John D. Bullough

Evaluating Light Source Efficacy under Mesopic Conditions Using Reaction Times

Rea, M. S., A. Bierman, Y. He and J. Bullough. 1995. Initial Research and Development of a New Light Source for Off-Axis Viewing at Night [report to New York State Energy Research and Development Authority]. Troy: Lighting Research Center, Rensselaer Polytechnic Institute. • Bruno, L. D. 1999. An Evaluation of High Pressure Sodium and Metal Halide Light Sources for Parking Lot Lighting [thesis]. Troy: Rensselaer Polytechnic Institute.

A proposed unified system of photometry

,!A unified system of photometry is proposed that is based on human vision and allows the specification of visual stimuli at all light levels. To be useful, however, photometry can never be entirely synonymous with vision. Additivity is an essential characteristic of photometry, yet many visual responses, such as brightness matching, incorporate visual channels that are inherently non-additive in responding to light. Following a review of models of mesopic photometry based on a number of experimental techniques, a system of photometry based on reaction times is proposed to bridge conventional photopic and scotopic luminous efficiency functions through the mesopic region. The basis for the unified system is a parameter, X, which describes the proportion of photopic luminous efficacy at any luminance, and luminance can be calculated from a simple closed-form equation. The utility of the system is described, including instrumentation for the measurement of unified luminance.

Circadian photobiology: an emerging framework for lighting practice and research

A century of research and practice have optimized the use of electric lighting in buildings to support human vision. However, recent lines of research show that light is also important to human circadian regulation, as reflected in such diverse phenomena as depression, sleep quality, alertness, and, perhaps, even health. Although light is essential to both vision and circadian regulation, research shows that the biophysical processes that govern circadian regulation are very different from those that govern vision. This growing body of research will probably influence the architectural lighting community and manufacturers to reoptimize the use of electric lighting in buildings to support both human vision and circadian functions. The present paper is concerned with establishing a framework for lighting practice and applied research that will assist lighting practitioners and manufacturers in interpreting this emerging research.

Several Views of Metal Halide and High-Pressure Sodium Lighting for Outdoor Applications

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.

What is Useful Life for White Light LEDs

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% light output level as the criterion for LED life. Although 50% light loss might be acceptable for noncritical 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 that 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 that 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 that 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. INTRODUCTION Light emitting diodes (LEDs) were first developed over three decades ago. Most of the early LEDs were narrow wavelength band emitters with light output predominantly in the red to yellow region. During the 1990s Nakamura and colleagues (1994, 1997, 1998, 1999) demonstrated a blue LED based on gallium nitride (GaN). The development of the blue LED made the creation of the broad band white LED possible. Presently, white light is generated by combining the GaN-based blue LED and Y3Al5O12 (yttrium aluminum garnet or YAG) phosphor or by grouping red, green and blue LEDs in the correct proportions. [Refer to Stringfellow and Craford (1997) for an indepth discussion of LED technology.] The potential for significant energy savings and the potential for long life are the two major factors that have attracted this technology to the general lighting community. Over the past few years the technology has advanced significantly and some white LEDs presently available in the marketplace are rated at 10 to 15 lumens per watt (lm/W). White-light LEDs are among the first signs of an evolving solid-state technology for architectural lighting applications. Many industry experts are optimistic that solid-state technology will revolutionize the architectural lighting industry. Although luminous efficacies have been steadily growing for these LEDs, the amount of light generated by a single device is still low, usually under 1 lm. Manufacturers are actively working towards developing larger light output LED devices. Some of the methods presently used for achieving this goal include, grouping several smaller LED devices together, increasing the size of the semiconductor device, enabling the device to be driven at higher drive currents, and improving the light extraction efficiencies by shaping or modifying the emitting surfaces of the semiconductor device to prevent the light from being trapped within the cavity by total internal reflection (Ochiai-Holcomb et al., 2000; Windisch et al., 2000). The development of LED technology is fueled by the electronics industry, and as a result, the advances of this technology have been much faster than most lighting technologies. In anticipation of its widespread use for architectural lighting applications, many original equipment manufacturers (OEMs) have begun to develop light sources using white LEDs for the marketplace. Developing a new light source for general lighting and achieving high levels of application depends upon industrys success at standardizing product performance and at designing cost-effective products that reliably produce light of acceptable color. Other promising lighting technologies,

Preliminary evidence for spectral opponency in the suppression of melatonin by light in humans.

Human adult males were exposed to light from blue light emitting diodes (18 lux; 29 μW/cm2) and from clear mercury vapor lamps (450 lux; 170 μW/cm2) 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.

Driver decision making in response to peripheral moving targets under mesopic light levels

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.

Simulated driving performance and peripheral detection at mesopic and low photopic light levels

This paper describes research conducted to measure peoples ability to perform a simulated driving task and respond to peripheral targets at mesopic and low photopic light levels under different spectral power distributions (SPDS). A computer-controlled driving simulator apparatus was employed. An achromatic projected driving scene could be controlled in terms of luminance and SPD using filters. Subjects drove simulated roadway courses; their average speed, frequency of crashing and subjective ratings of brightness were measured. As expected, driving performance and subjective ratings improved with photopic luminance. SPD had no measurable an driving performance or brightness ratings. Subsequent tests shawed, however, that the detection of targets located in the peripheral visual field (positioned where potential visual hazards to a driver might be located) was highly dependent on both luminance and SID. Rods and cones both appear to contribute to peripheral target detection even at luminances traditionally considered to be strictly photopic. The results and their implications are discussed in the context of previous research.

Toward a model of outdoor lighting scene brightness

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.

To illuminate or not to illuminate: Roadway lighting as it affects traffic safety at intersections

A two-pronged effort to quantify the impact of lighting on traffic safety is presented. In the statistical approach, the effects of lighting on crash frequency for different intersection types in Minnesota were assessed using count regression models. The models included many geometric and traffic control variables to estimate the association between lighting and nighttime and daytime crashes and the resulting night-to-day crash ratios. Overall, the presence of roadway intersection lighting was found to be associated with an approximately 12% lower night-to-day crash ratio than unlighted intersections. In the parallel analytical approach, visual performance analyses based on roadway intersection lighting practices in Minnesota were made for the same intersection types investigated in the statistical approach. The results of both approaches were convergent, suggesting that visual performance improvements from roadway lighting could serve as input for predicting improvements in crash frequency. A provisional transfer function allows transportation engineers to evaluate alternative lighting systems in the design phase so selections based on expected benefits and costs can be made.