Compiled by W. H. Lewis 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Main entry under title: 

Underground coal mine lighting handbook. 

(Information circularNniled States Department of the 

Interior, Bureau of Mines; 9073) 

Bibliographies. 

Supt. of Docs. no.: I 28.27: 

Contents: v. 1. Background-v. 2. Application. 

1. Mine lighting. 2. Coal mines and mining. 

I. Lewis, W. H. (William H.) 11. Series: Information circular (United States. Rurcau 

of Mines); 9073. 

TN309.U47 1986 
Other light cont. rol functions ............ 22 

Receiver-decoder system operation ......... 22 

Photoreceptor function ................ 22 

Light levels needed to stimulate photoreceptors ......................... 22 

analysis ......................... Unit of measure abbreviations .............. 

ILLUSTRATIONS 

1 . Radiant energy (electromagnetic) spectrum ............................................. 2 . Brightness sensitivity of the eye as a function of light wavelength ............................. 3 . Classification of reflectors ......................................................... 4 . Curved reflectors used to control light distribution ........................................ 5 . Use of surface patterns to control light distribution ....................................... 6 . Effect of interreflection on net light transmission for enclosed light fixtures ...................... 7 . Effect of changing transmission mediums on direction of light travel ........................... 8 . Use of curved lens to control light distribution .......................................... 9 . Illumination .................................................................. 10 . Determining angle of incidence (0) when applying the cosine law ............................. 11 . Basis of the cosine law ........................................................... 12 . Solid angle forms ............................................................... 13 . Beam angle of mining vehicle headlight ............................................... 14 . Concept of luminous intensity ...................................................... 15 . Basis of the inverse square law ..................................................... 16 . Candlepower curve for a typical incandescent fixture ...................................... 17 . Light measurement to establish candlepower curve for a mining vehicle headlight ................. 18 . Candlepower curve for a mining vehicle headlight ........................................ 19 . Geometry for performing illumination calculations ....................................... 


lamps. Research and observation, both qualitative and 

quantitative, indicate that improved lighting has in fact 

resulted in greater safety, increased production, and 

improved worker comfort. 

Underground coal mining is one industrial activity 

that has not kept pace with the application of improved 

lighting technology in the working environment. There 

are several reasons for this, including the following: 

1. Initially, mining lagged behind other industries 

because (1) early lamps had short service life in this application because of lack of mechanical strength, and (2) their 

light output was low, which provided little improvement 

over the open flame lamps then in use. 

Artificial lighting has always been a necessity in the 

otherwise totally dark underground mine environment. 

The types of light sources that have been used in underground coal mine lighting, summarized in table 1, range 

from oil soaked wood chips, reeds, and bulrushes to the 

present day incandescent and arc-discharge lamps. Developments such as the Spedding flint mill, the flame safety 

lamp, and the carbide lamp were aimed at providing a 

light source that would not ignite a gassy environment. 

Prior to perfection of devices that could accomplish this, 

hundreds of lives were lost to explosions initiated by light 

sources. Initial efforts to use incandescent electric lamps 

in the mines occurred in Europe as early as 1902, but 

Table 1 .-Light sources used in underground mining - .- 

Type Source Time per~od 

Saucer-type open flame lamps ........ Hanging open flame lamps .......... Candleholders ................. Carlisle Spedding flint mill ........... Spout lamps .................. Flame safety lamps .............. Carbide lamps ................. Arc lamps ................... Personal lamps ................ Cap lamps ................... Face lighting .................. 

Wood chips, reeds, and bulrushes soaked in oil . . Wick immersed in grease or oil ........... Wickcandles .................... Sparks from flint held against rotat~ng steel wheel. . Wick immersed in oil ................ Wick immersed in 011. naphtha, or gasoline ..... Calcium carbide and water ............. Carbon electrodes and electric current ....... Incandescent lamp ................. .. do ........................ Incandescent, fluorescent high-pressure sodium 

lamps. 

10.000 C.-16th century 

1500-1900. 

1800-1 900. 

1750-1813, 

1850-1920. 

1796-present.' 

1910-20 ' 

1878- 

1920-55. 

1925-presenL 

1975-present. 

' Currently used to check lor methane at the working lace. 

Still used in some developing countries and some 1- or 2-person mines 
9074. 

OBJECTIVES 

The general goal of industrial lighting is to improve 

safety and increase production. Statistics show that underground mining is one of the most hazardous industries and 

mining personnel should, therefore, benefit most significantly from the use of lighting that is consistent with the 

capabilities of modem technology. To realize these benefits, 

mine lighting must achieve specific goals as follows: 

1. Increase the visibility of hazards.-Because of the low 

luminance levels and poor contrast in coal mines, hazards 

have always been difficult to visually identify. A few examples of such hazards include frayed or cut cables, misplaced tools and timbers that may prevent a tripping 

hazard, and slips in roof rock. A primary goal of mine 

lighting is to increase the visibility of these objects which 

are visibly manifested by subtle details, and, hence, reduce 

injuries that may result if the hazards go undetected. 

2. Increase visual response of the peripheral field to enable 

early detection of hazards.-With the narrow-beamed cap 

lamp alone, movement of personnel, machines, and roof or 

rib material is difficult to detect when it occurs in a miner's 

peripheral field of vision, outside the localized main beam 

of the cap lamp. Another major goal of mine lighting is to 

allow miners to quickly detect even subtle movement anywhere in their normal field of vision. This will provide 

Although the studies described show wide variation 

in the magnitude of results and cannot be considered conclusive, the analyses consistently indicate improved safety 

and/or production in the lighted mine sections. 

A more reliable evaluation of mine lighting will 

require detailed studies similar to those conducted for lighting of roadways, offices, schools, and manufacturing facilities. It should be noted that at many mines there is a 

deficiency in accident reporting-the lack of operators' comments or descriptions of lighting conditions associated with 

mining operations or accidents. Lighting data should be 

included as part of accident reports to provide an additional basis for assessing the benefits of face lighting and 

in identifying shortcomings. 

SAFETY 

Halmos%eported that in a Hungarian mine a 60 pct 

decrease in the accident rate was recorded for a lighted 

section compared with the rate in a section where only cap 

lamps were used. In the same study, another test showed 

that increasing the lighting level by a factor of 12.5 (20 to 

250 lx) decreased the number of accidents by 42 pct. 

A 24-month study conducted in a West Virginia coal 

mine compared the major accident records for one lighted 

conventional section with records from five unlighted 

Influence of Lighting on Productivity and Safety in Mining. 
ment and face personnel unanimously approved of longwall 

lighting, citing many benefits and no major problems. 

Acceptance of room-and-pillar systems increased from the 

1979 survey markedly so for low seam height applications. Many equipment operators, despite an overall favorable opinion of the rwm-and-pillar systems, cited problem 

areas, indicating that careful implementation is required 

and general improvement is possible. 

MAJOR TECHNICAL CONSIDERATIONS 

The underground coal mine is one of the most difficult 

lighting applications encountered by industrial lighting 

designers. There are several unique technical considerations that must be carefully addressed in designing and 

implementing mine lighting systems. These considerations 

are reviewed briefly in the following and are discussed in 

more detail in other sections of this report and IC 9074. 

1. Mine lighting is a nonstationary application.-For 

most lighting applications, the object or area to be lighted 

is confined to a specific location and lighting hardware can 

be permanently installed. In coal mining the working face 

not only continually advances (or retreats), but several 

faces are being mined at the same time by the same 

A New Psychoengineering Strategy for Safety in Mines. 

Permissible Mine Lighting Systems Cut Down on Accidents and Increase Coal Output. 

require that light sources with relatively high output be 

utilized to achieve acceptable visibility. As discussed in 

item 6 of this section, this can lead to glare problems. 

Compromises between visibility levels and reducing the 

system's glare level must frequently be made in mine lighting system designs. 

3. Abusiveness of the enuironmen.t.-Few environments 

expose lighting hardware to the abuse found in the underground coal mine. The major hazards include impacts from 

striking rib or roof, roof rock falling on the hardware, 

mechanical shock produced by machine vibration and 

motion, and corrosion potential from moisture, salts, and 

other corrosive agents found in mines. These conditions 

require special consideration in the design and installation of hardware if adequate service life and acceptable 

maintenance costs are to be realized. 

4. Poorpower regulation.-Light sources and associated 

power control devices are sensitive to fluctuations in supply voltage, especially large and fast fluctuations, and to 

extended operation above or below design voltage. The 

effects of poorly regulated power include variation in light 

output, extinguishment of arc discharge lamps, destruction of lamps, and a general reduction in service life of 

power-conditioning components because of heat buildup. 

These problems have been particularly prevalent with dc 

power systems, which frequently experience extreme highvoltage transients and very poor voltage regulation. A system designer must, therefore, be familiar with the character of a mine's electrical system and take steps to protect 

against poor regulation andlor transients. 
understand lights physical properties and behavior. 

The following topics and associated key concepts will 

be addressed in this chapter. 

Nature of light . . . . . . Radiant energy, wavelength 

and frequency, and electromagnetic spectrum. 

Interpretation of visible Color, brightness, and the 

light . spectral luminous efficiency 

curve. 

Physical behavior of Absorption, reflection, and 

light . transmission. 

Lighting energy quanti- Luminous flux, illumination, 

fication, lighting intensity, luminance, cosine 

units . . law, and inverse square law. 

The section on lighting units presents example problems 

to illustrate the use of each major concept in design 

applications. 

Light is a form of radiant energy; one of several known 

energy forms, which include electrical, chemical, atomic, 

thermal, and mechanical energy. Light is called radiant 

energy because it travels through space from its point of 

origin, or source, in all directions as a repeating, or cyclical, 

wave pattern, much as the mechanical wave that results 

when a pebble is dropped in a pond. Light waves are made 

up of both electric and magnetic energy components, and 

therefore are called electromagnetic waves. 

esses that produce visible light generally occur on an 

atomic or molecular scale and involve acceleration of 

electrons. Some of these are (1) heated bodies (e.g., the 

common incandescent light bulb), (2) arc discharge (e.g., 

mercury lamp), (3) fluorescence (e.g., a glow-in-the-dark 

watch face), and (4) chemical process (e.g., the firefly). 

Because it consists of waves, radiant energy is characterized by two special properties: wavelength and frequency. Wavelength is the distance covered as an energy cycle 

repeats itself. Frequency is the number of times the cycle 

repeats itself during a unit of time, for instance, in 1 s. 

When wavelength is multiplied by frequency, the product 

is the speed at which the wave is carried forward from its 

source. For radiant energy, this speed is a constant in a 

given transmission medium. The speed equals 186,000 mils 

when the medium is a vacuum, and is slightly less in air. 

Visible light is only a narrow part of a broad spectrum 

of radiant (electromagnetic) energy. At one end of this 

electromagnetic spectrum are cosmic rays; electric power 

transmission waves are at the opposite end. Even though 

the effect of, uses of, and the generating processes for the 

different groups of radiant energy may differ, all are electromagnetic energy differing only in wavelength and 

frequency. The various categories of energy are arranged 

on the electromagnetic spectrum on the basis of wavelength and frequency, as shown in figure 1. 

Visually evaluated radiant energy, or visible light, is 

that portion of the electromagnetic spectrum with wavelengths between 380 and 780 nm. Longer or shorter wavelengths stimulate very little or no response in the eye. 
appear violet in color, while light wavelengths of approximately 600 nm appear yellow (see figure 1). 

When seen separately, individual visible wavelengths 

of light appear as distinct colors. When light consists of a 

blend of the wavelengths across the entire visible portion 

of the electromagnetic spectrum, it appears white. (Black 

is not a color, just the absence of light.) Certain sources, 

such as incandescent light bulbs and the Sun, give off 

relatively balanced amounts of all visible wavelengths and 

hence appear white. When light sources or reflectors give 

off higher or lower amounts of one or more visible wavelengths, our eyes distinguish such wavelength imbalances 

as colors. Because of the way our eyes interpret wavelength characteristics of light, mixtures of several wavelengths may appear the same as a certain individual 

wavelength. For example, blue and yellow when mixed 

appear green. 

Perception of brightness is affected by two physical 

characteristics of light--energy content of the light striking the eye and the wavelength composition of that light. 

The more energy at a given wavelength of light striking 

the eye, the greater the sensation of brightness produced. 

However, the eye does not respond to all wavelengths in 

the 380- to 780-nm range equally. It does not take nearly 

as much energy in a beam of 555-nm light for it to appear 

as bright as 650-nm light. Figure 2 shows the brightness 

response of the eye to different wavelengths of light in 

terms of a percentage of the response at the wavelength to 

which the eye is most sensitive (555 nm). This is an average curve; a particular individual's response might vary 

assessing light energy for illumination design. Light measuring instruments incorporate filtering systems andlor 

employ special sensing devices that discriminate against 

light wavelengths ifi accordance with this function. Moreover, the function is assumed in the definition of all standard systems of lighting units as will be discussed later. 

PHYSICAL BEHAVIOR 

As a light travels from a light source to a person's 

eyes and encounters objects in between, it can be altered 

in many ways. This section examines these alteration processes which are important to understanding both the basis 

of what is seen and the light control methods that may be 

employed in illumination design. 

When light or any other form of electromagnetic radiation encounters an object, it is transmitted, reflected, andlor 

absorbed. Transmitted light is the light that passes through 

an object. That is, it goes in one side and comes out another. 

Reflected light is light that does not penetrate an object; 

rather it bounces off the object's surface. Absorbed light is 

all the light that is neither transmitted nor reflected; rather 

it is "soaked up" by the material. The energy of absorbed 

light is changed to some other form such as heat. 

Three ratios-absorptance, reflectance, and transmittance--can be used to quantify how light energy striking a 

given material is distributed. These ratios define the proportion of absorbed, reflected, or transmitted light relative 

to the total amount of light striking an object. 
If an object selectively absorbs light at certain wavelengths, while allowing other wavelengths to be transmitted or reflected, the object is said to have discriminant 

absorption properties. The discriminant absorption properties create wavelength imbalances in the reflected or transmitted light. As noted earlier, these imbalances are disguished by the eye as various colors. Objects that appear 

red under a white light contain molecules, given the special name pigments, which absorb wavelengths in the bluegreen portion of the spectrum and reflect red light. Blue 

objects have pigments that absorb wavelengths in the 

green-red portion of the spectrum. Other colors, such as 

brown, purple. or pink, can be obtained through the proportionate mixing of pigments with different discriminant 

absorption properties. 

It is important to recognize that color rendering 

through the discriminant absorption of various wavelengths of light is a subtractive process. That is, the wavelength mixture in the reflected or transmitted light can 

only be a subset of the wavelength mixture in the light 

that struck the object. The total energy of reflectedtransmitted light in a given wavelength region cannot 

exceed thc energy of light in the wavelength region that 

composed the incident beam. However, the relative proportion of one wavelength region to other wavelength regions 

in the reflected or transmitted beam may be changed, which 

will change the beam's color. If an object that reflects only 

wavelengths from the red portion of the spectrum and 

absorbs all others is illuminated by a light consisting of 

blue and green wavelengths only, the object cannot reflect 

The eye does not see light that is traveling through 

air (space). Only the source of the light and the objects 

reflecting light from that source can be seen. For examplc. 

the sunlight striking the Moon at night cannot be seen. 

The space from the Sun to the Moon is completely dark. 

Only the light being reflected by the Moon's surface is 

seen. The reflected light, in turn, allows details of the 

Moon's surface to be discerned. 

Eyes sense the light that enters them, process the 

character of this light, and interpret this character back to 

the object that reflected it. This is accomplished by "focusing an image" on the light-sensitive surface of the eye (the 

retina). Focusing an image is process by which the light 

reflected from points on an object in the direction of the 

eye are. through optical mechanisms within the eye, 

directed or focused to form an image of the object on the 

retina. (See chapter 3 for more details.) In this respect, light 

acts as a coded signal carrying information about an object 

that reflects it to the eye where the signal is subsequently 

decoded and translated. Spatial relationships, brightness, 

and color can be discerned. Hence, it is reflected light that 

has greatest bearing on what is seen. Because of this, lighting designers must evaluate the environment to find out 

(1) the proportion of light the environment reflects and (2) 

how the reflected light is distributed. As noted previously, 

the proportion or ratio of light specific surfaces in the environment reflect compared to incident light on these surfaces is measurable and is called reflectance. The other 

variable important to designers is distribution of reflected 

light. 
I I 

DIFFUSE REFLECTION 

B. Circular reflector 

I 

SPREAD REFLECTION 

I 

MIXED REFLECTION 

Flgure 3.- Classltlcatlon of reflectors. 

C. Elliptical reflector 

Flgure 4.-Curved retlectors used to control light dlstrlbutlon. 
greater than the tfosted lamp. Diffusion always results in 

some reduction of light energy transmission and, thus, 

reduces efficiency of a light fixture. Through proper design 

of enclosed fixtures, the amount of energy loss can be 

reduced significantly by interreflection, shown in figure 6. 

Many transmitting mediums may selectively transmit some wavelengths while absorbing or reflecting others. 

This property can be used for removing certain wavelengths 

to obtain a desired wavelength composition of the transmitted beam. Such a material can change the color of light 

with little alteration of light distribution. The dichroic 

reflector, an example of this type of medium, is used in 

some headlights to reflect a beam of visible light forward 

but transmit infrared wavelengths backwards; if reflected 

fbiward, the infrared energy would tend to heat objects 

and people. 

The atmosphere is never a perfect transmitter of light 

(ti-ansinittance (TI + 1) even in what appears to be the 

clearest of conditions. This factor must be taken into 

account in some design problems, especially when fog or 

dust levels are significant and/or transmission distances 

are large. For such problems, transmittance is described 

by a transmissivity ratio, transmittance divided by unit 

distance. 

For instance, light transmissivity is about 0.94 per 

mile in a very clear atmosphere. That is. 94 pct of the light 

reaches a receiver 1 mile away and 88 pct (0.94 X 0.94) 

reaches a receiver 2 miles away. But in even a very light 

fog, the transmissivity reduces sharply to only 0.05 per 

Figure 5.-Use of surface patterns to control light dlstribution. 

A TYPICAL MATERIAL WITH 607. 

TRANSMISSION MIGHT HAVE AN 

85% EFFICIENCY IN TRANSMITTING 

LIGHT BECAUSE OF INTER-REFLECTION 

Flgure 6.-Effect of interreflection on net light transmission 

for enclosed llght flxtures. 
(IES) and International System of Units GI). The primary 

difference between IES and SI systems is that the IES 

system uses U.S. standard measures for linear dimensions 

in the unit definitions, while the SI system uses metric 

measures. Current U.S. coal mine lighting regulations customarily use IES units; therefore, these will be used primarily in this report. 

Systems of lighting units are unique in that they 

explicitly apply a human weighting function to the physical energy quantity they measure. That is, all unit systems take into account how the eye exhibits different sensitivities to various light wavelengths in terms of perceived 

brightness and weight the energy measurements according to the spectral luminous efficiency curve (fig. 2). 

Radiometric units are used to quantify other types of electromagnetic radiation. They are similar to light energy 

units, but do not include a weighting function. 

Figure 7.-Effect of changing transmission mediums on directlon of light travel. 

Each of these are discussed in detail. Examples are pro- 

! lumen tlm) Figure &-Use of curved lens to control light distribution. 
The IES and SI systems have different units for 

illumination. The lux, used by SI, is the average illumination produced by 1 lm of light distributed over a 1-m2 

receiving area. The footcandle, used by IES, is the average 

illumination produced by 1 lm of light distributed over a 

1-ft2 receiving area. 

As an example of the illumination concept, if light 

energy flowing at the rate of 9 lm is distributed over area 

A, which is equal to 1-ft2, the illumination of area A is 9 fc. 

If the same energy is subsequently distributed over larger 

area B, which is equal to 3-ft2, then the average illumination of area B is 3 fc, one-third as great as area A. 

It should be noted that the total illumination levels 

from two or more light sources shining on a surface are 

LUM FLUX 

, ,\I, , , 

p

ie.g., luminance), gnd (3) enable easy on-site verification 

that specifications are being met because of the relative 

simplicity of taking illumination measurements. 

Specifications presented in terms of illumination levels do not consider how a receiving surface reflects light, 

however. This is a definite shortcoming because, as noted 

previously, reflected light determines what is seen. Various properties of the light-receiving surfaces in the surrounding environment can affect reflectivity. 

Illumination and the Cosine Law 

The cosine law is one of two very useful lighting laws. 

(The other-the inverse square law-is discussed later.) 

Based on geometric principles, the cosine law states that 

the illumination of a surface varies as the cosine of the 

angle between an imaginary perpendicular line to the surface (i.e., the normal) and the actual direction of the incident light (i.e., angle of incidence, 8, see figure 10). To 

illustrate the cosine law, imagine a light beam consisting 

of uniformly distributed parallel rays traveling in a particular direction. Assume that 10 lm is incident upon surface 

A, with a surface area of 1-ft2, in figure 11. Note that 

surface A is perpendicular (normal) to the direction in 

which the light is traveling. The average illumination of 

this surface could be calculated according to the discussion 

in the previous section as follows: El = UA of 10 Im divided 

by 1 ft%quals 10 fc. 
10-lm beam intersects a surface rotated 0 degrees, as surface A' is in figure 11. The illumination is now uniformly 

distributed over a larger area (A" which is equal to A', 

divided by the cosine of 0. 

Therefore E2 = +/A2 which equals +/Al X cos 0 or 

El X cos 0. 

If 0 is 45", E2 = E, cos 45 = 10 (0.51 = 5 fc. 

Although the cosine law derived was for average 

illumination, the law also applies equally to point illumination. 

Luminous Intensity SymbolFI IES and SI unit 

candle (c) 

Luminous intensity is a concept used to describe how 

a light source (e.g., a lamp or luminaire) distributes the 

total luminous flux, or lumens, it emits into various portions of the space surrounding the source. The geometric 

concept of the solid angle is used to define the particular 

portion of surrounding space in question. Before luminous 

intensity is discussed, the geometric concept of solid angle 

must be explained. 

A solid angle is simply a three-dimensional angle. It 

is formed by a point at the center of a sphere and a surface, 

of any shape, which comprises a part of the surface of the 

sphere. Figure 12 illustrates various forms of solid angles. 

The dimension of a solid angle, w, is determinated by 

finding the area, A, of the sphere subtended, or enclosed, 

Problem.-A mining vehicle incandescent headlight with a spot beam has an effective beam angle 

of 30°, as shown in figure 13. What spherical surface 

area will the beam subtend at 10 and 20 ft? (Note, 

beam angle can be converted to solid angle by using 

the formula w = n (1 - cos 0) where 0 is the beam 

angle.) 

Solution.-Equation 2 (w = A$R2) defines the 

steradian. Therefore, any spherical subtended area, 

A,, at any distance, R, is A, = wR2. Substituting the 

equation given for converting plane angles to solid 

angles, A = 2n (1 - cos 0)R2. 

For 3o0%eam angle, 0, and radius of 10 ft, A, = 2~ 

(1 - cos 30")(102) or A, = (0.84)(100) which equals 84 

ft". When R equals 20 ft, A, = (0.84) (400), which 

equals 336 ft2. 

This problem shows how the light distributed from a 

luminaire might be described using the solid angle concept and how this concept conveniently describes beam 

spread with distance. 

Luminous intensity is defined mathematically as 

follows: 

I = Vw, (3) 

where I is equal to the luminous intensity of a point source 

of light in a direction defined by a particular solid angle, + 
0" to 10'-0.50 X 1,800 = 900 lm, 

10" to 20'-0.35 X 1,800 = 630 lm, and 

20" to 30"-0.15 X 1,800 = 270 lm, for a total of 

1,800 lm. 

The average intensity of each portion of the beam 

can be calculated using equation 3. 

Figure 13.-Beam angle of minlng vehlcle headllght. 0" to loo-I = 900/0.095, or 9,474 c; / 10. to 20"-1 = 63010.284, or 2,218 c; and 

LINES REPRESENT EOUAL 

AMOUNlS OF LUMINOUS POINT SOURCE 

FLUX PER LINE 

20" to 30"-I - 27010.463, or 583 c. 

This is a more accurate and useful description or 

distribution of light from the headlight than the 

2,143-c average intensity previously calculated. 

Nonetheless, the preceding example may have provided useful information if the light source was a 

more uniform emitter, as are some lamps. 

INTENSITY IN WA LESS lHAN 

_I INTENSITY IN Wg 

Inverse Square Law 

A common problem in lighting system design is determining the illumination on surfaces at various distances 

from a light source. This problem can be handled using the 

Flgure 14.-Concept of lumlnous lntenslty. inverse square law. 
sion of the light source. Consequently, using the law is 

practical for most purposes encountered in lighting design, 

except where long, tubular light sources (e.g., fluorescent 

lamps) are used. 

A second assumption inherent in the inverse square 

law is that the surface area is perpendicular to the direction of light flow. When this is not the case, the inverse 

square law can be combined with the cosine law as follows: 

POINT SOURCE 

OF LIGHT 

WR,,~ 

Figure 15.-Basis of the inverse square law. 

1/D 2 

Solving for E2 

Ez = El D21/D22 = El D21/D2. 

For the first problem, the value of E at 10 ft was 

94.7 fc. Using the alternative solution for E, at 15 ft 

E = 94.7 (10/15)2, 94.7 (0.44), or 42.1 fc. 

As shown previously, the illumination level on a 

surface oriented at angle 0 to the centerline of the light 

source is adjusted by a factor equal to the cosine of 8. 

Therefore, the alternative equation for E2 becomes 

E2 = E1(D1/D1)2 cos 0. At 8 = 30°, Ez = 94.7(10/15)" 

cos 30°, which is 94.7 (0.44)(0.866) or 36 fc. 

Candlepower Curves and Their Uses 

The most common way of presenting lighting data on 

luminaires and lamps is by the candlepower curve or some 

variation of this curve. A candlepower curve is a plot of 

the intensities of a light source in a particular plane, at all 

angles around it. Figure 16 illustrates a candlepower curve 

for a household incandescent fixture. As shown, these 

curves are usually plotted using polar coordinates. 

Candlepower curves are extremely valuable for various design calculations. They depict how a light fixture 

distributes its total luminous flux into the surrounding 

space. Also, because of the relationship between illumination and intensity-the inverse square law-these curves 
obtain the intensity in candles at that distance. 

0" . . . 96 fc . . 9,600 c 18" . . . 9 fc . . 900 c 

3" . . . 98 fc . . 9,800 c 21" . . . 4 fc . . 400 c 

6" . . . 89 fc . . 8,900 c 24" . . . 3 fc . . 300 c 

9" . . . 60 fc . . 6,000 c 27" . . . 2 fc . . 200 c 

12" . . . 42 fc . . 4,200 c 30" . . . 0 fc . . 0 c 

15" . . . 17 fc . . 1,700 c 

Figure 18 shows these values plotted on polar coordinate paper to obtain the candlepower curve. 

A candlepower curve applies only to the intensities in 

a single plane that passes through the light fixture. To 

fully describe a fixture's light distribution, several candlepower curves in different planes may be necessary, depending on the symmetry of the light distribution. 

One of the primary uses of candlepower curves is to 

utilize these data to calculate the surface illumination that 

the fixture would provide, given the geometry and dimensions of a particular setting. The following sample problem illustrates an application of this technique, which 

involves the inverse square and cosine laws. Note that the 

technique is also applicable to determination of illumination levels obtained from more than one luminaire. In this 

case, the calculations would be performed for each luminaire separately and the obtained illumination values 

would be added at each point. 

' 2000 10'00 

Flgure 18.--Candlepower curve for a mlnlng vehlcie headllght. 
Table 2.-Footcandle values for different dlstances from lamp 

axle and speclfled dlstances from coal face 

Because the candle power curve for the headlight 

is symmetric about the lamp axis, lines of equal illumination will form concentric rings as shown in figure 20. 

Distance from 

lamp axis, fl 

Distance from headlighl 

to face, ff8 1 12 I 16 

pg p

power curves can be used to obtain illumination patterns 

on a surface for various luminaire locations. When curves 

are not symmetric, as for a fluorescent lamp, similar 

calculations can be performed using candlepower curves 

for different planes and then drawing lines to connect the 

points of equal candlepower levels. 

Three conclusions can be drawn when comparing the 

values in table 2. As distance from the face is increased1. The luminaire distributes the light over a wider area; 

2. Peak illumination levels are decreased; and 

3. Differences in illumination between the various points 

tend to decrease; i.e., light distribution becomes more even. 

Depending upon their intended use, candlepower 

curves can be converted to other forms for the sake of 

convenience. A commonly used alternative form is the 

isofootcandle curve. The following problem illustrates the 

conversion to and the application of isofootcandle curves. 
qg p

applied in the preceding problem. 

Luminance Symbol - L IES unit - Candle per 

square inch (din2) 

SI unit - Candle per 

square meter (c/m2) 

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 an eye. The area of a light source, in practice, may 

be an area of a light reflecting surface, such as a wall or 

desk top; an area of a light emitter, such as a lamp; or an 

area of a light transmitter, such as a diffusing lens on a 

luminaire. The most common definition of luminance, L, 

is 

L = I I =- A, projected A, cos 0 

Referring to figure 23, I is the intensity of light produced 

by area A, of the source in the direction of the receiver, P, 

and A, projected is the projected area of the source when 

. . 

35' 30' Ib' 10' 

SCALE 1"=20' 

Figure 21.-lsofootcandle curve for a mlnlng vehlcle headlight. 

(FOOTCANDLE LEVEL) 

DISTANCE FROM 

BEAM AXIS 

v COAL FACE 

Flgure 22.-Pattern of llluminatlon wlth headlight 35 R from 

mlne face (determined by isofoatcandle curve). 
source while keeping the intensity of the source constant. 

In this case, envision 10 lighted wax candles first distributed over a 25-in2 area and then over a 10-in%rea. If the 

point of observation was distant from the two areas, the 

intensity in that direction would be approximately constant in both cases. However, the average luminance (see 

equation 6!, as well as the subjective evaluation of brightness would be greater when the candles were distributed 

over the smaller area. 

Finally, vary the distance of observation. In this case, 

consider observing the wall of a room from 8 and 12 ft. 

From equation 6 it can be seen that luminance is independent of distance. Likewise, the wall appears smaller, but 

not more or less bright as one moves away from it. 

To summarize, luminance and, therefore, the perception of brightnessVary directly with the intensity of a light source of constant area; 

Vary inversely with the area of a light source having a 

given intensity; and 

Are independent of distance of observation. 

A final variable of importance is the direction of 

observation. As noted previously, both light sources and 

light reflectors do not always distribute their flux uniformly; hence, their intensity varies with angle of observation. In addition, the projected area of the source also 

varies with angle of observation. Because of this, no generalizations can be made with respect to the effect of angle 

of observation except that angle of observation does have 

din" and L 9,400 1915.8 in2, or 594 clin2. 

Notice thg t=he distance to P or Q did not enter the 

calculations since luminance is independent of distance. 

As noted in a previous section, isointensity curves for 

luminaires are obtained by taking illumination (footcandle) 

measurements in a low-reflectance room and calculating 

intensity values using the inverse square law. Since coal 

mines and mine simulators are typically low reflectance, 

approximate luminance values of machine-mounted luminaires can be calculated directly from illumination measurements taken on-site without the necessity of having 

candlepower curves. The following problem explains this 

technique. 

Problem.-The measured illumination at an operator's station 6 ft from a single luminaire is 4 fc. For 

purposes of this measurement, the illumination 

meter was angled to the luminaire for maximum 

reading. The projected area of the luminous surface 

of the luminaire in the direction of the operator is 30 

in'. Solve for the average luminance of the lightemitting surface. 

Solution.-lo = ED2 and L = Io/Ap; therefore, 

L = ED2/~p, L = (4X62)130 or 4.8 c/in2. 
If the luminance at any angle H is determined, then Lo 

= I,/Ao = I,,, X cos HIA ,,,, , X cos 8 = I,,,/ 

A,,, = constant. 

For a small, flat surface, luminance is constant for 

any angle of observation. Such a surface is called a 

"perfectly diffuse" reflector or emitter. Many materials, 

such as a wall painted with a flat paint, approximate this 

distribution of emitted or reflected luminous flux. 

For such a perfectly diffuse reflector or transmitter, 

the ratio of the luminance, L, of the surface to the total 

lumens per unit area emitted by the surface can be shown 

to equal a constant, T; luminance divided by lumens emitted per unit areas equals the constant, T. 

Because of this relationship, illumination designers 

often consider lumens emitted per unit area to be the same 

as the luminance quantity. If the area is in square feet, 

the quantity lumens per square feet is given the special 

name of footlamberts. Luminance measured in footlamberts 

will be represented in this text by the symbol L'. Substituting into the constant equaling equation, L' = LIT. 

The conversion between footlamberts (an L' quantity) 

and candles per square inch (an L quantity) is 144 X n or 

452 fL = 1 din? The value of the footlambert concept 

becomes evident in the next section. 

use of the formula a practical matter in many cases. 

Although coal has a specular component in its reflection, 

the cleaved surfaces are generally not well oriented. In the 

main, coal can be considered a diffusing surface, and for 

practical purposes of analysis, perfectly diffusing. In some 

design problems, detailed reflectance measurements from 

many angles of observation are often made to assess the 

limits to which this equation may be applied. 

Problem.-Using the footcandle distribution derived for a surface 35 ft from a headlight, determine 

the luminance of the surface if it is coal with a 

reflectivity, p? of 4 pct or if it is rock-dusted coal with 

a reflectivity of35 pct. In both cases, assume that the 

surface is a perfectly diffuse reflector. 

Solution.-Because it is assumed that the surface 

is a perfectly diffuse reflector, luminance values in 

footlamberts can be determined by multiplying the 

footcandle values by the reflectivity-L = p X E. 

Therefore L' = 0.04 X E for coal and L' = 0.35 X E 

for rock-dusted coal. 
tioning of the visual sensory system, and (2) the light 

needed to establish an appropriate level of visibility necessary for safe, efficient work performance. 

The lighting design process begins by carefully determining these needs. Then practical, technical, and economic factors are considered in establishing an appropriate system design. 

This chapter explains how light and vision interact. It 

identifies the visual needs of coal miners and indicates, in 

general terms, what can be done to accommodate those 

needs. First, the functions of the eyes and the rest of the 

visual sensory system are examined. Then, various environmental factors that affect the visibility of surroundings 

are discussed. 

THE EYE AND HOW IT WORKS 

The eye (fig. 27) is the organ of sight. It senses the 

light that enters it and acts as the first processor of this 

light. It then provides this information to the brain information for determination of the form, size, shape, color, 

position, and motion of the objects in view. 

To understand how light and vision interact, it is helpful to consider the eye as a mechanism made up of two 

subsystems: (1) the light control system and (2) the receiver-decoder system. The parts of each system are outlined in the following tabulation. 

Cornea. which are therodsand cones. 

Iris-pupil. 

Lensand itsciliary muscle. 

Parts of the Light Control System 

The light control system focuses light on the light 

sensitive surface of the eye and controls the amount of 

light to which the receiverdecoder system is exposed. The 

parts of the light control system (see figure 27) are theEyelid: A flap of skin that covers and protects the eye. 

Under extreme brightness, it closes to reduce the amount 

of light entering the eye. 

Cornea: The clear, bulging, front portion of the sclera. It 

permits light to be transmitted into the eye and works in 

conjunction with the lens to focus light on the retina. 

Iris-pupil: The iris is the colored portion of the eye consisting of muscle tissue that extends over the lens. The iris 

defines the circular opening called the pupil (the pupil 

itself is not an actual structure). Light passes through the 

pupil to the lens. The iris automatically controls the size of 

the pupil and, therefore, acts as a diaphragm controlling 

the amount of light entering the eye. 

Lens-ciliary muscle: The lens is a flexible, transparent 

capsule surrounded by a ring of muscle tissue, called the 

ciliary muscle, directly behind the iris. The lens works in 

conjunction with the cornea to focus light on the retina. 
LIGHT CONTROL SYSTEM OPERATION 

Light Focusing and Accommodation 

The eye focuses an "image" of the surrounding environment on the retina by using two parts of the lightcontrol system-the cornea and lens-to bend the incoming light. The light is bent in such a way that all the 

"rays" of light reflected in the direction of the eye from a 

particular point, P, on an object being viewed are projected 

onto a corresponding single spot on the retina (see figure 

28A 1. Spatial relationships between the various points on 

the object are maintained in the image on the retina. The 

photoreceptors, in turn, respond to the focused image, and 

the response is integrated by the brain to perceive an 

object's form and shape. If the eye did not bend the light in 

such a manner, the light from a particular point on the 

object would spread over a relatively large area of the 

retina and only a blur would be perceived (fig. 28B). 

The amount of light bending necessary to obtain a 

focused image varies with the distance between an object 

and the eye. The ciliary muscle adjusts the shape of the 

eye's lens, and this changes the degree of light bending. 

The process of changing the lens shape when focusing 

objects at various distances is called accommodation (see 

figure 29). When looking at distant objects, less bending is 

required, the ciliary muscle is relaxed, and the lens is 

DISTANT VISION I NEAR VISION 

Figure 29.-Lens shape change when focusing (accommodation). 

flattened. When looking at nearby objects, more light bending is required, the ciliary muscle is tense, and the lens is 

rounded. This tension in the ciliary muscle is why some 

people get eye strain after looking at close objects after a 

period of time. 

Pupil Size 

Pupil size is another important light control function 

of the eye. As noted previously, pupil size is defined by the 

position of the iris on the lens. Pupil size serves the following two purposes: 

1. It automatically regulates the amount of light permitted to enter the eye under different light levels. When the 

retina is exposed to low light levels, pupil size gets larger 

(dilation). When the retina is exposed to high light levels, 

pupil size narrows (constriction). 

2. Pupil size prevents light that would pass through 

aberrations (i.e., deviations from appropriate curvature) 

on the outer edge of the cornea and lens from entering the 

eye, This is accomplished by opening only a limited central area of the lens for light passage. If not for the pupil, 

the aberrations would somewhat distort the image on the 

retina. This is why the pupil constricts for near vision. 
signals are interpreted merely as various shades of grey. 

Figure 30 illustrates this interpretation process. 

All the details of the process by which the photoreceptors interpret luminance and wavelength characteristics 

of the light are not precisely known. However, it is known 

that (1) the direct response of the photoreceptors to light 

relies on a photochemical reaction; and (2) there are various processes of neural interpretation of these photochemical reactions that enable brightness and color distinctions 

to be made by the brain. 

Operation of the rods is the simplest. Rods contain a 

light-sensitive chemical called "rhodopsin" or "visual 

purple." Rhodopsin breaks down into different chemical 

compounds when exposed to light. This results in a change 

in electrical energy of the rod cell, one or more of which are 

connected to a ganglion cell (see figure 31). The change in 

electrical energy accumulates to a point where the ganglion cell sends a nerve impulse (electrical) to the brain. 

The frequency of impulses is proportional to the energy of 

light incident upon the rod. On this basis, the brain 

responds with an interpretation of what is called brightness. 

Operation of the cones is more complex. The cones can 

be divided into three classes on the basis of the photochemical substances they contain. One class contains a chemical 

sensitive to short (blue) wavelengths, another to medium 

(green) wavelengths, and another sensitive to long (red) 

wavelengths. All three classes generate nerve impulses in 

a manner similar to the rods. The impulses from cones at a 

\ 

. . . . . . -a,- CONES 

PROCESSING IN IHE BRAIN 

Flgure 30.-Light interpretation process. 

RODS CONES 

GANGLION CELLS 

OPTIC NERVE FIBERS 

Figure 31 .-Rods, cones, and neural structure of the retlna. 
and one that is too late. Hence, shortening of reaction 

times is another important benefit that can be realized by 

increasing general illumination levels in mines. 

Photoreceptor Wavelength Sensitivity and 

the Purkinje Shift at Low Light Levels 

The luminous cfficiency curve was discussed previously 

Iscr figure 21. Also known as the eye sensitivity curve, this 

curve graphically illustrates how the eye exhibits different sensitivity to varictus wavelengths of light in terms of 

the brightness ptlrceived. This sensitivity varies because 

of wavelength selective characteristics of thc photochemicnls in the photoreceptors and because some of the transparent media in the eye tend to absorb or filter out certain 

wavelcngt,hs. The former factor is the most significant. 

The luminous efficiency curvc was del-ived for typical lighting conditions where cone functioning dominates; it., at 

luminance levels greater than 0.05 ft. Cone-dominant 

vision is called photopic vision. 

What about low luminance levels, say of less than 

0.01 fL, which nlay be found in some areas in underground 

mines? Remember that in such low luminance levels, primarily only the rnds arc functioning. Rod-dominant vision 

is called scotopic vision. 'I'hese rods contain different photochemicals than the cones. 'The effect is that the eye becomes 

more sensitive to shorter wavclength light. This change 

in sensitivity is called the Purkinje shift (named after its 

Figure 33.-Area of a coal surface 10 fl from observer (assumed 

reflectivity of 4 pct) that is illuminated above the minimum level 

for total cone functlon by a typical cap lamp beam. 

PURKINJE SHIFT 

I44 10 

0.8--NIGHT 

(scoToPlc) 

-ROD VISION 

CURVE 2 

0.6 

LIGHT WAVELENGTH IN NANOMETERS 

Figure 34.-Relative spectral iumlnous efficiency for scotopic 

end photopic vision depicting Purkinje shift. 
g

phol~chemicals in your eye to become too great to accommodate the sudden increase in the amount of light. After a 

while. you become comfortable and seeing is easy. 

On a sunny afternoon, you walk into a cinema. Initially, all you can see is the screen. Adaptation to the 

sunny outdoor env~ronment has caused the concentration 

of photochemicals in your eyes to become so low that you 

cannot see details you normally would in the low level of 

light in thc cinema. After a while, your eyes become 

adjusted and the surrounding details become visible. 

These examples illustrate that (1) changing adaptation from light to dark or vice versa requires time, and (2) 

during that time, the ability to see details is below that 

which will be achieved once adaptation is complete. 

Adaptation is reached at different rates by the cones 

and rods. The cones achieve adaptation much quicker. 

Thus, light adaptation (going from a dark environment to 

a light one) is relatively fast. Dark adaptation (going from 

light to dark) proceeds at a much slower pace. Figure 35 

illustrates dark adaptation. The first part of the curve 

represents the dark adaptation of the cones. This usually 

takes 1 ttr .5 min. The curve levels off at the point where 

the rods <ome into play. Another 10- to 30-min period is 

required for the rods to be completely adapted, as can be 

seen in the second drop in the curve. It may be a total of 30 

to 40 min until dark adaptation is complete. -. 

When comparlrlg paints of equal tohl reflectance, paints that reflect 

the shorter wavelenfihs should be used. 

pg y 

seconds, it is not so noticeable as local adaptation. 

To avoid visual loss from either of these two causes, 

the visual environment should be lighted with a reasonable degree of uniformity, avoiding excessively bright or 

dark areas, especially within the field of common visual 

tasks. 

0 10 1'5 2'0 25 3'0 3'5 4.0 

TIME IN DARK, MINUTES 

Flgure 35.-Effect of dark adaptation on seeing. 
cases, there is a separate passage for each photoreceptor. 

In others, several receptors are connected to a single passage through a ganglion cell. This "receptor-to-nerve" ratio 

is quite important. When the ratio of photoreceptors to 

nerve passages is low, sensitivity to light is relatively low, 

but ability to resolve detail is high. When many photoreceptor~ share the same passage, sensitivity to light is high, 

but detail resolution is impaired. The general relationship 

between photoreceptor-nerve ratio, depending on type of 

photoreceptor is shown in table 4. 

Table 4.--General relatlonshlp between 

photoreceptor-nerve ratio 

Because of these differences in sensitivity and detail 

resolution, the distribution of rods and cones across the 

retina determines how we see in different parts of the 

visual field. Figure 37 shows this distribution. Overall, 

there are 20 times as many rods as there are cones in the 

retina. The rods generally outnumber the cones except in 

one area-the fovea-which is a tiny region in the center 

of the retina. At the fovea, the cones are packed exceedingly close together. The distribution of the rods is more 

Photoreceptor-nerve ratio . . . . . L~ght sensitivity . . . . . . . . . . Ability to resolve detail 

' Generally. 

Cones 

Low' 

Low 

High 

Rods 

High 

Hlgh 

Low 

even though forms are not sharp, spatial reference is good. 

This is especially important for the performance of tasks 

where overall perception is necessary, such as maneuvering a wide roof bolter to its proper position in an entry 

crowded with timbers. 

The peripheral field of vision, because of its high sensitivity to light, is also sensitive to movement. Hunters know 

that their prey is often detected as movement seen "out of 

the corner of the eye," as opposed to detecting the details 

of the animal's appearance. When peripheral vision detects 

movement, a reflex is initiated which turns the eye to 

bring the object into central vision. Thus, the peripheral 

field of vision serves as an early warning device, which 

may alert a miner to on-the-job hazards. One instance 

where peripheral vision is often critical is in detecting 

rock dribble, which can precede a roof fall. Lighting systems that adequately illuminate areas in the peripheral 

field greatly enhance functioning of this hazard detection 

process. 

Boundaries of the Visual Field 

The boundaries of the human visual field are determined primarily by facial contours. Figure 38 shows these 

boundaries for a stationary head with eyes fixed at the 

center. The dark portions represent the cutoff by the eyebrows and cheeks. The gray portions, right and left, represent the regions seen by the respective eyes alone. The 

border between the gray and white areas represents the 
horizontally, receive somewhat different views. This small 

difference between the images is known as disparity. It is 

disparity that gives depth perception. In the eye, stereoscopic vision functions primarily for comparatively near 

objects (closer than approximately 110 yd). 

Eye and Head Movements 

The eyes do not get a detailed picture of an object just 

by focusing it upon the fovea. Eye movements are also 

essential to vision. When looking at a moving object, such 

as coal on a conveyor belt, the eyes follow along with 

smooth movements. When looking at a stationary object, 

the eyes scan the area of interest with a series of small 

rapid jerks. The function of eye movements is to keep 

sweeping the image over the retina so that the image does 

not fade away and thereby stop signaling the brain. 

Both the eyes and the head may be moved to look at 

an object of interest. If an object lies in the central 40" of 

the visual field (see figure 39), normally only the eyes are 

moved to look at it. Eye movement alone can also be used 

to fixate on an object within the central 90" of the visual 

field, but for extended direct viewing of any object within 

either the 40" or the 90" field, the head position will be 

readjusted. Beyond the central 90" of the visual field, head 

movement is used for looking at an object. 

In low-coal conditions, head movement is often restricted, as when drilling and installing roof bolts. Under 

Miner Nystagmus 

Under low light levels where the cones do not function, 

the point of focus shifts 20" to the side of the fovea where 

rod concentration is greatest (see figure 37). If the eyes 

work under such conditions for extended periods of time, 

they will not adjust to focusing on the fovea under normal 

light levels. This was a common problem for miners before 

the electric cap lamp was introduced. The condition is called 

miner's nystagmus. Spasmodic movement of the eyes, 

either rotary or from side, is a common symptom. The 

incidence of miner's nystagmus, once a very serious occupational health problem, has virtually disappeared since 

the introduction of the electric cap lamp. 

COMMON VISUAL DEFECTS 

This section discusses some common visual defects and 

the effects they may have on work performance. Some of 

these visual defects can be corrected through optical means. 

Others can be at least partially compensated for by proper 

illumination design. For others, no correction is available, 

and they may limit performance of certain tasks in mines. 

Refractive errors are problems with the cornea-lens 

system such that the eyes do not focus a sharp image on 

the retina. The net effect is that the light reflected from a 
measures. 

Concerning safe and eficient task performance, it is 

often the case that the work experience of the older worker 

mental contrast and vision is that, under given lighting 

conditions, the greater the contrast the "easier" it is to see 

objects, details, and spatial relationships. 

Table 5.-Common classes of refractive errors 

Class Definition 

Myopia ......... Images from distant objects are focused in front of the retina. Also called nearsightedness. 

Hyperobia ....... Images from distant objects are focused behind the retina. Also called farsightedness. 

Astigmatism ...... Unequal curvature of the refractive surfaces of the eye. As a result, a ray of light is not sharply 

focused on the retina, but is spread over a more or less diffuse area. 

Presbyopia ....... Farsightedness and impairment of vision due to advancing age. This occurs because of problems in bending the lens (accommodation). Causes problems for viewing near objects. 

Table 6.-Age-related eye defects and posslble correctlve measures 

Defect Possible corrective measure' 

Presbyopia ............... Cataract condition ............ Increased glare sensitivity ........ Decrease in ability to detect 

movement in peripheral field. 

lncreased llght scatter in eye 

(disability glare). 

Decrease in ability to see detail ..... Increased adaptation time ........ 

Bifocal lenses. 

Medical anention. 

Careful placement of lighting apparatus. etc. 

lncrease luminance levels. 

Careful placement of lighting apparatus. etc. 

Increase luminance levels. 

Gradient spacing of luminaires from light to dark areas. Also, minimize 

extreme brightness differences in the visual field. 

' ~he-best corrective measures required in a given mining situation could be considerably more extensive than those noted 

here. 
If you were to reduce the illumination on the pages of 

this report, it is likely that you would have more difficulty 

reading the text. You would not have changed the contrast 

between the letters and paper. Contrast, as noted above, 

remains constant under uniform illumination, regardless 

of the particular illumination level. However, by reducing 

illumination levels, you would have reduced your ability 

to detect or distinguish contrasts. 

Figure 40 shows "minimum perceptible contrast," the 

smallest contrast that can be distinguished, as a function 

of background luminance levels (which are directly proportional to level of illumination). This curve was derived 

from data collected under the following conditions: 

1. The object being observed was a small circular spot 

or disk that subtended a solid angle of 4' in the observer's 

field of vision. 

2. The spot had a higher luminance than its background. 

3. Observation time was 0.1 s. 

4. Each point on the curve represents the level of luminance contrast necessary for detection of the disk by an 

observer 50 pct of the times they were presented. 

The curve shows clearly that lower contrast can be 

detected as illumination levels are increased. At a background luminance of 0.1 fL, contrast of the spot has to be 

higher than 0.5; i.e., there had to be at least a 50 pct 

reflectivity difference between object and background to 

mance. 

The use of this relationship between contrast detection and illumination to define appropriate illumination 

levels is discussed later. The following sections discuss the 

other major variables that affect the ability to detect 

contrasts. 

Effect of Object Size 

It is accepted that larger objects are easier to see. Size 

is measured in terms of visual angle (fig. 421, usually in 

units of "minutes of arc" or degrees (60' equals lo). This 

accommodates for the effect of distance from the observer 

upon the apparent size of the object. Table 7 shows the 

visual angle subtended by objects of given overall linear 

dimensions tH in figure 42) at various distances from the 

eyes of the observer. 

Table 7.-Relatlonshlp between vlsual angle 

and object dlmenslon 

Apparent object slze at 2 

V~sual d~stances from observer, ~n 

angle 

' Approximately 1:64 in 
reflectance range (0 to 100 pct). Figure 43 shows the results 

of such an experiment. It is obvious that there is a minimum size object that can be discerned regardless of level of 

illumination. This limit occurs at a visual angle of 0.4' (28 

ten-thousandths of an inch when viewed from 2 ft) and 

is imposed by physiological characteristics of the eye (i.e., 

photoreceptor size and spacing). Also notice the asymptotical nature of the curve. If one were to increase light 

levels from 100 to 1,000 fL (a 10-fold increase in illumination probably accompanied by a 10-fold increase in energy 

costs), one would only gain about a 5-pct increase in ability to see detail at this contrast level. 

Object size, contrast, and illumination level are, of 

course, interrelated. A change in the magnitude of one 

variable affects the magnitude of the others necessary for 

perception of the object. The "spot on uniform background" 

experiment discussed in the previous section has been conducted with size of the spots a variable, and the results are 

shown in fi yre 44. 

It is evident from figure 44 that increased levels of 

luminance (obtained by increasing illumination) are necessary to maintain perception as object size decreases at any 

given level of contrast between object and background. 

Notice, also, that the curves are farther apart as object 

size decreases. Unless contrast is very high, extremely 

high levels of illumination (1,000 fL or more) may be necessary in performance of tasks involving discernment of small 

details (e.g., certain line assembly tasks). Localized illumination, concentrated on the task vicinity. is a practical 

method for attaining these levels. 

H 9 ARCTAN (E) 

8 H TAN - 2 

= - 20 

Figure 42.-Concept of visual angle. 

0.2 1 I I I I I 1 

0.01 0.1 1.0 10 100 1000 10.000 

BACKGROUND LUMINANCE IN FOOTLAMBERIS 

Figure 43.-Relatlonship between minimum perceptible object 

size and level of background luminance. 
3. Because of the steepness of the curve in the 0.001- to 

0.1-fL range (also, the range of luminance revels in many 

mines) time for perception may tend to become critical 

relative to reaction times for recognizing hazards even if 

contrasts are relatively high. For example, if contrast 

between the object and background were 0.5 (a 50 pct difference in reflectivity) and illumination levels were at 0.05 

fL, it would take at least one-third second to see the object. 

Although this example applies specifically to an object 

subtending an angle of 4', when designing or evaluating 

a lighting system, the general relationship shown by the 

curves of figures 44 and 45 can be applied to other sized 

objects to obtain an indi-ation of the "time to see." Curves 

equivalent to figures 44 and 45 are not currently available 

for a range of object sizes. 

Concept of Visual Acuity 

Thus far, the discussion has been directed at contrast 

detection and the major environment factorwontrast, illumination level, object size, and viewing time--which affect 

this ability. As may have been noticed, the experiments 

cited to explain these relationships involved very simple 

discriminations-was, or was not, the disk-shaped test 

object visible. What if the experiment were made a little 

more difficult and the observer was asked not only to distinguish the presence of the object but to discern whether 

it was a "C" or an "0." Such an experiment would yield a 

measure of "visual acuity" or ability to resolve fine details. 

quantified in terms of the visual angle subtended by the 

"critical detail." Figure 46 illustrates how the dimension 

of the critical detail is assessed in some common experimental test objects. Visual acuity is the concept employed 

in the Snellen chart used in standard vision tests. 

The difference between visual acuity and simple contrast detection is that perceptual processing in acuity 

experiments of visual information is considerably more 

complex. Moreover, the level of processing may differ from 

one acuity task to another. The level and complexity of 

processing involved in distinguishing between a hexagon 

and an octagon would differ substantially from distinguishing between a C and an 0. Hence, visual acuity cannot 

always be directly related to contrast detection, but experiments to date show that it does vary in a similar manner 

with luminance level and time of exposure. 

Quantifying and Measuring Visibility 

An attempt to quantify the visibility of an object is an 

attempt to quantify the degree of difficulty in seeing it. 

Lighting designers define the visibility quantity, V, as the 

ratio of the contrast between the object or detail in question and its background under a particular level of background luminance, CaCt, ,, to the minimum contrast 

between the object or detaiaand its background that would 

be necessary to just perceive the object at the same background luminance level, Cthreshold; i.e., V = C ,,,,, 11 

'threshold. 
Note that such an assessment is conveniently made with

out the direct measurement of contrast. 

Use of Visibility Measurements 

A measurement of the visibility of a particular task 

applies only to the level of background luminance under 

which the measurements were taken. One can estimate, 

however, how this visibility level will vary with changes 

in illumination level. The process is to relate field measurements to the relationship between visibility and background luminance defined by the spot on uniform background experiment. This relationship, as noted previously, 

has been carefully measured in the laboratory. In general 

terms, this is accomplished as follows. 

1. The critical detail of the task is reduced to threshold 

by a visibility meter. 

2. The threshold contrast of the 4' test object is identified for the particular background luminance of the visibility measurement. Since both the 4' test object and the 

critical detail being measured are at threshold, their visi- - bilities are equal. That is, Vtask = Vtest object - Vthmhold 

= 1. 

Now the amount of contrast reduction that was necessary 

to reduce the task to threshold can be utilized to determine the equivalent contrast the test object would have 

had if it had undergone the same contrast reduction (see 

figure 49). 

-8.0 

TRANSMI1TANCI.I. -Ioo7. ' VAlLlNG LUMINANCE 1. 

A Fu~~lI~~ OF REFLECTED DIFFUSELY OFF GLASS 

GLASS THICKNESS, I, 

Figure 48.-Principle of vlsibliity meter operation. 

I C 

z 

0 10 u 

w -4 

E 

,- 

E 1 u REDUCTION NECESSARY 10 .I Y 

L 

Z 0.1 

f 

0.01 , I I I 

0.001 0.01 0.1 1 10 100 1000 

BACKGROUND LUMINANCE IN FOOTLAMBERlS 

Figure 49.--Determining contrast of 4' test object which Is equlveient to real world teak critical deteli using vlslbiiity meaeurements. 
performance, including vision, processing of visual information, and motor skills. The significance of vision (and 

hence, illumination level) is difficult to resolve independent of the other factors and may vary from task to task. 

For example, the speed and accuracy of typing are easily 

measured, but how significant was lighting to the results? 

Were the errors a result of not seeing the words correctly 

or were they a result of deficiencies in motor skills? 

Similarly, is it necessary to distinguish every letter, 99 pct 

of the time when reading, since the tendency is to reorganize the letters in "chunks" called words or phrases. 

The procedure defined in the previous section provides 

valuable insight but not q precise definition of the optimum illumination level for performance of a particular 

task. The net effect on performance must be considered if 

the most cost effective level of illumination is going to be 

established. Much work is currently being carried out to 

define this interrelationship. 

TYPES OF ARTIFICIAL ILLUMINATION 

Lighting systems are frequently classified into three 

types on the basis of how they deploy and distribute luminous flux. 

1. General lighting. 

2. Localized general lighting. 

3. Local or supplementary lighting. 

Machine shops, rough machining .............. Rough assembly ...................... Central station coal conveyor. crusher, feeder, scale areas. 

pulverizer, fan area. transfer tower. 

Coal tipples and cleaning plants, breaking. screening, and 

cleaning. 

Railroad yards ........................ 

Defining appropriate illumination levels for underground coal mines is a complex task. In the early 1970's. 

the Bureau of Mines sponsored two studies to determine 

recommended levels of background luminance for underground coal mines. The following are major conclusions 

drawn from these studies, which had significant importance in the establishment of the current regulations. 

The threshold for photopic vision is approximately 0.05 

fL. The luminance in any position of the visual field within 

50" of the fovea (center of vision) should exceed 0.05 fL. 

In one of the studies, 90 mining tasks were evaluated 

(visibility measurements taken) to determine the minimum luminance required for mine workers to see adequately. A sampling of results is shown in table 9 along 

with the luminance provided by the central portion of a 

miner's cap lamp. Of 90 evaluated tasks, 16 pct required 

luminance levels greater than 0.07 fL. The maximum luminance required by a task was 0.819 fL and the minimum 

required was 0.004 fL. In most cases, illumination provided by direct viewing with a cap lamp was in excess of 

the minimum required for the tasks. Note that these values are a small, but statistical sampling. The luminance 
p, pg

ground shall be required to wear an approved personal cap 

lamp or an equivalent portable light. 

Current regulations often require the use of a complex combination of general, localized general, and supplementary types of lighting. The need for these systems has 

evolved as a result of numerous field evaluations and studies conducted by the Bureau of Mines and MSHA. The 

regulations also address factors including glare, equipment 

reflectance requirements, luminaire design, and methods 

for illumination measurement. The regulations are discussed in detail in IC 9074. 

COLOR IN MINING 

As noted in chapter 2, a color must be present in a 

light source if it is to be seen in the light reflected by an 

object. White light comes from a source (lamp) emitting 

radiant energy relatively balanced in wavelengths across 

the visible spectrum and permits all colors to be seen. 

Many light sources, however, emit only specific wavelengths in the visible spectrum. For example, high-pressure 

sodium lamps, often used in coal mines because of their 

high efficiency in converting electrical energy to light 

energy. emit wavelengths in the yellow and orange part of 

the spectrum. Because of this, color rendition is limited. 

Coal, even under white light, has an indistinguishable hue. Therefore, accurate color rendition when viewing coal itself is usually not important. But color rendition 

y, p 

tivity of the paint is generally a more important factor 

than color. In most instances, a highly reflective paint is 

preferable, regardless of color. 

Colors are, or can become, associated with certain 

meanings. Thus, color is often used as a means of coding, 

or signaling. When using color codes, it is important to be 

consistent. Workers are more likely to respond automatically in the event of an accident if hazards are uniformly 

color coded. Table 10 contains some recommended color 

codes for signs and for painting equipment where caution 

must be exercised in handling. 

Table 10.-Color codes 

Color Associated meaning 

Red ............... Danger or stop 

Yellow .............. Caut~on. 

Green .............. Safe or go. 

Black on yellow ......... Hazard. 

When using color codes, be sure the dimerrsion of the 

color coded area subtends at least a 0.5" solid angle. This 

requires assumptions to be made on viewing distance (fig. 

50). Smaller dimensions may make the color indistinguishable. This guideline may be applied to the marking 

of escape ways. 

Imagine that there is a light flashing on and off at a 

frequency of N or hertz cycles per second. If the frequency 

N is greater than the critical fusion frequency, the impression of perfectly constant luminance is perceived. The eyes 
old or defective fluorescent tubes fail in their attempts to 

start. The problem of flickering fluorescent lights can be 

minimized by proper lamp selection, maintenance, and circuit design, as discussed in IC 9074. 

(e.g., when the light flashes red it means one thing and 

flashes blue it means another) and the transmissivity (see 

chapter 2) of the media between the signal and observer is 

low (e.g.. dusty air), this factor may become important. 

CHAPTER 4.-DISABILITY AND DISCOMFORT GLARE 

Glare is addressed in this chapter from an engineering standpoint. Techniques to assess the merits of alternative illumination system concepts involving measurements 

and computations are presented. Application of specific 

methods to eliminate or reduce glare are presented in IC 

9074. 

Glare is a very significant factor in determining the 

design of underground coal mine illumination systems. It 

is the most frequent cause of complaint and dissatisfaction 

with mine illumination systems, and it may detract significantly from the benefit of these systems. It is so prevalent 

in the mine environment because luminaires must often 

be located close to the mine worker's line of sight and they 

are viewed against a very dark background, which results 

in a high contrast between the light source and its surroundings. 

Glare is a visual sensation that can result in annoyance, discomfort, loss in visual performance, or reduction 

of visibility. Some degree of glare is constantly experienced in an illuminated environment. Out of doors, the 

Sun can be an extreme source of glare. The headlights of 

an oncoming car at night is a classic glare example. Less 

obviously. when driving at night, even the dashboard lights 

inside the car are a source of glare since they reduce the 

ability to see the road and can also be a distracting source 

of discomfort. 

Glare is a complex subject. Its effect on visibility, visual 

performance, and comfort is dependent on many variables 

including the brightness of the glare source, general luminance level of the environment, sensitivity of an individual to glare. location of the glare source, and nature of the 

visual task. Unfortunately, no simple method is currently 

available to determine the acceptability of an illumination system from the standpoint of glare when light sources 

are in the field of view. The methods presented in this 

chapter are solely for the purpose of providing a means for 

the designer to determine the relative merits of alternative design approaches to eliminate or reduce glarc. 
pace a sed betweeyousead te uae a 

manner that does not change the illuminance on the face 

and again observe how well you can see the object. Visibility of the detail will likely improve. This difference in 

visibility is a measure of level of disability glare. In conducting such an experiment, it is important to realize that 

one should not "blame" the luminaire for a loss of visibility. 

Indeed, were it not there, visibility would be far less. Disability glare must be viewed as a tradeoff. Some level of 

disability glare is present in every illumination setting. 

The engineering feat is to minimize its effect while maintaining adequate net visibility levels. 

Although one generally associates discomfort with the 

term glare, discomfort does not necessarily accompany disability glare. Imagine standing on a sunny day near the 

base of a hill a few hundred yards or so from the opening of 

a tunnel into the hillside. The tunnel appears black and no 

detail within the opening can be discerned. One could, 

however, view the tunnel opening through a tube and, if 

the tube was long enough, details within the tunnel opening could be perceived. The effect of the tube is to reduce 

the amount of stray light that enters the eye and ultimately causes a veiling of the retinal image of the tunnel. 

In this example, disability glare exists without noticeable 

discomfort. This can also be the case with underground 

mine lighting systems. One should not rely on a complaint 

of discomfort as stimulus to investigate a glare problem. 

pp

cos 0. Substituting these relationships in the 

equation for veiling luminance gives L, = (KIT/O~)(I@~) 

cos 8. On the basis of this discussion it is evident that the 

effects of disability glare are greatest for high intensity 

luminaires that are close to the observer and, also, close to 

the line of sight. As the luminaire is moved away from the 

line of sight, the disability glare effect decreases rapidly 

since Lv is inversely proportional to both 8' and D2. 

PLANE PEIPENDICULAI 

10 LINE OF SlOHl 

OLARE SOUICE 

Figure 51 .-Geometric parameter tor dlsablllty glare. 
standpoint of minimizing disability glare. The following 

example illustrates their application. 

Application of the Disability Glare Relationships 

The machine depicted in figure 54 is a continuous 

miner for which five lighting arrangements have been 

established, each capable of providing the minimum required 2-fc face and roof illumination. In each system, a 

AGE (YEARS) 

Figure 52.-Dleablllty glare constant age factor. 

REPOSITIONED GLARE SOURCE 

CONCEPT (81 

CLARE SHIELD ADDED 

CONCEPT (D) 

HEADLAMP ADDED 

CONCEPT (E) 

Flgure 54.-Example of dlsablllty glare analysis. 
figure 47 was selected as a visibility criteria curve for this 

analysis. The equation for this curve is C' = [(0.482/L)0.4 

+ 1]2.5/16.846, where C' is the apparent contrast perceived by the machine operator and L is the apparent background luminance, Lb + L,, which equals 0.55 fL. 

Solving the above equation for C', C' = 0.315; therefore C' = 0.315, Ld = 0.0810.55, and Ld = 0.253. Ld is the 

threshold luminance of the object. The threshold contrast 

for the task is therefore Ctask = Ld - L,,lLb, 0.253 -0.081 

0.08 or 2.16, and the threshold reflectance of the object is 

pd = Ld/E, 0.25312.0 or 0.126. 

Using this procedure, values of threshold contrast, 

Eg;! and threshold object reflectance, pd, for the other four 

ing concepts can now be'computed and compared. Note 

that these computed values are not meaningful in themselves because of the many assumptions and simplification of the illuminated environment. However, on a comparative basis, the values are meaningful in determining 

the relative effect of the varied parameters on operator 

visibility. The four alternate concepts shown in figure 54 

are described as follows. 

Concept b 

The fluorescent luminaire was repositioned (rotated 

90"). The change reduced the illumination from the glare 

source on the operator. 

bility follow. 

Remove the glare source completely from the field of 

view or shield it from view, preferably with an opaqueglare 

shield. The advantage of shielding is shown when comparing the threshold object reflectances for concepts a and d. 

Adding the glare shield to concept a reduced the required 

object reflectance needed for visibility from 0.126 to 0.082. 

Reduce the illuminance from the glare source that 

impinges on the observer as much as possible. Consider 

alternative luminaire types and orientations that minimize the illuminance. Note the position of glare source in 

concept b significantly reduced the glare illuminance compared with the same luminaire in concept a. 

Place luminaires as far fromprincipal lines of sight as 

possible. Veiling luminance values are reduced significantly as the angle, 0, is increased since L, is a function of 

0 squared, L, = K IT E.l102. 

Note that when the angle was increased from 15" to 

30" in concepts b and c, L,, was reduced from 0.12 to 0.026 

fL. As 0 approaches zero, ;eiling luminance values become 

extremely high. These values are considered valid for val

- - ues of 0 as low as lo as long as the source does not impinge 

on the true line of sight. 

Increase the illuminance of the task object and background. For example, in concept e the task illumination 

was doubled as compared with illumination in concept d 

and the threshold reflectance of the object was reduced 
jg

mine illumination systems. A recent Bureau study, confirmed that, from the standpoint of discomfort glare, underground coal mine illumination systems can be very uncomfortable. In this study, discomfort glare was measured 

in a simulated mine on various illumination systems 

mounted on a continuous miner and roof bolting machine. 

The results of the study also present suggested modifications to the standard equations for evaluating discomfort 

glare. These modified equations are presented in this 

chapter. 

The basic relationships for quantitative discomfort 

glare analysis are outlined in the following discussion and 

a suggested method to evaluate and compare alternative 

illumination systems for the purpose of minimizing discomfort glare is presented. This method does not predict 

whether a particular design is acceptable from the standpoint of discomfort glare. To determine how much discomfort glare is acceptable is a complex task in underground 

coal mines since the overall benefit of improved visibility 

may at times outweigh the adverse effect of discomfort 

glare. For example, headlamps on continuous miners can 

at times be a source of extreme discomfort glare, yet their 

benefits in visibility usually outweigh the disability glare 

they intermittently impose on the mine worker. As in the 

case of disability glare analysis, the presented method of 

computing discomfort glare is solely for the purpose of 

providing the designer with a method of comparing the 

merits of alternative design approaches. The absolute values of computed discomfort glare are not precise because 

See figure 55. A is the projected area of glare source in 

the direction of tRe observer, in square feet. 

Q is a function of the solid angle, o, that subtends the 

glare source and extends to the observer as indicated in 

figure 55. The empirical definition of Q is Q = 20.4~ + 

1.520 0.2 - 0.075 and o = Ap/R2. 

r A GLABF SOURCE 

BACKGROUND 

. DISTANCE FROM 1HE OBSERVER 

TO rnl GLARE souacl 

W. SOLID ANGLE 

Flgure 55.--Geometric parameters involved In discomfort glare 

evaluation. 
nance on the observer's eyes on a plane that is normal to 

his or her line of sight. This method of measuring field 

luminance is shown in figure 57A. Note that although 

the measurement is in footcandles, if one assumes the background to be a perfectly diffuse reflector, this value is also 

the field luminance, in footlamberts. The meter must be 

cosine corrected to accommodate for the significant luminances that impinge on it from various angles. Remember 

the meter measures the lumens per square foot impinging 

on it from all directions over a solid angle of 2n sr (a 

hemisphere), Since a definition of the footlambert is a diffuse source that also emits 1 lm/ft2, one can imagine the 

measurement equivalent to that of a uniform diffuse field 

in front of the observer. Note that since the acceptance 

angle of the photometer is 180" (2n sr) and the field of view 

of an observer is less (approximately 5 sr) this approach 

introduces some error. Also, a meter with good cosine correction over all incident angles illumination is required. 

Method 2.-With this method, a diffuse reflecting card 

of known reflectance is placed at the observer's location as 

shown in figure 57B. The luminance of the card is proportional to the field illumination that would have been 

impinging on the observer. The field luminance, F, can be 

calculated by dividing the measured luminance of the card 

by the reflectance of the card surface or F = Lcard/pcard. 

The card surface should be normal to the line of sight of 

the observer. 

Method 3.-This procedure permits the calculation of 

field luminance by averaging the various luminances over 

GLARE SOURCE 

OBSERVER'S 

LINE OF SIGHT 

OBSERVER 

PHOTOMETER 

(A) DIRECT MEASUREMENT OF FIELD LUMINANCE 

WITH AN ILLUMINANCE PHOTOMETER I GLARE SOURCE 

BACKGROUND 

GLARE SOURCE 

OBSERVER'S 

LUMINANCE' 

METER 

DIFFUSE 

REFLECTOR 

OBSERVER 

(0) DIRECT MEASUREMENT OF FIELD LUMINANCE 

USING A DIFFUSE REFLECTOR AND LUMINANCE METER 

Flgure 57.-Direct measurement methods to evaluate average 

fleid lumlnance. 
VISUAL COMFORT PROBABILITY (VCP). PERCENT 

Figure 58.-Discomfort glare ratlng versus visual comfort 

probability for mlning populatlon. 

g

p

naire options to determine which one results in the lowest 

amount of discomfort glare in the location and surrounding luminance as shown in figure 59. Assume the following measured values for options 1 through 3: 

Area of light emitting surface . . ft' . . . . 0.25 0.5 0.5 

Measured illuminance on observer 

due to glare source alone . . . . fc . . . . 3.0 3.0 2.0 

In each case, the light emitting surfaces are flat diffuse 

surfaces. The background luminance is 0.06 fL. 

Solution.-The discomfort glare can be compared by computing and comparing the DGR and VCP rating for each 

approach. Since there is only one glare source in the field 

of view, the equation for DGR is DGR = M or LQIF'F*.~~. 

For option 1, L = nER2/Ap, source brightness. Ap = 0.25 

cos 45" or 0.176 ft2; R = 5tcos 45" or 7.07 ft; L = ~(3) 

(7.072)/0.176 or 2,676 n; Q = 20.4 ws + 1.52 wsO," 0.075; 

ws = A /R2, 0.176t7.07~ or 0.0035 sr; and Q = 20.4(0.035) 

+ 1.5283.0035~.~) - 0.075 or 0.487. 

To solve for the position factor, P, refer to figure 56. 

VIR = sin 45" or 0.707, L/R = 0, and position factor, P, is 

4.5. 

The average field luminance impingingon the observer 

is F = + Lsw, cos 8 15; wb + w, = 5 sr; w,, = 5 - 

0.0035 or 4.996 sr. Therefore, F = 0.06(4.996) + 2,676 

(0.0035)(0.707)15 or 1.38 fL, and DGR = 2,676(0.487)/4.5 

(1.38O.~~) or 261. 
Figure 60.-Discomfort glare analysis for multiple glare 

sources. 

VCP can be read directly fi-om figure 58-VCPlUmi- 

= 40 pct. The computed values of DGR and VCP 

for options 1 through 3 are as follows: 1-DGR 261, VCP 

40; 2-DGR 169, VCP 60; and LDGR 127, VCP 70. Option 

3 would result in the least amount of discomfort glare for 

the specified observer location. 

Multiple Glare Sources.-A continuous miner is depicted in figure 60. Evaluate the DGR and VCP rating for 

the indicated line of sight of the helper. Assume the following measured parameters of the system: 

Measured luminance of a diffuse reflector placed at 

the observer and facing his or her direction of sight is 1.0 

fL. The reflectance of the reflector is 80 pct. 

The locations of the glare sources as measured relative to the line of sight for luminaires 1 and 2 are as 

follows, in feet: 

Radius, R . . . . . . . . . . . . . . 6.0 5.5 

Vertical, V . . . . . . . . . . . . . -3.0 -2.0 

Lateral, L . . . . . . . . . . . . . . 1.0 4.0 

Projected areas, Ap, of the sources in the direction of 

the helper, in square feet, are 1-0.25 and 2-0.40. 

Illuminance on the helper, in footcandles, is 1-1.5 

and 2-3. 

when it is applied to underground mine illumination sys

tems follow. 

The field luminance is a complex function that, computed or measured, can be related to the basic laboratory 

measurements from which the glare equation was derived 

in an approximate manner only. 

The equation for computing Mt is based on multiple 

luminaires in ceiling arrangements. This equation may 

not accurately represent multiple luminaires that occur 

both above and below the line of sight. 

The luminaire size factor, Q, is for w values ranging 

from approximately 0.0001 to 0.13 sr. When solid angles 

exceed 0.13 sr for a single source it is suggested that the 

source be treated as multiple sources each having a size 

less than 0.13 sr. This method of handling large sources is 

an approximation only. 

Position factors, P, are not known for all points below 

the line of sight. The method recommended for positions 

below the line of sight is based on the broad assumption 

that position factors below the line vary in a manner similar to those above the line of sight. 

While this method of analysis does not, nor is it 

intended to, predict whether a lighting system is acceptable or not from the standpoint of discomfort glare, it provides the designer with a means to objectively evaluate 

alternate concepts to minimize discomfort glare. The 

method can significantly assist in assessment of this difficult problem.