Technological Aspects of Solid-State and
 

Incandescent Sources for Miner Cap Lamps
 

John J. Sammarco, Senior Member, IEEE, Jean Paul Freyssinier, John D. Bullough, Xin Zhang, and Miguel A. Reyes 

J. J. Sammarco and M. A. Reyes are with the National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA 15236 

USA (e-mail: jsammarco@cdc.gov). 

J. P. Freyssinier, J. D. Bullough, and X. Zhang are with the Lighting Research 

Center, Rensselaer Polytechnic Institute, Troy, NY 12180 USA. 

Abstract—Light-emitting diodes (LEDs) are emerging as viable replacements for incandescent (INC)-based cap lamps used 

in mining. The photometric and energy characteristics of these 

light sources differ in important ways. This paper describes the 

performance of LED and INC sources in cap lamps in terms 

of correlated color temperature, color rendering, light output, 

electric power, ambient temperature and air flow, and light source 

aging. Importantly, these characteristics can influence a miner’s 

ability to spot mining hazards thus impacting safety. Second, some 

of these characteristics interact with the operating life of the cap 

lamp’s battery power, such that differences between LED and 

INC sources can be magnified toward the end of a 10-h battery 

discharge cycle. Empirically, we have determined that after 8 h 

at an ambient temperature of 25 ◦C, the average light output of 

an INC 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 air flow on the light output of an LED 

and INC cap lamp were conducted in the National Institute for 

Occupational Safety and Health Safety Research Coal Mine. 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 INC 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 INC cap lamp with a lead-acid battery at 

the end of the 10-h driving cycle. Lastly, accelerated life tests were 

used to empirically determine light output depreciation as the INC 

light source age approached its useful life. There was about a 35% 

decrease in light output. This is quite considerable, particularly 

given that the light output will decrease an additional 30% to 45% 

over the period of a 10-h shift. The implications of the differences 

between LED and INC 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 INC cap lamp visual 

performance research. 

I. INTRODUCTION 

HE National 

initiated 

T Institute for Occupational Safety and Health 

(NIOSH), Pittsburgh Research Laboratory (PRL), has 

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 solidstate 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 lx [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 to 13.5 A · h [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 highcontrast 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. 

This 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 air 

flow, and as the light source ages. This information is crucial in 

determining how visual performance could be affected for realworld 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-V 

8-A · h nickel-hydride battery; 2) INC cap lamp with a 4-V 

13-A · h lead-acid battery; and 3) INC cap lamp with a 

6-V 8-A · h 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 h 

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 SPD 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 min 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 SPD 

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 SPD. 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 air flow 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. Fig. 1 shows 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-min intervals for the duration of the evaluation period. 

Fig. 1. Thermal image of an LED cap lamp at an ambient temperature of 

23.8 ◦C and negligible air flow. 

III. RESULTS AND DISCUSSION 

Fig. 2 shows the relative photopic light output over the 10-h 

discharge period for each system. 

Fig. 2. Average relative, photopic, light output of each system tested over a 

10-h discharge period. 

As shown in Fig. 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-h 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-h 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-h 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 SPD of INC lamps 

shifts toward increased long visible (“yellow”) wavelengths that 

could be considered detrimental to peripheral vision at mesopic 

light levels [8]. 

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 Fig. 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 min). 

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
 

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 II
 

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

All of these measurements were taken at room temperature 

(nominally 25 ◦C) with negligible air flow. All LED technologies have different sensitivities to temperature, both in the short 

and the long term. Thus, 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 air flow 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 INC 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 phosphor-converted 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 air flow 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 SRCM, with an ambient temperature of 10.7 ◦C and a 

recorded air flow of 178.4 m3/min, 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 versus 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 versus 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, particularly as the design relates to 

thermal management of the LED. For example, an 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 (on the order of 5000 K and higher). Fig. 3 shows 

the SPD of the phosphor-converted white 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. [7], 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 (ΔL) 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-h 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. 

Fig. 3. SPD of the LED and INC lamp tested. 

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 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 the following: 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, particularly given that the light output will 

decrease another 30% to 45% over the period of a 10-h 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 on the order of −50 K. Although 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. 

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, phosphor-converted 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 h for an LED’s light output to decrease 30% in contrast 

to 1800 h for an INC’s light output to decrease 30%. Given that 

an INC cap lamp would be expected to need approximately ten 

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 that the LED cap lamp used in this paper 

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, an LED could have a luminous 

efficacy of 100 lm/W. 

TABLE III
 

MESOPIC LUMINANCE CALCULATIONS FOR THE LED AND INC CAP LAMP SYSTEMS
 

TABLE IV
 

COMPARISON OF TWO INC LAMPS AT THE BEGINNING AND END OF LIFE
 

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 paper have not been formally disseminated by the NIOSH 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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