Jean Paul Freyssinier

Evaluation of light-emitting diodes for signage applications

This paper outlines two parts of a study designed to evaluate the use of light-emitting diodes (LEDs) in channel-letter signs. The first part of the study evaluated the system performance of red LED signs and white LED signs against reference neon and cold-cathode signs. The results show a large difference between the actual performance and potential savings from red and white LEDs. Depending on the configuration, a red LED sign could use 20% to 60% less power than a neon sign at the same light output. The light output of the brightest white LED sign tested was 15% lower than the cold-cathode reference, but its power was 53% higher. It appears from this study that the most efficient white LED system is still 40% less efficient than the cold-cathode system tested. One area that offers a great potential for further energy savings is the acrylic diffuser of the signs. The acrylic diffusers measured absorb between 60% and 66% of the light output produced by the sign. Qualitative factors are also known to play an important role in signage systems. One of the largest issues with any new lighting technology is its acceptance by the end user. Consistency of light output and color among LEDs, even from the same manufacturing batch, and over time, are two of the major issues that also could affect the advantages of LEDs for signage applications. To evaluate different signage products and to identify the suitability of LEDs for this application, it is important to establish a criterion for brightness uniformity. Building upon this information, the second part of the study used human factors evaluations to determine a brightness-uniformity criterion for channel-letter signs. The results show that the contrast modulation between bright and dark areas within a sign seems to elicit the strongest effect on how people perceive uniformity. A strong monotonic relationship between modulation and acceptability was found in this evaluation. The effect of contrast seems to be stronger than that of spatial frequency or background luminance, particularly for contrast modulation values of less than 0.20 or greater than 0.60. A sign with luminance variations of less than 20% would be accepted by at least 80% of the population in any given context.

White lighting for residential applications

The lighting industry relies on colour-rendering index (CRI) and correlated colour temperature (CCT) to communicate the colour properties of light sources. While advances have been made in recent years to develop new metrics for colour rendering, not much attention has been given to the validity or utility of CCT for communicating the appearance of illumination. Recently, it was shown that a wide range of chromaticities can appear ‘white’ or with minimal perceived ‘tint’. Importantly, these chromaticities do not follow the line of blackbody radiation, the reference for CCT. The present study builds on this previous research to determine whether ‘white’ illumination is preferred in simulated residential applications. Results presented here support the hypothesis that people prefer ‘white’ or minimally ‘tinted’ sources of illumination.

Reducing lighting energy use in retail display windows

A field study was conducted at three clothing stores to validate previous laboratory findings indicating that colored LEDs used as background display lighting could: 1) lower the power demand of accent lighting by up to 50 percent; and 2) provide greater attention capture and visual appeal than current lighting practice. Blue LEDs provided a colored background for window mannequins by illuminating white backdrops. Eliminating fluorescent general lighting and reducing the number and wattage of halogen accent lamps in the display windows reduced the lighting power demand by up to 50 percent. During an eight-week period, more than 700 shoppers rated the attractiveness, eye-catching ability, comfort, and visibility of four different lighting conditions. The results of this field study showed that by introducing color contrast between the displayed objects and the background, the power demand of the accent lighting could be reduced by up to 50 percent without sacrificing visual appeal, visibility, ability to capture the attention of shoppers, and the ability to see the colors of the objects on display. Furthermore, the sales of the products on display were not affected by the 50 percent reduction in lighting.

Effect of different coloured luminous surrounds on LED discomfort glare perception

Recently, there has been increased interest in energy-efficient lighting as energy resources become higher in demand. Anecdotal evidence suggests that certain populations believe light-emitting diodes (LED) produce more glare than traditional technologies. This may be due to a number of factors such as spectral power distribution (SPD), source luminance, or beam intensity distribution. A study was conducted to assess the effect of different SPDs on the perception of discomfort glare from an LED source. For the range of conditions evaluated, the presence of any luminous surround significantly reduced the perception of discomfort glare from the LED array. The blue luminous surround reduced discomfort glare perception significantly less than the white or the yellow luminous surrounds. The implications for solid-state lighting systems are discussed.

Determining contrast sensitivity functions for monochromatic light emitted by high-brightness LEDs

Light-emitting diode (LED) technology is becoming the choice for many lighting applications that require monochromatic light. However, one potential problem with LED-based lighting systems is uneven luminance patterns. Having a uniform luminance distribution is more important in some applications. One example where LEDs are becoming a viable alternative and luminance uniformity is an important criterion is backlighted monochromatic signage. The question is how much uniformity is required for these applications. Presently, there is no accepted metric that quantifies luminance uniformity. A recent publication proposed a method based on digital image analysis to quantify beam quality of reflectorized halogen lamps. To be able to employ such a technique to analyze colored beams generated by LED systems, it is necessary to have contrast sensitivity functions (CSFs) for monochromatic light produced by LEDs. Several factors including the luminance, visual field size, and spectral power distribution of the light affect the CSFs. Although CSFs exist for a variety of light sources at visual fields ranging from 2 degrees to 20 degrees, CSFs do not exist for red, green, and blue light produced by high-brightness LEDs at 2-degree and 10-degree visual fields and at luminances typical for backlighted signage. Therefore, the goal of the study was to develop a family of CSFs for 2-degree and 10-degree visual fields illuminated by narrow-band LEDs at typical luminances seen in backlighted signs. The details of the experiment and the results are presented in this manuscript.

Energy and user acceptability benefits of improved illuminance uniformity in parking lot illumination

This study set out to understand the benefits of improved illuminance uniformity in parking lots in terms of user perception and acceptability, as well as energy use, and to demonstrate that light-emitting diodes (LEDs) can achieve uniform distributions more efficiently than traditional light sources. The results from a field evaluation showed that more uniform illuminance distributions are favourably perceived by people in terms of goodness of illumination, ability to see around and at a distance, and perception of safety – all of this at a much lower average horizontal illuminance. Thus, improving uniformity alone can translate into lower energy use and potential for less glare and light pollution. Optical modelling showed that LEDs have a much greater potential to efficiently produce uniform illuminance distributions than larger light sources such as high pressure sodium or metal halide.

Implementing Semipermanent High-Mast Lighting for Highway Construction Projects

Semipermanent, high-mast pole-mounted lighting for nighttime road work construction and maintenance was implemented based on an installation along Interstate 90 in Albany, New York. The high-mast lighting system met state lighting performance specifications. There were no recorded accidents at the site during the construction period. Although that result is largely attributable to roadway closures allowed during part of the project, consensus of the contractor and transportation agency is that the lighting increased safety. The cost of the high-mast lighting system was approximately 16% higher than the estimated cost of the portable light towers. Despite the higher estimated cost of the high-mast lighting system, the economic and societal benefits seem considerable in this particular location, an urban, heavily traveled throughway. In particular, this lighting method probably shortened the time required for the construction, reducing the amount of risk to workers and drivers and reducing traffic delays for motorists. This paper identifies projects where this lighting technique might be appropriate.

Performance Evaluation of Semipermanent High-Mast Lighting for Highway Construction Projects

In 2005, the New York State Department of Transportation authorized the first-time use of semipermanent, high-mast, pole-mounted luminaires to illuminate nighttime road work construction and maintenance. This approach was used along a 3-mi stretch of Interstate 90 in Albany, New York, as an alternative to portable light towers. The performance of the high-mast lighting system is assessed according to the criteria of construction work quality and safety and visibility for workers and drivers. The high-mast lighting system was found to have met lighting performance specifications. Field measurements and an analytical comparison of the high-mast lighting with the portable light towers suggested that the high-mast lighting system provided sufficient illumination for performing maintenance and construction activities at the site, with few shadows and relatively low glare, thereby increasing visibility of hazards and improving performance of visual tasks for both workers and motorists. On the basis of these analyses, the high-mast lighting system should provide a higher level of safety than the portable light towers for the construction workers and for the drivers traveling through the construction zone for this project. The high-mast lighting system should also reduce the risk of injury to construction workers during high-exposure times when they would be setting up and removing portable lighting equipment. Other considerations, such as the implications for conditions of wet pavement and mesopic vision, are explored.

Brightness contrast perception in the mesopic region

Subjective evaluations of brightness contrast were obtained for Munsell chips of varying hues, lightness values and sizes against a white background illuminated in turn by four white illuminants at four background luminances in the mesopic region. Chip lightness was the strongest variable, resulting in a family of monotonic functions relating brightness contrast ratings to chip lightness. At the lowest light levels a nearly linear relationship was found between chip lightness and subjective ratings. As light levels increased, the relationship became curvilinear such that at the lowest contrast, chromatic brightness became important for subjective ratings. Within the level of precision for this experiment, chip hue and size as well as the spectral power distribution of the illuminants did not significantly impact the overall brightness contrast ratings. The subjective ratings of brightness contrast were analysed as a function of the luminance contrast of the stimuli as determined by conventional photometry and by three models of mesopic vision. The response functions relating subjective rating to the stimulus characterized in terms of these models were similar to those obtained with conventional photometry.

Luminance requirements for lighted signage

Light-emitting diode (LED) technology is presently targeted to displace traditional light sources in backlighted signage. The literature shows that brightness and contrast are perhaps the two most important elements of a sign that determine its attention-getting capabilities and its legibility. Presently, there are no luminance standards for signage, and the practice of developing brighter signs to compete with signs in adjacent businesses is becoming more commonplace. Sign luminances in such cases may far exceed what people usually need for identifying and reading a sign. Furthermore, the practice of higher sign luminance than needed has many negative consequences, including higher energy use and light pollution. To move toward development of a recommendation for lighted signage, several laboratory human factors evaluations were conducted. A scale model of a storefront was used to present human subjects with a typical red channel-letter sign at luminances ranging from 8 cd/m2 to 1512 cd/m2 under four background luminances typical of nighttime outdoor and daytime inside-mall conditions (1, 100, 300, 1000 cd/m2), from three scaled viewing distances (30, 60, 340 ft), and either in isolation or adjacent to two similar signs. Subjects rated the brightness, acceptability, and ease of reading of the test sign for each combination of sign and background luminances and scaled viewing distances.