Visual Performance for Incandescent and Solid-State
 

Cap Lamps in an Underground Mining Environment
 John J. Sammarco, Senior Member, IEEE, and Timothy Lutz

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◦ to about 60◦ off axis) 

or on axis (0◦). The objective of this research was to determine 

if there were visual performance improvements when using solidstate cap lamps with light-emitting 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◦ 

off axis. The objects were located near field (1.83 m) and far field 

(3.66 m). Upon presentation of the objects, the 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 were no missed objects when the subjects used 

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

occurrences when the subjects used the incandescent cap lamp. 

The mean detection time when using the incandescent cap lamp 

was 55.3% greater than that of the prototype LED cap lamp and 

43.5% greater than that of the LED cap lamp. It can be inferred 

from these data that the spectral distribution of LED-based cap 

lamps could enable significant visual performance improvements 

as compared to incandescent cap lamps. 

Index Terms—Cap lamps, mine illumination, mine safety, slip 

and fall hazards, visual performance. 

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 hazards, 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 2000–2004 indicate 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, particularly 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. Second, 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 about 60% when overall illumination was increased

from 20 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 Stevenson’s

Commission inquiry of the high incidence of fatalities at

Ontario mines, resulted in new legislation that caused a redesign

National Institute for Occupational Safety and Health

(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 light-emitting diode (LED) lamp with a visible spectrum containing more of the short wavelengths. At daytime and~ 

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.

is that it can increase glare if the illuminance at the eye of

of cap lamps. However, the downside of increasing illuminance
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 short 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 operating life, providing

relatively constant light color despite decreasing battery power,

and providing outstanding resistance to shock- or vibrationinduced 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 2700 K or 3000 K. 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 British thermal unit per hour. The

lumen is the unit of luminous flux used by 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

and lux [15], where 1 lx = 1 lm/m2 and 1 fc = 1 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 [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 candela per square meter.

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 nm).

II. METHODS

A. Experimental Design

A within-subjects design was employed in this study. Presentation of cap lamps was randomly assigned to subjects,

and a restricted randomization was used for stimulus order

presentation within cap lamps.

The independent variables were as follows:

1) three light sources: an LED cap lamp, an incandescent

cap lamp, and a prototype LED cap lamp;

2) four object locations: near field, far field, and two combinations of near field and far field.

The dependent variables quantifying visual performance

were as follows:

1) response time to detect objects;

2) 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 particularly 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 that 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 subjects 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 Subjects Review Board. The 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

MIL: The testing was conducted at the Mine Illumination

Laboratory (MIL) of NIOSH. The MIL is a simulated underground coal mine environment that is equipped with various test

equipment, data acquisition and controlsystems, and networked

computers. The interior is 4.88 m wide by 2.13 m high and is

coated with a rough-textured material that has color and reflectivity similar to that of a coal mine. The experimental layout

is shown in Fig. 1. Two location categories were established:

near field (1.83 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- and far-field

object locations were selected based on the risk that they pose

to a miner, and they were chosen given the physiology of the
Fig. 1. Experimental layout.

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. Second, 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 on

axis, 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.

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

density is at 20◦.

Four object location patterns were presented to each subject

as shown in Fig. 2. The presentation order was counterbalanced.

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

designed to allow all human subjectsto be tested at the same eye

height with reference to the floor.

Fig. 2. Four object location patterns.

Fig. 3. Observation station.

The eye height of 165.1 cm

is based on the 50th-percentile male standing [16]. The station

was required to allow test subjects ranging from the 5thpercentile 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–

84.8 cm. The seat was designed to rise 20.3 cm from the lowest

position to the highest to accommodate all test subjects. The

observation station uses an electric actuator for adjusting the

seat height. The height of the miner’s helmet with cap lamp and

ear protection is independently adjustable from the seat height

to accommodate the different torso heights of the subjects. The

helmet height is manually adjustable (up to 25.4 cm) with handoperated 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◦ 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

flat-black 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.

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 

The first was an MSHAapproved cap lamp using a single incandescent bulb as the

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

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 SPD.

Fig. 4 shows the SPDs for each cap lamp. I

Fig. 4. SPDs for each cap lamp.

t 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 green emission of the phosphor; thus, the

combination appears white to the eye.

Objects: The objects were made from sections of polyvinyl

chloride 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

that the object luminance was less than the floor luminance.

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 illuminance 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.

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 1.27 1.08 

4 (near field) 1.30 1.78 1.19 

5 (near field) 1.60 1.82 1.55 

6 (near 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 1.55 

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 hard hat and eye protection. The subjects were

given ample time (about 15 min) 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 that 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.

A. Missed Objects

Subjects were allotted 15 s to detect the objects. Those

objects not found within 15 s were classified as missed. Fig. 5

shows 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◦ 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 statistical significance (p).
Fig. 5. Missed-object location occurrences. No object misses occurred when

subjects used the LED or prototype LED cap lamp.

TABLE III
 

ANOVA SUMMARY
 

Source SS df MS F p 

Subject 3.05x108 8 3.814x107 

Cap lamp 1.128x108 2 5.640x107 5.04 0.0201* 

Subject x cap 

lamp 

1.79x108 16 1.119x107 

Object pattern 1.85x108 3 6.168x107 6.82 0.0004* 

Cap lamp x 

Object pattern 

9.134x107 6 1.523 x107 1.68 0.1378 

Subject x Cap 

lamp x Object 

pattern 

6.33x108 70 9049254 

*p<0.05 

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 shows the Bonferroni post hoc results for

the cap lamps, and Fig. 7 shows the results for the object

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

the standard error.

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

lamp.

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

location pattern.

The mean detection time of 6.7 s was the longest when

subjects used the incandescent cap lamp. This time was 55.3%

greater than that when subjects used the prototype LED cap

lamp. This difference was statistically significant; however,

no significant differences existed in a pairwise 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 s 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 particularly significant given that these misses

occurred in the near field. The distance between the subject

and the near-field object was only about two to three 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 particularly 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 illuminance 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 support 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 that of the prototype

LED cap lamp and 43.5% greater than that of the LED cap
lamp. The prototype LED cap lamp’s mean detection time was

faster than that of 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 that of 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 in the

near field and far field. The visual task consisted of multiple

components that included visual search of the forward field of

view, near-field 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.

ACKNOWLEDGMENT

The studies were conducted with assistance from J. Bartels,

T. Dubaniweicz, S. Gallagher, E. Rossi, and M. E. Nelson of the

National Institute for Occupational Safety and Health (NIOSH).

Vision testing was conducted by M. A. Rossi R.N.

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 NIOSH.

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John J. Sammarco (SM’91) received the B.S. degree in electrical engineering technology from The

Pennsylvania State University, University Park, in

1978, the M.S. degree in industrial engineering from

the University of Pittsburgh, Pittsburgh, PA, in 1986,

the M.S. degree in computer engineering from the

National Technical University, Fort Collins, CO, in

1986, and the Ph.D. degree in computer engineering from West Virginia University, Morgantown,

in 2003.

He is currently a Senior Research Engineer with

the National Institute for Occupational Safety and Health, Bruceton Research

Center, Pittsburgh. His federal service has been devoted to mine safety research

in a wide variety of areas, including mining machine navigation and guidance, control systems, and sensors. His recent research areas concern system

safety, hazard/risk analysis of processor-controlled mining systems, smart

wireless sensors, visual performance, and solid-state lighting systems for mine

applications.

Timothy Lutz received the B.S. degree in mechanical engineering from The Pennsylvania State University, University Park, in 1983.

He began his mining research career at the U.S.

Bureau of Mines in 1990 in a variety of safety areas.

He is currently a Research Engineer with the Mining Injury Prevention Branch, National Institute for

Occupational Safety and Health, Bruceton Research

Center, Pittsburgh, PA. Since 2005, his research

has included underground mine lighting using lightemitting diode technology which has emerged as an

efficient and durable alternative to existing lighting.

Mr. Lutz is a Registered Professional Engineer in Pennsylvania.