Evaluation of glare for incandescent and LED miner cap lamps in mesopic conditions

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 significant 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. Introduction The Illuminating Engineering Society of North America (IESNA) cites the working face of an underground coal mine as the most diffi cult environment in the world to illuminate (Rea, 1993). It is a dynamic environment that includes dust, confi ned spaces, low refl ective surfaces, low visual contrasts and glare. Light­ ing is critical to miners who depend heavily on visual cues to spot fall of ground, potential machinery-related pinning and striking incidents, and slipping and tripping hazards (Cornelius et al., 1998). Glare can be defined as the sensation from an uncom­ fortably or painfully bright light within a person’s visual field. Glare occurs from too much light and extremes that produce too broad a range of light levels compared to those for which the eyes are adapted.The effects of glare on workers include discomfort glare (annoying or pain­ ful sensation), disability glare (reduction of visibility), recovery or re-adaptation (visual performance returning to initial state) and photobiological (optical radiation effects on living systems). To assess visual performance, one must consider distinct parameters associated with the glare produced, the environment and the observer. The aspects of glare that affect visual performance include illuminance at the eye, angle of the glare source, lumi­ nance and size, spectral power distribution (SPD) and the duration of exposure. Additionally, visual performance is impacted by environmental and observer parameters, which include ambient conditions; complexity of the lighting environment; difficulty of location with light sources; and the observers, age and visual health (Rea, 2000; Van Der­ lofske and Bullough, 2006). Glare studies have been done in the past with underground coal min­ ers (Crouch, 1982; Guth, 1982). From a study of discomfort glare with un­ derground coal miners, Guth (1982) noted that miners are less sensitive to discomfort glare than offi ce work­ ers. The evaluation procedure used had been developed for interior lighting conditions (IES, 1973). Concerning disability glare, Crouch (1982) report­ ed in a joint study by Bituminous Coal Research Inc. and the Illuminating Engineering Research Institute that 78% of miners interviewed complained or questioned the lighting systems relative to discomfort and disability glare, veiling reflections and after-images. From the study results, Crouch estimated that miners working within the existing illuminated coal mining face environments could experience as much as a 40% or more loss of visibility. Trotter (1982) listed ten methods to reduce glare. Most of these methods resulted in decreasing the luminance at the observer’s eye or increasing the background lumi­ nance with respect to the task luminance. A number of nonmining studies have investigated glare. Most studied glare relative to various aspects of automobile headlamps while driving. For instance, Van Derlofske (2004) and Bullough (2002; 2003) concluded that the light source spectrum, as measured by the spec­ tral power distribution (SPD), played a signifi cant role in causing discomfort glare but did not play a signifi cant role for disability glare. Two studies (Collins and Brown, 1989; Scheiber, 1994) investigated glare recovery ac­ cording to age. Scheiber (1994) noted that the recovery time for older compared to younger subjects increased by a factor of three. FIGURE 1 Experimental layout: (a) plan view and (b) side view. (Figures not to scale.) Three situations indicate the need for new research addressing cap lamp glare. First, a miner’s cap lamp is typically the primary and most important source of light (Trotter and Kopeschny, 1997). However, cap lamps are often a source of discomfort or disability glare, which can impact both safety and task performance. Secondly, as stated above, age is a factor for glare. This is important to consider because the aging U.S. mine workforce has an average age of about 43 years. Thirdly, light emitting diodes (LEDs) are being used in new cap lamp designs. LEDs are an emerging technology for mine illumination and there is no prior research that addresses the safety of LEDs with respect to glare. Therefore, the primary objective of this study was to determine if new LED-based cap lamp technology has an impact on discomfort or disability glare for subjects in three distinct age groups. The authors’ approach was to focus on the spectral content of light, as measured by the SPD, because this can infl uence glare and because LEDs can have drastically different SPDs compared to the incandescent lights traditionally used in cap lamps, even though both types of lighting can provide the same luminance.