THE APPLICATION OF FIBER OPTICS TECHNOLOGY 

TO THE DESIGN OF MINE LIGHTING SYSTEMS 

William H. Lewis 

Pittsburgh Research Center 

U.S. Department of the Interior, Pittsburgh PA 

ABSTRACT 

This paper describes the Bureau of Mines 

efforts in the research and development of 

a new and innovative mine lighting system 

which is based on fiber optics technology. The new lighting concept offers 

signi ficant advantages over convent; onal 

systems in the areas of both safety 

and 1 ong- term mai ntenance costs. An 

overview of Bureau research into the 

problems and 1 imitations of present-day 

mine lighting systems is also presented. 

INTRODUCTION 

Congress recogni zed the essenti a1 need for 

better lighting in the mining workplace in 

the Federal Coal Mine Health and Safety 

Acts of 1969 and 1977. Today, 85 percent 

of required underground coal mining 

machinery is equipped with an approved 

lighting system, representing one of the 

most evolutionary changes to the mining 

environment in the last decade. Although 

cons? derabl e progress has been made 

towards providing better lighting and 

consequent 1 y increased visi bi 1 i ty and 

safety in mining environments, many 

problems and areas of investigation 

remai n . 

OVERVIEW OF MINE LIGHTING PROBLEMS 

Conceptually, the notion of providing 

lighting in underground coal mines appears 

simple, but its implementation is filled 

with subtle and complex problems. Recent 

efforts of the United Mine Workers of 

her; ca/ Bituminous Coal Operator Associ at? on (UMWA/BCOA) Joint Industry Health 

and Safety Committee in cooperation with 

Mine Safety and Health Administration 

(MSHA) personnel have involved surveys [I] 

of ill umination complaints and problems in 

low and high seam coal mines. Results 

of the surveys revealed that a large 

portion of the miners have complaints 

about the performance and design of 

existing lighting systems. Most of the 

compl ai nts are caused by vi sf on impedance 

(di scomfort glare, disability glare, 

vei 1 i ng ref1 ecti ons , and after images) 

from the light sources. In many cases the 

complaints are so severe (particularly in 

low-seam coal mines) that a large proportion of the miners prefer that the 

1 ighti ng systems be removed. Foll owup 

studies by the Bureau of the glare 

tolerance levels of miners 121 have shown 

that the level of glare created by a 

typical underground mine lighting system 

exceeds the tolerance level of the average 

miner. Other studies by the Bureau 133 

have shown that the cost of maintaining 

machine-mounted 1 ighti ng systems has been 

relatively high. The high maintenance 

costs can be primarily attributed to the 

cost of the luminaires, which are frequently damaged. Frequent l amp fai 1 ures 

coup1 ed with time consuming change-out 

procedures a1 so contributes to the 

problem. 

Most of the mine lighting systems on the 

market today were designed several years 

ago, either just prior to or shortly after 

the promulgation of the Federal lighting 

regulations. At that time, most of the 

design emphasis was placed on developing 

high 1 ight-output 1 uminai res that would 

provide the light levels required by the 

regulations, with 1 ittle or no emphasis 

was placed on the problem of glare. The 

high 1 ight-output fixtures were desi rabl e 

because fewer luminaires (lower cost) 

would be needed on each machine to bring 

them into compliance. As the number of 

1 ighti ng i nstall ati ons increased, more and 

more complaints were made by the miners 

about the 1 ighting systems causing vision 

impedance problems. In addition, complaints were made about the equipment 

being unreliable and costly to maintain. 

Over the years, some improvements have 

been made on the reliability of the 

equipment, but ma; ntenance costs are st? 11 

relatively high owi-ng to the frequent 

damaging of the luminaires, which out of 

necessity are placed in vul nerabl e 

locations on the machines. Few attempts 

have been made to resolve the glare 

problem. Approaches to the problem, to 

date, have involved changes in the 

regul at? ons ( increasing the assumed val ue 

of coal reflectance), which in effect 

1 owered the requi red 1 ighti ng 1 eve1 s . The 
net effect on resolving the glare problem 

has been minimal. 

STATUS OF PRESENT TECHNOLOGY 

Currently, six manufacturers (Crouse 

Hi nds, McJunki n , Mine Safety Appl i ance , Nati onal Mi ne Service, Ocenco , and Service 

Machi ne) offer MSHA-approved 1 ighting 

systems for underground coal mine applications [4]. A variety of light sources 

are available, including fluorescent, 

high-pressure sodium, mercury vapor, and 

incandescent . Popul ari ty of particular 

types of light sources and systems varies 

with the appl i cation and i ndi vidual 

preferences. 

All of the lighting -. systems can be 

categori zed into two bas? c types, i ntri nsically safe and explosion 

Fluorescent systems are most popular and 

more frequently used because of thei r 

re1 atively lower glare characteri stics, 

a1 though incandescent systems are gaining 

popularity because of their less sophfsticated powering requirements, lower cost, 

and availability of the lamps. Highi ntensi ty discharge (HID) 1 amp systems 

(e.g ., high-pressure sodi um and mercury 

vapor) are more widely used in high coal 

appl i cations, where these re1 atfvely high 

light-output fixtures are desirable. 

Presently, very few HID systems are 

installed in low coal because of the? r 

higher prof i 1 e design and higher gl are 

character; st; cs . 

COMPARISON OF FIBER OPTICS CONCEPT 

AND PRESENT MINE LIGHTING TECHNOLOGY 

Present studies of the Bureau have shown 

that the application of fiber optics to 

the design of mine lighting systems offers 

a potential solution to most of the 

identified problems. The fiber optics 

system, a1 though simple conceptually, 

offers an opportunity to significantly 

advance the state-of-the-art. To ful ly 

el ucidate the concept and its potenti a1 

advantages over conventional mine lighting 

systems, a comparative analysis is given 

be1 ow. 

A typical mine lighting system consists 

basically of five 1 uminai res distributed 

around the periphery of the main frame of 

a mining machine as shown in figure 1. 

The 1 uminai res are relatively expensive 

owi ng to thei r expl osi on-proof construction and are frequently broken and 

relatively time consuming and costly to 

replace. . Since the light sources are 

contained in the luminaires, the 

luminaires are heavy and large, and have a 

high profile, which further contributes to 

their vulnerability. Power to the light 

sources is suppl i ed through electrical cables and packing glands from a main 

power supply or ballast box. The 

electrical cables are routed around the 

machine and are subject to damage, which 

can present a potential shock hazard. A 

summary component analysis shows that a 

typical system consists of 23 components 

(6XP enclosures, 6 electrical cables, 11 

packing gl ands) that relate directly 

to the safety, cost and maintainability of 

the system. 

FIGURE 1. - Typical mine lighting system 

In comparison, the fiber optics lighting 

system (figure 2) consists of a single 

explosi on proof enclosure which houses the 

power supply and individual 1 ight sources. 

eeeee COUPO*LNl SuYUAnv 

.OI€R SU..LV AND Ill a. EWCLOSUII 

LIOMT SOURCE 

111 TOTAL 

FIGURE 2. - Fiber optics lighting system 

The light from the sources is piped out 

of the enclosure through fiber optics 

cables. These fiber optics cables 

are terminated with light-diffusing 

assembl Yes which in effect replace 

the explosion proof 1 uminai res containing 

1 i ght sources. Si nce the diffusing 

assemblies contain no light sources or 

electrical power sources, they need not be 

explosion proof and can be constructed 

inexpensively. Furthermore, they can be 

cast or molded of plastic and made in a 

variety of shapes and sizes w.ith a very 

low profile. Because of their low profile 

and sol id construction, they wi 11 be less 

vulnerable to damage. In event damage did 

occur, they could be replaced easily and 

13 
FIGURE 3. - Conceptual model fiber optic 

mine lighting system 

Off-the-she1 f, 80 watt tungsten halogen 

slide projector lamps (figure 4) were 

chosen as a light source and powered by a 

24 volt, constant-voltage stepdown 

transformer fed from a 120 VAC line 

(figure 5). These lamps were chosen 

because of their relatively high source 

brightness, small source size, and good 

projection efficiency. 

The enclosure design (figure 6) was of 

convent? onal expl osi on-proof box construction with a flange and bolted lid. The 

FIGURE 5. - 24 volt lamp power supply 

beam of light from the projection lamps 

exits the enclosure through explosionproof optical windows and is focused on 

the ends of the fiber optics cables, which 

are butted up against the windows. The 

fiber optics cables are 1/2 inch diameter 

and are the standard variety used in the 

photoelectric contra1 s industry. They are 

covered with a flexible steel sheath for 

protection (figure 71, much the same as 

armored electric cab1 es. The other end of 

the cable is terminated with a diffusing 

assembly which is used to spread the 

intense narrow beam of light which exits 

the cable and act as a low glare, relatively diffuse secondary source for area 

1ightin.g (figure 8). The diffusing 

assembl ~es are sol id cast using clear 

urethane resin. They were designed to 

have a low profile so as to minimize their 

projection height when mounted on a mining 

machine and reduce potential damage. The 

bottom surface is curved and painted white 

to act as a reflector (figure 9). The top 

14 surface was sanded with fine grain emery 
FIGURE 7. - Fiber optics cable with 

armored sheath -- 

FIGURE 8. - Light diffusing assembly 15 

system - Present lighting regulations for 

underground coal mines require that the 

surfaces of a defined workplace be 

illuminated to a value of -06 foot 

lambert . Assuming a nominal reflectivity 

of 4% for coal surfaces and a safety 

factor of .77 for lamp maintenance, the 

.06 foot lambert of reflected light 

from a surface equates to approximately 

1.95 foot candles of light incident on the 

surface as shown below: 

where E = illuminance in foot candles 

L = luminance in foot lamberts 

e = reflectance 

The conceptual model equipped with an 80 

watt tungsten halogen lamp and a 15-foot 

glass fiber optics cable and diffusing 

assembly is capable of ill uminating a 

surface 6 feet away to approximately .32 

foot candle. This is approximately 

one-sixth of the amount of incident light 

required to meet the regulations . 
(b) Overheating of the light source and 

power supply enclosure - As mentioned 

previous1 y, the present conceptual model 

consists of six 80 watt tungsten halogen 

lamps, which are contained in a single XP 

enclosure. This amounts to a total of 

approximately 480 watts of power (not 

incl uding the power consumption of the 

transformer) and its resultant heat that 

must be dissipated. As a consequence of 

that amount of power and the enclosure 

des i gn , the enclosure overheats. The 

overheating problem is not just a problem 

of the enclosure exceed? ng the overall 

surface temperature requirements of MSHA's 

schedule 2G (less than 150 C), but is also 

a problem of exceeding the maximum 

operating temperature of the components 

contained in the enclosure, such as lamp 

sockets, 1 eads, capacitors, transformer 

windings, etc. 

SOLVING THE PROBLEMS 

A major key to the solution of both 

problems (low light output and overheating) 1 ies in the development of a new 

light source that will produce a higher 

light output and at the same time generate 

less heat. To address the problem of the 

light source, a contract was awarded to a 

major 1 amp manufacturer to review existing 

1 amp technol ogy and make recommendations 

for the development of a lamp with 

sufficient 1 ight output and optical 

coup1 i ng efficiency for this appl i cation. 

On the basis of the technology review, it 

was determined that there were no off-theshe1 f 1 ight sources avai 1 able for this 

application, but that the technology did 

exist to develop such a lamp. 

Based on the application requirements of 

high brightness, low heat generation, long 

life, and compact and rugged design, 

short-arc metal halide technology was 

recommended. Tungsten ha1 ogen technol ogy 

was ruled out because of its high heat 

production and short operating life; 

this is particularly true for a mining 

environment , where it would be subjected 

to high shock and vibration. 

A c a l c u l ati on of the 1 amp output requi red 

using measurements of the efficiency es of 

the light loading assembly (optical 

window, 15 foot length of fiber optic 

cable, and diffuser) and assuming a 

coll ection or del i very ef f i ci ency of 35% 

for the lamp reflector indicates that an 

illumination of 2 foot candles at 6 feet 

away from the diffuser would require a 

6,600- 1 umen source. 

where 0 = luminous flux of the source 

E = illuminance at a distance of 6 

feet from the diffuser 

D = distance from the diffuser 

Zcol . = coll ection efficiency of the 

1 amp ref1 ector 

$win. = transmission efficiency of the 

1/2-inch-thick optical wi ndow 

?cable = transmission efficiency of a 

15-foot fiber optic cable 

dif. = transmission efficiency of the 

diffuser 

Assuming a coll ection efficiency of .35 

for the lamp reflector and measured values 

of T wi ndow = .95, fl cab1 e = .23, 

Zdiffuser = .45, then 

The search for a suitable lamp configuration and technology began with a 40-watt, 

short arc metal halide lamp. The 40 watt 

configuration was chosen because of our 

particular appl i cati on, which i s bas; cal ly 

a projection system requirement rather 

than a general or area illumination 

requi rement . Our appl i cati on requi res not 

only a high 1 men output (6,600 lumens) , but also a small source size (high source 

brightness), so that the output energy can 

be conveniently collected and projected 

into the small 1/2-inch aperture of the 

fiber optic cable. This dual requirement 

of both high lumen output and high source 

brightness presents a problem, since a 

40-watt, short-arc tube can generate (with 

normal design baundaries) 58 lunens per 

watt or a total of 2,320 lumens (58 

lumens/watt x 40 watts). This is approximately one-third of the lumen output 

required. One might ask, "Why not move up 

to a higher wattage arc tube configuration?" This sol ution is not satisfactory, 

since using the higher wattage arc tube 

means an increase in the size of the arc 

tube and in effect lowers the source 

brightness of the lamp and the collection 

efficiency. 

There are two basic areas of potential 

sf gni f i cant improvement by which the 

output of the system can be increased: 

The standard 40-watt arc tube design can 

be modified to improve the output, and the 

efficiency of the 1 ight-loading assembly 

(fiber optics cables and diffusers) 

can be improved. Considering the amount 

of improvement needed, both areas wi 11 

have to be addressed. 

(a) Lamp Modifications: As previously 

discussed, a 40-watt short-arc metal 

ha1 ide lamp of standard design is capable 

of generating approximately 2,300 1 unens, 

16 which is far short of the lunen output 
requi red for our appl ication . Normal ly, 

arc discharge lamps such as the metal 

halide have rather long operating lives, 

usually in excess of 10,000 hours. To 

improve the lumen output of the lamp, one 

has the option of overdriving the lamp, 

but the lamp operating life will be 

sac ri f i ced . The possi bi 1 i ty of overdriving the lamp at 60 watts, thus increasing the output to approximately 3,500 

1 unens, was considered, but the operati ng 

life would be far below the originally 

targeted 3,000 hours. In other applications with a similar dilemma, a doublewalled arc tube construction is used to 

improve the life when overdriving the 

lamp; this would be difficult to do 

conventionally in a small projection lamp 

such as ours. The idea then occurred that 

a lens or window fused to the front 

of the integral reflector could be used to 

form the basis of the second wall in a 

doubl e-wal 1 design . The doubl e-wall 

construction prevents water vapor, and 

consequently hydrogen, from migrating 

through the hot inner wall of the arc tube 

and contaminating the fill gases, which 

reduces the lamp life. The technique was 

successful in ma? ntai ni ng the necessary 

operating life. 

Expected performance parameters for the 

metal halide lamp and a comparison to the 

present tungsten halogen source used in 

the conceptual model follow: 

Tungsten Halogen Technology 

Lead- Wi re Conduct i on 2 % 

Gas Conduction & 10% 

Correction 

Infrared 81% 

Ultraviolet 0.5% 

heat 

(93%) 

Visible 6.5% 

Total.......... ...... 100% 

Metal Hal ide Technol ogy 

El ect rode Losses 12% 

Gas Conduction & 25% heat 

Wall Absorpti on (54% 1 

Infrared 17% 

Ul t rav i 01 et 19% 

Visible 27% 

Total ................ 100% 

From the above information, it can be 

deduced that the present 80 watt tungsten 

halogen lamp used in the conceptual model 

will generate approximately 74 watts of 

heat and 5 watts of light. In comparison, 

the 60-watt standard fill metal halide 

will generate 32 watts of heat and 16 

watts of light. 

The resultant design improvements are - 

Heat reduction - 57% 

Light production - 320% 

As can be seen in the preceding analysis, 

significant improvements have been made in 

the areas of heat reduction and light 

production. Unfortunately, this is not 

enough in terns of 1 ight production. 

The present 60 watt prototype metal halide 

l amp is capable of producing approximately 

3,480 lumens (60 watts times 58 

1 umenslwatt) . Using equation 2 and 

rearranging the terms to solve for E 

yields 

where @ = 3500 lumens 

D = distance from diffuser = 6 feet 

Let gtp = present total system efficiency 

wherettp = (iJcol .) (g wi n .) (?cab1 e)#di f .) 

Assume 35% collector efficiency and a 

15-foot fiber optics cable 

Then Ztp = (.35)(.95)(.23)(.45) = .0344 

Itp - 3500 ( .0344) 119 E= - 2 - 2 = 1.06 F.C. 

RD 3.14(6) 

As shown in the above analysis, the 

60-watt metal halide lamp with the 

present 1 ight-loading assembly is capable 

of producing 1.06 foot candles at a 

distance of 6 feet from the diffuser. 

This is a little more than one-half of the 

2 foot candles required by the 

reg ul at i ons . 

In addition to overdriving the lamp, 

another option is avai 1 able for increasing 

the lunen output. This option involves 

changing the metallic salt mixture that is 

added to the arc tube. It is known that 

the addition of thallium iodide to the 

fill mixture can increase the lunen 

output of the standard metal halide lamp 

from 58 1 umenslwatt to 75 or 80 

l umenslwatt . This is accompl ished by 

altering the spectral distribution 

of the lamp and, in effect, providing more 

emission in the yellow-green portion of 

the spectrum and less emission in the blue 

end. This technique does alter the color 

rendering properties of the lamp, however, 

and for this reason, it is normally not 

used because most appl i cat i ons requi re 

1 i ghti ng with good color-rendering 

propertTes. It is be1 ieved by the author 

that in mining applications, color 

rendition is not an essential consideration, particularly when you have the use 

of a caplamp for detailed task work. In 

this case, the a1 tered color rendition 

produced by the thalium iodide doping is 
not very severe and is a far cry from that 

of the monochromatic low pressure sodium 

lamp. Visually, the lamp appears a 

greenish-white because of the enhanced 

ye1 1 ow-green emission and decreased bl ue. 

Another important aspect or benefit of 

this effect is that the resultant spectrum 

shift will allow more light to be transmitted through the fiber optics cables 

because the new spectral distribution is 

more compatible with the spectral transmission of the cables. In effect, we are 

impedance matching to obtai n maximum power 

transfer. The net magnitude of this 

effect is not known at this time. 

If the thall ium iodide studies prove 

successful , the followi ng light levels 

could be achieved : 

where 0 = 4500 lunens (60 watts x 75 

1 wens/ watt) 

4550(.0344) 154.8 Then: E = 2 1.37 F.C. 

Note: The above levels do not take into 

account the impedance-matchi ng effect 

prev i ousl y ment i oned . 

(b) Efficiency of the Light Loading 

Assembly 

The preceding analysis is based on 

projected or expected lumen output 

levels, and it would be unrealistic to 

assue that all of our expectations would 

come to fruition. In addition, if the 

levels were achieved, they would only be 

sufficient for thin seam mining appl ications. Ideally, one would like the new' 

lighting system to be applicable to all 

mining environments, so that its inherent 

benefits could have widespread use. 

In view of the fact that we are operating 

at the state-of-the-art of source technology, if additional light output from the 

system is required, it can only be 

obtained by improving the efficiency of 

the 1 i ght 1 oadi ng assembly . 

As can be seen from equation 3, there are 

three parameters that can affect the 1 ight 

output of the system: , tp, and D. 

is the luninous flux' from the source; 

efforts to maximize this parameter have 

been discussed in the previous section. D 

is the distance from the diffuser to the 

mine surface and is determined by the 

seam width or height, and the machine 

dimensions, which are beyond our control . ztp is the present total efficiency of 

the light loading assembly and can be 

expressed as the product of four component 

efficiencies: 

 collector is the collection or delivery 

efficiency of the ref1 ector assembly of 

the lamp and is determined by the design 

limits of the lamp designer; it is 

estimated at .35. 

Note: Some control of the delivery 

FfFici ency can be obtai ned by i ncreasi ng 

the numerical aperture (N.A.) of the fiber 

optics cable and by increasing the 

diameter of the cable. This will be 

discussed in a later section. 

ewi ndow is the transmission efficiency of 

the 1/2-inch-thick optical window used in 

the XP enclosure and has been measured at 

0.95. 

Y cable is the transmission efficiency of 

mot glass fiber optics cable and has 

been measured at approximately .23. 

diffuser is the transmission efficiency 

of the diffusion assembly and has an 

average measured val ue of .45. 

Substituting the above values into 

equation 4 for tp yields the present 

total system effic7ency: 

It is anticipated that continuing efforts 

on improving the system efficiency could 

lead to the followi ng increases: 

gcol lector = .35 (no improvement) 

wi ndow = .98 (increased from .95) 

Zcable = .29 (increased from .23) 

diffuser = .65 ( increased from .45) 

Substituting the projected improvements 

into equation 4 yields 

= ( .35) ( .98)( .27) ( -65) = -060 

where at1 = improved total system 

efficiency 

Substituting the new 3 t~ into equation 3 

yields the following increases in the 

system output 1 eve1 s : 
which means it is physically small and 

that both electrode leads are exited at 

one end of the lamp. Because of this, it 

becomes a difficult design task to prevent 

the hi gh-vol tage starting pulses from 

tracking between the leads, rather than at 

the electrode tips. To date, a prototype 

ballast has been designed (figure 10). 

Also, a design for a lamp has been 

completed that can withstand the highvoltage starting pulses (figure 11). The 

ballast design is conventional technology 

(choke, transformer) and should be more 

reliable than a solid state configuration, 

particular1 y in a mining environment 

where it will be exposed to high 

transients. 

(2) Fiber Optics Cables: Considerable 

efforts have been focused on improving the 

efficiency of the glass fiber optics 

cables. These efforts have resulted in a 

10% increase in the transmission efficiency of the cables and involved working 

with the manufacturer to improve the 

qua1 i ty control of the manufacturing 

process. Initially, the cables were 

approximately 20% to 25% low in their 

efficiency when compared to their advertised specifications. Studies of the 

FIGURE 11. - New metal halide projection 

1 amp 

problem showed that the pack+ ng fractions 

of the cables were low, which was the 

result of a reduced number of fibers in 

the cable. In addition to the low number 

of fibers, the actual control of the 

amount of fibers varied from cable to 

cable, causing large variations in the 

transmission of cables of the same 

length. This was brought to the attention 

of the manufacturer, and some improvements in the efficiency and consistency 

were obtained. In addition, the particular epoxy that the manufacturer was 

using did not lend itself to a good finish 

on the ends of the cables, resulting in 

higher- than-expected entrance and exit 

losses. The epoxy formulation has been 

changed, and some improvements have been 

achieved 'n this area. 

In add? tion, the numerical aperture 

(N.A.) of the cables has been increased 

from -55 to .68. This has been accomplished by changing the core and clad 

materials and consequent1 y the4 r respec19 tive indices of refraction. The larger 
STATUS OF PRESENT RESEARCH 

To date. a prototype fiber optics mine 

lighting system has been designed and 

fabricated according to the requ'rements 

of MSHA's Schedule 26 for explosion-proof 

enclosures (figure 13). The system uses 

the new standard fill metal halide lamps 

and ballasting system and is capable 

of providing 1.65 foot candles at distance 

of 6 feet away from the diffusing 

assembly. This is accomplished with a 

15-foot-long fiber optics cable. The 15 

foot cable is used for testing purposes 

because this is a worst case condition. 

In practice, most applications will use 

shorter cable lengths, and according1 y the 

output wi 11 be higher. 

In the near future, the thalium doped 

lamps will be completed. It is anticipated that the new thalium lamps will 

br:ng the output close to the 2-footcandle target. 

Research to further improve the system 

output is continuing. Attempts will be 

made to improve the efficiency of the 

d'ffusing assembly, which at present is 

FIGURE 13. - Advanced prototype fiber 

optic mine lighting system 

approximatel y 50%. It is anticipated that 

designs can be made that will have an 

efficiency in the 75% to 80% range. 

CONCLUSIONS 

If ongoing research proves successful, the 

new lighting concept will provide the 

mining industry with a machine-mountable 

lighting system that offers significant 

improvements over conventional mine 

1 ighting systems in the following areas: 

glare (discomfort and disabi 1 ity) 

safety (electrical and expl osion-proof 

integrity) 

reliability 

economy 

ease of ma: ntenance and instal 1 at: on 

Additional research is needed to improve 

the overall system efficiency so that its 

inherent benefits can be extended from low 

seam to high-seam coal mine applications 

where a greater light output would be 

20 required. 
REFERENCES 

1. UMWAIBCOA Joint Industry Health and 

Safety Committee, Report on Coal 

Mine I1 1 mination Survey, 1979. 

2. Lindahl , P. O., K. L. Whitehead, and 

R. D. Sal tsman, "Psychophysical 

Investigation of Di scomfort and 

Di sabf 1 ity Gl are From Underground 

Coal Mi ne I1 1 urnination Systems," 

Interim Report of Bureau of Mines 

Contract J0308090, September 13, 

1982. 

3. Whitehead, K. L., J. C. Yingling, 

"Development of Guide1 i nes for Instal 1 at? on and Mai ntenance of Mi ne 

I1 1 urnination Systems," Final 

Report of Bureau of Mines Contract 

30308040, April 1981. 

4. ~ewi s, W. H., 3. C. Yingl ing, and 

M. H. Leon, "Longwall Lighting," 

Chapter 5 of "Longwall-Shortwall 

Mi ni ng , State-of-the-Art ," Society 

of Mining Engineers Handbook, 

R. V. Ramani , Editor, 1981.