Arguably the most important element of creating a great projected display is in the combination of a projector’s brightness and a screen’s ability to adequately reflect light. The overall result is known as Image Luminance. Simply put, if you want a good projected image, the image needs to be bright enough to be easily seen. It sounds simple enough but there are various rules of physics that must be followed in order to get that perfect image. In order to successfully acquire the right projector an projection surface, you need to be familiar with these basic concepts:
The American National Standards Institute (ANSI) averaged multiple readings of visible light from various angles to determine a standardized unit of measure known as ANSI Lumens. Lumens are the units of measure for the amount of light that a projector is reportedly able to produce. To put it into easier perspective, a candela is about the equivalent of the light of one candle. The candela actually replaces the obsolete measurement of the “Candlepower (cp)” and is equivalent to 1.02 cp units of measured visible light.
Foot Lamberts are the measured brightness of a projector’s output spread over the surface of your projection screen. According to The Society of Motion Pictures and Television (SMPTE), the range of foot lamberts for a darkened room would be 12 foot lamberts for the minimum, 22 foot lamberts for the maximum, and 16 foot lamberts as the ideal output for home theaters and commercial cinemas. In bright rooms 50 foot lamberts is more appropriate.
· Measure the screen width and height in inches
· Divide the length and height measurements each by 12 to get the foot measurements
· Multiply the width and height together (w*h= Area in ft²)
· Divide the projector’s brightness (lumens) by the screen area in square feet (ex. 1000 lumens/43 ft² is about 23 foot lambert)
Image luminance is found by multiplying the Foot Lamberts times the projection screen’s reflectivity or gain. The image luminance is the overall image brightness created by the combination of projector output and screen reflectivity. Projection screens vary in their reflective properties from Matte White (1.0-1.4 gain) to negative-gain (0.5 to 0.9). Or they could be Ambient Light Rejecting or ALR that can range from negative to over 1.8 gain despite their “dark” appearance from using heavy contrast layers incorporated to enhance darkness levels.
Take the foot lamberts (ex. of Section 2 = 23 foot lamberts) and multiply it by the “gain” of the projection screen material.
· Example 1, 23 foot lamberts times 1.0 gain has 23 candelas per ft²
· Example 2, 23 foot lamberts times 1.5 gain has 34.5 candelas per ft²
· Example 3, 23 foot lamberts with a negative gain 0.5 is 11.5 candelas per ft²
The formula is simple. You need a certain level of screen brightness to present superb picture clarity and it corresponds directly with the amount of atmospheric light that you are presenting in. The factors of projector brightness and your screen’s ability to adequately reflect light are key in helping you get the big picture.
Want more technical details – check out http://elitescreens.com/support/tech-tips-of-the-month/2435-tech-tip-march2016-2
Dave Rodgers Marketing Mgr.
About the Author: Dave Rodgers is a guest writer for numerous publications and the Marketing Manager for Elite Screens Inc., with 20-years experience in the AV and wireless communications industries. He travels worldwide playing an active role with international business development and is regularly consulted in matters of branding and new business development. David has made numerous television, radio and editorial appearances providing installers and Do-It-Yourselfers with easy solutions toward creating larger-than-life big screen applications.
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