Technological Aspects of Solid-State and 

Incandescent Sources for Miner Cap Lamps 

J. J. Sammarco, Ph.D., Senior Member 

M.A. Reyes 

National Institute for Occupational Safety and Health
 

Pittsburgh Research Laboratory
 

Pittsburgh, PA 

jsammarco@cdc.gov
 

J. P.Freyssinier 

J. D. Bullough, Ph.D. 

X. Zhang 

Lighting Research Center 

Rensselaer Polytechnic Institute 

Troy, NY 12180 

Abstract— Light emitting diodes (LEDs) are emerging as viable 

replacements for incandescent-based cap lamps used in 

mining. The photometric and energy characteristics of these 

light sources differ in important ways. The present paper 

describes the performance of LED and incandescent sources in 

cap lamps in terms of correlated color temperature, color 

rendering, light output, electric power, ambient temperature 

and airflow, and light source aging. Importantly, these 

characteristics can influence a miner’s ability to spot mining 

hazards thus impacting safety. Secondly, some of these 

characteristics interact with the operating life of the cap lamp’s 

battery power, such that differences between LED and 

incandescent sources can be magnified toward the end of a 10­

hour battery discharge cycle. Empirically, we have determined 

that after 8 hours at an ambient temperature of 25 °C, the 

average light output of an incandescent cap lamp can decrease 

to about 69% of its initial value when powered by a lead-acid 

battery and it can decrease to about 65% of its initial value 

when powered by a nickel-hydride battery. An LED-based cap 

lamp using a constant current drive circuit can maintain about 

96% of its initial value when powered by a nickel-hydride 

battery. Real-world tests addressing the effects of ambient 

temperature and airflow on the light output of an LED and 

incandescent cap lamp were conducted in the NIOSH Safety 

Research Coal Mine (SRCM). The LED cap lamp yielded a 

vertical average illuminance improvement of approximately 

9.5% and the INC cap lamp yielded a vertical average 

illuminance degradation of approximately 4%. The differences 

between LED and incandescent cap lamps were further 

quantified by the calculation of “mesopic luminance” data that 

indicated for the same photopic luminance (i.e., as measured 

using a conventional light meter) the LED cap lamp could be 

up to 38 % more efficient than the incandescent cap lamp with 

a lead-acid battery at the end of the 10-hour driving cycle. 

Lastly, accelerated life tests were used to empirically determine 

light output depreciation as the incandescent light source age 

approached its useful life. There was about a 35% decrease in 

light output. This is quite considerable, especially given that 

the light output will decrease an additional 30% to 45% over 

the period of a 10-hour shift. The implications of the 

differences between LED and incandescent sources are 

discussed. This information is crucial in determining how 

visual performance could be affected for real-world conditions 

where batteries discharge during the work shift and as the 

light source ages. To date, only idealized conditions have been 

used for LED and incandescent cap lamp visual performance 

research. 

I. INTRODUCTION 

The National Institute for Occupational Safety and 

Health (NIOSH) Pittsburgh Research Laboratory (PRL) has 

initiated a project to improve mine illumination such that a 

miner’s visual performance improves to better recognize 

mine hazards. The project’s main objective is to determine if 

solid-state lighting technology (i.e., light emitting diodes 

(LEDs)) enables visual performance improvements as 

compared to traditional mine illumination technology (i.e., 

incandescent (INC) lamps). A miner’s visual performance 

is critical for spotting mine hazards [1]. Typically, 80% of 

our perception is obtained visually. Human process visual 

data at about four times the rate of audible information. For 

mining, important audible data (i.e., verbal communications 

and audible warning alarms) are less useful given the noisy 

mine environment. 

Prior cap lamp research aimed to improve safety by 

increasing the light output as defined by quantifying 

illuminance. Canadian researchers noted dramatic increases 

in the ability of miners to see loose rock as illuminance 

increased from 500 to 1500 lux [2]. Indian researchers 

conducted a comparative study of cap lamp light output 

verses battery discharge for various battery capacities. The 

light output increased 20% to 22% during the period of a 

working shift when batteries increased 35% from 10 ampere 

hours (Ah) to 13.5 Ah [3]. 

Other research has focused on the spectral 

characteristics of light. Lighting research indicates that 

LEDs with a visible spectrum containing more of the shorter 

wavelengths can enable significant improvement in 

peripheral visual performance at mesopic conditions for 

automotive applications [4]. Recent NIOSH mining research 

findings indicated that, in comparison to INC lighting for 

cap lamps, cool-white LEDs do enhance peripheral visual 

performance from 15% to 20% for high-contrast targets [5]. 

Cool white LED-based miner cap lamps can also enable 

visual performance improvements with respect to 

slip/trip/fall hazard detection [6]. It was inferred in these 
 

 

 

 

studies that the light source’s spectral distribution was a 

statistically significant main effect. These NIOSH studies 

focused on a miner’s cap lamp because this is a miner’s 

primary light source [2]. 

The LED and INC cap lamps used by NIOSH 

researchers were operated at optimal conditions [5] [6]. New 

cool-white LEDs and new INC bulbs were used, and the cap 

lamps were powered at the levels for a fully charged, new 

cap lamp battery. It would be expected that the visual 

performance improvements enabled by the LED cap lamps 

would be even greater under real-world conditions. 

The present paper addresses the real-world mining 

conditions for INC and LED cap lamps. Presented are the 

results of an analytical study conducted to understand and 

quantify the photometric and spectral characteristics of INC 

and LED light sources used in miner cap lamps as a function 

of battery discharge (i.e., over the length of a day’s work 

shift), ambient temperature and airflow, and as the light 

source ages. This information is crucial in determining how 

visual performance could be affected for real-world 

conditions that would be encountered during a work shift. 

II. METHODS 

We tested the relative lumen (lm) maintenance curve 

and spectral power distribution (SPD) of three cap lamp 

systems: 1) LED cap lamp with an internal heat sink and a 6 

volt (V), 8 Ah nickel-hydride battery, 2) INC cap lamp with 

a 4 V, 13 Ah lead-acid battery; and 3) INC cap lamp with a 

6 V, 8 Ah nickel-hydride battery. The light sources of each 

system were new at the time of test. Each system was tested 

for two discharge cycles. Each discharge cycle lasted 10 

hours without interruption during the discharge period. An 

automated, computer controlled data acquisition system was 

set up for the purpose of this test and collected data every 

minute. A photometrically calibrated photosensor measured 

the relative light output of each cap lamp for the duration of 

each test. A three-channel power analyzer measured the 

electric current, voltage, and power of the light source in 

each system. After the second discharge cycle was finished, 

the data between the two cycles were compared. Since the 

difference between the two cycles in all cases was less than 

5%, the data were averaged for the purpose of characterizing 

each system photometrically. A 152.4 cm photometrically 

calibrated integrating sphere was used to measure the 

absolute light output and spectral power distribution of each 

system. The averaged electric current and voltage were used 

to power each system under four conditions corresponding 

to 0, 10, 300, and 600 minutes of battery discharge time. 

To understand the changes in spectral composition of 

INC lamps over time, ten 6 V lamps were subjected to an 

accelerated life test. For traditional light sources, including 

INC lamps, life is defined as the median time to failure of a 

large group of samples under controlled conditions. 

Accordingly, for this test, “life” was defined as the time to 

failure of the first five lamps. To reduce the time needed to 

complete the test the lamps were operated at 7.2 V, which is 

20 % higher than nominal voltage. By using empirical 

relationships between the operating voltage and life [8], the 

expected life of the samples could be estimated if needed. 

However, in this case knowing the actual lamp life in hours 

was not as important as analyzing the spectral shifts at the 

end of life. In order to characterize spectral changes, the 

initial light output and spectral power distribution of two 

new lamps were measured before the life test. After the life 

test, two out of the five remaining lamps were tested for 

light output and spectral power distribution. Because of the 

small differences in light output (less than 4 %) and spectral 

composition expected among the ten test lamps, it was not 

necessary to characterize all lamps at the beginning of the 

life test. 

Real-world tests addressing the effects of ambient 

temperature and airflow on the light output of an LED and 

INC cap lamp were conducted in the PRL Safety Research 

Coal Mine (SRCM). The cap lamps were powered by fully 

charged Ni-MH battery for the purpose of replicating typical 

cap lamp use. The cap lamps remained on until the 

temperature stabilized as measured by thermocouples 

positioned internal (near the LED) and external to the cap 

lamp. Positioning of the external thermocouples was 

dictated by the hotspots identified using a thermal imaging 

camera. Figure 1 depicts a thermal image of the LED cap 

lamp given an ambient temperature of 23.8 °C and 

negligible air flow. The thermal image depicts a maximum 

surface temperature of 44.8°C. Temperature and light 

output measurements were recorded at 15 minute intervals 

for the duration of the evaluation period. 

Figure 1. Thermal image of an LED cap lamp at an ambient 

temperature of 23.8 °C and negligible air flow. 
 

 

 

 

 

III. RESULTS AND DISCUSSION 

Figure 2 shows the relative, photopic light output over 

the 10-hour discharge period for each system. As evidenced 

in Figure 2, the INC systems show a relatively sharp drop in 

light output after just a few minutes, particularly with the 

lead acid battery system. The INC-based cap lamp systems 

tested do not have any regulation circuitry, thus the light 

output of the lamps follow the discharge characteristic of the 

battery used in each case. By the end of the 10-hour cycle, 

the light output levels of the INC systems were 

approximately 65 % (nickel-hydride battery) and 56 % 

(lead-acid battery) of the initial value. In contrast, the LED 

based system tested uses a current regulating device that 

compensates for the voltage drop of the battery, resulting in 

a light output drop of only 4 % over the 10-hour period. 

This 4% light output drop can be attributed to a typical light 

reduction experienced as the LED junction temperature (Tj) 

increases, rather than a light output reduction as a function 

of battery voltage drain. Once the thermal equilibrium is 

reached, an LED will experience a relative stable photopic 

light output % over the 10 hour period. 

A second effect of the lack of regulation in the INC 

systems is the change of spectral composition in the light 

over the test period. At lower operating voltages, the 

spectral power distribution of INC lamps shifts toward 

increased long visible ("yellow") wavelengths that could be 

considered detrimental to peripheral vision at mesopic light 

levels [8]. 

Figure 2. Average relative, photopic, light output of each system tested over a 10-hour discharge period. 

As the light output of the cap lamp drops, the adaptation 

level of the miner will shift toward the shorter wavelengths, 

i.e., toward a scotopic adaptation. At these lower light levels 

(approximately 1 cd/m2

 and lower), the short wavelength 

content of the light source becomes more useful than the 

longer wavelengths. In the case of INC lamps, as the voltage 

drops the content of short wavelength decreases, resulting in 

potentially lower visibility. It is worth emphasizing that the 

LED discharge curve of Figure 2 could be representative of 

the tested system only because there are different current 

limiting methods that could be used in commercial systems 

to achieve lower cost or intrinsic safety approvals. For 

example, it would be reasonable to expect that an LED 

system regulated with a simple resistor would behave 

similarly to an INC based system. Such systems would need 

to be evaluated on a one by one basis in order to compare 

their relative benefits to INC systems. Although there are 

some benefits of using a resistor to regulate the current of 

LED cap lamps, there are significant disadvantages with 

respect to light output and the potential for thermal runaway 

conditions that could greatly reduce light output or result in 

catastrophic failure of the LED cap lamp. 

Table I summarizes the electrical and photometric 

characteristics of the LED based cap lamp and the INC with 

lead-acid battery cap lamp at the four conditions tested (0, 

10, 300, and 600 minutes). Table II shows for reference only 

the photometric characteristics of the LED based prototype 

cap lamp developed during the 2006 joint research efforts of 

LRC and NIOSH. 
 

 

 

 

 

 

 

 

 

 

 

 

TABLE I. ELECTRIC AND PHOTOMETRIC CHARACTERISTICS OF AN LED SYSTEM WITH A NICKEL-HYDRIDE BATTERY AND AN INC SYSTEM WITH 

LEAD-ACID BATTERY AT 25°C.
 

Source

Time 

 Electric characteristics Photometric characteristics 

Voltage 

(V) 

Current 

(A) 

Power 

(W) 

Luminous flux 

(lm) 

Efficacy 

(lm/W) 

CIE 1931 chromaticity 

(x,y) 

Correlated color 

temperature (K) 

General color 

rendering 

index (Ra) 

LED 

0 min 6.82 0.365 2.49 36 14.5 0.3252 0.3283 5851 73 

10 min 6.66 0.375 2.50 35.7 14.3 0.3251 0.3281 5857 74 

300 min 5.99 0.424 2.54 35.2 13.9 0.3245 0.3272 5890 74 

600 min 5.74 0.444 2.55 35.0 13.7 0.3243 0.3269 5905 74 

INC 

0 min 3.39 1.088 3.69 38.7 10.5 0.4411 0.4067 2951 100 

10 min 4.28 1.055 4.52 30.8 6.8 0.4463 0.4082 2880 100 

300 min 4.13 1.04 4.3 27.7 6.4 0.4489 0.4089 2847 100 

600 min 4.03 1.025 4.13 24.9 6.0 0.4513 0.4094 2815 100 

TABLE II. ELECTRIC AND PHOTOMETRIC CHARACTERISTICS OF THE LED BASED PROTOTYPE CAP LAMP DEVELOPED BY LRC DURING 2006. 

Source 

Electric characteristics Photometric characteristics 

Current (A) Power 

(W) 

Luminous 

flux (lm) 

LED efficacy 

(lm/W; does not 

include LED 

driver) 

Average 

CIE 1931 

chromaticity (x,y) 

Average 

correlated 

color 

temperature 

(K) 

General color 

rendering 

index (Ra) 

LED Main plus 

peripheral beam 

0.350 3.75 155.3 41.4 

0.3134 0.3290 6490 82 

0.700 8.24 255.2 31.0 

All of these measurements were taken at room 

temperature (nominally 25°C) with negligible airflow. All 

LED technologies have different sensitivities to 

temperature, both in the short and the long term. Thus, 

junction temperature (Tj) is usually the main determinant of 

life, light output, power, and color shift of LEDs. Note that 

Tj is partially determined by the forward current and 

partially by the ambient temperature and airflow of the 

environment in which the LED is operating. Measurements 

for an INC cap lamp over a wide ambient temperature of 

25°C ±10°C will yield relatively consistent results over this 

temperature range because for incandescent lamps the effect 

of ambient temperature is limited. However, ambient 

temperature can play a major role in LED performance 

because the temperature at the junction depends in part on 

the ambient conditions. Thus, measurements over the same 

ambient temperature range for an LED cap lamp will result 

in a wide range of performance and inconsistent results 

across tests. Considering that a typical ambient temperature 

in a coal mine is 13°C, it would be reasonable to expect a 

slight increase in the performance of LED systems and 

almost no change for INC systems [8]. 

The light output of phosphor-converted white LEDs 

changes from about -0.25%/°C to -0.33 %/°C depending on 

the manufacturer [9][10]. Thus, the theoretical estimate of 

light output increase for a white LED would be of the order 

of 4 %. This calculation assumes that Tj drops at the same 

rate as the ambient temperature (i.e., Tj at 25°C ambient is 

12°C higher than at 13°C ambient). Following the same 

assumption, it would be reasonable to expect a benefit in 

terms of the estimated life of an LED system. Recent 

research has shown that the depreciation rate of phosphorconverted white LEDs doubles with every 10°C increase in 

junction temperature [11]. Thus, the projected life of an 

LED system operating in an environment at 13°C could be 

significantly longer than that when operating in an 

environment at 25°C. In reality, the practical implications of 

a low ambient temperature would include the opportunity to 

optimize an LED cap lamp in terms of light output, battery 

discharge time, LED life, and heat sink requirements. 

Testing conducted at NIOSH research facilities 

compared the effects of ambient temperatures and airflow 

on the performance of both LED and INC lamps. The Mine 

Illumination Laboratory (MIL), with an ambient 

temperature of 23.8 °C and negligible air flow, was used for 

room temperature evaluations. The Safety Research Coal 

Mine (SRCM), with an ambient temperature of 10.7 °C and 

a recorded air flow of 178.4 cubic meters per minute, was 

used for underground coal mining environment evaluations. 

Cap lamp performance was evaluated by conducting a light 

survey of the cap lamps’ central beam (hot spot). 

Measurements were made on a vertical illuminance test grid 
 

 

 

 

 

located 1.83 m from the cap lamps. The LED cap lamp 

yielded a vertical average illuminance increase of 

approximately 9.5% (MIL: 1520 lx vs. SRCM: 1664 lx) 

after extended use in cooler ambient conditions whereas the 

INC cap lamp yielded a vertical average illuminance 

decrease of approximately 4% (MIL: 1382 lx vs. SRCM: 

1327 lx) after extended use in cooler ambient conditions. 

The results for other LED cap lamps will vary depending on 

the individual LED cap lamp design, especially as the 

design relates to thermal management of the LED. For 

example, a LED cap lamp with an external heat sink yielded 

a vertical average illuminance increase of approximately 

14.4% in the SCRM. 

Phosphor converted white LEDs offer an obvious 

advantage over INC lamps for mesopic vision because, 

being based on blue LEDs, they have a greater proportion of 

short-wavelength light than INC lamps. LED SPDs are rich 

in short-wavelength energy as evidenced by the typically 

high correlated color temperatures (in the order of 5000 K 

and higher). Fig. 3 shows the SPD of the phosphor 

converted white LED and INC lamp tested. 

Figure 3. Spectral power distribution of the LED and INC lamp tested. 

To further quantify the implications of the spectral 

change as a function of battery discharge, and to compare 

among the two systems, we conducted mesopic luminance 

calculations using the SPD of each light source at the 

beginning and at the end of each test cycle. By using the 

model of unified photometry described in Rea et al. (2004), 

the “mesopic” luminance can be estimated for a given SPD 

and light level. Table III shows the “mesopic” luminance of 

the LED and INC cap lamps for different photopic 

luminances relevant to mine conditions. It is worth noting 

that the luminances of specific objects in a scene will 

depend upon the illuminance on each object’s surface and 

its reflectance. For the mesopic calculations, several 

representative luminance (L) values were selected and the 

respective illuminance levels (E) were calculated 

considering an average diffuse reflectance of 0.26 for 

common surfaces found in coal mines. The results are 

shown in Table IV. As the light level decreases, the relative 

effectiveness (DL) of the LED system compared to the INC 

one is more evident. In other words, for the same photopic 

luminance (i.e., as measured using a conventional light 

meter) the LED cap lamp could be up to 38 % more efficient 

than the INC cap lamp with a lead-acid battery at the end of 

the 10-hour driving cycle. It is also worth noting that these 

calculations are relevant to peripheral vision, where light 

levels would be expected to be the lowest due to the typical 

low intensity of most cap lamps at wide angles. 

The implications of these results are that an LED system 

could be optimized to provide the same visibility at mesopic 

light levels as an INC system while reducing the battery 

requirements or, it could be optimized to increase the 

reducing the battery requirements or, it could be optimized 

to increase the visibility of miners throughout the duration 

of a work shift for the same battery discharge time. 

Table IV summarizes the electrical and photometric 

characteristics of the INC lamps at the beginning and at the 

end of the accelerated life test. The two main points to 

consider from this test are a large decrease in light output 

and the spectral shift. In terms of light output depreciation, 

the observed value was approximately 17%. This 

depreciation is considerable, especially given that the light 
 

 

output will decrease another 30% to 45 % over the period of 

a 10 hour shift. The main implication of the light output 

depreciation over the life of an INC lamp is that even when 

a cap lamp may pass all the photometric tests when new, it 

is quite possible the minimum intensity requirement of 1 cd 

[12] would not be met at some point as the lamp ages, 

resulting in reduced visibility of miners. Again, this problem 

is compounded by the light output reduction over the length 

of each shift. In terms of spectral shift, the observed change 

was in the order of -50 K. Though this value may seem 

small, it is an indication that in general the relative 

effectiveness of the LEDs over INC lamps calculated for 

mesopic conditions (Table III) will only increase as INC 

lamps age. 

TABLE III. MESOPIC LUMINANCE CALCULATIONS FOR THE LED AND INC CAP LAMPSYSTEMS. 

L (cd/m2

) E (lux) 

Mesopic luminance 

from LED (cd/m2) 

Mesopic luminance 

from INC (cd/m2) ΔL 

0 hrs 10 hrs 0 hrs 10 hrs 0 hrs 10 hrs 

1 12.1 1 1 1 1 0% 0% 

0.1 1.21 0.149 0.15 0.127 0.123 17% 22% 

0.01 0.121 0.0182 0.0184 0.0142 0.0136 28% 35% 

0.001 0.0121 0.00188 0.0019 0.00144 0.00138 31% 38% 

TABLE IV. COMPARISON OF TWO INC LAMPS AT THE BEGINNING AND END OF LIFE. 

 Electric characteristics Photometric characteristics 

Voltage 

(V) 

Current 

(ma) 

Power 

(W) 

Luminous flux 

(lm) 

Luminous 

efficacy (lm/W) 

CIE 

1931 chromaticity (x,y) 

Correlated color 

temperature (K) 

Beginning of life test 

(samples 1 and 2) 

6.0 603.4 3.62 46.06 12.7 0.4403 0.4067 2965 

6.0 614.8 3.69 48.11 13.0 0.4399 0.4063 2969 

End of estimate life 

(samples 1 and 2) 

6.0 588.3 3.53 39.81 11.3 0.4448 0.4089 2910 

6.0 588.4 3.53 38.70 11.0 0.4445 0.4087 2914 

In the interest of putting this information in a certain 

context, it would be necessary to compare these results to 

LED light output depreciation and spectral shift over time. 

Different LED technologies have different degradation and 

failure mechanisms. However, in general terms, phosphorconverted LEDs have a much slower depreciation rates and 

a smaller spectral shift over time. As an example, it would 

be reasonable to take 50,000 hours for an LED’s light output 

to decrease 30% in contrast to 1,800 hours for an INC’s 

light output to decrease 30%. Given that an INC cap lamp 

would be expected to need approximately 10 lamp 

replacements before an LED system would be considered 

depreciated, the relative benefits of LED systems over INC 

ones hold true for each replaced INC lamp. 

Lastly, we note the LED cap lamp used in this research 

was purchased in 2005 at which time LED luminous 

efficacies were about 20 lm/W. This value does not include 

the efficiency of the LED driver circuitry. LED luminous 

efficacies have significantly increased in the past two years 

for LEDs. Given ideal laboratory test conditions, a LED 

could have a luminous efficacy of 100 lm/watt. 

IV. SUMMARY 

Although the performance tests were conducted using 

relatively dated LED technologies, the results indicate the 

potential benefits of LED systems over INC systems. LED 

systems can provide higher system efficacy, resulting in 

improved lighting conditions for the same battery run time, 

or equal lighting conditions for a longer battery run time. 

This can potentially result in more compact system designs 

encouraged by the use of smaller batteries. Additionally, by 

using controlling circuitry, the lighting conditions provided 

by an LED system can remain almost constant over the 

duration of each shift while providing the evident 

advantage, from the spectral composition of phosphor-based 

cool white LEDs, for peripheral vision under mesopic 

conditions. An LED system’s inherent characteristics and 

design advantages can potentially lead to improvements in 

miner’s safety and productivity. 

A. Disclaimer 

The findings and conclusions in this report have not 

been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed 

to represent any agency determination or policy. Mention of 

any company or product does not imply endorsement. 

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