Andrew Bierman

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.

Impact of dimming white LEDs: chromaticity shifts due to different dimming methods

The goal of this study was to characterize the chromaticity shift that mixed-color and phosphor-converted white LED systems undergo when dimmed. As light-emitting diodes continue to rapidly evolve as a viable light source for lighting applications, their color shift while being dimmed should meet the current requirements of traditional lighting sources. Currently, LED system manufacturers commonly recommend pulse-width-modulation or PWM dimming schemes for operation of LED systems. PWM has the ability to achieve lower intensity levels and more linear control of light intensity compared to continuous current dimming methods. However, little data has been published on the effect dimming has on chromaticity shift of white LED lighting systems. The primary objective of this study was to quantify chromaticity shifts in mixed-color and phosphor-converted white LED systems due to continuous current dimming and pulse-width-modulation dimming schemes. In this study, the light output of the LED system was reduced from 100% to 3% by means of continuous current reduction or PWM methods using a PC white LED system and a mixed-color RGB LED system. Experimental results from this study showed that the PC white LED system exhibited very little chromaticity shift (less than a 4-step MacAdam ellipse) when the light level was changed from 100% to 3% using both dimming schemes. Compared to PC white LEDs, the mixed-color RGB LED system tested in this study showed very large chromaticity shifts in a similar dimming range using both dimming schemes. If a mixed-color RGB system is required, then some active feedback system control must be incorporated to obtain non-perceivable chromaticity shift. In this regard the chromaticity shift caused by the PWM method is easier to correct than the chromaticity shift caused by the continuous current dimming method.

Modelling the spectral sensitivity of the human circadian system

It is now well established that the spectral, spatial, temporal and absolute sensitivities of the human circadian system are very different from those of the human visual system. Although qualitative comparisons between the human circadian and visual systems can be made, there still remains some uncertainty in quantitatively predicting exactly how the circadian system will respond to different light exposures reaching the retina. This paper discusses attempts to model the spectral sensitivity of the circadian system. Each of the models discussed here varies in terms of its complexity and its consideration of retinal neuroanatomy and neurophysiology. Future testing to validate or improve any of these computational models will require a targeted hypothesis, as well as a suitably high level of experimental control before one model can be rejected in favour of another. Until specific hypotheses are formulated and tested, it would be premature to recommend international acceptance of any model or system of circadian photometry.

The Daysimeter : a device for measuring optical radiation as a stimulus for the human circadian system

Optical radiation incident on the human retina stimulates vision as well as provides time-of-day information to the brains circadian clock. The visual and circadian systems respond very differently to optical radiation. A device, the Daysimeter, was developed and tested to help progress towards a system of circadian dosimetry. The Daysimeter is a lightweight, head-mounted device that records radiation exposure estimates for both the visual and circadian systems, and is specifically designed for field use. In addition to logging spectrally weighted radiation measurements, it records head position and motion to be utilized as a representation of human circadian activity. This paper provides background on the differences between radiation for the visual and circadian systems, as well as a description of the development and testing of this prototype device.

A system of mesopic photometry

Nearly all the studies to develop mesopic luminous efficiency functions employ brightness matching. Brightness matching elicits parvocellular-channel response, which does not follow Abneys law of additivity as required by photometry. A reaction-time methodology appears to isolate magnocellular-channel response, which seems to be additive. In this study, a set of mesopic luminous efficiency functions were obtained using a novel psychophysical method, which measured reaction time differences between the two eyes. The luminous efficiency functions are well described by a simple model that linearly combines the spectral sensitivities of rods and cones. Based on this model, a preliminary system of mesopic photometry was defined and adapted for practical use.

Preliminary evidence that both blue and red light can induce alertness at night

BackgroundA variety of studies have demonstrated that retinal light exposure can increase alertness at night. It is now well accepted that the circadian system is maximally sensitive to short-wavelength (blue) light and is quite insensitive to long-wavelength (red) light. Retinal exposures to blue light at night have been recently shown to impact alertness, implicating participation by the circadian system. The present experiment was conducted to look at the impact of both blue and red light at two different levels on nocturnal alertness. Visually effective but moderate levels of red light are ineffective for stimulating the circadian system. If it were shown that a moderate level of red light impacts alertness, it would have had to occur via a pathway other than through the circadian system.MethodsFourteen subjects participated in a within-subject two-night study, where each participant was exposed to four experimental lighting conditions. Each night each subject was presented a high (40 lx at the cornea) and a low (10 lx at the cornea) diffuse light exposure condition of the same spectrum (blue, λmax = 470 nm, or red, λmax = 630 nm). The presentation order of the light levels was counterbalanced across sessions for a given subject; light spectra were counterbalanced across subjects within sessions. Prior to each lighting condition, subjects remained in the dark (< 1 lx at the cornea) for 60 minutes. Electroencephalogram (EEG) measurements, electrocardiogram (ECG), psychomotor vigilance tests (PVT), self-reports of sleepiness, and saliva samples for melatonin assays were collected at the end of each dark and light periods.ResultsExposures to red and to blue light resulted in increased beta and reduced alpha power relative to preceding dark conditions. Exposures to high, but not low, levels of red and of blue light significantly increased heart rate relative to the dark condition. Performance and sleepiness ratings were not strongly affected by the lighting conditions. Only the higher level of blue light resulted in a reduction in melatonin levels relative to the other lighting conditions.ConclusionThese results support previous findings that alertness may be mediated by the circadian system, but it does not seem to be the only light-sensitive pathway that can affect alertness at night.

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,

Comparisons of three practical field devices used to measure personal light exposures and activity levels

This paper documents the spectral and spatial performance characteristics of two new versions of the Daysimeter, devices developed and calibrated by the Lighting Research Center to measure and record personal circadian light exposure and activity levels, and compares them to those of the Actiwatch Spectrum. Photometric errors from the Daysimeters and the Actiwatch Spectrum were also determined for various types of light sources. The Daysimeters had better photometric performance than the Actiwatch Spectrum. To assess differences associated with measuring light and activity levels at different locations on the body, older adults wore four Daysimeters and an Actiwatch Spectrum for seven consecutive days. Wearing the Daysimeter or Actiwatch Spectrum on the wrist compromises accurate light measurements relative to locating a calibrated photosensor at the plane of the cornea.

Retinal mechanisms determine the subadditive response to polychromatic light by the human circadian system

Light is the major synchronizer of circadian rhythms to the 24-h solar day. The intrinsically photosensitive retinal ganglion cells (ipRGCs) play a central role in circadian regulation but cones also provide, albeit indirectly, input to these cells. In humans, spectrally opponent blue versus yellow (b-y) bipolar cells lying distal to the ganglion cell layer were hypothesized to provide direct input to the ipRGCs and therefore, the circadian system should exhibit subadditivity to some types of polychromatic light. Ten subjects participated in a within-subjects 3-night protocol. Three experimental conditions were employed that provided the same total irradiance at both eyes: (1) one unit of blue light (lambda(max)=450 nm, 0.077 W/m(2)) to the left eye plus one unit of green light (lambda(max)=525 nm, 0.211 W/m(2)) to the right eye, (2) one unit of blue light to the right eye plus one unit of green light to the left eye, and (3) 1/2 unit of blue light plus 1/2 unit of green light to both eyes. The first two conditions did not differ significantly in melatonin suppression while the third condition had significantly less melatonin suppression than conditions 1 and 2. Furthermore, the magnitudes of suppression were well predicted by a previously published model of circadian phototransduction incorporating spectral opponency. As was previously demonstrated, these results show that the human circadian system exhibits a subadditive response to certain polychromatic light spectra. This study demonstrates for the first time that subadditivity is due to spectrally opponent (color) retinal neurons.