Abstract: Light quantities based on the photopic luminous efficiency function do not predict brightness perceptions of lighted outdoor scenes such as streets, parking lots and plazas. This paper summarises a series of experiments conducted using scale-model outdoor scenes illuminated by different light sources to assess judgements of brightness. From the results and from previously published literature on the relative increase in short-wavelength spectral sensitivity for brightness, a tentative model for brightness perception of outdoor scenes is proposed. The model can serve as a starting point for efficiently testing future hypotheses regarding brightness perception in lighted outdoor scenes.
Abstract: A provisional model of brightness perception for illuminated outdoor scenes was recently proposed based upon a combination of photopic and short-wavelength cone spectral sensitivities, where short-wavelength cone sensitivity increases with radiance. An experiment was undertaken to determine whether coloured objects within the illuminated scenes affected overall scene brightness perception and, in so doing, to provide a test of the provisional model. The results suggest that overall scene brightness perceptions are, for the range of light sources and light levels selected, consistent with the provisional model and that scene brightness perception is independent of object colours within the illuminated scenes.
Abstract: Visual displays used in nighttime driving and some other applications require low luminances. Visual performance under mesopic light levels is not well characterized using conventional photopic photometric quantities. Response times to flashed decrement targets against colored backgrounds on a computer monitor display were systematically shorter for blue than for red backgrounds. Responses were consistent with a unified system of photometry that models the relative contributions of rod and cone photoreceptors at mesopic light levels. The empirical data and unified photometric system can have practical application for the design of displays in low-light applications.
Abstract: Unlighted highway signs, which use newly developed retroreflective materials, were installed along the major expressway in an urban area by the local department of transportation. Photometric measurements of the signs were used to assess their legibility applying the relative visual performance model, in comparison to lighted signs, conforming to recommended illumination practices. The calculated visibility of the measured unlighted signs was similar to that of the signs equipped with exterior sign illumination. The practical significance and limitations of the relative visual performance approach are discussed.
Abstract: Photometrically accurate lighting software and a model of visual performance were used to evaluate realistic roadway lighting and driver scenarios. A great deal of data, both virtual photometric quantities and values of relative visual performance, were generated. A method for simplifying these data was developed to create practical insights about the value of different fixed roadway lighting systems. In particular, it was concluded that it is important to illuminate both high- and low-speed intersections, to provide high illumination for older drivers on high-speed roadways with intersections, and perhaps to incorporate refined photosensor-based control of lighting in highly developed areas. The approach used here enables officials responsible for public cost and public safety to make more sophisticated and rational decisions about roadway lighting.
Abstract: ABSTRACT: The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.
Abstract: Light-emitting diodes (LEDs) are emerging as viable replacements for incandescent (INC)-based cap lamps used in mining. The photometric and energy characteristics of these light sources differ in important ways. This paper describes the performance of LED and INC sources in cap lamps in terms of correlated color temperature, color rendering, light output, electric power, ambient temperature and air flow, and light source aging. Importantly, these characteristics can influence a miner's ability to spot mining hazards thus impacting safety. Second, some of these characteristics interact with the operating life of the cap lamp's battery power, such that differences between LED and INC sources can be magnified toward the end of a 10-h battery discharge cycle. Empirically, we have determined that after 8 h at an ambient temperature of 25 degrees C, the average light output of an INC cap lamp can decrease to about 69% of its initial value when powered by a lead-acid battery, and it can decrease to about 65% of its initial value when powered by a nickel-hydride battery. An LED-based cap lamp using a constant current drive circuit can maintain about 96% of its initial value when powered by a nickel-hydride battery. Real-world tests addressing the effects of ambient temperature and air flow on the light output of an LED and INC cap lamp were conducted in the National Institute for Occupational Safety and Health Safety Research Coal Mine. The LED cap lamp yielded a vertical average illuminance improvement of approximately 9.5%, and the INC cap lamp yielded a vertical average illuminance degradation of approximately 4%. The differences between LED and INC cap lamps were further quantified by the calculation of "mesopic luminance" data that indicated for the same photopic luminance (i.e., as measured using a conventional light meter) the LED cap lamp could be up to 38% more efficient than the INC cap lamp with a lead-acid battery at the end of the 10-h driving cycle. Lastly, accelerated life tests were used to empirically determine light output depreciation as the INC light source age approached its useful life. There was about a 35% decrease in light output. This is quite considerable, particularly given that the light output will decrease an additional 30% to 45% over the period of a 10-h shift. The implications of the differences between LED and INC sources are discussed. This information is crucial in determining how visual performance could be affected for real-world conditions where batteries discharge during the work shift and as the light source ages. To date, only idealized conditions have been used for LED and INC cap lamp visual performance research.
Abstract: Light treatment has been used as a non-pharmacological tool to help mitigate poor sleep quality frequently found in older people. In order to increase compliance to non-pharmacological light treatments, new, more efficacious light-delivery systems need to be developed. A prototype personal light-treatment device equipped with low brightness blue light-emitting diodes (LEDs) (peak wavelength near 470 nm) was tested for its effectiveness in suppressing nocturnal melatonin, a measure of circadian stimulation. Two levels of corneal irradiance were set to deliver two prescribed doses of circadian light exposure. Eleven older subjects, between 51 and 80 yrs of age who met the selection criteria, were exposed to a high and a low level of light for 90 min on separate nights from the personal light-treatment device. Blood and saliva samples were collected at prescribed times for subsequent melatonin assay. After 1 h of light exposure, the light-induced nocturnal melatonin suppression level was about 35% for the low-light level and about 60% for the high-light level. The higher level of blue light suppressed melatonin more quickly, to a greater extent over the course of the 90 min exposure period, and maintained suppression after 60 min. The constant exposure of the low-light level resulted in a decrease in nocturnal melatonin suppression for the last sampling time, whereas for the high-light level, suppression continued throughout the entire exposure period. The present study performed with healthy adults suggests that the tested personal light-treatment device might be a practical, comfortable, and effective way to deliver light treatment to those suffering from circadian sleep disorders; however, the acceptance and effectiveness of personal light-treatment devices by older people and by other segments of the population suffering from sleep disorders in a real-life situation need to be directly tested.
Abstract: Metal halide (MH) lighting systems are gaining in acceptance relative to high-pressure sodium (HPS) lighting systems for many night time applications. The present paper describes a series of studies carried out to address some of the 'quality' issues associated with MH and HPS lighting systems. HPS and MH installations were compared in terms of perceptions of brightness and safety, acceptability for social interaction, facial recognition and eyewitness identification. At equal photopic light levels, a street scene illuminated at night by an MH lighting system was reliably seen as brighter and safer than the same scene illuminated by an HPS system. In terms of acceptability for social interaction, facial recognition and many aspects of eyewitness identification, the measured differences between lighting systems were not as clear.
Abstract: 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.
Abstract: Light emitting diodes (LEDs) are emerging as viable replacements for incandescent-based cap lamps used in mining. The photometric and energy characteristics of these light sources differ in important ways. The present paper describes the performance of LED and incandescent sources in cap lamps in terms of correlated color temperature, color rendering, light output, electric power, ambient temperature and airflow, and light source aging. Importantly, these characteristics can influence a miner's ability to spot mining hazards thus impacting safety. Secondly, some of these characteristics interact with the operating life of the cap lamp's battery power, such that differences between LED and incandescent sources can be magnified toward the end of a 10-hour battery discharge cycle. Empirically, we have determined that after 8 hours at an ambient temperature of 25 degrees C, the average light output of an incandescent cap lamp can decrease to about 69% of its initial value when powered by a lead-acid battery and it can decrease to about 65% of its initial value when powered by a nickel-hydride battery. An LED-based cap lamp using a constant current drive circuit can maintain about 96% of its initial value when powered by a nickel-hydride battery. Real-world tests addressing the effects of ambient temperature and airflow on the light output of an LED and incandescent cap lamp were conducted in the NIOSH Safety Research Coal Mine (SRCM). The LED cap lamp yielded a vertical average illuminance improvement of approximately 9.5% and the INC cap lamp yielded a vertical average illuminance degradation of approximately 4%. The differences between LED and incandescent cap lamps were further quantified by the calculation of "mesopic luminance" data that indicated for the same photopic luminance (i.e., as measured using a conventional light meter) the LED cap lamp could be up to 38% more efficient than the incandescent cap lamp with a lead-acid battery at the end of the 10-hour driving cycle. Lastly, accelerated life tests were used to empirically determine light output depreciation as the incandescent light source age approached its useful life. There was about a 35% decrease in light output. This is quite considerable, especially given that the light output will decrease an additional 30% to 45% over the period of a 10-hour shift. The implications of the differences between LED and Incandescent sources are discussed. This information is crucial in determining how visual performance could he affected for real-world conditions where batteries discharge during the work shift and as the light source ages. To date, only idealized conditions have been used for LED and incandescent cap lamp visual performance research.
Abstract: In addition to sky glow and light trespass, discomfort glare from outdoor lighting installations is a growing concern to the public. A series of experimental investigations was performed to assess the relative impacts of light source photometric characteristics on subjective ratings of discomfort glare. The results converge, demonstrating the influence of light source illuminance, surround illuminance and ambient illuminance on subjective judgements of discomfort glare. A simple model relating these photometric quantities is proposed for making predictions of discomfort glare from outdoor lighting installations. This model can be readily incorporated into existing frameworks for evaluating light pollution as well as into lighting calculation software.
Abstract: Outdoor Site-Lighting Performance (OSP) is a comprehensive method for predicting and measuring three different aspects of light pollution: glow, trespass and glare. OSP is based upon the philosophy that a rational framework is necessary for optimising private and public desires for and against night-time lighting. Results are presented from over one hundred outdoor lighting installations that provide an empirical foundation for acknowledging the benefits of night-time lighting while establishing limits on light pollution. Recommended limits for glow, trespass and glare are offered to stimulate discussion among all stakeholders concerned with night-time lighting.
Abstract: Aviation signal lights using light emitting diodes (LEDs) are commonly perceived as brighter than those using incandescent sources, even at the same measured intensity. In general, saturated colors, like those produced by LEDs, appear brighter than less saturated lights, like those produced by incandescent sources.
Abstract: As presently practiced and applied, photometry does not reflect the response of the human visual system at many light levels commonly used for nighttime lighting. Vision and photometry have never been synonymous, but this disconnect could obscure prudent light source specification for outdoor illumination. The unified system of photometry, which links photopic and scotopic luminous efficiency at nighttime light levels, provides a framework for communicating information about light sources considered for exterior lighting. Despite methodological differences, work from a recent research consortium closely matches the system of unified photometry and exactly matches its framework. This correspondence should encourage decision-making bodies as they move toward a consensus basis for specifying and communicating luminous quantities.
Abstract: A field study was conducted to extend fundamental findings about visual performance at mesopic light levels to a driving context. Subjects participating in this field study drove a vehicle along a lighted street while performing a high-order decision-making task. Subjects identified the direction of an off-axis target, toward or away from the street, and braked or accelerated, accordingly. Two sets of light sources were compared: a set of ceramic metal halide light sources and a set of high-pressure sodium light sources. The same study was also performed during the day-time. The results demonstrated that both braking and acceleration response times decreased monotonically as unified luminance increased, suggesting that unified luminance is a suitable rectifying variable for characterizing light levels for different light sources with respect to a complex visual task.
Abstract: Light exposure at night increases alertness; however, it is not clear if light affects nocturnal alertness in the same way that it affects measures of circadian regulation. The purpose of this study was to determine if a previously established functional relationship between light and nocturnal melatonin suppression was the same as that relating light exposure and nocturnal alertness. Four levels of narrow-band blue light at the cornea were presented during nighttime sessions. The ratio of electroencephalographic alpha power density with eyes closed to eyes open (alpha attenuation coefficient, AAC) and the Norris mood scale were used. The AAC and ratings of alertness increased monotonically with irradiance and were highly correlated. Both measures of alertness were highly correlated with model predictions of nocturnal melatonin suppression for the same circadian light stimulus, consistent with the inference that the suprachiasmatic nuclei play an important role in nocturnal alertness as well as circadian regulation.
Abstract: There is a growing interest in the role that light plays on nocturnal melatonin production and, perhaps thereby, the incidence of breast cancer in modern societies. The direct causal relationships in this logical chain have not, however, been fully established and the weakest link is an inability to quantitatively specify architectural lighting as a stimulus for the circadian system. The purpose of the present paper is to draw attention to this weakness.
Abstract: Nocturnal rodents are frequently used as models in human breast cancer research, but these species have very different visual and circadian systems and, therefore, very different responses to optical radiation or, informally, light. Because of the impact of light on the circadian system and because recent evidence suggests that cancer risk might be related to circadian disruption, it is becoming increasingly clear that optical radiation must be properly characterized for both nocturnal rodents and diurnal humans to make significant progress in unraveling links between circadian disruption and breast cancer. In this paper, we propose a quantitative framework for comparing radiometric and photometric quantities in human and rodent studies.
Abstract: 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/m(2) to 1512 cd/m(2) under four background luminances typical of nighttime outdoor and daytime inside-mall conditions (1, 100, 300, 1000 cd/m(2)), 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.
Abstract: Recently, the Consensus Committee to Develop Recommended Standards for Newborn Intensive Care Unit (NICU) Design has developed recommended lighting standards for the NICU. One objective of these standards are to propose lighting standards that reflect the general principles of good lighting, and to chart new directions for lighted environments in the NICU that are consistent with the latest scientific literature. Rationales for the latest recommended standards in NICUs as they pertain to the ambient lighting in infant care areas, procedure lighting in infant care areas, illumination of support areas, daylighting and floor surfaces are discussed. This article is intended to serve as a bridge between health care and building professionals interested in quality lighting in the NICU environment.
Abstract: 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.
Abstract: Forty subjects participated in a study to test the accuracy of a recent model of human circadian phototransduction for predicting the relative effectiveness of two polychromatic light sources at suppressing nocturnal melatonin. Brief exposures to four different light levels (30, 100, 300 and 1000 photopic lux at the cornea) and two different "white" lamp spectra (4100 and 8000 K) were used. Results suggest that the model can properly order the relative magnitudes of the two circadian stimuli, but that nocturnal melatonin suppression follows a rate-limited response to light that cannot be predicted from the magnitude of the suppressing light stimulus alone. Some practical implications of these results are discussed.
Abstract: Published data, both on nocturnal melatonin suppression in humans and on widely accepted retinal structure and function, suggest that spectral opponency plays a role in human circadian phototransduction. We directly test subadditivity, implied by spectral opponency, in human circadian phototransduction in response to nearly monochromatic and to polychromatic light.
Abstract: The absolute and spectral sensitivities to light by the human circadian system, measured through melatonin suppression or phase shifting response, are beginning to emerge after a quarter century of active research. The present paper outlines a hypothesized model of human circadian phototransduction that is consistent with the known neuroanatomy and physiology of the human visual and circadian systems. Spectral opponency is fundamental to the model, providing a parsimonious explanation of some recently published data. The proposed model offers a framework for hypothesis testing and subsequent discussion of the practical aspects of architectural lighting with respect to light and health.
Abstract: It is well established that the absolute sensitivity of the suprachiasmatic nucleus to photic stimulation received through the retino-hypothalamic tract changes throughout the 24-hour day. It is also believed that a combination of classical photoreceptors (rods and cones) and melanopsin-containing retinal ganglion cells participate in circadian phototransduction, with a spectral sensitivity peaking between 440 and 500 nm. It is still unknown, however, whether the spectral sensitivity of the circadian system also changes throughout the solar day. Reported here is a new study that was designed to determine whether the spectral sensitivity of the circadian retinal phototransduction mechanism, measured through melatonin suppression and iris constriction, varies at night.
Abstract: Light sources used in signal lights for transportation applications have a variety of temporal onset characteristics, including a wide range of onset times. These characteristics, along with luminous intensity and color characteristics, can have important impacts on the ability to detect and respond to colored signal lights. Studies of the impact of these factors on responses to colored signals are reviewed, along with potential implications for the selection of light sources used in traffic and vehicle signals. The onset characteristics of recently developed light sources might offer significant potential to improve visual detection of signal lights. Nonetheless, it is important to understand the context in which a signal light is presented to determine whether such improvements in visual detection have practical significance.
Abstract: Additivity in the circadian phototransduction system of the mouse has not been tested directly. Because of this, accurate prediction of circadian phase shifts elicited by polychromatic light stimuli cannot be derived from the results of studies using monochromatic light stimuli. This limitation also makes it impossible to deduce the relative contributions of the photoreceptive mechanisms (rods, cones and melanopsin-containing retinal ganglion cells) underlying circadian phototransduction in the mouse. Using nearly monochromatic light stimuli of different spectral composition, and combinations thereof, we demonstrated that murine circadian phototransduction exhibits additivity. Based on the locomotor activity phase shifts elicited by these stimuli, we developed the first quantitative assessment of the relative contributions of conventional and novel photoreceptive mechanisms for circadian functioning in the mouse.
Abstract: Light exposure regulates several circadian functions in normal humans including the sleep-wake cycle. individuals with Alzheimer's Disease (AD) often do not have regular patterns of activity and rest, but, rather, experience random periods of sleep and agitation during both day and night. Bright light during the day and darkness at night has been shown to consolidate activity periods during the day and rest periods at night in AD patients. The important characteristics of bright light exposure (quantity, spectrum, distribution, timing and duration) for achieving these results in AD patients is not yet understood. Recent research has shown that moderate (similar to18 lx at the cornea) blue (similar to470 nm) light is effective at suppressing melatonin in normal humans. It was hypothesized that blue light applied just before AD patients retire to their beds for the night would have a measurable impact on their behavior. A pilot study was conducted for 30 days in a senior health care facility using four individuals diagnosed with mild to moderate levels of dementia. Four AD patients were exposed to arrays of blue light from light emitting diodes (max wavelength = 470 nm) in two-hour sessions (18:00 to 20:00 hours) for 10 days. As a control, they Were exposed to red light (max wavelength = 640 nm) in two-hour sessions for 10 days prior to the blue light exposure. Despite the modest sample size, exposure to blue LEDs has shown to affect sleep quality and median body temperature peak of these AD patients. Median body temperature peak was delayed by approximately 2 hours after exposure to blue LEDs compared to exposure to red LEDs and sleep quality was improved. This pilot study demonstrated that light, especially LEDs, can be an important contribution to helping AD patients regulate their circadian functions.
Abstract: Human adult males were exposed to light from blue light emitting diodes (18 lux; 29 microW/cm) and from clear mercury vapor lamps (450 lux; 170 microW/cm) during night-time experimental sessions. Both conditions suppressed nocturnal melatonin concentrations in blood plasma with the blue light more effective than mercury at melatonin suppression. No additive model incorporating opsin photopigments either alone or in combination could explain the results, but a model incorporating an opponent mechanism was consistent with the present data as well as data from previously published studies.
Abstract: Mesopic vision corresponds to luminances in which both the rods and cones contribute to vision. Many light levels for roadways, parking lots, and other exterior locations result in luminances within the mesopic vision region. Research is summarized that shows that the potentially large magnitude of spectral effects under mesopic conditions cannot be predicted from orthodox models of visual response without additional information about the contrast, size, and context of a visual stimulus. If peripheral detection is a relevant visual task for a roadway lighting installation that operates at mesopic conditions, then characterizing light levels in terms of photopic luminous efficiency, as is conventionally done, will not properly characterize the effectiveness of those light levels for this response. Consideration of the visual periphery can result in different selection of light sources than would result from a consideration of on-axis vision alone.
Abstract: Protan observers have lower spectral sensitivity than color-normal observers at long wavelengths of the visible spectrum. Responses of protan and color-normal subjects to light emitting diode (LED) and incandescent traffic signals of red and yellow nominal color during simulated daytime viewing conditions are described. Reaction times, missed signal percentages, and color-identification accuracy were measured. The results indicate that for protans, detection performance to red LED signals was enhanced when the dominant wavelength was moved toward shorter wavelengths, but this shift also appeared to decrease their ability to correctly identify the signal color. Although the yellow LED signal used in this study provided similar detection performance for protans as a yellow incandescent signal, its dominant wavelength was sufficiently long for it to sometimes be confused with red. In general, the results of this study are consistent with the 1994 recommendations of the Commission Internationale de l'Eclairage for signal colors to be seen by color-normal and protan observers. Nonetheless, neither detection nor color identification for protans approached that of color-normal observers.
Abstract: We present the design and calibration of a retinal flux density (RFD) meter. This device can serve as a standard illurninance meter or can measure flux density on the retina, using both photopic and scotopic spectral responses. In addition, through post processing, the instrument can determine standard illuminances and RFDs at mesopic levels. This device is an extension of an earlier instrument, reported on by us, which replicated the spatial efficiency function of the human eye to determine retinal exposure. In this paper we describe the need for such a measurement device, detail its design, and report on its performance.
Abstract: Human adult males were exposed to combinations of two illuminances and two broadband spectral power distributions over the course of four night-time sessions. Results showed that melatonin suppression is dominated by short visible wavelengths (420-520 nm), consistent with recently published studies. Although the authors of these recent studies suggest that a novel opsin underlies melatonin suppression, the present paper offers a more conservative interpretation of the data based on what is known about existing photoreceptors and associated neuroanatomy and neurophysiology.
Abstract: The goal of this paper is to initiate a discussion within the lighting community regarding standardized measurement procedures and a definition for useful life for light emitting diode (LED) technology. In general, LEDs do not fail catastrophically, but instead their light Output slowly decreases over their operating period. Presently, some manufacturers use a 50 percent light output level as the criterion for LED life. Although 50 percent light loss might be acceptable for non-critical signage applications using monochromatic LEDs, it might not be acceptable for general lighting applications. It is important to develop a method for rating lamp life and a definition of "useful life" for LEDs so, when reported by manufacturers, the lighting community can compare LEDs to traditional light sources. The "useful life" definition for LEDs should consider light loss and color shift. Therefore, an experimental study was conducted to investigate light loss and color shift patterns of white LEDs as a function of operating time. The 5 mm type white InGaN +YAG LEDs evaluated in this experiment, representing technology commercially available in 1999, exhibited high light output degradation rates and color shifts as a function of operating time. It is further shown, using a simple mathematical fit to the data gathered during a short life-test study, and extrapolating it to predict the life of white LEDs, depends on the initial data collection period. Therefore, an alternate method for projecting LED life is investigated by overdriving the LEDs at different currents. Using their degradation patterns at higher drive currents, the life of these LEDs was predicted at normal drive current values. The results show excellent correlations between predicted light loss and actual measured losses at 20 and 30 mA drive currents for the LEDs tested. The authors believe this technique is applicable for accurately predicting life of any type of LED and hope to verify this using future configurations. This study adds information to the knowledge needed for the lighting community to develop standardized measurement procedures and a definition for useful life for LED technology.
Abstract: The visual discomfort from simulated traffic signals under nighttime viewing conditions was assessed. Discomfort increased with increasing signal luminance and decreasing viewing distance. Color identification of signals was very high for all signal colors and luminances studied. Red signals meeting luminous intensity specifications of ITE and specifications being drafted in Europe were unlikely to cause visual discomfort under the conditions tested. Green and yellow signals meeting ITE and European specifications were more likely to cause visual discomfort. Dimming signals using light-emitting diodes might be a feasible strategy for minimizing discomfort under some nighttime viewing conditions.
Abstract: Human adult males were exposed to combinations of two illuminances and two spectral power distributions over the course of four nighttime sessions. A dose-dependent response of acute melatonin suppression to light was found, but photopic (cone-based) illuminance did not adequately predict suppression. When melatonin suppression was plotted against scotopic (rod-based) illuminance, the data formed a nearly monotonic function, implicating rods, or a rod-dominated mechanism, in the human melatonin regulation system. The results do not, however, rule out mechanisms other than rods, including novel photoreceptors, as candidates for melatonin regulation in humans.
Abstract: The primary measure of success in achieving lighting energy efficiency has conventionally been improvements to the luminous efficacy of lamps. However, in architectural lighting, some specifiers recognize that the most effective lamp and luminaire combination for a given application is often not the one with the highest lamp luminous efficacy. The widespread use of tungsten halogen sources in display and downlighting applications, despite the low lamp luminous efficacy of these technologies relative to others, suggests that lamp luminous efficacy is only partially related to the effectiveness of a lighting installation. The purpose of this paper is to suggest a new measure of efficacy for lighting applications that is, first, based upon the lamp and luminaire combination rather than, as usually considered, solely on lamp luminous efficacy. Second, and more importantly perhaps, this new measure is concerned specifically with delivering light where it is needed in the most energy efficient manner. It will be argued that the proposed metric, termed application efficacy, which can be approximated by the maximum luminous intensity (in candelas) from a luminaire per unit power (in watts), is a practical and general measure that helps ensure the most energy efficient luminaire will be employed for a given application.
Abstract: A primary purpose of electric lighting is to aid vision in conditions when natural light (daylight, sunlight) is unavailable. Such conditions occur at night and indoors. Lighting recommendations of organizations such as the IESNA are almost entirely based on requirements for visibility.1 An increasing body of evidence exists to support the effects of light and lighting in psychological and physiological areas of life. Light affects moods, metabolism and even communicates meaning. Light is used therapeutically to combat depression, to rid the body of toxins, and to reset the body's internal circadian clock.(2) With such capacity to impact human life, it is reasonable to suppose the use of light might also carry certain risks. This brief review discusses one specific type of potential risk associated with light - namely, the ability of short-wavelength ("blue") light of sufficient intensity and duration to cause photochemical damage to title visual system.
Abstract: The lumen is based on the CIE photopic luminous efficiency function. It was derived at high light levels and for on-axis (foveal) vision. This definition is adequate for specifying the luminous stimulus for foveal tasks, but it does not strictly apply to certain nighttime applications where low-light level, off-axis vision is most critical (e.g., night driving). This paper compares visual performance, using reaction time (RT), for two light sources, metal halide (MH), and high pressure sodium (HPS). A small, 2-degree target was flashed either on-axis or off-axis at background luminances ranging from 0.01 to 10 cd/m(2). The results of the experiment showed that RT increases exponentially with decreasing background luminance. There was no difference between light sources when the target was flashed on-axis, or when the target was seen at high background luminances (above 1 cd/m(2)). When the target was flashed off-axis, reaction times th HPS became systematically longer than MH as background luminance decreased below 1 cd/m(2). Our results support the conclusion that rods play an increasing role in peripheral vision as background luminance drops below photopic levels, but play little or no role in the fovea at any background luminance. These data led to the development of a preliminary mesopic luminous efficacy function for off-axis detection. Under a representative asphalt luminance of 0.1 cd/m(2), MH was calculated to be approximately 60 percent more efficacious than HPS.
Abstract: Effects of light and electromagnetic fields (EMFs) on pineal function could have implications for long-term risk of breast cancer, reproductive irregularities, or depression. Health-care workers in a neonatal intensive care unit (NICU) were interviewed to determine the tasks, work locations, and practices in their work environment as well as the care provided to the infants. After an initial visit, methods for measuring illuminance, luminance, and broadband resultant magnetic fields throughout the NICU were developed. Measurements were made of one nursery during a daytime (1:00 p.m.) and a night-time (12:30 a.m.) visit. Measurements relevant to both nurses and premature infants in the NICU were made. Some measurements could not be completed so as not to interfere with nurses' duties in the NICU. Illuminances measured during the daytime and nighttime averaged 184 and 34 lux (lx), respectively, much lower than those reported in other studies of illuminance in NICUs, with a maximum illuminance of 747 lx. Peak levels may be consistent with those thought to suppress melatonin. There was a high degree of variability in EMF levels, which exceeded 1,000 mG close to certain hospital equipment but averaged 1-2 mG at the nurses' workstation. Fields within incubators exceeded 10 mG.
