AV Technology - February/March 2009 - (Page 42) MULTI-TASKING WITH TECHNOLOGY DISPLAY CONSIDERATIONS Since many command/control installations operate 24/7, operators may spend an entire shift in front of the displays. So it’s important that the information displayed is easy to read with minimal eyestrain. The system designer must match videowall components to the environment to achieve the optimal level of brightness and contrast. FACTORS IN DETERMINING BRIGHTNESS REQUIREMENTS The brightness (or luminance) of a videowall is measured in foot-lamberts or candelas per square meter (nit). One footlambert is equivalent to 3.426 nits. Many videowalls are designed using tiled projection displays (or “cubes”) to encase individual projectors utilizing higher performance Digital Light Projection (DLP) technology. These structures allow for “seamless” imaging of several projection devices to create one continuous display wall. The factors that affect the brightness requirements of a videowall system include: Ambient light conditions Projection system brightness Contrast ratio AMBIENT LIGHT CONDITIONS Room lighting should be balanced with the luminance of the videowall in order to minimize operator fatigue and eyestrain. It’s important to have a moderate level of ambient light. Typically it is recommended that control room lighting is between 20 to 30 foot-candles (200 to 300 lux). A good practice in control room lighting is to use diffusive lighting. If spot lighting is required, it should be directed away from the display wall, not toward it; otherwise contrast aberrations and reflections may be visible on the videowall. CONTRAST RATIO There are two methods for measuring contrast ratio: “full field” and ANSI. For the full field measurement, an entirely black image is displayed and its brightness is measured. Next, an entirely white image is displayed and its brightness is measured. The ratio of white measurement over the black is the full field contrast ratio. Typically, the full field contrast ratio measurement should be at least 1,000:1. However, a more useful and practical PROJECTION SYSTEM BRIGHTNESS contrast ratio measurement is the ANSI method. The ANSI In a rear-projection display wall, each projector has a specified method measures the black and white levels but does this brightness level, measured in ANSI lumens. The total combined with a “checkerboard” pattern. Actual contrast values may projectors lumen level, along with screen size and screen gain, be much lower than the full field method because of the are used to calculate the videowall brightness level. This meas“light spill” from the white “checkers” onto the black. This urement is specified in foot-lamberts or candelas/m2 (nits). measurement better represents the actual contrast perScreen gain is a factor in determining brightness because formance of the system. A level of >450:1 ANSI is the miniof the unique transmissive characteristics of rear-projection mum recommended considering curVideowall brightness (footlamberts) = projector brightness (lumens) X screen gain rent technologies. See figure 1. 2 screen surface area (ft ) Another important design considVideowall brightness (cd/m2) = Videowall brightness (footlamberts) X 3.426 eration is “system contrast ratio.” System contrast ratio takes into account the overall room display screens. Screens with higher gain factors will exhibit situation, considering both the videowall itself and the higher brightness. Also, the screen brightness drops as you ambient light at the wall. The ambient light at the wall is move off-axis, and this drop may be far more abrupt in a measured in foot-candles (imperial) or lux (metric). This higher gain screen. Typically lower gain screens have a wider information is used along with the videowall contrast ratio viewing range, making the drop-off less abrupt. to calculate the final system contrast ratio. The higher the Therefore, depending on the screen types used and the ambient light level, the lower the system contrast ratio. A position of the operators in the control room relative to the minimal acceptable system contrast ratio is 10:1. Typically a screen, brightness and contrast can vary significantly. Typical design target is 100:1. display brightness in 24/7 control rooms is between 150 nits [Source: by Jim Gavloski, Christie Digital] (minimum) and 500 nits. www.avtechnologyonline.com 42 | AV TECHNOLOGY | february | march 2009 http://www.avtechnologyonline.com
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