Visual Performance for Incandescent and Solid-State Cap 

Lamps in an Underground Mining Environment
 

John J. Sammarco Ph.D., P.E., Senior Member 

Timothy Lutz P.E., Mechanical Engineer 

National Institute for Occupational Safety and Health
 

626 Cochrans Mill Road, PO Box 18070
 

Pittsburgh, PA 15236
 

Jsammarco@cdc.gov 

Tlutz@cdc.gov
 

Abstract— Miners depend most heavily on visual cues to 

recognize underground mining hazards; consequently,

illumination plays a critical role in miners’ safety. Some hazards 

are located in the miners’ peripheral field-of-view (10 degrees to 

about 60 degrees off-axis) or on-axis (0 degrees). The objective of 

this research was to determine if there were visual performance 

improvements when using solid-state cap lamps with lightemitting diodes (LEDs) as compared to incandescent light bulbs 

commonly used in miner cap lamps. Recent research has 

indicated that an increased short-wavelength content of the 

spectral power distribution of LEDs relative to incandescent 

lamps improves peripheral visual performance for low-light 

(mesopic) conditions. The visual performances of nine subjects 

were quantified by measuring the subjects’ speed and accuracy in 

detecting floor objects located on axis and at ±20 degrees off axis. 

The objects were located near field (1.83 meters) and far field 

(3.66 meters). Upon presentation of the objects, subjects would 

count and point to each object using a red-laser pointer. The 

object detection response time and number of missed objects 

were recorded. The results of the visual performance comparison 

for an LED, a prototype LED, and an incandescent cap lamp are 

presented. There we no missed objects when subjects used the 

LED-based cap lamps but, there were three missed object 

occurrences when subjects used the incandescent cap lamp. The 

mean detection time when using the incandescent cap lamp was 

55.3% greater than the prototype LED cap lamp and 43.5% 

greater than the LED cap lamp. It can be inferred from this data 

that the spectral distribution of LED-based cap lamps could 

enable significant visual performance improvements as compared 

to incandescent cap lamps1

. 

Keywords- mine illumination; visual performance; slip and fall 

hazards; cap lamps; mine safety 

I. INTRODUCTION 

An underground mine is the most difficult environment to 

illuminate according to the Illuminating Engineering Society of 

North America (IESNA) [1]. It is a dynamic environment that 

includes dust, confined spaces, low reflective surfaces, and low 

visual contrasts. Lighting is critical to miners since they depend 

heavily on visual cues to spot fall of ground, pinning and 

striking, and slipping and tripping hazards [2]; consequently, 

illumination greatly affects miners’ ability to perform their jobs 

safely. Typically, a miner’s cap lamp is the primary and most 

important source of light [3]. 

It can be reasoned that the quality of underground mine 

illumination is problematic based on Mine Safety and Health 

Administration (MSHA) accident data, an aging workforce, 

and recent assessments of mine illumination. First, MSHA 

injury data for 2000-04 indicates that there are a high number 

of accidents involving falls of ground, slips and falls, and 

powered haulage. These accident categories rank within the top 

four highest rates of lost-workday injuries. Slips and trips have 

the second highest rate of 1.14 injuries per 100, full-time 

equivalent workers. Lighting, especially from a cap lamp, plays 

a critical role for miners as they visually inspect the mine roof, 

ribs, back and floor for hazards. Cap lamp lighting is critical 

for inspecting the mine floor for slip, trip, and fall hazards. 

Objects associated with these hazards are typically of very low 

contrast and reflectivity. Secondly, there are age-related factors 

that require a better quality of light. Diminished night vision is 

one of the most common problems experienced by older people 

because there are changes in the eye that include decreased 

pupil size, cloudier lens, and fewer rod photoreceptors that are 

very sensitive to light [4]. The average age of a U.S. miner is 

about 43 years. Lastly, researchers from Canada [3], Australia 

[5], India [6], and South Africa [7] [8] have conducted 

assessments of mine illumination and identified the need to 

improve the quality of illumination to improve safety. 

Prior lighting research approaches have focused on 

increasing illumination to improve safety. Mine accident rates 

decreased to about 60% when overall illumination was 

increased from 20 lx to 250 lx where 20 lx served as the 

statistical base [9]. Canadian researchers noted dramatic 

increases in the ability of miners to see loose rock as the cap 

lamp illuminance increased [3]. The Canadian study, prompted 

by the Stevenson’s Commission inquiry of the high incidence 

of fatalities at Ontario mines, resulted in new legislation that 

caused a redesign of cap lamps. However, the downside of 

increasing illuminance is that it can increase glare if the 

illuminance at the eye of other people increases. The level of 

illuminance is a dominant factor in glare produced by an 

oncoming headlamp [10]. Also, increasing the illuminance 
 

increases cap lamp battery loading; thus, requiring larger 

battery capacities. 

NIOSH research investigates a different approach that does 

not increase illuminance to improve visual performance. Our 

research objective was to determine if visual performance, with 

respect to slip/trip/fall hazard detection, could be improved by 

using a LED lamp with a visible spectrum containing more of 

the shorter wavelengths. At daytime and interior light levels, 

the cone photoreceptors of the eye dominate vision, but as light 

levels decrease, the rod receptors of the eye that have greater 

short-wavelength spectral sensitivity than cones, play an 

increasing role in vision. Cool white LEDs emit a “whiter” 

light with a greater proportion of short-wavelength energy in 

comparison to the yellowish light of incandescent lighting 

commonly used for miner cap lamps. Lighting research 

indicates that at low-light (mesopic) conditions where rods and 

cones both contribute to vision, the spectral content can 

improve peripheral visual performance [11]. LEDs with a 

visible spectrum containing more of the shorter wavelengths 

can enable considerable improvement in peripheral visual 

performance at mesopic conditions for automotive applications 

[12]. However, research indicates that for perturbed 

environments (i.e. dust, snow, and fog) there is not a 

statistically significant difference in visual performance when 

the light is white rather than yellowish [13]. 

The use of white LED technology for cap lamps has many 

other advantages such as requiring about half the battery 

power, having more than 25 times the operating life, providing 

relatively constant light color despite decreasing battery power, 

and providing outstanding resistance to shock or vibration 

induced failures because there is not a filament that can break 

[14]. Currently, incandescent bulbs are commonly used for cap 

lamps. This lighting is characterized by a yellowish light that 

becomes more yellow as the luminaire ages and as the source 

voltage decreases, as in the case of a battery powered cap lamp. 

The positive aspects of LED technology are outside the scope 

of this paper although they are likely to have numerous safety 

benefits. 

It is important to note that there is a wide range of “white” 

available for LEDs. Warm white LEDs are similar to 

incandescent lamps where the correlated color temperature 

(CCT) is 2700K or 3000K. Cool white LEDs have a CCT 

higher than 3500 K. 

A.	 Terminology 

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 flux used by the IESNA and the 

International System of Units (SI) [15]. 

Illuminance – The measure of the density of luminous flux 

striking a surface. The IESNA and SI units are footcandle (fc) 

and lux (lx) [15] where lx = lm/m2 and fc = lm/ft2. 

Luminous intensity– The term describes how a light source 

distributes the total luminous flux, or lumens, it emits into 

various portions of the space surrounding the light source. The 

IESNA and SI unit is the candela (cd) [15]. 

Luminance – In physical terms, luminance is a concept used 

to quantify the density of luminous flux emitted by an area of a 

light source in a particular direction toward a light receiver 

such as a human eye [15]. Luminance is closely correlated with 

a person’s perception of brightness. The IESNA and SI unit is 

cd/m2

. 

Luminance contrast – The relationship between luminance 

of an object and its immediate background [1] 

Spectral power distribution (SPD) – The radiant power 

emitted at each wavelength in the visible region of light (360­

770 nanometers). 

II. METHODS 

A. Experimental Design 

A within-subjects design was employed in this study. 

Presentation of cap lamps were randomly assigned to subjects 

and a restricted randomization was used for stimulus order 

presentation within cap lamps. 

The independent variables were: 

•	 3 Light sources: LED cap lamp, incandescent cap 

lamp, and a prototype LED cap lamp; 

•	 4 Object locations: near field, far field, and two 

combinations of near field and far field. 

The dependent variables quantifying visual performance 

were: 

•	 Response time to detect objects; 

•	 Number of objects missed (not detected). 

B. Subjects 

NIOSH personnel from the Pittsburgh Research Laboratory 

(PRL) were the subjects. Miners were not used because of 

potential expectancy biases that could confound empirical data. 

Expectancy biases are especially challenging for lighting 

research because the variable of study (visible light) is usually 

observable to the subjects. Miners could easily detect that the 

light from the LED-based cap lamps is very different from the 

camp lamp light they use on a daily basis. 

NIOSH subjects that passed vision tests for distance visual 

acuity, contrast sensitivity, color vision deficiency, and 

peripheral vision were accepted for the study. Eight male and 

one female subject participated. The average age was 47 years 

which is comparable to the average U.S. coal miner age of 43 

years. 

The protocol for this study was approved by the NIOSH 

Human Subject Review Board. Subjects signed an informed 

consent form and were instructed about their right to withdraw 

freely from the research at any time without penalty. 

C. 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 

4.88 meters (m) wide by 2.13 m high and is coated with a 

rough-textured material that has a color and reflectivity similar 

to that of a coal mine. The experimental layout is depicted by 

Fig. 1. Two location categories were established: near-field 

1.83 meters (m) which is about the distance of two strides for 

the average male and far-field which is at a distance that the 

floor receives most of the cap lamp light due to the cap lamp 

mounting angle on the miner’s helmet. Near-field and far-field 

object locations were selected based on the risk they pose to a 

miner, and they were chosen given the physiology of the eye. 

The on-axis location was selected because it poses the greatest 

risk for a trip/slip/fall given that it is directly in the path of a 

walking person. Secondly, the on-axis location assumes that 

foveal vision is employed to produce a person’s visual 

perception. When a person’s eyes are directed to an image onaxis, the image is projected onto the foveal area of the retina. 

This area is filled with cone photoreceptors that play a critical 

role for color. The rod photoreceptors are well-suited for very 

dark lighting conditions but do not detect color or fine detail. 

Figure 1. The four object location patterns. 

Figure 2. The experimental layout. 

The highest density of rods is between 10 and 30 degrees; the 

peak density is at 20 degrees. 

Four object location patterns were presented to each subject 

as depicted by Fig. 2. The presentation order was 

counterbalanced. 

Observation Station: The observation station (Fig. 3) was 

designed to allow all human subjects to be tested at the same 

eye height with reference to the floor. The eye height of 165.1 

centimeters (cm) is based on the 50th percentile male standing 

[16]. 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 eye height when in the seated 

position. Torso heights for the specified test subjects have a 

range of 68.6 cm to 84.8 cm. The seat was designed to rise 20 

cm 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 cm of travel. The helmet height is manually 

adjustable (up to 25.4 cm) with hand operated clamps. The 

helmet adjusts fore and aft manually up to 15.2 cm. The seat is 

adjustable fore and aft and has foldable arm rests. A swivel was 

incorporated in the station to allow the seat to rotate 180 

degrees to conduct two different tests in the MIL without 

moving the station. There is a fixed foot rest at each end of the 

platform. Three tabs are located on the helmet post to mount 

cap lamps for easy changes during the tests. 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 flatblack color to help eliminate any reflections or distractions 

during testing. 

Cap lamps: Three cap lamps were used. The electrical data 

for each cap lamp are listed in Table I. The first was a MSHAapproved cap lamp using a single incandescent bulb as the 

primary light source. This served as the reference. The second 

Figure 3. The observation station. 
TABLE I. CAP LAMP ELECTRICAL DATA 

Cap Lamp Battery supply 

voltage 

 (Vdc) 

Current 

(amps) 

Power 

(watts) 

Incandescent 6.1 0.63 3.84 

LED 6.1 0.42 2.56 

Prototype LED 12.0 0.113 1.56 

Figure 4. The spectral power distributions for each cap lamp. 

was an MSHA-approved cap lamp with a single phosphorwhite LED as the primary light source. The third cap lamp was 

a laboratory prototype that was jointly developed by NIOSH 

and the Lighting Research Center of Rensselaer Polytechnic 

Institute. This prototype uses two phosphor-white LEDs as the 

primary light source. The prototype LED cap lamp meets the 

photometric requirements specified by MSHA [17]. The cap 

lamp housing and internal components were selected to have a 

very high reflectance to mitigate discomfort glare. 

Each cap lamp was characterized with respect to its spectral 

power distribution (SPD). Fig. 4 depicts the SPDs for each cap 

lamp. It is evident that the LED and prototype LED cap lamps 

have a greater proportion of short-wavelength light than the 

incandescent cap lamp. Both cap lamps use a phosphor-white 

type of LED. Basically, this type is a blue LED with phosphor 

deposited on top of the die. The phosphor passes a portion of 

the blue light that mixes with the yellowish-greenish emission 

of the phosphor; thus, the combination appears white to the 

eye. 

Objects: The objects were made from sections of PVC 

electrical conduit. Each object was 6.4 cm long with an outer 

diameter of 3.3 cm and an inner diameter of 2.2 cm. The 

objects were painted a dark color such that they would have a 

very low contrast and a reflectivity very similar to an object 

(mine cable, pipe, or tool) that was coated with the material on 

the mine floor. This choice also enabled us to easily have a 

consistent set of test objects with respect to size, shape, 

contrast, reflectance, and shadow rendering. 

The luminance contrast of the objects ranged from -0.11 to 

0.09 with respect to the floor. The negative contrast indicates 

the object luminance was less than floor luminance. 

TABLE II. CAP LAMP ILLUMINANCE AT THE OBJECT LOCATIONS 

Object location Incandescent 

(lux) 

LED 

(lux) 

Prototype LED 

(lux) 

1 (near field) 1.29 1.19 1.14 

2 (near field) 1.41 1.27 1.31 

3 (near field) 1.15 1.07 1.08 

4 (far field) 1.30 1.78 1.19 

5 (far field) 1.60 1.82 1.55 

6 (far field) 0.92 0.88 1.05 

Average (near field) 1.28 1.17 1.17 

Average (far field) 1.27 1.49 1.26 

Average (all) 1.28 1.34 1.22 

Illuminance distributions: The illuminance levels were 

measured at each object location (Fig. 1) for each cap lamp. 

Illuminance is an important factor for visual performance 

because, in general, visual performance increases as 

illuminance increases. In this study, it was important that the 

illuminance distributions be comparable otherwise major 

illuminance differences would be a confounding factor. The 

initial illuminance distributions had significant variation. For 

instance, the illuminance at object location 6 was 42% greater 

with the prototype LED cap lamp than the incandescent cap 

lamp. Therefore, a diffusion filter was placed over the lens of 

each cap lamp. This filter was successfully used to realize a 

more uniform illuminance distribution among the three cap 

lamps. Table II lists the vertical illuminace distributions for all 

object locations and cap lamps with diffusion filters. The 

illuminance measurements were taken perpendicular to the line 

of view of the subjects. 

D. Procedure 

The subjects were seated on the observation station and 

adjustments were made such that each person had the same eye 

height of 165.1 cm from the floor. While seated, the subjects 

wore a hardhat and eye protection. The subjects were given 

ample time (about 15 minutes) to adjust to the darkened 

environment. 

A practice (warm-up) session was initially conducted to 

help subjects learn how to conduct the tests and to become 

familiar and comfortable with the test apparatus. The procedure 

was to close a black, electrically powered curtain located 0.91 

m in front of the subject. When the subject was ready, the 

curtain was opened and the PC-based data acquisition started 

recording time. The subject identified the objects by counting 

each out loud while pointing to it with a red-laser pointer that 

was fitted with an image filter that resulted in the image of a 

hand to appear on the object. This image was about 5.1 cm 

high and made it easier for the subject to “hit” the object. Two 

researchers determined an object was detected once they saw 

the red laser-pointer light on the target. The curtain was closed 

and the time was recorded when the subject detected the last 

object. 

III. RESULTS 

The visual performance results were objectively measured 

in terms of missed objects and detection time. Twelve data 

points were collected for each of the nine subjects. 
 

 

 

 

TABLE III. ANOVA SUMMARY 

Source SS df MS F p 

Subject 3.051x108

 8 3.814x107 

Cap lamp 1.128x108 2 5.640x107 5.04 0.0201* 

Subject x Cap 1.791x108 16 1.119x107 

lamp 

Object pattern 1.850x108

 3 6.168x107 6.82 0.0004* 

Cap lamp x Object 9.143x107 6 1.523x107 1.68 0.1378 

pattern 

Subject x Cap 6.334x108

 70 9049254 

lamp x Object 

pattern 

*

p<0.05 

Figure 5. Missed object location occurrences. No object misses occurred 

when subjects used the LED or prototype LED cap lamps. 

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Object pattern 

Figure 7. Bonferroni post hoc results for the mean detection time for each 

object location pattern. 

A. Missed objects 

Subjects were allotted 15 seconds to detect the objects. 

Those objects not found within 15 seconds were classified as 

missed. Fig. 5 depicts the number of missed objects for all cap 

lamps and all object locations. There were no occurrences of 

missed objects for the LED or prototype LED cap lamp. There 

were three occurrences of missed objects for the incandescent 

cap lamp. Each miss occurred for object location 2 located at 0 

degrees in the near field. 

B. Object Detection Time 

An analysis of variance (ANOVA) was used to evaluate 

whether there were significant differences for the independent 

variables. Table III summarizes the ANOVA results that 

include degrees of freedom (df), sums of squares (SS), mean 

square (MS), F-ratio (F value), and the statistical significance 

(p). 

Two main effects were considered significant (p<0.05): the 

cap lamps and the object patterns. A Bonferroni post hoc 

analysis was conducted at α = 0.05 level for the significant 

main effects. Fig. 6 depicts the Bonferroni post-hoc results for 

the cap lamps and Fig. 7 depicts the results for the object 

patterns. The y-error bars above and below the means indicate 

the standard error. 

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7 

neco6 se (m5 i tno4 iect 3 et d2 eanM

1

Incandescent 

LED 

Prototype LED 1 

0 

Cap lamp type 

Figure 6. Bonferroni post hoc results for the mean detection time for each 

cap lamp. 



The mean detection time of 6.7 seconds was the longest 

when subjects used the incandescent cap lamp. This time was 

55.3% greater than when subjects used the prototype LED cap 

lamp. This difference was statistically significant; however, no 

significant differences existed in a pair-wise comparison 

between the LED and prototype LED cap lamps. There was a 

difference between the incandescent and LED cap lamps; 

however, it did not approach statistical significance (p < 0.05). 

A significant difference existed among object patterns. The 

mean detection time was 6.4 seconds for object pattern 1. This 

time was 113.3% slower than the mean detection time for 

object pattern 4, which consisted of two objects in the far field. 

The means for object patterns 1, 2, and 3 were found not to be 

statistically different. 

No significant (p< 0.05) interaction existed between the cap 

lamp and object pattern factors. 

IV. DISCUSSION 

Our research objective was to determine if LED technology 

could be used to improve visual performance with respect to 

slip/trip/fall hazard detection. The missed object and detection 

time data seem to support that the higher content of shorter 

wavelengths as found with cool white LEDs could offer 

significant visual performance improvements. 
 

 

 

 

 

 

 

 

 

In terms of missed objects, the LED-based cap lamps had 

no missed object occurrences; however, the incandescent cap 

lamp had three occurrences, all of which were object location 2 

(on axis). It is especially significant given these misses 

occurred in the near field. The distance between the subject and 

near field object was only about 2 to 3 walking strides away; 

thus, this object location poses the highest safety risk of all 

locations. It is noted that the on-axis object location is 

especially challenging given foveal vision limitations in low 

light. The object illuminance does not seem to be a factor 

because the illuminance from the LED cap lamp was 11% less 

and the prototype LED cap lamp was 8% less than the 

illuminance from the incandescent cap lamp. The other on axis 

location was in the far field at object location 5. No objects at 

this location were missed for the incandescent cap lamp. This 

far-field location is a more common visual attention location 

for a walking person and this could be one reason why objects 

at this location were not missed; however, it is noted that the 

incandescent cap lamp illuminance was 14% higher at this 

location. 

The object detection time data also supports that LED 

technology is likely to enable significant improvements in 

visual performance. The mean detection time when using the 

incandescent cap lamp was 55.3% greater than the prototype 

LED cap lamp and 43.5% greater than the LED cap lamp. The 

prototype LED cap lamp’s mean detection time was faster than 

the LED cap lamp but these differences did not have statistical 

significance given the data from nine subjects. The LED cap 

lamp SPD (Fig. 4) had proportionately more longer 

wavelengths resulting in a “warm white” light compared to the 

prototype LED which is a “cool white” light with 

proportionately more shorter wavelengths. A larger and more 

detailed study is needed to explore visual performance with 

regard to warm white and cool white LEDs. 

The visual performance testing was conducted in terms of 

on-axis and peripheral visual tasks for objects located near field 

and far field. The visual task consisted of multiple components 

that included visual search of the forward field of view, nearfield detection, far-field detection, and the combined near and 

far field detection. Therefore, the visual performance was an 

aggregate of all visual task components. A more detailed study 

is needed to determine the on-axis and peripheral visual 

performances separately. 

It is important to note that all cap lamps were powered by 

laboratory grade power supplies so that the cap lamp power 

remained constant. The power levels were set to that of a fully 

charged battery, this is a best-case scenario. The SPD of the 

incandescent cap lamp would significantly shift to longer 

wavelengths (reds and yellows) as the battery power decreased 

during the mine shift while the SPDs of the LED-based cap 

lamps would be largely unchanged; hence, we would anticipate 

a greater decline in visual performance for the incandescent cap 

lamp in comparison to the LED cap lamps. 

The results of the visual performance comparisons between 

LED and incandescent cap lamps provide important data for 

improving the design of future cap lamps and should positively 

affect the safety of employees in the underground mining 

industry. It is apparent that LED technology has the potential to 

improve safety by improving visual performance with respect 

to detecting slip, trip and fall hazards. 

ACKNOWLEDGEMENTS 

The studies described in this paper were conducted with 

assistance from J. Bartels, T. Dubaniweicz, S. Gallagher, E.W. 

Rossi, and M.E. Nelson of NIOSH PRL. Vision testing was 

conducted by M. A. Rossi R.N. 

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