Lighting Fundamentals 1

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ightingundamentals

Lighting basics

Light sources-- Lampcharacteristics

Photometry

Calculations

Lighting quality

Index ntroduction

Illumination is light falling on a surfacemeasured in footcandles. Distributedwith an economic and visual plan, it

Page 1 of 36School of Lighting / Lighting Fundamentals / HL-862

2/4/2006http://www.holophane.com/School/HL-862.htm


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IntroductionHolophane Research andDevelopmentLighting BasicsLuminous FluxLuminous IntensityIlluminanceLuminanceMetric conversions

Light Sources-Lamp CharacteristicsIncandescentFluorescentHigh Intensity DischargeMercuryMetal HalideHigh Pressure SodiumLow Pressure SodiumQuartz

PhotometryCandlepower Distribution CurveCoefficient of UtilizationIsofootcandle ChartSpacing Criteria

Methods of Calculating Levels ofIlluminanceThe Zonal Cavity Method of Calculating

Average Illuminance LevelsCalculating Average Illuminance using theUtilization CurvePoint Calculations using Candlepower DataPoint Calculations using Isofootcandle Chart

Lighting QualityVisual Comfort IlluminationEquivalent Sphere Illumination

Selection of Level of Illuminance

becomes engineered lighting andtherefore, practical illuminance.

A lighting designer has four majorobjectives:

1. Provide the visibility required basedon the task to be performed and theeconomic objectives.

2. Furnish high quality lighting byproviding a uniform illuminance leveland by minimizing the negative effectsof direct and reflected glare.

3. Choose luminaires estheticallycomplimentary to the installation with

mechanical, electrical and maintenancecharacteristics designed to minimizeoperational expense.

4. Minimize energy usage whileachieving the visibility, quality andaesthetic objectives.

There are two parts to the solution of adesign problem. One is to selectluminaires which are designed to

control the light in an effective andenergy efficient manner. The other is toapply them to the project with all theskill and ingenuity the designer canbring to bear from his own knowledgeand all the reliable sources at hisdisposal.

This primer has been developed to givethe designer a useful summary of basiclighting principles. It gives importantdata and practical information on howto apply them. It offers the assistance ofthe Holophane technical sales force whohave CALAPro application softwareand LSAC!" economic analysis softwareat their disposal. The facilities and staffof the Holophane Technical SupportGroup are also available.

In addition, it prefaces a selection of




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quality lighting products that use thebest design and manufacturingtechniques of illumination science andtechnology available today. Their useassures the ultimate in lighting quality,economy, light distribution, energyefficiency and glare control.

easearch & Development

The high caliber performancecharacteristic of Holophane

luminaires is a result ofquality in concept, research,develop-ment and execution.This depends on a staff withability and integrity, alongwith the physical plant andequipment, to carry on theirwork. The following are somebrief aspects of the moreimportant activities andfacilities vital to the creation

of quality Holophane lightingproducts.

Photometers (A/B)A fullscale radial photometer (A)with a radius of 25' that willaccom-modate up to an 8'long or 5' square luminaire.There are photocells along thearc at every 2 1/2, starting at0 (nadir) up to 180 and a

single cell spinning mirrorphotometer with an effectivetest distance of 25'. Eachluminaire that is tested isrotated to measure up to 72planes of data. The systemsare fully automated so thephotocell readings are sentdirectly to an inhousecomputer (B) which generates




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Photometric Test Reportsused for calculation andanalysis. Photometric data isavailable in IESNA format ondisks for use in CALAPro andother lighting applicationprograms.

Electric and ballastlaboratoryA heavy currentlaboratory to simulate variousfield power and loadsituations. Ballasts aredesigned and tested to ensurethat they operate withinapplicable American NationalStandards design limits. Aproperly designed ballast will

optimize its own life whilepro-viding full lamp life andoutput.

Thermal laboratory (C)Heat testing facility whereluminaires and componentsare subjected to heatconditions well in excess oftheir normally expectedexposure under field use.

While this laboratory is usedfor research and developmentof luminaires, a significantpart of its activities is directedto the meeting andmaintenance of Underwriters'Laboratories requirements.


Sound laboratory (D)An




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anechoic (non-echoing) soundroom that has been isolatedfrom extraneous sounds. Thesound power is measured overeach 1/3 of an octave bandthrough the audible spectrumfrom 20 to 20,000 hertz. The

values are weighted according toa "standard hearer", then aLighting System Noise Criterion(LSNC) is established for a givenroom and layout.

ibration laboratory (E)Stability of equipment under avariety of vibration loadings isrigorously tested to meetspecifications and field-use

conditions. This assures productreliability when luminaires andpoles are subjected to variouswind conditions.

Water spray facilities (F)Resistance to water penetrationis evaluated in this closed cyclewater spray system. Luminairescan be tested for standard ULwet-location and outdoor

marine suitability; also, a special100 gallon per minute, 100 psicapacity can be used to test suchsevere conditions as those foundin tunnels.

CAD system (G)A ComputerAided Design system is used forthe precise design of optical andfixture components to assureprecise light control and

manufacturing tolerances fromall the elements which make upthe luminaire assembly.

Electronics laboratoryAcomplete facility for the design,development and testing ofelectronic components of aluminaire. All designs are




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thoroughly life tested to assurefull service life and performance.

Light and Vision institute(H)A facility for teachingprinciples of lighting design andcalculation as well as a center forthe consideration of lightingproblems in consultation withrecognized experts in the field.

Seminars on energyconservation, lighting for retailand roadway lighting areconducted together with schoolsfor electrical consultants,distributors and utilitypersonnel. Contact your local

Holophane representative forschedule.


Lighting demonstrationcenter (I)In thislaboratory, completeluminaires and systems areinstalled for measurementand visual evaluation ofperformance. The room ishighly flexible andmounting heights can be

altered to duplicate variouslighting conditions.

Outdoor lightinglaboratory (J/K)A streetand parking lot areaarranged for the measure-ment and visual evaluationof a variety of lightingsystems including signage.




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Outdoor architectural,historical and municipalluminaires may also beexamined in an adjacentpark-like setting

Technical SupportGroup (L)A computerequipped department,staffed with professionallighting designers andengineers, to aid consultantsand users in reaching theirlighting decisions. Thedepartment uses theCALAPro lighting analysisprogram for all of theirlighting designs.

Optical laboratory (M)Avisual evaluation facility toaid in the optical design ofhigh quality light controlelements of Holophaneluminaires.

Materials laboratory (N)A facility for the testing ofmaterials for strength,

corrosion resistance andother properties related toluminaires.

Model shop (O)Acomplete wood and metalworking shop for thepreparation of models - andworking prototypes ofluminaires under design.

Lighting Basics




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An understanding of some of the fundamental terms in lighting technology is basic togood design practice. The more important terms and concepts are reviewed here forthis purpose.

Luminous fluxLuminous flux is the time rate of flow of light as measured in lumens.

It is a measure of the total light emitted by a source and is most commonly used formeasurement of total lamp output.

Luminous intensityThe candela is the unit of intensity (I) and is analogous topressure in a hydraulic system. it is sometimes called "candlepower" and describes theamount of light (lumens) in a unit of solid angle. This unit of solid angle is called thesteradian. It will be seen from figure 1 that while the light travels away from the sourcethe solid angle covers a larger and larger area; but the angle itself remains the same, asdoes the amount of light it contains. Intensity therefore, in a given direction is constantregardless of distance.

Illuminance (E)Illuminance is the quantity of light reaching a unit area of surfaceand is measured in footcandles or lux. It is defined by intensity (), in candelas, directedtoward point P divided by the square of the distance (D) from the source to the surface.

As the area covered by a given solid angle becomes larger with distance from the source,the included light flux remains the same. The illumination density of light on thesurface decreases, therefore, as the inverse square of the distance. This forniula holdsonly if the receiving surface is perpendicular to the source direction. If light is incidentat some other angle, the formula becomes:

I=(lumens)

(steradians)

E=I

D2

E=I cos 0

D2




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where E = illumination in footcandles (fC) or lux

I = intensity in candela (cd) toward point P

D = distance in feet or meters

0 = angle of incidence

Luminance (L)Luminance, often called "brightness", is the name given to what wesee. "Brightness" is a subjective sensation varying from very dim or dark to very bright.Objectively it is referred to as luminance, defined as intensity in a given directiondivided by a projected area as seen by the observer. Luminance is usually referred to inone of two ways, either pertaining to a luminaire or to a surface.

The direct luminance or brightness of luminaires at various angles of view is a majorfactor in the visual comfort evaluation of an installation using those luminaires. Ingeneral, it is desirable to minimize the brightness of ceiling mounted luminaires at thehigh vertical angles, 60-90. When the intensity is in candelas, and the projected areais in meters, the unit of luminance is candelas per square meter (cd/m2).

Exitance (M)It is often desirable to calculate the amount of light reflected from roomsurfaces. Many room surfaces are diffuse in nature and as a result the correct term touse is Exitance (M), Where: Existance = illuminance x reflection factor

M = E x p

Where E = Illuminance in footcandles

p = the reflection factor of the surface expressed as the fraction of light reflected over

incident light

M = the resulting exitance in footcandles

Metric systemAs the U.S.A. moves toward conversion to the metric system toconform with the scientific fields and the rest of the world, our illuminationengineering, will convert to the International System of Units (SI). Only the termsinvolving length or area, illuminance and luminance, are affected. Illuminance (E) isstated in lux in the metric System. lfc= 10.76 lux. Luminance (L) is stated in nits in themetric system.

ight Sources amp Characteristics




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One of the first decisions in thedesign of a good lighting system isthe choice of a light source. Anumber of light sources areavailable, each with its own uniquecombination of operatingcharacteristics. A few of the lampcharacteristics that a lightingdesigner should consider whenchoosing a light source includeefficacy, or lumens per watt; color;lamp life; and lamp lumendepreciation, or the percent ofoutput that a lamp loses over its life.

Although there are hundreds of lamps on the market today, they can be categorized

by construction and operating characteristics into three groups: incandescent,fluorescent and high intensity discharge (HID). HID lamps can be grouped into fourmajor classes: high pressure sodium, metal halide, mercury and low pressure sodium.

IncandescentAn incandescent filament lamp is the light source most commonlyused in residential lighting. Light is produced in this source by a wire or filamentbeing heated to incandescence by a flow of current through it. The short life and lowefficacy (lumens per watt) of this source limits its use mostly to residential anddecorative commercial lighting. Efficacy varies with wattage and filament type, butgenerally ranges from 15 to 25 lumens per watt for general service lamps.

The incandescent source does, however, produce a highly accepted warm colorrendition. It is more convenient than other light sources because it can be run directlyon line current and therefore does not require a ballast. It can also be dimmed usingrelatively simple equipment. It is also available in different bulb sizes, shapes anddistributions to add a decorative touch to an area.

FluorescentThe fluorescent lamp produces light by activating selected phosphorson the inner surface of the bulb with ultraviolet energy which is generated by amercury, arc. Because of the characteristics of a gaseous arc, a ballast is needed tostart and operate fluorescent lamps.

The advantages of the fluorescent light source include improved efficacy and longerlife than incandescent lamps. Efficiencies for fluorescent lamps range anywhere from45 to 90 lumens per watt. Their low surface brightness and heat generation makethem ideal for offices and schools where thermal and visual comfort are important.

The disadvantages of fluorescent lamps include their large size for the amount of lightproduced. This makes light control more difficult which results in a diffuseshadowless environment. Their use in outdoor areas becomes less economicalbecause light output of a fluorescent source is reduced at low ambient temperatures.




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Also, although fluorescent efficacy is higher than that of an incandescent lamp, higherlumens per watt often can be achieved by high pressure sodium or metal halidelamps.

High Intensity Discharge (HID) High intensity discharge sources includemercury, metal halide, high pressure sodium (HPS) and low pressure sodium lamps.Light is produced in HID sources through a gaseous arc discharge using a variety ofelements. Each HID lamp consists of an arc tube which contains certain elements ormixtures of elements which when an arc is created between the electrodes at eachend, gasify and generate visible radiation.

The major advantages of HID sources are their high efficacy in lumens per watt, longlamp life and point source characteristic for good light control. Disadvantages includethe need for a ballast to regulate lamp current and voltage as well as a starting aid forHPS and the delay in restriking instantly after a momentary power interruption.

ight Sources amp Characteristics

Mercury (MV)The mercury source was the first HID lamp developed, filling theneed for a more efficient, yet compact, high output lamp. When first developed, themajor disadvantage of this lamp was its poor color rendition. The color of the deluxewhite lamp, is greatly improved through use of a phosphor coated bulb wall.

The life of mercury lamps is good, averaging 24,000 hours for most larger wattagelamps. However, because the output diminishes so greatly over time, economicoperational life is often much shorter. Efficacy ranges from 30 to 60 lumens per watt,with the higher wattages being more efficient than the lower wattages.

Like other HID lamps, starting of a mercury lamp is not immediate. Starting time isshort, though, taking 4-7 minutes to achieve maximum output depending upon theambient temperature.

Metal halide (MH) Metal halide lamps are similar in construction to mercury

lamps with the addition of various other metallic elements in the arc tube. The majorbenefits of this change are an increase in efficacy to 60 to 100 lumens per watt and animprovement in color rendition to the degree that this source is suitable forcommercial areas. Light control of a metal halide lamp is also more precise than thatof a deluxe mercury lamp since light emanates from the small arc tube, not the totalouter bulb of the coated lamp.

A disadvantage of the metal halide lamp is its shorter life (7,500 to 20,000 hrs) ascompared to mercury and high pressure sodium lamps. Starting time of the metalhalide lamp is approximately the same as for mercury lamps. Restriking, after a




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voltage dip has extinguished the lamp, however can take substantially longer, rangingfrom 4 to 12 minutes depending on the time required for the lamp to cool.

High pressure sodium (HPS)Inthe 1970's, as increasing energy costsplaced more emphasis on the efficiencyof lighting, high pressure sodiumlamps (developed in the 1960's) gainedwidespread usage. With efficaciesranging from 80 to 140 lumens perwatt, these lamps provide about 7times as much light per watt asincandescent and about twice as muchas some mercury or fluorescent. Theefficacy of this source is not its only,advantage. An HPS lamp also offers thelongest life (24,000 hrs.) and the bestlumen maintenance characteristics of

all HID sources.

The major objection to the use of HPSis its yellowish color. It is ideal for most industrial and outdoor applications.

Low pressure sodium (LPS)Low pressure sodium offers the highest initialefficacy of all lamps on the market today, ranging from 100 to 180 lumens per watt.However, because all of the LPS output is in the yellow portion of the visiblespectrum, it produces extremely poor and unattractive color rendition. Control of thissource is more difficult than other HID sources because of the large size of the arctube. Average life of low pressure sodium lamps is 18,000 hours. While lumen

maintenance through life is good with LPS, there is an offsetting increase in lampwatts reducing the efficiency of this lamp type with use.

hotometry

The term "Photometry" is used to define any test data whichdescribes the characteristics of a luminaire's light output. Themost common type of photometric data includes candlepowerdistribution curves, spacing criteria, luminaire efficiency,isofootcandle charts, coefficient of utilization and luminancedata. The purpose of photometry is to accurately describe theperformance of a luminaire to enable the designer to select




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the lighting equipment and to design a fixture layout whichbest meets the needs of the job.

Following is a review of the more frequently used types ofphotometric data.

Candlepower distribution curve (Figure 1)Thephotometric distribution curve is one of the lighting designersmost valuable tools. It is a cross sectional "map" of intensity(candelas) measured at many different angles. It is a twodimensional representation and therefore shows data for oneplane only. If the distribution of the unit is symmetric, thecurve in one plane is sufficient for all calculations. Ifasymmetric, such as with street lighting and fluorescentunits, three or more planes are required. In general,incandescent and HID reflector units are described by asingle vertical plane of photometry. Fluorescent luminairesrequire a minimum of one plane along the lamp axis, one

across the lamp axis and one at a 45 angle. The greater thedeparture from symmetry, the more planes are needed foraccurate calculations.

Coefficient of utilization (Figure 2)A coefficient ofutilization refers to the ratio of lumens which ultimatelyreach the work plane to the total lumens generated by thelamp. CU figures are necessary for calculating averageilluminance levels, and are provided in one of two ways: a CUtable or a utilization curve. A utilization curve is usuallyprovided for units intended for outdoor use or units with a

distribution radically asymmetric. A CU table is provided forunits which are used primarily indoors, where the zonalcavity method of calculation applies. Use of CU data will bediscussed in the section covering calculation methods.

Isofootcandle chart (Figure 3) Isofootcandle charts areoften used to describe the light pattern when a fixtureproduces a distribution other than symmetric. These chartsare derived from the candlepower data and show exact plotsor lines of equal footcandle levels on the work plane when thefixture is at a designated mounting height. Use of

isofootcandle charts in determining illuminance atdesignated points will be discussed in the point calculationssection.

Spacing criteria Spacing criteria provides the designer withinformation regarding how far apart to space luminaires andmaintain acceptable illumination uniformity on the workplane. Criteria for spacing is generally conservative i.e., ittakes into account the direct component of illumination only

Candlepower curveFigure 1

Figure 2

Figure 3




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and ignores the indirect component of light which cancontribute significantly to the uniformity. However, usedwithin its limits, Spacing Criterion can be useful. To use thespacing criterion, multiply the net mounting height(luminaire to work plane) by the spacing criteria number.This ratio is used predominantly with the zonal cavity methodof calculation. Since there are many assumptions built into

the zonal cavity method, the designer should be aware of theassumptions.

Calculations




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Methods of calculating illuminance

In order to design a luminaire layout which best meets the illuminance anduniformity requirements of the job, two types of information are generallyneeded; average illuminance levels and illuminance levels at a given point.Calculation of illuminance at specific points is often done to help the designerevaluate the lighting uniformity especially when using luminaires wheremaximum spacing recommendations are not supplied or where task lightinglevels must be checked against ambient.

If average levels are to be calculated, two methods can be applied.

1. For indoor lighting situations, the zonal cavity method is used with datafrom a coefficient of utilization table.

2. For Outdoor lighting applications, a coefficient of utilization curve isprovided and the CU is read directly from the curve and the standard lumenformula is used.

The following two methods can be used if calculations are to be done todetermine illuminance at one point.

1. If an isofootcandle chart is provided, illuminance levels may be read

directly from this curve.

2. If sufficient candlepower data is available, illuminance levels may becalculated from this data using the point to point method.

The following sections describe these methods of calculation.




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onal Cavity Method

The zonal cavity method is the currently accepted method for calculating

average illuminance levels for indoor areas unless the light distribution isradically asymmetric. It is an accurate hand method for indoor applicationsbecause it takes into consideration the effect that interreflectance has on thelevel of illuminance. Although it takes into account several variables, thebasic premise that footcandles are equal to flux over an area is not violated.

The basis of the zonal cavity method is that a room is made up of three spacesor cavities. The space between the ceiling and the fixtures, if they aresuspended, is defined as the "ceiling cavity"; the space between the workplane and the floor, the " floor cavity"; and the space between the fixtures andthe work plane, the "room cavity".

Once the concept of these cavities is understood, it is possible to calculatenumerical relationships called "cavity ratios", which can be used to determineeffective reflectance of the ceiling and floor and then to find the coefficient ofutilization.

There are four basic steps in any calculation of illuminance level:

1.Determine cavity ratio 2.Determine effective cavity reflectances

3.Select coefficient of utilization 4.Compute average illuminance level

Step 1: Cavity ratios may be determined by calculating using the followingformulas:

CeilingCavityRatio(CCR) =

5 hcc (L+W)L x W

Room




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Where:

hcc = distance in feet from luminaire to ceiling

hrc= distance in feet from luminaire to work plane

hfc= distance in feet from work plane to floor

L= length in feet of room

W= width in feet of room

An alternate formula for calculating any cavity ratio is:

Step 2:Effective cavity reflectances must be determined for the ceiling cavityand for the floor cavity. These are located in Table A (pg. 12) under theapplicable combination of cavity ratio and actual reflectance of ceiling, wallsand floor. Note that if the luminaire is recessed or surface mounted, or if thefloor is the work plane, the CCR or FCR will be 0 and then the actualreflectance of the ceiling or floor will also be the effective reflectance. Theeffective reflectance values found will then be pcc (effective ceiling cavityreflectance) and pfc (effective floor cavity reflectance) .

Step 3:With these values of pcc, pfc, and pw (wall reflectance), and knowingthe room cavity ratio (RCR) previously calculated, find the coefficient ofutilization in the luminaire coefficient of utilization (CU) table. Note thatsince the table is linear, linear interpolations can be made for exact cavityratios or reflectance combinations.

The coefficient of utilization found will be for a 20% effective floor cavityreflectance, thus, it will be necessary to correct for the previously determinedpfc. This is done by multiplying the previously determined CU by the factor

CavityRatio(RCR) =

5 hrc (L+W)L x W

FloorCavityRatio

(FCR) =

5 hfc (L+W)L x W

Cavity Ratio=

2.5 x height of cavity xcavity perimeter

area of cavity base




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from Table B (pg.13).

CU final = CU (20% floor) x Multiplier for actual pfc. If it is other than 10% or30% then interpolate or extrapolate and multiply by this factor.

Step 4:Computation of the illuminance level is performed using thestandard lumen method formula.

# of fixtures x lamps per fixture x lumens perFootcandles = lamp x CU xLLF (maintained) area in square feet

onal Cavity Method

When the initial illuminance level required is known and the number of fixturesneeded to obtain that level is desired, a variation of the standard lumen formula isused.

The total light loss factor (LLF) consists of two basic factors, lamp lumendepreciation (LLD) and luminaire dirt depreciation (LDD). If initial levels are to befound, a multiplier of 1 is used. Light loss factors, along with the total lamp lumenoutput vary with manufacturer and type of lamp or luminaire and are determined byconsulting the manufacturers published data.

Occasionally, other light loss factors may need to be applied when they areapplicable. Some of these are, ballast factor, luminaire ambient temperature, voltagefactor and room surface dirt depreciation.

# ofluminaires =

maintainedfootcandlesdesired

x area in sq. ft.

lamp/fixture xlumen/lamp x

CU x LLF




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onal Cavity Example




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Example:A typical lecture hall is 60' long and 30' wide with a l4' ceiling height.Reflectances are ceiling 80%, walls 30%, floor 10%. Four lamp Prismawrap(coefficients of utilization shown below) is to be used on 4' stems and the work planeis 2' above the floor. Find the illuminance level if there are 18 luminaires in the room.

Solutions:

(l) Calculate cavity ratios as follows:

(2) In Table A, look up effective cavity reflectances for these ceiling and floor cavities,pcc for the ceiling cavity is determined to be 62% while pfc for the floor cavity is 10%.

(3) Knowing the room cavity ratio (RCR), it is now possible to find the coefficient ofutilization for the Prismawrap luminaire in a room having an RCR of 2.0 andeffective reflectances as follows:

pcc = 62%; pw = 30%; pfc = 20%. By interpolation between boxed numbers in thetable this CU is .55. Note that this CU is for an effective reflectance of 20% while theactual effective reflectance of the floor pfc is 10%. To correct for this, locate theappropriate multiplier in Table B for the RCR already calculated (2.0). It is .962 and

CCR=

5(4)(30+60)

30 x 60 =1.0

RCR=5(8)(30+60)

30 x 60=2.0

FCR=5(2)(30+60)

30 x 60=5.0




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levels when CU's are taken from a utilization curve.

To calculate the number of luminaires needed to produce the desired footcandles,the following formula is used:

A variation of this formula, which is used mostly for roadway lighting, calculateshow far apart the fixtures must be spaced to produce the necessary averageilluminance.

A utilization curve shows the percent of light which falls onto an area having adesignated width and an infinite length. This width is expressed on the utilizationcurve in terms of a ratio of the width of the area to the luminaire mounting height.

A CU is found by reading across the bottom axis to this ratio, up until thedashed CUline is intersected, thenacross to the right hand axis, to read the value of the CU.Separate CU's are given for the area to the street side and area to the house side ofthe fixture and may be used to find illumination on the roadway or sidewalk areas oradded to find the total light on the street in the case of median mounted luminaires.

Example:A roadway 24 ft. wide is to be lighted to an average maintained illumination level of1.0 fc. Holophane Mongoose MV400HPNC6 is to be used. They will be mountedon 30 ft. poles which are set back 36 ft. from the road. Find the spacing required.

Footcanales=(maintained)

lumens/lamp xlamps/

luminaire x #luminairesx CU x LLF

area in square feet

# of luminaires=

maintained footcandlesdesired x area in sq. ft.lumens/lamp x lamps/

luminaire x CU x LLF

Spacing =lamp lumens x CU x LLFAvg. MTD FC x width of

Road

Spacing=

lamp lumens x CU x LLF

Avg. MTD FC x width ofRoad




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SolutionThe CU is determined by reading from the chart #l the intersection of thedistance across/mounting height with the CU and hence horizontally to the CU axis.

Chart 1The CU for the roadway area only is determined by subtracting the CU of the setbackarea from the CU of the total area of both roadway and setback. The width of thetotal area is 60 feet ( 2.0 M.H.) and the width of the setback is 36 feet (1.2 M.H.).From the CU curve (see chart 1 ) we find that the corresponding CU's are .52 and .3.Deducting the second from the first we get a CU of .22. Inserting this CU into thestandard lumen method formula results in a spacing of 371 feet.

Spacing =50,000 x .22 x .81

1.0 x 24= 371 ft.

oint Calculations andxample

Point calculations using candlepower data

This method is especially useful in the determination of variation of illumination

levels and the uniformity of illumination provided by a lighting design. It is mostfrequently used in heavy industrial and design where interreflections are not aconsideration.

The point-by-point method accurately computes the illuminance level at any givenpoint in an installation by summing up the illumination contributions to that pointfrom every luminaire individually. It does not account for contributions from othersources such as reflection from walls, ceiling, etc. For accuracy the calculationdistance from source to point of calculation should be at least five times the




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maximum luminaire dimension. Using the photometric distribution for the unit wemay calculate values for specific points as follows for horizontal surfaces.

Example:

A single 400W HPS Prismpack luminaire is mounted 26' above a work plane. it isdesired to find the initial horizontal illumination at a point 15' to one side of theluminaire. See figure 2.

Solution:

we need to determine the angle y and look up the cp at this angle. We also mustdetermine the distance D.

Since D2 = a2 + h2D2 = (15)2 + (26)2D = 30'

Then we can determine the candlepower of this luminaire from the cp curve, figure3, to be, 18936 (cp).

fc =candlepower x Cosq

D2

Since fc =candlepower x Cosq

D2

and tangent g=

ah

y= arctangent ah

y= 30




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The illumination then is:

When many point calculations must be done by hand a variation of the basicformula is somewhat more useful.

This version of the formula lets us deal with only the net mounting height of thefixtures and candlepower angles and eliminates the necessity to calculate eachseparate distance "D".Point calculations using the isofootcandle chart Theisofootcandle chart can also be used to find the illumination at a specific point. It isfound by defining the horizontal distance from the fixture to that point in terms of aratio of distance to mounting height, then looking up that ratio on the chart. If theactual mounting height of the fixture is different than the isofootcandle chartmounting height, a correction factor must be applied using the following formula:

Example:Using the same layout and fixtures as were used in example on page 14 determinethe illuminance level, between the two units, on the outside edge of the road usingChart 1.

Solution:From either fixture, point A is 60 feet to the street side (2.0 M.H.) and 143 feet downthe street (4.8 M.H.). Looking at the isofootcandle curve, we find that the footcandle

fc = 18936 x Cos 30(30)2

= 18.2 fc

fc =Candlepower x

cos30(30)2

correction factor=

chart MH2actual MH2




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line at hat point is the .30 fc curve. This is the contribution from one luminaire andshould be summed with other contributions for total footcandles. Since theisofootcandle chart mounting height is the same as our mounting height, no furthercorrection is necessary.

Computer programsPoint by point calculations can be time consuming. Several computer programs areavailable that perform such calculations for many analysis points and luminaires ina fraction of the time necessary to do the same calculations by hand.

ighting Quality

Achieving the required illuminance level does not necessarily ensure good lightingquality. The quality as well as the quantity of illuminance is important to produce acomfortable, productive, aesthetically pleasing lighting environment. The quality ofthe lighting system refers, but is not limited to, aspects of lighting such as propercolor, good uniformity, proper room surface luminances, adequate brightnesscontrol and minimal glare.

Research has suggested that the lighting system can effect user's impressions of

Prismatic Glass (left) Aluminum Reflector (right)




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The following conditions are factored into this method:1. The task to be performed2.The details of the object to be viewed 3. The age of the observer 4.Theimportance of speed and/or accuracy for visual performance 5. The reflectance ofthe background material

This method, then, allows the designer to use his own evaluation of theenvironmental conditions to select the target illuminance level.

Step 1: Determine the type of activity for which the level of lighting is to be selected.

Step 2: Select the appropriate illuminance category by one of the followingmethods:

A.When the visual task is defined by one of the typical task categories, choose theappropriate illuminance category from Table E.

B.If a specific task cannot be established, the illuminance category may bedetermined from the generic task descriptions listed in Table C.

Step 3: Establish illuminance target value. Once the illuminance category is chosen,an exact illuminance level may be determined from within this range. These levelsare established on Table D by matching the appropriate user, room, and taskcharacteristics with the previously determined illuminance category.

Because the intention of this method is to factor in the previously listed fiveconditions, it is not applicable to certain areas. Therefore, it will be noted thatspecific footcandle levels, rather than ranges, are given for these environments.

These levels should be used as a guide for the designer. Absolute values cannot andshould not be assigned to cover all situations. It is recognized that other installationcircumstances may alter the necessary level to higher or lower figures; the finaldiscretion resting with the designer.




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Selection of level oflluminance




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Holophane Corporation, 214 Oakwood Ave., Newark, OH 43055 / Holophane Canada, Inc. 294Walker Drive, Unit #3, Brampton, ON Canada L6T 4Z2 / Holophane Europe Limited, BondAve., Milton Keynes MK1 1JG, England./ Unique Lighting Solutions, 13/30 Heathcote Road,Moorebank, NSW 2170 Australia/ Holophane, S.A. de C.V., Apartado Postal No. 986,




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Contact your local Holophane sales representativefor application assistance, andcomputer-aided design and cost studies. For information on other Holophane products andsystems, call the Customer Service Center at 740-345-9631. In Canada call 905-793-3111 or fax905-793-9597.

Limited Warranty and Limitation of Liability Refer to the Holophane limited materialwarranty and limitation of liability on this product, which are published in the "Terms andConditions" section of the current price schedule, and is available from our local Holophanesales representative.

HL-862 7/99 Copyright Holophane Corporation 1999Visit our website at www.holophane.comPrinted in USA


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Lighting Fundamentals 1

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