applicable,” she said.
However, the one-sixth rule works best in rooms where mostly traditional Power Point presentations and video are delivered. In rooms where there are lots of screen presentations that include small numbers, detailed graphs and reading, the size of the screen might be determined using the “one-fourth rule,” so the content on the screen is easier to view. Again, if the furthest seat from the screen is 30 feet, the height of the screen should be the result of 30 divided by four, or 7.5 feet.