Evaluation of glare for incandescent and 


LED miner cap lamps in mesopic conditions
 

J. SAMMARCO, A. MAYTON, T. LUTZ 

AND S. GALLAGHER 

J. Sammarco, A. Mayton, T. Lutz and S. Gallagher are 

senior research scientist, lead research scientist, lead research 

scientist and senior research scientist, respectively, with the 

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, PA. 

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 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 appears 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, confined spaces, low refl ective surfaces, 

low visual contrasts and glare. Lighting 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 uncomfortably 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 painful 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, luminance 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 Derlofske and Bullough, 2006). 

Glare studies have been done in 

the past with underground coal miners (Crouch, 1982; Guth, 1982). From 

a study of discomfort glare with underground coal miners, Guth (1982) 

noted that miners are less sensitive 

to discomfort glare than offi ce workers. The evaluation procedure used 

had been developed for interior lighting conditions (IES, 

1973). Concerning disability glare, Crouch (1982) reported 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 luminance 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 spectral 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 according 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 influence 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. 

Table 1 

 

Cap lamp electrical and photometric data. 

Electrical characteristics 

Supply Supply 


voltage, current, 


Cap lamp Vdc amps 


Incandescent 6.1 0. 63 


LED 6.1 0.42 


Prototype LED 12.0 0.113 


Photometric characteristics

 Supply Peak Correlated color

 power, wavelength, temperature, 

watts nm K 

3.84 780 2,880 

2.56 448 5,855 

1.36 444 6,844 

Terminology 

The following terms are based on Whitehead and 

Bockosh (1992): 

• 	 Correlated color temperature (CCT): The phrase 

used to describe the temperature at which a Planckian black body radiator and an illumination source’s 

appear to match (usually specified in Kelvin (K)). 

• 	 Illuminance: The measure of the density of luminous flux striking a surface. The IESNA and the 

International System of Units (SI) units are footcandle (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 concept used to quantify the density of luminous fl ux 

emitted by an area of a light source in a particular 

direction toward a light receiver such as a human 

eye. Luminance is closely correlated with a person’s 

perception of brightness.The IESNA and SI unit is 

candela (cd)/m2

. 

• 	Luminous flux: The time flow rate of light energy 

similar in concept to horsepower or Btu per hour. 

The lumen (lm) is the unit of luminous fl ux used 

by the IESNA and the SI. 

• 	 Spectral power distribution (SPD):The radiant power 

emitted at different wavelengths in the visible light 

spectrum. For most practical purposes, it includes the 

range from 380 to 780 nanometers (nm). 

Methods 

Experimental design. A 3 x 3 (age group x glare 

source) split-plot factorial design was used, and two replications of each light source condition were performed 

for each subject. Age group represented the whole-plot 

factor and light source represented the split-plot factor. 

The interaction of age group and light source was part of 

the split-plot analysis.An analysis of variance (ANOVA) 

was used to evaluate whether there were signifi cant differences for the independent variables. 

The primary independent variables were: 

•	 Age categories: Group A = 18 to 25 years old, 

Group B = 40 to 50 years old and Group C = 50+ 

years old; and 

•	 Glare sources: LED cap lamp, incadescent cap lamp 

and a NIOSH prototype LED cap lamp. 

The dependent variables were: 
•	 Subjective discomfort glare ratings: Qualitative 

using a de Boer scale 1 through 9; and 

•	 Contrast sensitivity score: Quantitative using a 

Mars Contrast Sensitivity test score. 

The presentation order for cap lamps was counterbalanced 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 accepted method for measuring discomfort glare.The de Boers 

scale is a nine-point subjective scale including qualifi ers 

at the odd points: 1 = unbearable, 3 = disturbing, 5 = just 

acceptable, 7 = satisfactory and 9 = just noticeable. The 

even numbers (2, 4, 6 and 8) designated a degree more or 

less of the odd numbered qualifi ers. 

Disability glare was quantified by measuring a subject’s visual performance for contrast sensitivity while 

a given glare source was present. As glare increases, the 

ability to detect low contrast object decreases. Age is 

another factor that directly affects contrast sensitivity, 

as age increases the ability to detect low contrast object decreases. The Mars Letter Contrast Sensitivity test 

was used to measure visual performance with respect to 

contrast sensitivity. This is a standardized test chart that 

measures 23 x 35.5 cm (9.1 x 14 in.) and is printed with 

48 letters that are 1.75 cm (0.7 in.) high. The letters are 

arranged in eight rows of decreasing contrast. 

Subjects. NIOSH personnel from the Pittsburgh Research Laboratory (PRL) were the subjects. No NIOSH 

personnel involved with cap lamp research were used. 

Only the subjects that passed vision tests for distance 

visual acuity, contrast sensitivity, color vision defi ciency 

and peripheral vision were accepted for the study. Most 

NIOSH personnel had very little if any familiarity with 

cap lamps so as to reduce expectancy biases. Miners were 

not used because of potential expectancy biases that could 

confound empirical data. Miners could immediately determine that the bluish-white light from an LED cap lamp is 

very different from the yellowish light of an incandescent 

cap lamp; thus, a negative bias could exist because the light 

is not what they are accustomed to, or a positive bias could 

exist if the person perceives something new as better. 

Twenty-four male and six female subjects participated. 

There were ten subjects each in the age groups of younger 

(18 to 25 years), middle (40 to 50 years) and older (50+ 

years). The mean ages where 22.6, 47.3 and 57.6 years, respectively.The age group from 26 to 39 years was not used 

because there are generally minimal changes in vision 

for those ages. The subjects’ contrast sensitivity eye test 

scores were of prime interest because disability glare was 

empirically quantifi ed in terms of contrast sensitivity. For 

the youngest to oldest age groups, the mean scores were 

1.748, 1.748 and 1.649, respectively, where a higher score 

is better. The mean value on this test for young (mean of 

about 20 to 25 years) and older (mean of about 55 to 60 

years) subjects with normal vision is about 1.7 for both age 

groups. Older subjects classifi ed as having low vision had 

mean scores of about 1.3 (Dougherty et al., 2005). 

Subjects signed an informed consent form and were 

instructed about their right to withdraw freely from the 

research at any time without penalty. The NIOSH Human Subject Review Board approved the protocol for 

this study. 

FIGURE 2 

The spectral power distributions for each cap lamp: (a) 

incandescent, (b) LED and (c) NIOSH prototype LED. 

Experimental layout and apparatus 

Mine illumination laboratory. The testing was conducted at the Mine Illumination Laboratory (MIL) of 

NIOSH PRL. The MIL is a simulated, underground coal 

mine environment that is equipped with various test 

equipment, data acquisition and control systems, and 
 

networked computers. The interior is 488 cm (192 in.) 

wide by 213 cm (84 in.) high and is coated with a roughtextured material that has a color and refl ectivity similar 

to that of a coal mine.The experimental layout (Fig. 1) for 

glare testing was arranged to place the test subject in the 

observation station facing the test cap lamps to simulate 

glare from a coworkers cap lamp. The cap lamps were 

located 183 cm (72 in.) away from the test subject at -10 

° off axis from the charts directly in front of the test subject. The cap lamp was placed at the same height as the 

eye height of the test subject (165.1 cm or 65 in.). Each 

of the three test cap lamps was tested twice in a different 

order with the same contrast sensitivity chart. 

Table 2	 

ANOVA summary for contrast sensitivity. 

Source SS df MS F 

S (Subjects) 2.438 9 0.271 – 

R (Reps) 0.024 1 0.024 0.28 

A (Age Group) 5.721 2 2.861 33.661 

AR 0.011 2 0.006 0.07 

SAR (WP error) 3.825 45 0.085 – 

B (Cap Lamp) 0.319 2 0.159 12.862 

AB 0.188 4 0.047 3.803 

Bψ1(A)

Bψ2(A)

Aψ1(B)

Aψ2(B)

Aψ3(B)

ψ ψ 1(A) 1(B)

ψ ψ 1(A) 2(B)

ψ ψ 1(A) 3(B)

ψ ψ 2(A) 2(B) 

 BR

 0.128

 0.060

 0.053

 0.156

 0.074

 0.049

 0.125

 0.017

0.004

 0.031 

2 

2 

2 

2 

2 

1 

1 

1 

1 

2 

0.064 

0.030 

0.027 

0.078 

0.037 

0.049 

0.125 

0.017 

0.004 

0.016 

5.1554 

2.433 

2.146 

6.2724 

2.965 

3.976 

10.0915 

1.398 

0.316 

1.26 

 ABR 0.016 4 0.004 0.33 

 SABR (SP error) 1.339 108 0.0124 – 

Total 13.9125 179 

 Treatment A coefficients Treatment B coeffi cients 

 Young Middle Old Incandescent LED 	 NIOSH 

 prototype LED

 C = 1 -1 0 C = -1 -1 1j	 1k

 C = 1 0 -1 C = -1 1 2j	 2k 

C = 0 1 -1 3j

2

0

1

F = 3.21 (For observed F test statistic: p < 0.001) .05,2,45 2

F = 3.08 (For observed F test statistic: p < 0.001) .05,2,108 3

F = 2.45 (For observed F test statistic: p < 0.05) .05,4,108 4

(4F. )/2 = 4(2.45)/2 = 4.90(For observed F test statistics: p < 0.01) 05,4,108 5

(4F )/1 = 4(2.45) = 9.80 (For observed F test statistic: p < 0.05) .05,4,108

Observation station. The observation station (Fig. 

1 (b)) was designed to enable all human subjects to be 

tested at the same eye height with reference to the fl oor. 

The eye height of 165.1 cm (65 in.) is based on the 50th 

percentile male standing (USDOT, 2003).The station was 

required to allow test subjects ranging from the 5th percentile female to the 95th 

percentile male to be adjusted to the 165.1 cm (65 

in.) eye height when in 

the seated position.Torso 

heights for the specifi ed 

test subjects have a range 

of 68.6 cm (27 in.) to 84.8 

cm (33.4 in.). The seat 

was designed to rise 20.3 

cm (8 in.) from the lowest position to the highest 

to accommodate all test 

subjects. The height of 

the miner’s helmet with 

cap lamp and earphones 

is independently adjustable from the seat height 

to accommodate the different torso heights of 

the subjects.The observation station uses an electric actuator for adjusting 

the seat height with 20.3 

cm (8 in.) of travel. The 

helmet height is manually adjustable up to 25.4 

cm (10 in.) with hand-operated clamps. The helmet adjusts fore and aft 

manually up to 15.2 cm 

(6 in.). The seat is adjustable fore and aft and has 

foldable arm rests. The 

platform includes a fi xed 

foot rest. The platform is 

constructed of wood and 

steel and outlined with 

yellow reflective tape to 

minimize tripping when 

the human subjects are 

preparing to be tested. 

All of the components 

are a flat-black color to 

help eliminate any reflections or distractions during testing. 

Cap lamps. Three cap lamps were used. Each cap lamp 

was brand new and powered at levels for a fully charged 

battery. The first was an MSHA-approved cap lamp using 

a single incandescent bulb as the primary light source. 

This served as the reference. The second was an MSHAapproved cap lamp with a single phosphor-white LED as 

the primary light source.The third cap lamp was a laboratory prototype that was jointly developed by the Lighting 

Research Center of Rensselaer Polytechnic Institute and 

NIOSH. This prototype uses two phosphor-white LEDs 

as the primary light source. The NIOSH prototype LED 

cap lamp meets the photometric requirements specifi ed 

by MSHA (CFR 30, 2005). Each cap lamp had a unique 

beam profile such that the beam spots were of varying 

size and intensity; therefore, the illuminance at the subject’s eye varied as follows: incandescent cap lamp was 50 

lux, LED cap lamp was 100 lux and the NIOSH prototype 
 

 

 

 

LED cap lamp was 70 lux. None of the cap lamps were 

focusable. 

Each cap lamp was characterized in terms of its electrical data and its SPD. For each cap lamp, the electrical 

and photometric data are listed in Table 1. Each cap lamp 

was energized from a regulated power supply rather than 

a cap lamp battery to eliminate voltage fl uctuations as 

the battery discharged. The power supply voltages for 

the different glare source cap lamps were set at 4, 6 and 8 

V according to the specifications for the particular make 

and model of cap lamp.These voltages are representative 

of fully charged batteries. 

At daytime light levels (photopic conditions), the 

eye’s cone photoreceptors dominate vision, but as light 

levels decrease, the rod receptors of the eye have an increasing role in night vision. Rods have greater shortwavelength spectral sensitivity than cones. The spectral 

content of visible light can be characterized by the spectral power distribution (SPD). Lighting research indicates that at low-light (mesopic) conditions, where rods 

and cones both contribute to vision, a short-wavelength 

spectral content can improve visual performance. Therefore, the spectral content for each cap lamp was characterized by measuring the SPD. Figure 2 depicts the 

SPD for each cap lamp.The LED and NIOSH prototype 

LED cap lamps have a “bluish-white” color of light because the cool-white LEDs have a greater proportion of 

short-wavelength energy in comparison to the yellowish 

light of incandescent lighting commonly used for miner 

cap lamps. Both LED cap lamps use a cool-white type 

of LED characterized by a correlated color temperature 

(CCT) between 5,000K and 10,000K. It is important to 

note that there is a wide range of “white” available for 

LEDs. There are neutral white (CCT is typically 3,700K 

to 5,000K) and warm white (CCT is typically 2,600K to 

3,700K). The CCT for warm-light LEDs is similar to incandescent lamps. The CCTs for each cap lamp after a 

five minute warm up are listed in Table 1. 

Procedure. The subjects sat in a darkened environment until their eyes adapted. Next, subjects were seated 

on the observation station and adjustments were made 

such that each person had the same eye height of 165.1 

cm (65 in.) from the floor.While seated, the subjects wore 

a miner’s hardhat with an incandescent cap lamp light illuminated for a low ambient light level. Researchers then 

explained the glare experiments and how they would be 

conducted. 

Prior to the start of the glare experiments, researchers gave an overview of the experiment to the subjects, 

explaining that the experiment included tests for assessing subjects’ responses to discomfort and disability glare. 

Subjects were directed to focus their eyes on a center 

display target at all times while seated on the observation station. Next, one of the cap lamp glare sources was 

turned on and given time to stabilize (warm up). The 

subject’s response to the discomfort glare was obtained 

followed by their responses to the disability glare tests. 

Two datasets were collected for each of the three cap 

lamps. The cap lamp sequences were the incandescent, 

LED and NIOSH prototype LED for Dataset 1 and the 

reverse cap lamp sequence for Dataset 2. 

The first test measured discomfort glare and focused 

on subjects providing a subjective rating of each glare 

source using the de Boer scale shown on the chart in 

front of them. While sitting in the observer’s chair, subjects were asked to think about the discomfort ratings 

relative to the designated cap lamp as it was switched 

on. Subjects subsequently gave a numerical rating from 

1 to 9 for the de Boer scale chart. Immediately following the discomfort glare test, the disability glare test was 

conducted.The Mars Letter Contrast Sensitivity test was 

utilized by switching out the de Boer scale target with the 

appropriate contrast sensitivity chart. Three charts with 

different lettering orders were used to minimize learning 

effects. Each chart had the letters C, D, H, K, N, O, R, S, 

V and Z in varying contrasts and sequences.The subjects 

were asked to read aloud the letters on the chart from 

left to right for each row of letters beginning at the top 

row and moving down the chart. Researchers explained 

that selections would only include the letters C, D, H, 

K, N, O, R, S, V and Z. When in doubt about any letter, 

subjects made their best guess from this set of letters.The 

test continued until they failed to correctly identify two 

consecutive letters.After completing both discomfort and 

disability tests, the glare source cap lamp was switched 

off and the subjects were asked to relax for about one 

minute to allow their eyes to readapt to the ambient light 

level provided by the incandescent cap lamp fitted to the 

a miner’s hardhat that they were wearning. 

FIGURE 3 

The interaction between age group and glare source in 

terms of contrast sensitivity. 

Test chart illuminance. Illuminance in the vertical 

plane was measured at the test charts for test conditions, 

i.e., for each cap lamp separately with ambient lighting 

and for ambient lighting only. Illuminance levels were 

produced from light refl ected from the surrounding test 

environment including the subject and observer station. 

Ambient lighting conditions were produced from the cap 

lamp mounted to the subject’s helmet. Measurements 

were made on the test chart at three locations (top, mid­
 

dle and bottom). The average illuminance measured in 

the absence of any cap lamp glare source and for the ambient condition only was 4.76 lux. For the incandescent, 

LED and NIOSH prototype LED test cap lamp glare 

sources, the average illuminances were 6.26, 5.17 and 7.01 

lux, respectively. 

Table 3	 

ANOVA summary for de Boer ratings. 

Source SS df MS F 

S (subjects) 78.450 9 8.717 – 

R (reps) 0.939 1 0.939 0.27 

A (age group) 66.078 2 33.039 9.411 

AR 1.878 2 0.939 0.27 

SAR (WP error) 158.050 45 3.512 – 

B (cap lamp) 5.344 2 2.672 1.92 

AB 11.556 4 2.889 2.07 

BR 4.478 2 2.239 1.61 

ABR 3.556 4 0.889 0.64 

SABR (SP error) 150.400 108 1.392 

Total 480.728 179 

1

F = 3.21 .05,2,45

Multiplicative Sidak Pairwise comparisons for deBoer ratings2

 by age group. Age groups containing the same letter are not signifi cantly different from one another. 

Age Group: Young Middle Old 	

4.47 3.77 2.98 

 A AB B 

2

The de Boer scale is as follows: 1 = unbearable, 3 = disturbing, 5 = just acceptable, 7 = satisfactory, 9 = just noticeable. 

Results 

Tables 2 and 3 summarize the ANOVA results for the 

contrast sensitivity tests and the de Boer ratings, respectively.These tables include degrees of freedom (df), sums 

of squares (SS), mean square (MS) and F-ratios (F value 

for the main effects and interactions). As can be seen in 

Table 2, a significant interaction was detected between 

age group and light source for contrast sensitivity. The 

nature of the interaction is depicted in Fig. 3. 

To determine the nature and sources of the non-additivity resulting in this interaction, treatment-contrast 

and contrast-contrast interactions were examined (Kirk, 

1995). Results of these tests are also contained in Table 2. 

This analysis indicated that the specific contrasts responsible for the interaction compared the NIOSH prototype 

LED cap lamp versus the two other cap lamps and compared age Group A with age Group C. Specifi cally, the 

NIOSH prototype LED cap lamp resulted in signifi cantly 

better contrast sensitivity scores in the older age group 

compared to the other cap lamps, whereas the youngest age group showed no difference in contrast sensitivity between any of the cap lamps (FS 4,108 = 10.091, p 

< 0.05). Followup contrasts examining whether differences existed between 

the LED and incandescent cap lamp indicated 

no statistically signifi cant 

difference in contrast 

sensitivity across age 

groups (FS 2,108 = 1.10, p 

> 0.05). A test examining 

differences in contrast 

sensitivity between the 

young and middle age 

groups indicated a decrement in performance in 

the middle age group (FS 

2,108 = 10.97, p < 0.05). 

Figure 3 discloses an 

even more rapid drop in 

contrast sensitivity in the 

older age group. However, a formal test between 

these age groups was not 

possible (Kirk, 1995). 

The qualitative de 

Boer ratings were affected by age group (F 2,45 

= 9.41, p < 0.001) but not 

by light source. Post hoc 

Multiplicative Sidak tests 

indicated a signifi cant difference in de Boer rating 

between young and old 

age groups. However, the 

middle age groups were 

not different in terms 

of de Boer rating from either the young or old groups. 

Analysis of the mean de Boer ratings for each cap lamp 

suggests that ratings were highest (with a higher rating 

meaning less discomfort glare) for the NIOSH prototype 

LED than for the other cap lamps. However, this qualitative assessment did not achieve statistical signifi cance 

between cap lamps. 

Discussion 

The results of the glare comparisons between LED 

and incandescent cap lamps provide important data for 

improving the design of future cap lamps and have the 

potential to positively affect the safety of employees in 

the underground mining industry. It is apparent from the 

authors’ findings that spectral content of light from coolwhite LEDs does not appear to increase discomfort glare 

and it has the potential to significantly improve safety by 

reducing disability glare, especially among older workers 

(who are most susceptible to glare and its effects). 

While the benefits of the NIOSH prototype LED design in contrast sensitivity were significant only when 

comparing youngest versus oldest age groups, it is notable that the NIOSH prototype LED also scored highest 

in the middle age group. In fact, contrast sensitivity for 

the NIOSH prototype LED was little changed between 

the young and middle age groups. Meanwhile, both the 

incandescent and the commercial LED declined in contrast scores between these age groups. This suggests the 

possibility of a small benefit for the NIOSH prototype 

design even among middle age workers, which becomes 
 

 

increasingly apparent in older subjects. 

An interesting result from the analysis of the interaction was that the authors were able to test whether contrast sensitivity was different between the incandescent 

and the commercially available LED light source. No 

statistically signifi cant difference was observed between 

these two light sources in terms of contrast sensitivity 

scores. Thus, the only light source that had any positive 

impact on contrast sensitivity was the NIOSH prototype 

LED and that result was age dependent. Also, age had a 

signifi cant negative impact on contrast sensitivity with a 

slight decline seen from the young to middle age groups 

and a steep decline between middle and older age groups. 

The authors reasoned that the prototype cap lamp improved contrast sensitivity for older subjects because it 

provided the most short-wavelength energy, which should 

help older subjects. As a person ages, the eye changes 

such that the muscles of the iris weaken, which limits how 

large the pupil expands, and the lens of the eye becomes 

less opaque.Thus, less light reaches the retina for an older 

person. Secondly, an older person has fewer rod photoreceptors in the eye compared to a young person. These 

rods play a critical role in night vision, and they are more 

sensitive to short-wavelength energy. Therefore, older 

adults would respond better to light with a higher proportion of short-wavelength energy. High contrast and high 

illumination are not typical in underground mining, so 

age and contrast are important factors for mining. 

There are other factors that could have helped the 

prototype LED cap lamp to have a positive impact on 

contrast sensitivity. The illuminance at the eye was 70 

lux for the prototype LED cap lamp and 100 lux for the 

LED cap lamp. Contrast sensitivity would decrease as 

the illuminance at the eye increases, so the prototype cap 

lamp would have some advantage. But note that the incandescent cap lamp provided only 50 lux at the eye, yet 

this decrease did not improve contrast sensitivity for any 

age group. Secondly, contrast sensitivity would improve 

as the test chart illuminance increased. The prototype 

cap lamp provided 7.01 lux of test chart illuminance compared to 6.26 lux for the incandescent cap lamp and 5.17 

lux for the LED cap lamp. Note again that no statistically 

significant difference was observed between the incandescent and LED cap lamp. The authors would expect 

that contrast sensitivity would improve for the LED cap 

lamp if the illuminance at the eye decreased and if test 

chart illuminance increased to the levels of one of the 

other cap lamps. However, the authors do not know if the 

improvement would be statistically signifi cant. 

The qualitative assessment using the de Boer discomfort glare scale provided additional confirmation of the 

increasing glare discomfort associated with increasing 

age. There was a clear linear trend in discomfort ratings 

indicating the increasing discomfort with age (Table 3). 

De Boer ratings were not significantly affected by the 

light source. However, mean ratings were highest for the 

NIOSH prototype LED followed by the incandescent 

and LED light sources.Again, note that discomfort glare 

increases as illuminance at the eye increases; the illuminances at the eye were 50 lux (incandescent cap lamp), 

100 lux (LED cap lamp) and 70 lux (prototype LED cap 

lamp). There is the possibility to reduce discomfort glare 

for the LED and prototype LED cap lamps if the illuminance at the eye decreased to 50 lux. One can infer from 

these results that the LED-based cap lamps can provide 

significantly more illumination without increasing discomfort glare. This increased illumination would likely 

aid a miner’s ability to spot hazards in their forward fi eld 

of vision. 

Again, the general results indicate that, given the conditions of this study, the LED-based cap lamps do not 

increase discomfort glare and they have the potential to 

decrease disability glare for older workers. These fi ndings differ from other research concerning automotive 

headlamp glare, where the light source spectrum played 

a signifi cant role in causing discomfort glare but did not 

play a significant role for disability glare (Bullough et al., 

2003).The conditions of this research differed in that the 

illuminance at the eye for normal automotive driving was 

much less (0 to 10 lux) and that the glare sources included halogen and high-intensity discharge headlamps that 

have different spectral power distributions in comparison 

to the LEDs used in miner cap lamps. 

Taken together, the results of this study provide evidence that the NIOSH prototype LED design has several benefits in terms of improving visual performance 

(especially among older workers) and may help decrease 

glare discomfort compared to currently available light 

sources. As mentioned above, the current average age of 

the mining population is 43 years.As a result, many mine 

workers could be positively impacted by implementation 

of the NIOSH prototype LED cap lamp design. For older 

workers, there was a 45% improvement in contrast sensitivity compared to the other light sources studied. Because contrast sensitivity is an important aspect of visual 

performance and is a critical factor for many work tasks, 

implementation of the prototype cap lamp design could 

provide a major benefit for the aging mining population. 

Lastly, this study was limited to cool-white LEDs and 

new cap lamps that functioned at optimal performance at 

the power levels for a fully charged battery. A larger and 

more detailed study is needed to explore discomfort and 

disability glare with regard to warm-white and neutralwhite LEDs. It would be expected that warm-white LEDs 

would result in glare findings similar to the incandescent 

cap lamp. Another study would also be needed to determine the effects of battery discharge over a typical mine 

shift. It would be expected that the glare fi ndings would 

change because the relative light output and SPDs would 

change as the battery discharged. 

Acknowledgments 

The studies described in this paper were conducted 

with assistance from R. Unger, M. Reyes, and T. Dubaniweicz of NIOSH PRL. M.A. Rossi, R.N., conducted the 

vision testing. 

Disclaimers 

Mention of any company or product does not constitute endorsement by NIOSH. The findings and conclusions in this report are those of the authors and do not 

necessarily represent the views of the National Institute 

for Occupational Safety and Health. 

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