Handbook of Mine Lighting

8/3/2019 Handbook of Mine Lighting

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Bureau of Mines Information Circularll986

Underground Coal MineLighting Handbook(In Two Parts) 1. Background

Compiled by W. H. Lewis

UNITED STATES DEPARTMENT OF THE INTERIOR


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Information Circular 9073

Underground Coal MineLighting Handbook(In Two Parts) 1. BackgroundCompiled by W. H. Lewis

UNITED STATES DEPARTMENT OF THE INTERIORDonald Paul Hodel, SecretaryBUREAU OF MINESRobert C. Horton, Director


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Library of Congress Cataloging-in-Publication Data

Main entry under title:Underground coal mine lighting handbook.(Information circularNniled States Department of theInterior, 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.) 1. Series: Information circular (United States. Rurcauof Mines); 9073.

TN309.U47 1986


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CONTENTSPage

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2Chapter 1.-Coal mine ligh ting . . . . . . . . . . . . . 2History . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 3Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3P~oductivity . . . . . . . . . . . . . . . . . . . . . . . 4. . . . . . . . . . . . . . . . . . . . .pinion surveys 4Major technical considerations . . . . . . . . . . . . 4Chapte r 2....Light physics . . . . . . . . . . . . . . . . . 5Perception . . . . . . . . . . . . . . . . . . . . . . . . . . 6Physical behavior . . . . . . . . . . . . . . . . . . . . . 6Absorption . . . . . . . . . . . . . . . . . . . . . . . . 7. . . . . . . . . . . . . . . . . . . . . . . . .eflection 7Transmission . . . . . . . . . . . . . . . . . . . . . . . 9Quantifying light energy . . . . . . . . . . . . . . . . 10Illumination and t he cosine law . . . . . . . . . . 11Inverse square law . . . . . . . . . . . . . . . . . . . 13

Candlepower curves and their uses . . . . . . . 14Average luminance a nd candlepower curves . . 18Perfectly diffuse reflectors a nd emitte rs . . . . 19Relationship among reflectance,illumination. and luminance . . . . . . . . . . . 19Limits of agreement between luminance. . . . . . . . . . . . . .nd perceived brightness 20. . . . .hap ter 3.-Light and vision relationships 20The eye and how it works . . . . . . . . . . . . . . . 20. . . . . . . . .arts of the light control system 20. . . . . . .art s of the receiver-decoder system 21. . . . . . . . . . . .ight control system operation 21. . . . . . . .ight focusing an d accommodation 21. . . . . . . . . . . . . . . . . . . . . . . . .upil size 21. . . . . . . . . . . .ther light cont. ol functions 22. . . . . . . . .eceiver-decoder system operation 22Photoreceptor function . . . . . . . . . . . . . . . . 22Light levels needed to stimulate photo-. . . . . . . . . . . . . . . . . . . . . . . . .eceptors 22

. . . . . . . . .ime and photoreceptor responsePhotoreceptor wavelength sensitivity and. . . . .he Purkinje shift a t low light levels. . . . . . . . . . . . . . . . . . . . . . . .daptation. . .isual field an d perception within th e field. . . . . . .mpact of photoreceptor distributionBoundaries of the visual field . . . . . . . . . . .Binocular a nd stereoscopic vision . . . . . . . . .Eye and head movements . . . . . . . . . . . . . .Miner nystagmus . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .ommon visual defectsEnvironmental factors affecting vision . . . . . .. . . . . . . . . . . . . . . . . .ontrast and seeingRelationship between contrast detection. . . . . . . . . . . . . . .nd illumination levelsEffect of object size . . . . . . . . . . . . . . . . . . .Effect of time . . . . . . . . . . . . . . . . . . . . . .Concept of visual a cuity . . . . . . . . . . . . . . .. . . . . .uantifying and measurin g visibility

Use of visibility measurements . . . . . . . . . .Consideration of performance . . . . . . . . . . .. . . . . . . . . . . .ypes of artificial illuminati on . . . . . . . . . . .ypical task illumination levelsColor in mining . . . . . . . . . . . . . . . . . . . . . .. . . .hapte r 4.-Disability and discomfort gla reDisability glare . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .isability glare relationshipsApplication of the disability glarerelationships . . . . . . . . . . . . . . . . . . . . . .Discomfort glare . . . . . . . . . . . . . . . . . . . . . .

Discomfort glar e equati ons . . . . . . . . . . . . .Visual comfort probability . . . . . . . . . . . . .Limitations of th e discomfort glareanalysis . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .ni t of measure abbreviations

ILLUSTRATIONS1 Radiant energy (electromagnetic) spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Brightnes s sensitivi ty of the eye a s a function of light wavelength3 Classifi cation of reflector s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Curved reflectors used to control light distribu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Use of surface patterns to control light distribution6 Effect of interreflection on ne t ligh t transmission for enclosed light fixtures . . . . . . . . . . . . . . . . . . . . . .7. Effect of changing transmis sion mediums on direction of light travel . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Use of curved le ns to control ligh t distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Illumination10 Determini ng angle of incidence (0 ) when applying the cosine law . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Basis of th e cosine law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Solid ang le forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Beam angle of mining vehicle headlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Concept of luminous intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Basis of the inverse squ are 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 mini ng vehicle headlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 Geometry for performing illumination calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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UNDERGROUND COAL MINE LIGHTING HANDBOOK(In Two Parts)1. Background

Compiled by W.H. ~ e w i s '

ABSTRACTThis Bureau of Mines report and its companion report (Information Circular 9074)have been prepared as a complete reference on underground coal mine lighting. Thisreport discusses the fundamentals of light and its interrelationship with the visualprocess. The purpose of the report is to insure an understanding of the numerouscomplex and interrelated factors that must be considered to design and implement amine lighting system tha t will satisfy human needs for good vision and comfort. Topicsinclude history, objectives, and technical considerations of coal mine lighting; lightphysics; light an d vision relationships; an d disability and discomfort glare.

'Electrical engineer. Pittsburgh Research Center , Bureau af Mines, Pittsburgh. PA


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INTRODUCTIONThis Bureau of Mines report and its companion report,Information Circular 9074, have been prepared as a com-plete reference on underground coal mine lighting. Thereports are intended to assist those persons who design,install, and/or maintain mine lighting systems in makingappropriate decisions. The reports include system designcriteria and procedures, da ta and specifications to aid inselection of suitable mine lighting ha rdware, and guide-lines for system installation and maintenance.This report discusses the fundamentals of light andits interrelationship with t he visual process; necessarybackground information for anyone involved with mine

lighting. The report provides information to insure anunderstanding of the numerous, complex, and interrelatedfactors th at m ust be considered to design and imp lement amine lighting system that will satisfy human needs forgood vision and comfort.The design of good lighting systems for undergroundcoal mines is no easy task because of the unique environ-

ment and work procedures encountered i n coal mines. Theprimary objective of these reports is, therefore, to identifythe major problems encountered i n th is lighting applica-tion and to provide guidance in the solution of theseproblems. If they a re carefully designed and implemented ,lighting systems provide mine workers improved visibil-ity and contribute to improved safety, productivity, andmorale. Properly designed lighting systems can prove tobe a very cost-effective inves tment for the mine operator.

This report was prepared by BCR National Laboratory(BCRNL), Monroeville, PA, under Bureau of Mines contract50113063. The following contributed significantly to th eproduction of the report: J. Yingling and K. Whitehead,BCRNL; A. Szpak, ADS Engineering and Design, Inc.; andpersonnel of th e CIE Mine Lighting Committee, th e Illum-inating Research Institute, and Applied Science Associates,Inc.

CHAPTER 1 -COAL MINE LIGHTINGHISTORY

The theory t ha t better lighting results in safer, moreproductive operations has motivated the development andgpplication bf artificial light sources for theindustrialenvironment. After Edison ~ at en te d he first ~r act ica lincandescent lamp in 1879, industrial lighting systemsevolved rapidly to the modem l ight ing systems tha t employseveral types of light sources including incandescent,fluorescent, mercury vapor, sodium, and metal halidelamps. Research and observation, both qualitative andquantitative, indicate that improved lighting has i n factresulted in greater safety, increased production, andimproved worker comfort.Underground coal mining is one industrial activitythat has not kept pace with the application of improvedlighting technology in the working environment. Thereare several reasons for this, including the following:

1. Initially, mining lagged behind other industriesbecause (1) early lamps had short service life in this appli-cation because of lack of mechanical strength, and (2) heirlight output was low, which provided little improvementover the open flame lamps the n in use.

2. New electrical equipment had to be introduced intocoal mines with particular care because of the potentialtha t i t could entail for explosions and mine fires.3. Systems could not be permanently installed but hadto be moved as the mine expanded and advanced.4. The abusive and hazardous mine environment re-quired the development of special and expensive hardwareand circuitry; the limited market did not provide the incen-tive to develop th is special equipment unt il m ine lightingwas required by law.Artificial lighting ha s always been a necessity in th eotherwise totally dark underground mine environment.The types of light sources that have been used in under-ground coal mine lighting, summarized in table 1, rangefrom oil soaked wood chips, reeds, and bulrushes to thepresent day incandescent and arc-discharge lamps. Devel-opments such as th e Spedding flint mill, th e flame safetylamp, and the carbide lamp were aimed at providing alight source that would not ignite a gassy environment.Prior to perfection of devices that could accomplish this,

hundreds of lives were lost to explosions initi ated by l ightsources. Initial efforts to use incandescent electric lampsin the mines occurred in Europe as early a s 1902, bu tTable 1 -Light sources used in underground mining- .-

Type Source Time per ~od. . . . . . . .aucer-type open flame lampsHanging open flame lamps . . . . . . . . . .Candleholders . . . . . . . . . . . . . . . . .Carlisle Spedding flint mill . . . . . . . . . . .. . . . . . . . . . . . . . . . . .pout lampsFlame 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 teel wheel. .Wick immersed in oil . . . . . . . . . . . . . . . .Wick immersed in 011. naphtha, or gasoline . . . . .Calcium carbide and water . . . . . . . . . . . . .Carbon electrodes and electric current . . . . . . .Incandescent amp . . . . . . . . . . . . . . . . .. .do . . . . . . . . . . . . . . . . . . . . . . . .Incandescent, fluorescent high-pressure sodium

lamps.

10.000 C.-16th century1500-1900.1800-1900.1750-1813,1850-1920.1796-present.'1910-20'1878-1920-55.1925-presenL1975-present.

' Currently used to check lor methane at the working lace.Still used in some developing countries and some 1- or 2-person mines


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electric lighting was not successfully used a t the workingface for another 25 yr.The batt ery powered personal lamp was developed inth e 1920's and required t he miner to carry a heavy, cum-bersome battery that tended to leak acid. After smallerand lighter weight batteries were developed, the lamp'swere mounted on miners' caps instead of being handheldas originally used. By 1935, the cap lamp was in commonuse and is still a primary source of light in coal mines.In 1969, Congress passed the Federal Coal MineHealth and Safety Act, which included a mandate that"directs and authorizes th e secretary to propose and pro-mulgate standards under which all working places in amine shall be illuminated by permissible lighting whilepersons are working in such places." As a result of thismandate the Bureau initiated a research and developmentprogram to develop (1) nformation to be used in establish-ing lighting regulations, and (2) lighting hardware tha tcould be safely used to gain compliance with these regula-tions. Major results of this research included-

1. Establishment of 0.06 & a s the. minimum reflectedlight level on a ll surfaces required by t he regulations to belighted:2. Development and successful demonstration of permis-sible lighting hardware for underground application.The Federal lighting regulations, 30 CFR 75.1719through 75.1719-4, were proposed in 1976 and, followingpublic hearings and comments, were amended and pub-lished in 1978. These regulations prescribe the require-ments for illuminat ion of working places, in undergroundcoal mines, while persons ar e working in such places, andwhile self-propelled mining equipment i s operated in suchplaces. A discussion of these requirements and the MineSafety and Health Administrat ion (MSHA) policies inenforcement are presented in Information Circular (IC)9074.

OBJECTIVESThe general goal of industrial lighting is to improvesafety and increase production. Statistics show tha t under-ground mining is one of the most hazardous indus tries andmining personnel should, therefore, benefit most signifi-cantly from the use of lighting tha t i s consistent with thecapabilities of modem technology. To realize these benefits,mine lighting must achieve specific goals as follows:

1. Increase the visibility of hazards.-Because of the lowluminance levels and poor contrast in coal mines, hazardshave always been difficult to visually identify. A few exam-ples of such hazards include frayed or cut cables, mis-placed tools and timbers that may prevent a trippinghazard, and slips in roof rock. A primary goal of minelighting is to increase the visibility of these objects whichare visibly manifested by subtle details, and, hence, reduceinjuries tha t may r esult if the hazards go undetected.2. Increase visual response of the peripheral field to enableearly detection of hazards.-With the narrow-beamed caplamp alone, movement of personnel, machines, and roof orrib material is difficult to detect when i t occurs in a miner'speripheral field of vision, outside th e localized main beamof the cap lamp. Another major goal of mine l ighting is toallow miners to quickly detect even subt le movement any-where in their normal field of vision. This will provide

additional time for personnel to react and thereby avoidinjuries.3. Improcred vision for performance of tasks.-If minerscan clearly see the detail s of tasks, they can perform themfaster and with fewer mistakes. Studies in many otherindustries have conclusively shown that improved light-ing increases productivity and work quality. Cost-effectiveinvestments in li ghting systems capable of producing veryhigh luminance level have been made a t many industrialplants. Hence, an important goal of mine lighting is toilluminate work areas consistent with the needs for opti-mum performance of underground tasks.4. Zrzcreasegeneral comfort and reduce fatigue.-Workingin poorly lighted environments causes worker fatigue,reduces comfort, and adversely affects morale. Improvedlighting offers considerable potential for improving thepsychological aspects of working in a mine environmentand should produce corresponding improvements in relatedareas such as productivity, absenteeism, etc.

STUDIESHow close have modern mine lighting systems cometo achieving these goals? This question can only be an-swered conclusively through controlled studies of miningoperations. Unfortunately, there are many interrelatedfactors, such as geologic conditions, seam height, and gasemissions, which can affect safety and production in themine environment. Because these conditions change con-tinually and are virtually impossible to precisely controlor to isolate, dete rmining the effect of a s ingle factor, suchas light levels, is very difficult. Unlike other safety tech-nology such as cabs and canopies, "saves" (i.e., preventedinjuries or fatalities) attributable to lighting systems ar edifficult to document or count.Although the studies described show wide variation

in the magnitude of results a nd cannot be considered con-clusive, the analyses consistently indicate improved safetyand/or production in t he lighted mine sections.A more reliable evaluation of mine lighting willrequire detailed studies similar to those conducted for light-ing of roadways, offices, schools, and manufacturing facili-ties. It should be noted that at many mines there is adeficiency in accident reporting-the lack of operators' com-ments or descriptions of lighting conditions associated withmining operations or accidents. Lighting data should beincluded as part of accident reports to provide an addi-tional basis for assessing the benefits of face lighting andin identifying shortcomings.

SAFETYHalmos%eported that in a Hungarian mine a 60 pctdecrease in the accident rate was recorded for a lightedsection compared with t he rate in a section where only caplamps were used. In the same study, another test showedtha t increasing the lighting level by a factor of 12.5 (20 to250 lx) decreased the number of accidents by 42 pct.A 24-month study conducted in a West Virginia coalmine compared the major accident records for one lightedconventional section with records from five unlighted

Influence of Lighting on Productivity and Safety in M ining.


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sections. Results showed no accidents in the lighted sec-tion and a tota l of 10 accidents (five in one section) in t hefive unlighted sections.Mishra an d Dixit3 reported t ha t 35 pct of all minoraccidents in India's coal mines can be attributed to poorlighting.

Ilalmos describes two studies on productivity. In one2-month study, the par t of the working are as illuminatedwith general lighting showed a 5 pct increase per workerin production compared with production in those sectionsusing only cap lamps. In a second mine, a similar studyshowed a 26 pct increase in production per worker.In a l-y r study at a U.S. mine: a test section equip-ped with ge nera l stope lighting showed a 1 7 pct increase intons per worker-shift over the next highest producingsection.

Opinion SurveysA joint committee with representativ es from th e

United Mine Workers of America (UMWA), BituminousCoal Operators Association (BCOA), and MSHA conductedan extensive opinion survey of room-and-pillar face person-nel in 1979, shortly afte r implementation of the light ingstandards. The survey indicated a n overall favorable re-sponse to systems implemented in seam heights greatertha n 42 in and a n unfavorable response to systems in lowerseam height mines, primarily from glare problems. Sev-eral chan ges in official enforcement of th e lighting s tand -ards resulted from the particular personnel acceptanceproblems identified in th e survey.Bituminous Coal Research, Inc., conducted a similarstudy in 1980, interviewing management and face per-sonnel at longwall and room-and-pillar mines. Both manage-ment and face personnel unanimously approved of longwalllighting, citing many benefits and no major problems.Acceptance of room-and-pillar systems increased from the1979 survey markedly so for low seam height applica-tions. Many equipment operators, despite a n overall favor-able opinion of th e rwm-and-p illar systems, cited problemarea s, indicating th at careful implementation is requiredand general improvement is possible.

MAJOR TECHNICAL CONSIDERATIONSThe underground coal mine i s one of th e most difficult

lighting applications encountered by industrial lightingdesigners. There ar e several uniq ue technical considera-tions that must be carefully addressed in designing andimplementing mine lighting systems. These considerationsar e reviewed briefly in the following and a re discussed inmore detail in other sections of this report and IC 9074.

1. Mine lighting is a nonstationary application.-Formost lighting applications, the object or area to be lightedis confined to a specific location and lighting hardware canbe permanently installed. In coal mining the working facenot only continually advances (or retreats), but severalfaces are being mined at the same time by the sameA Ne w Psychoengineering Strategy for Safety in Mines.Permissible Mine Lighting Systems Cut Down on Accidents and In-crease Coal Output.

equipment. The designer, therefore, has two choices. Oneis to mount the hardware on the machines so the areaaround the machine is lighted regardless of where it isoperating. The second choice is to mount a system semiper-manently a t each active face and move the system a s theface is advanced. Both systems pose unique problems wit hrespect to electrical design and logistics. At the presenttime, mine lighting systems are almost exclusively ma-chine mounted.2. Lighting i n a low-reflectivity, low-contrast enuiron-m.ent. -Under equal illumination, th e brightness of sur-faces with in a person's field of vision is dependent on thereflectivity of these surfaces. Reflectivity, or reflectance,is a measure of the ability of a surface to reflect incidentlight supplied by a light source (see chapter 2). In mostligh ting applications, exposed surfaces reflect a relative lyhigh percentage of light, which helps in providing goodvisibility of objects andior surface details. In t he coal mine,nearly all surfaces have low reflectivity, especially theribs an d face which have a n avera ge reflectivity of only 4pct. Therefore, to provide a given general level of surfacebrightness, the output of light sources used in mine light-ing systems would hav e to be much higher (15 to 20 times)th an would be required in applications with high reflectiv-ity surfaces.Another important lighting design consideration isthe relationship between the light level required for goodvisibility of detail, and th e contrast between th e detail andthe background aga ins t which i t is viewed. For example, ifcritical details such as roof slips or cracks were whiteagainst the dark coal or rock surfaces, very low light lev-els would be required to see them . However, there is verylittle contrast between a slip and its background (oftendistinguishing between shades of gray), so higher lightlevels ar e required to m ake them acceptably visible. Thisapplies to many othe r signs of serious mine hazards.Both low coal-surface reflectivity and low contrastrequire that light sources with relatively high output beutilized to achieve acceptable visibility. As discussed initem 6 of this section, this can lead to glare problems.Compromises between visibility levels and reducing thesystem's glar e level must frequently be made in mine light-ing system designs.3. Abusiveness of the enuironmen.t.-Few envi ronmentsexpose lighting hardw are to t he abus e found in th e under-ground coal mine. The major hazards include impacts fromstriking rib or roof, roof rock falling on the hardware,mechanical shock produced by machine vibration andmotion, and corrosion potential from moisture, salts, andother corrosive agents found in mines. These conditionsrequire special consideration in th e design and insta lla-tion of hardware if adequate service life and acceptablemaintenance costs ar e to be realized.

4. Poorpower regulation.-Light sources and associatedpower control devices are sensitive to fluctuations in sup-ply voltage, especially large and fast fluctuations, and toextended operation above or below design voltage. Theeffects of poorly regulated power include variation in lig htoutput, extinguishment of arc discharge lamps, destruc-tion of lamps, and a general reduction in service life ofpower-conditioning components because of heat buildup.These problems have been p articularly prevalent with dcpower systems, which frequently experience extreme high-voltage tran sien ts and very poor voltage regulation. A sys-tem designer must, therefore, be familiar with the charac-ter of a mine's electrical system and ta ke ste ps to protectagainst poor regulation andlor tra nsients.


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5. Increased electrical hazards.-Any time additionalelectrical equipment is added to a mining machine, thepotential for an electrical fault and an associated shockhazard is increased. This makes it imperative that thesystem designer comply with all Federal electrical designrequirements, that all electrical devices be of adequatecapacity, and that cables be adequately protected frommechanical damage. Additionally, lighting systems addanother potential source of hea t and electrical arcing which,in a gassy mine, could trigger a disastrous methaneexplosion. Federal and State regulations provide specificrequirements for design of electrical equipment to preventsuch occurrences. A system designer must comply withthese standard s, discussed in IC 9074 (chapter 21, assurethat a total system or i ts individual components will notignite an explosive gas-air mixture.

6. Glare potential.-A lighting system is effective onlyif it improves a person's ability to ascertain visual informa-tion from his or her surroundings. Experience with minelighting has shown th at i n thi s regard the effectiveness ofmany systems is impaired by glare. Factors such as thehigh light-source output ( as required by low coal-surfacereflectivity), the extreme contrast between light sourcesand their low-reflectivity surroundings, the frequent neces-sity of locating ligh t sources near the line of sight to meetprescribed light levels , and job procedures that place the

CHAPTER 2.-

Light is a form of energy tha t enables us to see; it isknown as oisible energy. It is th e tool most readily a t thedisposal of the lighting system designer to improve theseeing environment. To use it most effectively; one mustunderstand light's physical properties and behavior.The following topics and associated key concepts willbe addressed in this chapter.

Nature of light . . . . . . Radiant energy, wavelengthand frequency, and electro-magnetic spectrum.Interpretation of visible Color, brightness, and thelight . spectral luminous efficiencycurve.Physical behavior of Absorption, reflection, andlight . transmission.Lighting energy quan ti- Luminous flux, illumination,fication, lighting intensity, luminance, cosine

units . . law, and inverse square law.The section on li ghting un its p resents example problemsto illustrate the use of each major concept in designapplications.Light is a form of radiant energy; one of several knownenergy forms, which include electrical, chemical, atomic,thermal, a nd mechanical energy. Light i s called radiantenergy because i t travels through space from its point oforigin, or source, in al l directions as a repeating, or cyclical,wave pattern, much a s the mechanical wave that resultswhen a pebble is dropped in a pond. Light waves ar e madeup of both electric and magnetic energy components, andtherefore are called electromagnetic waves.

light sources in or near the line of sight of workers atvarious times in the work cycle, result in glare problemsthat are difficult to resolve. The system designer must befamiliar with the factors that determine the nature, level,and impact of glare and must also be familiar with designtechniques tha t can minimize glare potential. These con-siderations are discussed in Chapter 4, "Disability andDiscomfort Glare" and in IC 9074 (chapter 5) .

In addition to these major technical considerations,each type of machine, lighting hardware, and mine mayhave unique requirements that should be considered indesign. It is important t ha t each application be consideredindividually to assure the design for that application isoptimum.Even though problems still exist, the at titude towardmine lighting has become more positive as miners gainexperience with working i n lighted sections. Significantly,the results of the survey completed in 1980 showed that,overall, 74 pct of the miners liked mine ligh ting comparedwith 60 pct that expressed favorable opinions during thesurvey conducted in 1979. Improved lighting system de-sign should result in greater acceptance of mine lighting.The information in thi s report and IC 9074 is presented toassist the coal indust ry personnel in achieving better light -ing of their workplaces and its at tenda nt benefits.

.LIGHT PHYSICS

Electromagnetic radiati on i s a byproduct of any proc-ess where an electrical charge is accelerated. Manyprocesses, both natural and artificial, accelerate chargesand hence produce electromagnetic radiation. Those proc-esses that produce visible light generally occur on anatomic or molecular scale and involve acceleration ofelectrons. Some of these are (1) heated bodies (e.g., thecommon incandescent light bulb), (2) arc discharge (e.g.,mercury lamp), (3) fluorescence (e.g., a glow-in-the-darkwatch face), and (4 )chemical process (e.g., th e firefly).Because it consists of waves, rad ian t energy is charac-terized by two special properties: wavelength and frequen-cy. Wavelength is the distance covered as an energy cyclerepeats itself. Frequency is the number of times the cyclerepeats itself during a unit of time, for instance, in 1 s.When wavelength i s multiplied by frequency, the productis the speed at which th e wave is carried forward from itssource. For radiant energy, this speed is a constant in agiven transmission medium. The speed equals 186,000 milswhen the medium is a vacuum, and is slightly less in air.

Visible ligh t is only a narrow pa rt of a broad spectrumof radia nt (elec tromagnetic ) energy. At one end of th iselectromagnetic spectrum are cosmic rays; electric powertransmission waves are a t the opposite end. Even thoughthe effect of, uses of, and the generating processes for thedifferent groups of radi ant energy may differ, all ar e elec-tromagnetic energy differing only in wavelength andfrequency. The various categories of energy are arrangedon the electromagnetic spectrum on the basis of wave-length and frequency, as shown in figure 1.Visually evaluated radia nt energy, or visible light, istha t portion of the electromagnetic spectrum with wave-lengths between 380 and 780 nm. Longer or shorte r wave-lengths stimu late very litt le or no response in the eye.


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FREQUENCY I N HEETZ10 107: loM 10'8 )o16 lo1* 1oI1 1o1O 10' lo2 1

t . m - r I - I 1 ' I 1 7 .C?SMIC a 9 - ,".&


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VlOlET GREEN REDBLUE YELLOW

WAVELENGTH OF RADIANT ENERGY IN NANOMETERSFigure 2.-Brightness sensltlvity of tt

PHOTOMETRICRADIANT ENERGY METER BRIGHTNESS METER

5 5 0 N M LIGHT=\The sum of the three ratios always equals 1, but forany material none of the individual ratios will equal 1.

For example, a material t ha t transmits light never has atransmittance equal to 1. Rather, its transmittance isreduced by positive reflectance and absorptance values.Many objects exhibit zero transmittance, but their reflec-tance will be less than 1, reduced by a positive absorptancevalue. It is evident that an object's absorptance acts toreduce the amount of light energy leaving the object'ssurface.Absorption

If a n object selectively absorbs light a t certain wave-lengths, while allowing other wavelengths to be transmit-ted or reflected, the object is said to have discriminantabsorption properties. The discriminant absorption proper-ties create wavelength imbalances in the reflected or trans-mitted light. As noted earlier, these imbalances are dis-guished by th e eye a s various colors. Objects that appearred under a white light contain molecules, given the spe-cial name pigments, which absorb wavelengths in the blue-green portion of the spectrum and reflect red light. Blueobjects have pigments that absorb wavelengths in thegreen-red portion of the spectrum. Other colors, such asbrown, purple. or pink, can be obtained through the propor-tionate mixing of pigments with different discriminantabsorption properties.It is important to recognize that color renderingthrough the discriminant absorption of various wave-lengths of ligh t is a subtractive process. Tha t is, the wave-length mixture in the reflected or transmitted light canonly be a subset of the wavelength mixture in the lightthat struck the object. The total energy of reflected-transmitted light in a given wavelength region cannotexceed thc energy of light in the wavelength region thatcomposed the incident beam. However, the relative propor-tion of one wavelength region to other wavelength regionsin the reflected or transmitted beam may be changed, whichwill change the beam's color. If an object tha t reflects onlywavelengths from the red portion of the spectrum andabsorbs all others is illuminated by a light consisting ofblue and green wavelengths only, the object cannot reflect

ie eye as a tunctlon of light wavelength.any red wavelengths and would appear much darke r andessentially colorless. Hence, regardless of the d iscriminantabsorptive properties of an object, the eye cannot see col-ors that are not present in the source light that illumi-nat es the object. This has impact on the selection of artifi-cial light sources in cases where color discrimination isimportant, as with color coding of wire, pipes, etc.

ReflectionSpecial Importance of Reflected Light

The eye does not see light that is traveling throughair (space). Only the source of the light and th e objectsreflecting light from tha t 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 isseen. The reflected light , in turn , allows deta ils of theMoon's surface to be discerned.Eyes sense the light that enters them, process thecharacter of this light, and interpret this character back tothe object th at reflected it. This is accomplished by "focus-ing an image" on the light-sensitive surface of the eye (th eretin a). Focusing an image is process by which t he li ghtreflected from points on an object in the direction of theeye are. through optical mechanisms within the eye,directed or focused to form an image of the object on theretina. (Seechapter 3 for more details. ) n this respect, lightacts as a coded signal carry ing information about an objecttha t reflects it to th e eye where the signal is subsequentlydecoded and translated. Spatial relationships, brightness,and color can be discerned. Hence, it is reflected light thathas greatest bearing on what i s seen. Because of this , light-ing designers must evalua te the environment to find out(1) he proportion of light th e environment reflects and (2)how the reflected lig ht is distributed. As noted previously,the proportion or ratio of light specific surfaces in t he envi-ronment reflect compared to incident light on these sur-faces is measurable and is called reflectance. The othervariable importan t to designers is distribution of reflectedlight.


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HOWObjects Distribute Reflected Light Diffuse ref lectors represent the opposite extreme. Here.the t exture of the reflecti ng surface is rough. Light is scat-Objects distr ibute reflected light in different ways, tered in all directions. Such a surface would tend to appeardepending upon texture of the surface or near-surface equally bright from any direction of observation. A walllayers of th e object. The simplest case of reflection is shown painted with flat paint is an examp le of a diffuse reflector.bv a ~er fec t lvmooth reflector. such as a mirror. For such.reflectors. th e direction of the rkflected ligh t is determinedby the direction of the source of light, as shown in fi gure 3 .The beam angl e of th e light source and the beam angle ofthe reflected light are equal in size, but on opposite sidesof a line perpendicular to the surface. Objects tha t reflectlight in this man ner a re called specular ref lectors .

T h e Lisr of spec-u larRefZec tors n Con tro l l ing L igh t .-The principle of specular reflectance is used in thedesign of reflectors for control of light emissionfrom lamps and luminaires. In brief, the curvatureof th e reflector can be adjusted to control th e spreadof the beam from a point source of light. The threereflectors shown in fi y r e 4 indicate distribution tha tcan be obtained with three different shapespara-bolic, circular, and elliptical.

A . P a r a b o l i c r e f l e c ' t o r

IS P E C U L A R R E F L E C T I O N

I ID I F F U S E R E F L E C T I O N

B. C i r cu l a r r e f l e c t o r

IS P R E A D R E F L E C T I O N

IM I X E D R E F L E C T I O N

Flgure 3.- Classltlcatlon of reflectors.C. E l l i p t i c a l r e f l e c t o r

Flgure 4.-Curved retlectors used to control light dlstrlbutlon.


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Surfa ces may reflect in combinations of diffuse andspecular m anners. A polished coal surface, for example, isprimarily a diffuse reflector that reflects light uniformlyover a wide range of directions but with an increase inlight energy reflected a t t he specular angle of reflectance.Figure 3 illustrates various classifications of reflectors.Th e type of reflector, in combi nation with th e angl e ofobservation and th e incident angle of th e source light stri k-ing the object. ultimately determine the perceived bright-ness of an object and the measured light energy reflectedby a n object. Thi s leads to complications in reflectancemeasu remen t and in design procedures for reflectors, whichare a combination of diffuse and specular.

Transmission'rransmission of light thro ugh a m edium is affected by

various properties of the medium. Transparent materials(e.g., clear glas s) transm it light without scatter. Objectson the other side or within t he transp arent object can beseen in sha rp detail. Trarzslucenl objects (e.g., frosted glas s)also transmit light, but w ith some degree of scatter . Objectson the other side or within a ppear blurred in form.Light can be controlled by using a ma teria l with cer-tain transmission capabilities to cause scatter. This isknown as diffusion (see figure 5). Diffusion is extremelyimpor tant in th e design of lighti ng system s for visua l com-fort ti.e. normal glare reduction). It is achieved throughvarious means of tre ati ng th e trans mitti ng object, includ-ing surface etching, incorporation of light-scattering par ti-cles withi n th e medium, a nd application of surface coatings.The effect of' diffusion ;; to make the light sourceappear larger and less bright. Consider clear, frosted, andsoft-white household incandescent lamps. Th e two diffusebulbs make the light source appear to be larger a nd, hence,reduce the perceived brightness per un it are a. In fact, theclear bulb has a brightness per unit a re a over seven timesgreate r than the tfosted lamp. Diffusion always results insome reduction of light energy transmission and, thus,reduces efficiency of a l ight fi xture . Through proper designof enclosed fixtures, the amount of energy loss can bereduced significantly by int erreflection, shown in figure 6.Many transmitting mediums may selectively trans-mit some wavelengths while absorbing or reflecting others.This property can be used for removing certain wavelengthsto obtain a desired wavelength composition of the trans-mitted beam. Such a mater ial can change t he color of lightwith little alteration of light distribution. The dichroicreflector, a n examp le of thi s type of medium, is used insome headlights to reflect a beam of visible light forwardbut transmit infrared wavelengths backwards; if reflectedfbiward, the infrared energy would tend to heat objectsand people.

The atmosphere is never a perfect trans mitte r of light(ti-ansinittance (TI + 1 ) even in what appears to be theclearest of conditions. This factor must be taken intoaccount in some design problems, especially when fog ordust levels are significant and/or transmission distancesar e large. For such problems, transmit tance is describedby a transmissivity ratio, transmittance divided by unitdistance.For instance, light transmissivity is about 0.94 permile in a very clear atmosphere. Th at is. 94 pct of the lightreaches a receiver 1 mile away and 88 pct (0.94 X 0.94)reaches a receiver 2 miles away. But in even a very lightfog, the transmissivity reduces sharply to only 0.05 per

mile. Only 0.25 pct of th e light energy would reach areceiver 2 miles away. The concept of transmissivity rat iois primarily used in si gnal design problems (c.g. , n sclec-tion of a fan-on signal a t a mine).The direction of light backscatter is another veryimportant factor to consider when working with atmo-spheric conditions such a s dense fog or dust. Rackscatteroccurs when the particles in the a ir reflect t he light backtoward an observer looking through the medium. Back-scatt er in undesir able directions can impair visibility. Forexample, to prevent undesirable backscatter, i t is neces-sary to use low-beam headlights while driving in a densefog.

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

A TYPICAL MATERIAL WITH 6 0 7 .TRANSMISSION M IGHT HAVE AN8 5 % EFFICIENCY IN TRAN SMITT INGLIGHT BECAUSE OF INTER-REFLECTION

Flgure 6.-Effect of interreflection on net light transmissionfor enclosed llght flxtures.


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When light passes from one transmitting medium(such as ai r) to anoth er (s uch as glass), its speed willchange. Figu re 7 shows the progression of the lig ht-wavefronts traveling throu gh air into glass. Each line repre-sent s the position of th e wave front after equal timeintervals. As th e light leaves the less dense medium (air)and en ters th e denser medium (glass), it slows down andthe distance traveled in a given amo unt of time is reduced.The net eftect is a bending. or deflecting, of the light waves,which is called refraction. This principle is illustrated by astraw in a glass of water; the straw appears to be bent atthe point where it en ters the water. The degree of bendingis determined by t he r atio of th e speed of light in th e twomediums.The principle of refraction impacts upon lighting intwo primary ways.

1 . Lenses may be designed utilizing th e refraction prin-ciple to control the dist ribut ion of light froma light source.This is accomplished by adjusting the lens curvature (seefigure 8). These lenses may be used alone or in conjunctionwith specular reflectors to control lig ht.

2. The eye utilizes th e principle of refraction to obtain afocused imag e on the reti na. (T his is discussed in detail inchapter 3.)

QUANTIFYING LIGHT ENERGYThis section discusses the fu ndamental concepts th atare employed by illum ination engineers in the quantifica-

tion of light energy for design purposes and explains theirinterrclationships. These concepts enable the design er toevaluate, and thu s devise means to control, light energylevels and their distribution.TWOmajor systems of units a re current ly used for thequantification of light: Illumination Engineering Society(IES) and International System of Unit s GI).The primarydifference between IES and SI systems is that the IESsystem uses U.S. standard measures for linear dimensionsin the unit definitions, while the SI system uses metricmeasures. Cur ren t U.S. coal min e lighting regulations cus-tomarily use IES units; therefore, these will be used pri-marily in th is report.

Systems of lighting units are unique in that theyexplicitly apply a h uma n weighting function to the physi-cal energy quantity they measure. That is, all unit sys-tems tak e into account how the eye exhibits different sensi-tivities to various ligh t wavelength s in term s of perceivedbrightness and weight the energy measurements accord-ing to the spectral luminous efficiency curve (fig. 2 ).Radiometric un its are used to q uanti fy other types of elec-tromagnetic radiation. They are similar to light energyunits, but do not include a weighting function.

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

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

! lumen tlm) Figure &-Use of curved lens to control light distribution.


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Luminous flux is the ti me ra te flow of light energy. Flux isa power quantity i n th e same manner as horsepower orBtu per hour. The unit of luminous flux, the lumen, ismost frequently used to describe the total lighting powerof ligh t sources. Other l ight energy concepts (e.g., illumi-nation, luminous intensity, and lumi nance) use th e lumenin conjunction wi th various geometric quantit ies to describeth e distribution of light energy flow to th e surroundings.Light sources are often evaluated for their total lumenoutput. For example, a 100-W incandescent l amp producesabout 1,740 lm. Two or more lig ht sources can be comparedon the basis of the ir total luminous flux, or lumen, ratings.This is analagous to comparing two or more motors on thebasis of the ir horsepower r ating .Total lumen ratings of light sources are extremelyvaluable for use in m aking preliminary approximations inlighting design problems. The number of lumens neededin a given situation will help determine the size and/ornumber of lighting fixtures necessary. These rating s ar eusually available from hardware manufacturers.

Illumination Symbol-E IES u n i tfootcandle (fc)SI unit-lux (1x1

Illumination (illuminance ) s a measure of the densityof luminous flux striking a surface. Mathematically, illu-mination may be defined as:

where E is the illumination produced by the luminousflux, +, falling on a light-receiving surface of area AR (seefigure 9).The IES and SI systems have different units forillumination. The lux, used by SI, is th e average illumina-tion produced by 1 lm of light distributed over a 1-m2receiving area. The footcandle, used by IES, is t he av erageillumination produced by 1 lm of light distributed over a1-ft2 eceiving area.As an example of the illumination concept, if lightenergy flowing a t th e ra te of 9 lm is distributed over areaA, which is equal to 1-ft2, he illumination of area A is 9 fc.If the sam e energy is subsequently distributed over largerarea B, which is equal to 3-ft2, h en th e average illumina-tion of area B is 3 fc, one-third as great as area A.It should be noted that the total illumination levelsfrom two or more light sources shining on a surface are

LU M F L U X , , \ I , , ,

obtained by adding t he illumination level produced by eachsource separately. If light A illumina tes the surface to 5 fcand light B illuminates the surface to 3 fc, the surfaceillumination will be 8 fc if both lights operate simulta-neously.When determining illumination, C$n equation 1con-cerns the total lum ens strik ing the receiving surface area,regardless of the originating direction of the luminous flux.Until now, only average illumination over the ent irereceiving surface ha s been discussed. The illumination a tany point on t he receiving surface may also be determinedsimply by considering a very small ar ea ar ound th e pointas th e receiving area. Point illumination for all points on asurface equals av erage illumination if th e luminous fluxis uniformly distributed over a surface. If the illuminationis nonuniformly distribu ted, point il luminatio n, Ep, wouldvary for each point on the surface.Average illumination is a n easy quantity to measure.Instruments are available that contain light-sensitivematerials th at convert total light energy striking them toproportional electrical energy levels. The resultant electri-cal energy levels can the n be measured and read on stand-ard (although sensitive) electrical meters. Since the areaof exposed, light-sensitive material is known, the electri-cal energy values can readily be equated to illumination(energy per un it are a) values (footcandle or lux). The sig-nificance of the fact th at illumination is an easy quanti tyto measure is tha t illumination measureme nts can be con-verted to other li ght quantities (e.g., luminous intensity),which are meaningful but more difficult to measuredirectly. Such conversions are illustrated in this report.Lighting design specifications are often presented inter ms of illumi nation (footcandle or lux) levels. Specifica-tions given in such terms (1)enable a good description tobe made of light levels and how light should be distributedin a given environment, (2)mak e design calculations sim-pler than if the s~ecif icati ons re made in other termsie.g., luminance), gnd (3) nable easy on-site verificationth at specifications are being met because of th e relativesimplicity of taking illumination measurements.Specifications presented in terms of illumination lev-els do not consider how a receiving surface reflects light,however. This is a defini te shortcoming because, as notedpreviously, reflected light determines wha t is seen. Vari-ous properties of the light-receiving surfaces in the sur-rounding environment can affect reflectivity.

Illumination and the Cosine LawThe 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 thatthe il lumination of a surface varies as the cosine of theangle between an imaginary perpendicular line to the sur -face (i.e., the normal) and th e actua l direction of the inci-dent light (i.e., angle of incidence, 8, see figure 10). Toillustrate the cosine law, imagine a light beam consistingof uniformly distributed parallel rays traveling in a particu-lar direction. Assume that 10 lm is incident upon surfaceA, with a surface area of 1-ft2, in figure 11. Note thatsurface A is perpendicular (normal) to the direction inwhich th e ligh t is traveling. The average illumination ofthi s surface could be calculated according to the discussionin t he previous section a s follows: El = U A of 1 0 Im dividedby 1 ft%quals 10 fc.


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Figure 10.-Determining angle of incidence (0) when applyingthe cosine law.

Figure 12.-Solid angle forms.

Figure 11.-Basis of the cosine law.

Now, imagine that instead of intersecting a surfaceperpendicular to the light's direction of travel, this same10-lm beam intersects a surface rotated 0 degrees, a s sur-face A is in figure 11. The illumination is now uniformlydistributed 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 orE l X cos 0.

If 0 is 45", E2 = E, cos 45 = 10 (0.51 = 5 fc.Although the cosine law derived was for averageillumination, the law also applies equally to point illumi-nation.

Luminous Intensity SymbolFI IES and SI u n i tcandle (c)Luminous intensity is a concept used to describe howa light source (e.g., a lamp or luminaire) distributes thetotal luminous flux, or lumens, it emits into various por-tions of the space surrounding the source. The geometricconcept of the solid angle is used to define the pa rticularportion of surrounding space in question. Before luminousintensity is discussed, the geometric concept of solid anglemust be explained.A solid angle is simply a three-dimensional angle. Itis formed by a point a t the center of a sphere and a surface,of any shape, which comprises a par t of the surface of thesphere. Figure 12 illus trate s various forms of solid angles.The dimension of a solid angle , w, is determina ted byfinding the area, A, of the sphere subtended, or enclosed,

by the angle divided by the radius, R, of the sphere squared.Hence,

The unit of the solid angle is the steradian (sr). A sub-tended surface area of 1 t h n a sphere of 1-ft radius formsa solid angle of 1 sr .Note th at th e spherical area subtended by a solid anglevaries with the radius squared. For example, a 1-s r solidangle would subtend a 1-ft' area on a 1-ft radius sphere.The same solid angle would subtend 4 f t2 on a 2-ft radiu ssphere, or 9 ft2 on a 3-ft radius sphere. The followingsample problem shows one use of the solid angle concept ina lighting design problem.Problem.-A min ing vehicle incandescent head-light with a spot beam has an effective beam angleof 30, a s shown in figure 13. What spherical surfaceare a will t he beam subtend a t 10 and 20 ft? (Note,beam angle can be converted to solid angle by usingthe formula w = n (1 cos 0) where 0 is the beamangle.)Solution.-Equation 2 (w = A$R2) defines thesteradian. Therefore, any spherical subtended area,A,, at any dis tance, R, is A, = wR2. Substituting th eequation given for converting plane angles to solidangles, A = 2n (1 - cos 0)R2.For 3o0%eamangle, 0, and radius of 1 0 ft, A, = 2~

(1 cos 30")(102) r A, = (0.84)(100)which equals 84ft". When R equals 20 ft, A, = (0.84) (400) , whichequals 336 ft2.

This problem shows how the light distributed from aluminaire might be described using the solid angle con-cept and how this concept conveniently describes beamspread with distance.Luminous intensity is defined mathematically asfollows:

I = Vw, (3)where I is equal to the luminous in tensi ty of a point sourceof light in a direction defined by a particular solid angle, +


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is the total luminous flux (lumens) emana ting from th epoint source within the specified solid angle, and w is thedimension of th at solid angle i n steradians. The candle isthe luminous intensity uni t (both the SI and IES systems)and equals 1 Im/sr.Figure 14 illus trates the concept of luminous intensity.The lines in the figure may be thought of as representingequa l amount s of lumens, or light energy flow. As shown,solid angle A and solid angle B ar e the sam e size, but thedensity of luminous flux in solid angle B is greater than insolid angle A. Therefore, the luminous intensity is higherin B than it is in A.Luminous intensity can be compared to the sprayintensity of a g arden hose with a n adjustable nozzle. If acons tant water flow is assumed, t he nozzle can be adjustedto spray over a wide angle in a low-intensity spray or overa narrow angle in a high intensity spray. Luminousintensity differs from spray intensity in that the densityof ligh t energy flow (lumens) in a par ticul ar solid angle isconsidered rathe r t han the density of water droplets in thesolid angle.As noted, intensity is a directional quantity, with th edirection in question being defined by the lin e th at formsthe axis of the solid angle. The intensity of the light sourcemay, indeed, vary with direction. Intensity, a s calculatedfrom equation 3, is an average intensity over the entiresolid angle, w. As w is subdivided into sm aller and s mall ersolid angles, the direction of the intensity is better definedand th e dis tribution of light from t he source is more accu-rately established. Th e following problems illus trate t heconcept of luminous intensity.

ALL LUM INO 'JS FLUXEMITTED WITHINT H I S BOUNDARY

Problem.-Assume th at the manufacturer's speci-fications for a headlight show th at i t emits light a t arate of 1,800 lm. What is the average intensity of theheadlight beam?Equation 3 (I = +/w) is used to calculate an averageintensity over the e ntire solid angle formed by thebeam. If w equals 0.84 sr, the solution is I equals1,800 lm divided by 0.84 sr or 2,143 c.Problem.-Now assume tha t the light energy dis-tribution varies within t he beam of this headlight a sfollows:Portion of Share of to-beam angle tal lumens, pct0" to 10" . . . . . . . . . 5010" o 20" . . . . . . . . . 3520"to 30" . . . . . . . . . 15

Using equation 3, calculate the average intensitywithin each portion of th e beam. (Note.-Such a dis-tribution might be found with typical "spot" 150-Wincandescent headlight fixtures.)Solution.-In order to use equation 3, w must becalculated for each portion of the beam. This may bedone using t he formula w = A$Z2 and subtractingthe solid angle equivalent of the lower beam angledefining th e ran ge i n question from th e solid angleequivalent of the larger beam angle in this range.Using the formula for converting beam angle to solidangle (w = 2rr (1-cos 9)) results in the following:0"to 10"= 0.095 - 0, or 0.095 sr ;10" o 20" = 0.379 - 0.095,or 0.284 sr; and20"to 30' = 0.824 - 0.379,or 0.463 sr.

To determine lumens within each portion, multi-ply t he total lumens by the appropriate percentage,as follows:0" o 10'-0.50 X 1,800 = 900 lm,10"to 20'-0.35 X 1,800 = 630 lm, and20" to 30"-0.15 X 1,800 = 270 lm, for a total of1,800 lm.

The average in tensi ty of each portion of th e beamcan be calculated using equation 3.Figure 13.-Beam angle of minlng vehlcle headllght. 0" o loo-I = 900/0.095,or 9,474 c;/ 10. to 20"-1 = 63010.284,or 2,218 c; and

LINES REPRESENT EOUALA M OU N l S OF LU M IN OU SPOINT SOURCEF L U X PER LINE

20"to 30"-I - 27010.463,or 583 c.This is a more accurate and useful description ordistribution of light from the headlight than the2,143-c average intensity previously calculated.

Nonetheless, the preceding example may have pro-vided useful information if the light source was amore uniform emi tter, a s ar e some lamps.INTENSITY IN WA LESS l H A N

_IINTENSITY IN W g Inverse Square Law

A common problem in l ight ing system design is deter-mining the illumination on surfaces at various distancesfrom a light source. This problem can be handled using theFlgure 14.-Concept of lumlnous lntenslty. inverse square law.


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Given the intensity of the light source depicted infigure 1 5 in th e direction defined by the i llustr ated solidangle, the flow of luminous flux within that solid anglecan be calculated:

I = +/w, therefore 4 = Iw.The illumination of the depicted surface subtended by thesolid angle would be equal to this flux, 4 divided by thearea of the surface, A,.Thus E - +/As - IwIA,, since + = Iw.The solid ang le concept allows this a rea to be definedin te rms of the dist ance from the source-w = As/R2, here-fore A, = wR2Substituting A, = wR2 into the illum ination equation,it is seen th at

E = IwIA,, Iw/wR2, an d I/R2. (4)This relationship among illumination, intensity, and dis-tance between the source and light receiving surface isknown as th e inverse sq uare law. It enables illuminationof a surface to be calculated if the intensity of the lightsource and the distance between the light source and thesurface are known.In gen eral practice, it is common to calculate th e illu-mination a t a point on a surface rather th an a n area on asphere. The inverse square law can then be modified touse the distance, D, between the light source and the pointrather than the radius, R, of the sphe re. Equation 4 th enbecomes E - UD2.The inverse squ are law assumes a point source of light.Most real light sources are not point sources, however.Nevertheless, the law can be applied, with negligible error,if the distance between the light source and the illumi-nated area i s greater than five times the maximum dimen-sion of the light source. Consequently, using the law ispractical for most purposes encountered in lighting design,except where long, tubul ar light sources (e.g., fluorescentlamps) are used.

A second assumption inh erent in the inverse squarelaw is th at t he surface area is perpendicular to the direc-tion of light flow. When this is not the case, the inversesqua re law can be combined with the cosine law as follows:

POINT SOURCEOF LIGHT

W R , , ~Figure 15.-Basis of the inverse square law.

The following problem illus trates uses of the inve rsesquare law.The average int ensi ty for the central 10" of a head-light beam was calculated to be 9,474 c. Assume tha tthis is the intensity a t the beam axis.Problem.-To what level of illumination would the

headlight illuminate an area perpendicular to thelamp axis at a distance of 10 f t? At a distance of 15f t?Solution.-Using E = I/D2, when D = 10 ft, E =9,474 c/(10 ftI 2o r 94.7 fc. When D = 15 ft, E = 9,474c/(15 ft)' or 42.1 fc.Problem.-What would happen to the illumina-

tion of each surface in th e first problem if their nor-mals (with respect to the direction of the light) weretilted a t a n ang le of 30" from the lam p axis?Solution.-Equation 5 can be used to solve for Eo

of the surface titled 0" (in this case 0 equa ls 30").At 10 ft , LID" 94.7 fc, therefore, E, = 94.7 cos30" (0.866). or 82 fc.At 15 ft , UD" 42.1 fc, therefore, Eo = 42.1 cos30" (0.866), or 36 fc.Alternative solutions ar e available. If an illumina-tion level measurement, Ew, is taken a t distance Dl,the illumination level. E2 , at distance D2, can be

calculated using a variation of the inverse squarelaw show in the following.At distance Dl the illumination level is given byEl = UDZl, and a t distance D2 by E 2 = IID",. Set-ting up th e ratio of the two illumination values per-mits solving for E2:

1/D2Since I is constant, El/E2 = -'and D22/D211/D 2Solving for E2 Ez = El D21/D22= El D21/D2.

For the first problem, the value of E at 10 f t was94.7 fc. Using the a lterna tive solution for E, at 15 f tE = 94.7 (10/15)2,94.7 (0.44), or 42.1 fc.As shown previously, the illumina tion level on a

surface oriented at angle 0 to the centerline of the lightsource is adjusted by a factor equal to th e cosine of 8.Therefore, the alter native equation for E2 becomesE2 = E1(D1/D1)2os 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 UsesThe most common way of presenting lighting dat a on

luminaires and lamps is by the candlepower curve or somevariation of this curve. A candlepower curve is a plot ofthe intensities of a light source in a particular plane, at allangles around it. Figure 16 illustrates a candlepower curvefor a household incandescent fixture. As shown, thesecurves are usually plotted using polar coordinates.Candlepower curves are extremely valuable for vari-ous design calculations. They depict how a light fixturedistributes its total luminous flux into the surroundingspace. Also, because of the relationship between illumina-tion and intensity-the inverse squa re law-these curves


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may be viewed as depicting a light fixture's abili ty to illu-minat e in various directions. The higher th e intensity in agiven direction, the greater the ability of that source toilluminate surfaces in that direction. The curves areobtained by making illumination measurements a t vari-ous orientations with respect to the lighting fixture andutilizing the inverse squar e law to calculate the intensitya t tha t orientation, a s is illustrated in the followingproblem.

Problem.-Assume th at the headlight is suspendedin the center of a large room. (The room is paintedblack to minimize any secondary light reflectionsfrom th e walls an d ceiling.) An ill umination meteris used to measure footcandle levels on the horizon-tal plane around the fixture. The measurements aretaken at 3' intervals starting at the lamp centralaxis. A cons tant distance of 10 ft between the lightand meter is maintained while the measurementsare ta ken (fig. 17). The results of the measurementsare a s follows, in footcandles:0" . . 96 12" . . 42 24" . . 33" . . 98 15" . . 17 27" . . 26" . . 89 18" . . 9 30" . . 09" . . 60 21" . . 4

Because the beam is symmetrical about the beamaxis, these data can be used to plot a candlepowercurve (light intensity versus ang le of observation).Solution.-The inverse squ are law permits eachillumination measurement to be converted to anintensity meas ureme nt by multiplying it by thesquare of the distance to the point of observation-E= I/D2, therefore I = E X D2.Since all measurements were made at 10 f t , th efootcandle read ings can be multip lied by 100 (10" toobtain the intensity in candles at that distance.0" . . . 96 fc . . 9,600 c 18" . . . 9 fc . . 900 c3" . . . 98 fc . . 9,800 c 21" . . . 4 fc . . 400 c6" . . . 89 fc . . 8,900 c 24" . . . 3 fc . . 300 c9" . . . 60 fc . . 6,000 c 27" . . . 2 fc . . 200 c12" . . . 42 fc . . 4,200 c 30" . . . 0 fc . . 0 c15" . . . 17 fc . . 1,700 cFigure 18 shows these values plotted on polar coor-dinate paper to obtain th e candlepower curve.

A candlepower curve applies only to the intens ities ina single plane that passes through the light fixture. Tofully describe a fixture's lig ht distribution, several candle-power curves in different planes may be necessary, depend-ing on the symmetry of the light distribution.One of the primary uses of candlepower curves is toutilize these dat a to calculate the surface illumination tha tthe fixture would provide, given the geometry and dimen-sions of a particular setting. The following sample prob-lem illustrates an application of this technique, whichinvolves th e inverse squ are and cosine laws. Note th at thetechnique is also applicable to determination of illumina-tion levels obtained from more than one luminaire. In t hiscase, the calculations would be performed for each lumi-naire separately and the obtained illumination valueswould be added at each point.

I I S 0Flgure 16.-Candlepower curve for a typlcal Incandescent

fixture.

Figure 17.-Llght measurement to establlsh candlepowercurve for a mlnlng vehicle headlight.

' 2 0 0 0 10'00Flgure 18.--Candlepower curve for a mlnlng vehlcie headllght.


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Problem.-Usingthecandlepowercurve establishedfor the mining vehicle headlight in the previousproblem, determine the illumination levels on thecoal face if th e headlight is mounted on th e center ofthe inby end of the cutting machine and t he machineis 8, 12. and 16 ft from the face.Solution.-The inverse square law and the cosinelaw, coupled with the candlepower curve, enable cal-culation of illumination a t any point on the coal face.Perform the calculations for 1-ft intervals from thebeam axis, a s shown in figure 19.Illumination on a surface normal to the directionof light travel is defined by the inverse square law asEn,,,, = I$DZ. When the surface is not normal toD, as is the case in this problem, the actual illumina-tion i s reduced according to the cosine law- EaCtual-- En X cos 0 = IdD2 cos 0.~ o t K and 0 can be determined by using trigo-nometry. Referring to figure 19, it can be seen tha tD2 = X2+ Y2. lso, tan 0 = X/Y,herefore, 0 = arctan XIY.Perform the calculations for Y = 8 ft and X = 2 ftfrom beam axis: D2 = X2 + Y2, 22 + 8' = 68;D = m o r .25 ft; and 0 = arc ta n X/Y = ar ctan 218 or 14".To calculate EaCtunl, a value must be first ob-tained for 18, when 0 equals 14". This is done byconsulting the candlepower curve, where the valueshown for I8 is 2,300 c. Hence, EaCtual I$D2 cos 0,2,300168 cos 14",or 33 fc.

The footcandle values obtained from the calcula-tions in the preceding problem for different distances fromthe lamp axis and for the specified distances from theface are presented i n table 2.Table 2.-Footcandle values for different dlstances from lampaxle and speclfled dlstances from coal face

Because the candle power curve for the headlightis symmetric about the lamp axis, lines of equal illumina-tion will form concentric rings as shown in figure 20.

Distance fromlamp axis, f l

F O O T C A N D L E L E VE L S2

Distance from headlighlto face, ff-8 1 12 I 16

H E A D L I G H T 8 'F R O M F A C E

C O A L F A C EFlgure 20.-Pattern of lllumlnatlon wlth headllght 8 R frommlne face.

The preceding problem has illustrated how candle-power curves can be used to obtain illumination patternson a surface for various luminaire locations. When curvesare not symmetric, as for a fluorescent lamp, similarcalculations can be performed using candlepower curvesfor different planes an d then drawing lines to connect thepoints of equal candlepower levels.

Three conclusions can be drawn when comparing thevalues in table 2. As distance from the face is increased-1. The luminaire distributes the light over a wider area ;2. Peak illumination levels ar e decreased; and3. Differences in illumination between the various pointstend to decrease; i.e., light distribution becomes more even.

Depending upon their intended use, candlepowercurves can be converted to other forms for the sake ofconvenience. A commonly used alternative form is theisofootcandle curve. The following problem il lustrates theconversion to and the application of isofootcandle curves.


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An isofootcandle curve shows the distance a sourcewill illuminate to a certain footcandle level in thevarious directions around t he source. A typical draw-ing includes a family of curves a t different footcan-dle levels.Problem.-Using th e inverse squ are law, convertthe headlig ht candlepower curve to a n isofootcandlecurve. Use thi s curve to determine t he illuminationlevels of a su rface normal to the lamp axis and 35 ftaway.Solution.-To convert the candlepower curve to anisofootcandle curve, firs t read t he candela values fora sampling of angles. For the purposes of this prob-lem, the values will be read a t 5" ntervals. (To insuremore accu rate curves, one would read t he values a tsmaller intervals, particularly where intensity val-ues change rapidly.)After th e readings are taken, the inverse squarelaw is used to calculate the distance the luminairewill light to the various footcandle values. The resultsof these calculations ar e summarized in table 3.Assume th e 2- , 4-, and 6-fc isolines are to be plotted.By takin g the distance values for the various footcan-dle values of interest (2 ,4 , and 6 fc), the isofootcandlecurves can be plotted on polar-coordinate graph paperas is shown in figure 21.Illumination a t a surface 35 ft along the lamp axismay be determined simply by drawing such a sur-face on the isofootcandle curve (fig. 21). Distancesfrom the axis where th e isolines intersect the sur-face may be scaled from the drawing. Because theheadlight isofootcandle curve is symmetric about it saxis, the isofootcandle lines would appear a s shownin figure 22.It is evident that use of an isofootcandle curvemakes determination of footcandle levels much eas-ier than the technique using candlepower curvesapplied in the preceding problem.

Luminance Symbol- IES unit- andle persquare inch (din 2)

SI unit - andle persquare meter (c/m2)

In physical term s, luminance is a concept used to quan-tify the density of luminous flux emitted by an area of alight source in a part icula r direction toward a l ight receiversuch as an eye. The area of a light source, in practice, m aybe an are a of a ligh t reflecting surface, such as a wall ordesk top; a n ar ea of a light emitter, such a s a lamp; or anarea of a light tra nsmit ter, such a s a diffusing lens on aluminaire. The most common definition of luminance, L,isL = I I=-A, projected A, cos 0

Referring to figure 23, I is th e intensity of light producedby area A, of the source i n the direc tion of the receiver, P,and A, projected is th e projected are a of the source when

Table 3.-lllumlnatlon dlstances at varlous degrees from lampaxis and varlous levele of lumlnoue IntensityDegreesfrom

NAp Not applicable.

axis

. .3 5 ' 30' Ib' 1 0 'S C A L E 1"=20'

Luminousinten- or 6 fc, A2fc I 4f c I 61c

Figure 21.-lsofootcandle curve for a mlnlng vehlcle head-light.

Distance of illumi-nation of 2, 4.

( F O O T C A N D L E L E V E L )

D I S T A N C E FROMB E A M A X I S

C O A L F A C EvFlgure 22.-Pattern of llluminatlon wlth headlight 35 R frommlne face (determined by isofoatcandle curve).


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7% I G H T S O U R C E~ ~ ~ ~ 1 1 1 ~ ~ .R A N S M I l I E R .

L I G H T R E C E I V ER

v r l N l i N S I l Y O f A s IN D I R E C T I O N OF PFigure 23.-Concept of iumlnance.

viewed from th e receiver. A, projected is equal to A, cos 0where 0 is the angle between th e normal to A, and theangle of observation. Figure 23 illustrates the concept.When A, is large, L is th e average luminance of A,; a s A,decreases, L approaches a value of point luminance.The luminance concept is extremely import ant in illu-mination design because it is a physically measurablequantity th at correlates with the subjective evaluation of"brightness" when viewing a surface or object. The ma the-matical definition of luminance (equation 6) does not,perhaps, ap pear meaningful a t first. However, it s signifi-cance becomes evident when one manipulates the vari-ables involved and compares th e effect on the magnitudeof luminance with the effect on the subjective evaluationof brightness, if the same manipulation were to be observedin an actual lighting application.First, vary the inte nsity, I, of the source while keep-ing the are a, A,, constant. For example, envision lookingat a n incandescent light bulb connected to a dimmer switchand th e switch being manipulated to increase or decreaseintensity. As th e intens ity of th e bulb increases, so doesthe subjective evaluation of brightness of the a rea observed,and, in accordance with eq uation 6, so does the luminance .Next, consider the effect of varying the area of thesource while keeping the intensity of the source constant.In thi s case, envision 10 lighted wax candles first distri-buted over a 25-in2ar ea and then over a 10-in%rea. If thepoint of observation was distant from the two areas, theintensity in that direction would be approximately con-sta nt i n both cases. However, the average luminance (seeequation 6!, a s well as the subjective evaluation of bright-ness would be great er when the candles were distributedover the smaller area.Finally, vary the dis tance of observation. In thi s case,consider observing the wall of a room from 8 and 12 ft.From equation 6 it can be seen th at luminance is indepen-dent of distance. Likewise, th e wall appears smaller, butnot more or less bright as one moves away from it.

To summarize, luminance and, therefore, the percep-tion of brightness-Vary directly with the inten sity of a light source of con-stant area;Vary inversely with th e area of a light source having agiven inten sity; andAre independent of distance of observation.

A final variable of importance is the direction ofobservation. As noted previously, both light sources andlight reflectors do not always distribute their flux uni-formly; hence, their intensity varies with angle of ob-servation. In addi tion , the projected a rea of the source alsovaries with angle of observation. Because of this, no gen-eralizations can be made with respect to th e effect of angleof observation except th at angle of observation does have

bearing. Candlepower curves (in the case of lamps andluminaires) and accurate reflectance measurements canbe used to establish these effects.

Average Luminance and Candlepower CurvesA candlepower curve of a luminaire can be used in

conjunction with the physical dimensions of the sourceand the definition of luminance to calculate the averageluminance of the source when observed from a particularpoint. This calc ulati o~i s useful in assessing the visualcomfort (i.e., level of glar ej of lighting designs. The follow-ing sample problem illus trate s calculation of the averageluminance observed a t certa in positions.

Problem.-Assume tha t the lens of the miningvehicle headlight has a 4.5-in diameter. If an indi-vidual looks directly at the fixture from position P(fig. 241, determine t he average observed luminanceby using the candlepower curve derived previously.What is th e average luminance observed a t positionQ?Solution:-Average luminance is equa l to inten-sity divided by projected sur face area . Projected sur-face area may be determined by multiplying theactual ar ea by the cosine of the angle of incidence, 0:Ap = Aa cos 8 . From geometry, actual area =~ ( d i a m / 2 ) ~ ,( 4 . 5 1 2 ) ~r .15.9 in2: projected a rea in Pdirection = 15.9 X cos 15" or 15.4 in2, and projectedarea in Q direction = 15.9 X cos 5" or 15.8 in2.From the candlepower curve (fig. 18), it can beseen t hat the intensity a t 15", in the P direction,equals 1,700 c. Also, the intensity at 5", in the Qdirection, equals 9,400 c.It is known from equation 6 that L,,,,,,,-I/Aprojeeted.herefore, Lp = 1,700 c11.54 in" or 110din" and L 9,400 1915.8 in2, or 594 clin2.Notice t h g =he distance toP or Q did not ente r thecalculations since luminance is independent of dis-tance.

As noted in a previous section, isointensity curves forluminaires ar e obtained by taking illumination (footcandle)measurements in a low-reflectance room and calculatingintensity values u sing the inverse squ are law. Since coalmines and mine sim ulators are typically low reflectance,approximate luminance values of machine-mounted lumi-naires can be calculated directly from illumination mea-surements taken on-site without the necessity of havingcandlepower curves. The following problem explains th istechnique.

Problem.-The measured illumination at an opera-tor's statio n 6 ft from a single lumin aire i s 4 fc. Forpurposes of this measurement, the illuminationmeter was angled to the luminaire for maximumreading. The projected area of the luminous surfaceof the lu mina ire in the direction of the operator is 30in'. Solve for the average luminance of the light-emitting surface.Solution.-lo = ED2 and L = Io/Ap; therefore,L = E D 2 / ~ p , = (4X62)130or 4.8 c/in2.


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P , Q . A N D H E A D L I G H T A R E INn S A M E H O R I Z O N T A L P L A N E

IS' . Q/soB E A M A X I S

Flgure 24.-Locatlon of observation polnts P and Q for deter-mlnlng luminance of headllght.

P E R F E C TL Y D I F F U S E R E F L E C T I N G S U R F A C EFlgure 25.-Relatlonahlp between lntenslty and angle of obser-vatlon for a perfectly diffuse reflector.

Perfectly Diffuse Reflectors and EmittersConsider a small flat surface emitting or reflecting

likht such that t he intensity va ries with direction of obser-vation a s I. = I,,, X cos H (see figure 25).As mentioned earlie r, th e projected ar ea of the surfaceat any angle 8 s A,, = A cos 0.If the luminance a t 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 forany ang le of observation. Suc h a surfac e is called a"perfectly diffuse" reflector or emitter. Many materials,such a s a wall painted with a flat paint, approximate thi sdistribution of emitted o r reflected luminous flux.

For such a perfectly diffuse reflector or transmitter,the ra tio of th e luminance , L, of the surface to the totallumens per unit are a emitted by the surface can be shownto equal a constant, T; luminance divided by lumen s emit-ted per unit are as equals the constant, T.Because of this relationship, illumination designersoften consider lumens emitted per un it a rea to be th e samea s the luminance quantity. If the a rea i s in square feet,the q uantity lumens per squ are feet is given th e specialname of footlamberts. Luminance measured in footlambertswill be represented in th is text by t he symbol L'. Substitut-ing into the constant equaling equation, L' = LIT.

The conversion between footlamberts (a n L' quantity)and candles per squ are inch (an L quantity) is 144 X nor452 fL = 1 din? The value of the footlambert conceptbecomes eviden t in t he next section.

Also note th at since the luminance of a perfectly dif-fuse reflector has a constant relationship with the lumensemitted per uni t area, this luminance could be easily mea-sured directly with a photoelectric device similar to anillumination me ter th at converts light energy to electricalenergy. The only requisite is th at th e field of acceptance ofthe device must be totally subtended by the area for whichthe luminance is being measured. Hence, such a devicewould be inadequate for smal l objects, but would work finefor large surfaces such a s interior walls, coal ribs and roof,etc.

Relationship Among Reflectance, Illumination, andLuminanceReflectance, p, is the rat io of reflected to incident lightenergy, which may be defined as lume ns emitted per uni tarea divided by lumens incident per unit area.For perfectly diffuse reflectors, lumens emitted per

unit ar ea is luminance, L', an d lumens incident per unitarea is illumination, E, therefore,

This is a n extremely important equation for illumina-tion design because it ca n be used to determine t he illumi-nation t ha t should be provided for a n environmen t, giventhe desired luminance (i.e., brightness) level and the reflec-tance of the environment. Th e following sample problemutilizes equation 7 .The simple relationship among luminance. illumina-tion, and reflectance represented by equation 7 appliesonly to perfectly diffuse reflectors. Although no surfaceexactly meets this criterion, such a relationship oftenapplies over a wide range of viewing angles making theuse 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 themain , coal can be considered a diffusing surface, and forpractica l purposes of ana lys is, perfectly diffusing. In somedesign problems, detailed reflectance measurements frommany angles of observation are often made to assess thelimits to which this equation may be applied.

Problem.-Using the footcandle distribu tion de-rived for a surface 35 ft from a headlight, determ inethe luminance of the surface if it i s coal with areflectivity, p ?of 4 pct or if it i s rock-dusted coal witha reflectivity of35 pct. In both cases, assume t hat thesurface is a perfectly diffuse reflector.Solution.-Because it is assumed th at the surfaceis a perfectly diffuse reflector, luminance values i nfootlamberts can be determined by m ultiplying thefootcandle values by the reflectivity-L = p X E.Therefore L' = 0.04 X E for coal an d L' = 0.35X Efor rock-dusted coal.


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Figure 26 shows conversion of th e following illumin a-tion levels to luminan ce levels.

Coal Rock-dusted coal

Limits of Agreement Between Luminanceand Perceived BrightnessAs noted earl ier, luminance is a physical concept th atha s been defined to correlate with th e perceived brightness .

It i s important to recognize th is correlation i s not an abso-lut e one, howevei-. If a series of objects were obsenred und erth e same level of background ill umination, they could eas-ily be arrang ed i n order of perceived brightness and th isorder would correspond to the ordering that would beobtained if the level of luminance from each object wasmeasured with instruments.In th is case luminance and the subjective evaluationof brightness agree perfectly. Now imagine maki ng a com-parison of th e subjective brightness of a miner's cap lampif it was observed in the dark surroundings undergroundand outside on a su nny day. The cap lamp would not appearas bright when observed outside on a sunn y day as t wouldunderground. However, if th e luminance of th e cap lampwas measured, i t would be th e same regardless of where itwas measured.

Figure 26.-Calculated lumlnance levels for coal and rock-dusted coal with headlight 35 f l from mine face.

The purpose of thi s discussion i s to illustrate the lim-its of the agreement of luminance and the subjectiveevaluation of brightness. In t he case of the cap lamp, theeye adaptation sta te differed in th e two instances and thi saffected the way brightness was evaluated. Luminancemeasurements and the subjective evaluation of bright-nesses correlate only when t he circumstances of seeing,primarily the eye adaptation state, remain constant.

CHAPTER 3.-LIGHT AND VISION RELATIONSHIPSThis chap ter addresses the visual needs of the worker,which a re th e ultimate basis for illumination design. These

needs are defined by (1 ) th e requiremen ts for optimal func-tioning of the visual sensory system, and (2) the l ightneeded to establish a n appr opriate level of visibility neces-sary for safe, efficient work performance.Th e lighting design process begins by carefully deter-mining these needs. Then practical, technical, and eco-nomic factors ar e considered i n establishing a n appropri-at e system design.This chap ter explains how light an d vision interact. Itidentifies th e visual needs of coal miner s and indicates, ingeneral terms, what can be done to accommodate thoseneeds. First, t he functions of the eyes and th e rest of thevisual sensory system are examined. Then, various envi-ronmental factors th at affect the visibility of surroundingsar e discussed.

THE EYE AND HOW IT WORKSThe eye (fig. 27) is the organ of sight. It senses thelight that enters it and acts as the first processor of thislight. It the n provides this information to the brain infor-

mation for determination of the form, size, shape, color,position, and motion of the objects in view.To understand how lig ht an d vision interact, it is help-ful to consider th e eye as a mechan ism made u p of twosubsystems: (1 ) the light control system and (2 ) the re-ceiver-decoder system. The parts of each system are out-lined in th e following tabulation .

Lig ht control system Receiver-decoder systemEyelid. Retina and itsphotoreceptors,Cornea. which ar e hero dsan d cones.Iris-pupil.Lens

Handbook of Mine Lighting

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