LRC evaluates LEDs for use in outdoor signage
An LED test sign constructed for the study.Rapidly evolving LEDs are making headway in the world of general illumination, and the next big thing for solid-state lighting may soon be seen at the mom-and-pop store around the corner. As a follow-up to the light-emitting diode’s success in the traffic signal market, the next major application for this progressive light source may be commercial outdoor signage.
As a result, LRC researchers have conducted a number of performance measurements and human factors studies to evaluate the use of LEDs in backlighted outdoor signs. These studies looked at system characteristics, long-term performance, and human perceptions of luminance uniformity—all in an effort to understand how LEDs can be tailored and implemented for signage applications.
Test sign and digital imaging system used to evaluate luminance uniformity.Outdoor signs have typically employed neon and cold-cathode fluorescent light sources, said Jean Paul Freyssinier, a lighting design specialist with the LRC. However, advancing technology and the promise for energy savings have quickly propelled LEDs toward use in applications requiring colored light, such as commercial retail signs. In the first part of the year-long study, LRC researchers sought to understand and compare the performance of backlighted signs using LED, neon, and cold-cathode fluorescent lamps.
Performance comparisons of LED, traditional signs
Researchers tested signs constructed with red and white LEDs from various manufacturers and compared their performance against neon and cold-cathode fluorescent signs. The results were promising for red LEDs. “Depending on the configuration, we found that a red LED sign could use 20 percent to 60 percent less power than a neon sign at the same light output,” said Freyssinier, making the current red LED technology more efficacious than neon for use in signage.
White LEDs currently available cannot match the performance of the standard cold-cathode lamps, however. “The best white LED system from our study was 40 percent less efficient than the cold-cathode system tested,” said Yutao Zhou, an LRC research specialist who worked on the energy management and long-term performance studies of this project. However, he adds that white LED signs will eventually surpass cold-cathode fluorescent signs in efficiency.
Experiments evaluate uniformity of light, acceptance of non-uniformity
The second part of the study involved two human factors experiments to understand how people perceive luminance uniformity with the goal of developing evaluation methods and uniformity criteria for outdoor signage.
The first experiment studied sensitivity to contrast. “In order to understand how humans perceive uniformity, it helps to know how the visual system senses contrast and spatial frequency,” said Vasudha Ramamurthy, an LRC graduate student whose Master’s thesis became an integral part of the study. Sensitivity to these variables is typically described by a contrast sensitivity function (CSF), which exists for many light sources but not for narrowband LEDs at typical light levels and viewing conditions common for outdoor signs.
The second human factors study evaluated the acceptability of non-uniform light within signs. The team experimented with a variety of non-uniform light configurations, spatial frequencies, viewing distances, and background light levels, and recorded the conditions at which people rated non-uniformities as not acceptable. Subjective ratings of sign uniformities were also recorded.
From these experiments, LRC researchers developed a number of quantitative tools for evaluating signs, including a series of CSFs for red, green, and blue LEDs and contrast modulation values for acceptability.
Freyssinier said the next step will be to refine a metric using a CCD camera for analyzing brightness uniformity, a criterion that defines the uniformity of light throughout a sign’s face. Further studies will include determining the acceptance standards for different applications, such as how people perceive lighted signs at high-end vs. low-end retail establishments.
“Presently, there are no standards for signage in terms of luminance, brightness uniformity, or life,” said Freyssinier. “With these studies, we were able to estimate not only how well LEDs would perform over time, but also begin to set some criteria for the appearance of backlighted signs.”
Details of these studies will be published this fall in the proceedings of the Third International Conference on Solid-State Lighting (SPIE Proceedings 5187). For more information, visit the LRC’s Solid-State Lighting web site:
Evaluation of LEDs for Outdoor Signage Applications (Link to completed study Jean Paul on SSL Completed Research)
Determining Contrast Sensitivity Functions for LEDs (Link to completed study Vasudha on SSL Completed Research)
The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and the environment