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Evaluation of Glare for Incandescent and LED Miner Cap Lamps in Mesopic Conditions Introduction J

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Evaluation of Glare for Incandescent and LED Miner Cap Lamps in Mesopic Conditions Introduction J Evaluation of glare for incandescent and LED miner cap lamps in mesopic conditions Introduction J. SAMMARCO, A. MAYTON, T. LUTZ which include ambient conditions; The Illuminating Engineering AND S. GALLAGHER complexity of the lighting environ- Society of North America (IESNA) ment; difficulty of location with light cites the working face of an under- J. Sammarco, A. Mayton, T. Lutz and S. Gallagher are sources; and the observers, age and ground coal mine as the most diffi cult senior research scientist, lead research scientist, lead research visual health (Rea, 2000; Van Der­ environment in the world to illumi- scientist and senior research scientist, respectively, with the lofske and Bullough, 2006). nate (Rea, 1993). It is a dynamic Pittsburgh Research Laboratory, National Institute for Occupa- Glare studies have been done in environment that includes dust, con- tional Safety and Health (NIOSH), Pittsburgh, PA. the past with underground coal min­ fi ned spaces, low refl ective surfaces, ers (Crouch, 1982; Guth, 1982). From low visual contrasts and glare. Light­ a study of discomfort glare with un­ ing is critical to miners who depend derground coal miners, Guth (1982) heavily on visual cues to spot fall of noted that miners are less sensitive ground, potential machinery-related to discomfort glare than offi ce work­ pinning and striking incidents, and ers. The evaluation procedure used slipping and tripping hazards (Cornelius et al., 1998). had been developed for interior lighting conditions (IES, Glare can be defi ned as the sensation from an uncom­ 1973). Concerning disability glare, Crouch (1982) report­ fortably or painfully bright light within a person’s visual ed in a joint study by Bituminous Coal Research Inc. fi eld. Glare occurs from too much light and extremes that and the Illuminating Engineering Research Institute that produce too broad a range of light levels compared to 78% of miners interviewed complained or questioned those for which the eyes are adapted.The effects of glare the lighting systems relative to discomfort and disability on workers include discomfort glare (annoying or pain­ glare, veiling reflections and after-images. From the study ful sensation), disability glare (reduction of visibility), results, Crouch estimated that miners working within the recovery or re-adaptation (visual performance returning existing illuminated coal mining face environments could to initial state) and photobiological (optical radiation experience as much as a 40% or more loss of visibility. effects on living systems). To assess visual performance, Trotter (1982) listed ten methods to reduce glare. Most one must consider distinct parameters associated with the of these methods resulted in decreasing the luminance glare produced, the environment and the observer. The at the observer’s eye or increasing the background lumi­ aspects of glare that affect visual performance include nance with respect to the task luminance. illuminance at the eye, angle of the glare source, lumi­ A number of nonmining studies have investigated nance and size, spectral power distribution (SPD) and the glare. Most studied glare relative to various aspects of duration of exposure. Additionally, visual performance automobile headlamps while driving. For instance, Van is impacted by environmental and observer parameters, Derlofske (2004) and Bullough (2002; 2003) concluded Abstract The U.S. National Institute for Occupational Safety and Health (NIOSH) is conducting mine illumination research to determine if light-emitting diode (LED) cap lamps can improve safety by reducing glare. Glare can impede a miner’s ability to see hazards and to safely perform their work.Another objective is to determine if a person’s age is a factor.This is important because the workforce is aging — the average miner is now about 43 years old. Three cap lamps were used to evaluate glare: an incandescent cap lamp, a commercially available LED cap lamp and a NIOSH prototype LED cap lamp.Thirty NIOSH personnel from the Pittsburgh Research Laboratory (PRL) served as test subjects.Three age groups were established with ten subjects in each group. Testing was conducted in the Mine Illumination Laboratory (MIL) of NIOSH PRL. The results indicate no statistically signifi cant difference in discomfort glare among the incandescent and LED cap lamps. However, an analysis of variance for disability glare indicates that the LED cap lamps were superior for the older subjects. Disability glare scores for the oldest subject group improved 53.8% when using the NIOSH prototype LED cap lamp compared to the incandescent cap lamp and 36.5% compared to the commercial LED cap lamp. It ap­ pears that, given the conditions of this study, LED cap lamps will not increase discomfort glare and can enable signifi cant improvements in disability glare for older workers. It is also evident that not all LEDs are created equal. The disability glare improved the best for older workers when they used the NIOSH prototype LED cap lamp, which has a different spectral power distribution (SPD) (more short wavelength energy) than the commercial LED cap lamp. Therefore, for disability glare, the results suggest that the SPD is an important factor to consider in cap lamp design. FIGURE 1 Terminology Experimental layout: (a) plan view and (b) side view. The following terms are based on Whitehead and Bockosh (1992): (Figures not to scale.) • Correlated color temperature (CCT): The phrase used to describe the temperature at which a Planck­ ian black body radiator and an illumination source’s appear to match (usually specified in Kelvin (K)). • Illuminance: The measure of the density of lumi­ nous flux striking a surface. The IESNA and the International System of Units (SI) units are foot- candle (fc) and lux (lx), where lumen (lm) is the unit of luminous flux and lx = lm/m² and fc = lm/ft². • Luminance: In physical terms, luminance is a con­ cept used to quantify the density of luminous fl ux emitted by an area of a light source in a particular that the light source spectrum, as measured by the spec­ direction toward a light receiver such as a human tral power distribution (SPD), played a signifi cant role eye. Luminance is closely correlated with a person’s perception of brightness. The IESNA and SI unit is in causing discomfort glare but did not play a signifi cant 2 role for disability glare. Two studies (Collins and Brown, candela (cd)/m . 1989; Scheiber, 1994) investigated glare recovery ac­ • Luminous fl ux: The time flow rate of light energy cording to age. Scheiber (1994) noted that the recovery similar in concept to horsepower or Btu per hour. time for older compared to younger subjects increased The lumen (lm) is the unit of luminous fl ux used by a factor of three. by the IESNA and the SI. Three situations indicate the need for new research • Spectral power distribution (SPD): The radiant power addressing cap lamp glare. First, a miner’s cap lamp is emitted at different wavelengths in the visible light typically the primary and most important source of light spectrum. For most practical purposes, it includes the (Trotter and Kopeschny, 1997). However, cap lamps range from 380 to 780 nanometers (nm). are often a source of discomfort or disability glare, which can impact both safety and task performance. Methods Secondly, as stated above, age is a factor for glare. This Experimental design. A 3 x 3 (age group x glare is important to consider because the aging U.S. mine source) split-plot factorial design was used, and two rep­ workforce has an average age of about 43 years. Thirdly, lications of each light source condition were performed light emitting diodes (LEDs) are being used in new for each subject. Age group represented the whole-plot cap lamp designs. LEDs are an emerging technology factor and light source represented the split-plot factor. for mine illumination and there is no prior research The interaction of age group and light source was part of that addresses the safety of LEDs with respect to glare. the split-plot analysis. An analysis of variance (ANOVA) Therefore, the primary objective of this study was to was used to evaluate whether there were signifi cant dif­ determine if new LED-based cap lamp technology has ferences for the independent variables. an impact on discomfort or disability glare for subjects The primary independent variables were: in three distinct age groups. The authors’ approach was to focus on the spectral content of light, as measured by • Age categories: Group A = 18 to 25 years old, the SPD, because this can infl uence glare and because Group B = 40 to 50 years old and Group C = 50+ LEDs can have drastically different SPDs compared to years old; and the incandescent lights traditionally used in cap lamps, • Glare sources: LED cap lamp, incadescent cap lamp even though both types of lighting can provide the same and a NIOSH prototype LED cap lamp. luminance. The dependent variables were: Table 1 Cap lamp electrical and photometric data. Electrical characteristics Photometric characteristics Supply Supply Supply Peak Correlated color voltage, current, power, wavelength, temperature, Cap lamp Vdc amps watts nm K Incandescent 6.1 0. 63 3.84 780 2,880 LED 6.1 0.42 2.56 448 5,855 Prototype LED 12.0 0.113 1.36 444 6,844 • Subjective discomfort glare ratings: Qualitative FIGURE 2 using a de Boer scale 1 through 9; and • Contrast sensitivity score: Quantitative using a The spectral power distributions for each cap lamp: (a) Mars Contrast Sensitivity test score. incandescent, (b) LED and (c) NIOSH prototype LED. The presentation order for cap lamps was counterbal­ anced where rep 1 was the baseline presentation order and rep 2 was the reversed presentation order. The glare sources were treated as a within-subjects variable with each subject rating the discomfort glare based on the de Boers scale, which is a commonly accept­ ed method for measuring discomfort glare.
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