•••••••••••••••••••••••••••••••••••••••••••••• 37 Visual Performance as a Function of Spectral Power Distribution of Light Sources at Luminances Used for General Outdoor Lighting Alan L. Lewis The need to provide outdoor lighting for orientation, The interest in the role of spectral power distribution identification, and safety at a cost that is reasonable in on visual performance, originally of concern only to the- terms of both dollars and energy has resulted in the use oreticians trying to understand visual physiology, became of light sources that differ significantly in spectral power of practical significance with the introduction of gaseous distribution from those used in interiors and for which discharge sources for general illumination. Because of almost all data on visual performance have been accu- the obvious advantages of using such energy efficient mulated. Furthermore, the illuminances used in most and long-lived lamps for general illumination, studies outdoor lighting applications (0.01 cd/m2.....10 cd/rn") were performed to evaluate their performance relative are vastly lower than those under which the greatest to the more conventional incandescent and fluorescent amount of visual data is available. Under such condi- illuminants. Mercury, sodium (both low and high pres- tions, the visual system is, at best, in a mesopic state for sure), and metal halide lamps have all been studied in which the standard definition of "light," the lumen, is some detail at high illuminances with the result that almost certainly inappropriate. suprathreshold visual performance, measured in terms The lumen is defined as visually evaluated electro- of speed and accuracy on achromatic tasks, is relatively !.I magnetic radiation. Its magnitude was initially derived independent of light source color or type as long as con- using viewing conditions that limited the visual response trasts are equated.' Although the color characteristics of to one that was mediated exclusively by the cone recep- HID lamps have reduced their use in interior applica- tor systems of the human eye. Most critically, a high level tions, they are widely employed for outdoor lighting. of retinal illuminance and a central 2 degree visual field In the last few years two separate, but related, groups was used, which effectively eliminated any significant of investigations have raised new questions about the contribution by the rod system which is largely absent effects of both spectral power distribution (SPD) and from that area of the retina. The relative spectral sensi- light source color on performance. Berman et al., at the tivity of the eye under those conditions is nominally Lawrence Berkeley Labs, have shown that both pupil size described by the 1924 erE Standard Observer for Photometry (VI).1 For larger fields, or when retinal illu- CONTRAST THRESHOLD minances are insufficient to adequately stimulate the All Spatial Frequencies cone systems, the use of the lumen to predict the human response to light is questionable. As a result, additional ~1~~:~ spectral sensitivity functions have been described for 13 §E larger fields (1964 erE Observer for 10 degree •... 1.015.1r=============~-......::::::~ fields-VlO),)2 and for very low retinal illuminances (1951 ~ -~ ~ 1 , ~~----==~ eIE Scotopic Observer=-V'j )." o 0.95 Although spectral sensitivity functions are adequate to ~ 0.9 _--------- w I,: predict the amount of energy required to stimulate ~ 0.85 +.1---r-----,,-.,.......,......,...,....,. -t-1..----..--...,........,........,..... -r+r- ..-i0 0 1 vision, they are very poor at describing the perceived LUMINANCE (cd/sq m) effect of suprathreshold lights. For example, it is well Ii known that perceived brightness differs significantly from photometric luminance under all but very restrict- -- INC -+- HPM -- HPS 1 Ii" 1 ---- LPS -- MH I: ed viewing conditions." The discrepancy between per- l" ceived effects and photometric quantities is especially Figure I-Relative contrast threshold as a function of adapted sensitive to conditions where lights are of different color luminance (mean luminance of sinusoidal gratings) for five illumi- or where viewing conditions are markedly different from nants (incandescent, low pressure sodium, high pressure sodium, those under which the light units were defined. high pressure mercury, and metal halide). Thresholds are plotted relative to those obtained with an incandescent source. Luminances Author's Affiliation: Michigan Collegeof Optometry, Ferris State were 0.1, 1.0, 3.0, and 10.0 c/m2. University, Big Rapids, Michigan. JOURNAL of the illuminating Engineering Society Winter 1999 38 •••••••••••••••••••••••••••••••••••••••••••••• REACTION TIME Indeed. Kinney showed a spatial frequency dependent Realistic ask effect in her study. Such results are not inconsistent with 1200 the known properties of the dual visual system (known variously as the transient/sustained, XjY, or magnocel- ~ 1100 s lular/parvocellular systems) found in most vertebrate ..,§. 1000 animals, including humans . 900 E•• F aoo a General experimental parameters 700 a~: 600 Subjects 500 , 0.1 1 10 Subjects were five paid students each of whom met the LUMINANCE (cdl sq m) following criteria: (1) no current ocular disease or anatomical anomaly; (2) refractive error less than ± 0.50 -- INC -+- HPM ......- HPS D in any meridian without correction; (3) normal color --=- LPS --- MH vision (Ishihara PIP); and (4) normal visual fields (Humphrey 30-2). Figure 2-Mean time to correctly identify the orientation of a grating There were three men and two women aged 20-23. as a function of adapted luminance for five illuminants (incandescent, They were informed of the nature of the experiments, low pressure sodium, high pressure sodium, high pressure mercury, and metal halide). Luminances were 0.1, 1.0, 3.0, and 10.0 c/m2. but not of the ultimate experimental questions. All testing was performed on the same five subjects and brightness are differentially affected by SPD;8.9i.e., who were carefully selected and highly trained in psy- environments that produce equal photopic luminances, chophysical measurements. Training on threshold and but which differ in SPD, produce unequal responses-even reaction time determinations was conducted for each under nominally photopic conditions. They attribute the subject for several weeks until asymptotic performance differences to the fact that both pupil size and brightness was achieved. Training was conducted in the same appa- sensation are mediated, in part, by the rod system which ratus used in the experiment, but was accomplished has a different spectral sensitivity than does the cone sys- using different gratings (e.g., square waves instead of tem. The differences exist even when the sources are sine waves) and sources (e.g., fluorescent) from those metameric. employed in the experiments. The decision to use a few Kelly'? found up to a 40 percent increase in brightness highly trained subjects rather than a large number of sensation using yellow filters as compared to an equilu- untrained persons was made to reduce the noise in the minous white light at luminances between 7 and 40 data so that small performance differences would be cd/m2. She also attributed the effect to rod activity more apparent. Subjects served as their own controls. All because the effect was present only under conditions testing was done monocularly using the subject's right where rods are presumably active. eye; all subjects were right eye dominant. Data-gathering Kinney et al." showed an increase in visual perfor- sessions were of approximately 20 min duration after mance (defined as a decrease in reaction time) for some which a minimum of 15 rnin of rest was permitted. spatial frequencies when lights were viewed through yel- low filters. Kinney's group did not investigate the cause, Sources but speculated that it was due to reduced inhibition in Five sources were used for each condition of the the chromatic processing system. experiments, chosen because they are commonly used in While there is no doubt that SPD has important ram- outdoor lighting installations. They were incandescent, ifications for the perception of lights, it is not so clear high pressure mercury, high pressure sodium, low pres- that these effects translate into benefits that are of engi- sure sodium, and metal halide. neering significance. Indeed, if the effects are as large as Spectral power distributions were measured using an have been reported by Berman and Kelly, it is surprising Opticon Spectroradiometer that was calibrated with a to some that most controlled and properly analyzed stud- standard traceable to NIST. Relative spectral power dis- ies have shown little or no effect of SPD on visual per- tributions for each source are given in the Appendix. formance with achromatic tasks. However, since several Luminances were adjusted to account for the actual investigations have suggested that the effects are rod- SPDs rather than relying on filtered photometers mediated, and since most of the performance tasks stud- (although a photometer was used to monitor for possible ied have been dependent primarily on the resolution of changes in luminance during each experimental ses- high spatial frequencies (for which the rods are not par- sion). Luminances were calculated using the formula: ticularly sensitive), perhaps it isn't so strange after all. Winter 1999 JOURNAL of the illuminating Engineering Society •••••••••••••••••••••••••••••••••••••••••••••• 39 REACTION TIME cpd) were tested