Lighted signage has become a mainstay for most retailers, restaurants, and other businesses. However, no luminance standards exist in North America for signs based on visibility. Instead, businesses often install brighter signs to compete with those of adjacent stores. In many cases, the brightness of a sign may far exceed what people need to identify and read it. This practice has drawbacks of higher energy use, light trespass, and light pollution.
With LEDs growing as a popular replacement for neon and fluorescent lamps in signage, the Lighting Research Center investigated the luminance requirements for backlighted, channel-letter signs.
LEDs: An option for channel-letter signs
Channel-letter signs consist of individual letters that house a lighting system inside a metal case topped with a plastic diffuser. LEDs offer the potential for energy and maintenance savings in channel-letter signs, particularly red LEDs, says Jean Paul Freyssinier, LRC research assistant professor. Yet to become a successful replacement for neon and fluorescent lamps in this application, LEDs have to meet the subjective standards of viewers, he says.
“Brightness and contrast are perhaps the two most important elements of a sign that determine its attention-capture capability and its legibility,” Freyssinier says.
|Scale model storefront used in experiment.
Freyssinier’s team conducted a human-factors study to identify a suitable range of luminances for red channel-letter signs. Using a scale model of a strip plaza storefront, subjects evaluated the brightness acceptability and ease of reading of several sign luminances under different background lighting conditions commonly found outdoors at night and during the day inside a shopping mall. Three viewing distances were considered: 9.1 meters (30 feet); 18.3 meters (60 feet); and 103.6 meters (340 feet). Subjects also considered how the presence of adjacent signs affected their perceptions of brightness.
Brighter backgrounds, brighter signs
The team found statistically different preferences for sign brightness at each of four background light levels, with brighter backgrounds calling for brighter signs.
For stand-alone signs, the most acceptable sign luminances fell between 40 cd/m2 and 190 cd/m2. When adjacent signs were present, subjects preferred a higher sign brightness. For these conditions, the most preferred sign luminance ranged from 65 cd/m2 to 230 cd/m2.
“Most likely," said Freyssinier, "adding the adjacent signs increased the overall luminance of the background, which led to a preference for a brighter sign.” Similarly, an increased viewing distance indicated a preference for brighter signs, he added.
In a limited local survey, LRC researchers found that the luminance of typical red channel-letter signs ranged between 100 cd/m2 and 450 cd/m2, depending on the size and light source used.
Freyssinier said that while this study investigated just one sign color at three viewing distances for a limited number of backgrounds, it is a good start to furthering our understanding of appropriate sign brightness and ways to reduce energy use and light pollution for lighted signs.
For more information. . .
This study, which was funded by the New York State Energy Research and Development Authority (NYSERDA) and RPC Photonics, was published in the SPIE proceedings for the Sixth International Conference on Solid State Lighting. A preprint of the paper, “Luminance requirements for lighted signage,” is available on the LRC Solid-State Lighting Web site (www.lrc.rpi.edu/programs/solidstate/).