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Lighting Fundamentals • Lighting basics • Light sources-- Lamp characteristics • Photometry • Calculations • Lighting quality Index Introduction Illumination is light falling on a surface measured in footcandles. Distributed with an economic and visual plan, it Page 1 of 36 School of Lighting / Lighting Fundamentals / HL-862 2/4/2006 http://www.holophane.com/School/HL-862.htm

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Page 1: Lighting Fundamentals - Lighting Associates, · PDF fileLighting Fundamentals • Lighting basics • Light sources-- Lamp characteristics • Photometry • Calculations • Lighting

Lighting Fundamentals

• Lighting basics

• Light sources-- Lamp characteristics

• Photometry

• Calculations

• Lighting quality

Index Introduction

Illumination is light falling on a surface measured in footcandles. Distributed with an economic and visual plan, it

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Introduction Holophane Research and Development Lighting Basics Luminous Flux Luminous Intensity Illuminance Luminance Metric conversions

Light Sources-Lamp Characteristics Incandescent Fluorescent High Intensity Discharge Mercury Metal Halide High Pressure Sodium Low Pressure Sodium Quartz

Photometry Candlepower Distribution Curve Coefficient of Utilization Isofootcandle Chart Spacing Criteria

Methods of Calculating Levels of Illuminance The Zonal Cavity Method of Calculating Average Illuminance Levels Calculating Average Illuminance using the Utilization Curve Point Calculations using Candlepower Data Point Calculations using Isofootcandle Chart

Lighting Quality Visual Comfort Illumination Equivalent Sphere Illumination

Selection of Level of Illuminance

becomes engineered lighting and therefore, practical illuminance.

A lighting designer has four major objectives:

1. Provide the visibility required based on the task to be performed and the economic objectives.

2. Furnish high quality lighting by providing a uniform illuminance level and by minimizing the negative effects of direct and reflected glare.

3. Choose luminaires esthetically complimentary to the installation with mechanical, electrical and maintenance characteristics designed to minimize operational expense.

4. Minimize energy usage while achieving the visibility, quality and aesthetic objectives.

There are two parts to the solution of a design problem. One is to select luminaires which are designed to control the light in an effective and energy efficient manner. The other is to apply them to the project with all the skill and ingenuity the designer can bring to bear from his own knowledge and all the reliable sources at his disposal.

This primer has been developed to give the designer a useful summary of basic lighting principles. It gives important data and practical information on how to apply them. It offers the assistance of the Holophane technical sales force who have CALAPro® application software and LSAC!" economic analysis software at their disposal. The facilities and staff of the Holophane Technical Support Group are also available.

In addition, it prefaces a selection of

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quality lighting products that use the best design and manufacturing techniques of illumination science and technology available today. Their use assures the ultimate in lighting quality, economy, light distribution, energy efficiency and glare control.

Reasearch & Development

The high caliber performance characteristic of Holophane luminaires is a result of quality in concept, research, develop-ment and execution. This depends on a staff with ability and integrity, along with the physical plant and equipment, to carry on their work. The following are some brief aspects of the more important activities and facilities vital to the creation of quality Holophane lighting products.

Photometers (A/B) A full scale radial photometer (A) with a radius of 25' that will accom-modate up to an 8' long or 5' square luminaire. There are photocells along the arc at every 2 1/2°, starting at 0° (nadir) up to 180° and a single cell spinning mirror photometer with an effective test distance of 25'. Each luminaire that is tested is rotated to measure up to 72 planes of data. The systems are fully automated so the photocell readings are sent directly to an inhouse computer (B) which generates

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Photometric Test Reports used for calculation and analysis. Photometric data is available in IESNA format on disks for use in CALAPro and other lighting application programs.

Electric and ballast laboratory A heavy current laboratory to simulate various field power and load situations. Ballasts are designed and tested to ensure that they operate within applicable American National Standards design limits. A properly designed ballast will optimize its own life while pro-viding full lamp life and output.

Thermal laboratory (C) Heat testing facility where luminaires and components are subjected to heat conditions well in excess of their normally expected exposure under field use. While this laboratory is used for research and development of luminaires, a significant part of its activities is directed to the meeting and maintenance of Underwriters' Laboratories requirements.

Reasearch & Development

Sound laboratory (D) An

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anechoic (non-echoing) sound room that has been isolated from extraneous sounds. The sound power is measured over each 1/3 of an octave band through the audible spectrum from 20 to 20,000 hertz. The values are weighted according to a "standard hearer", then a Lighting System Noise Criterion (LSNC) is established for a given room and layout.

Vibration laboratory (E) Stability of equipment under a variety of vibration loadings is rigorously tested to meet specifications and field-use conditions. This assures product reliability when luminaires and poles are subjected to various wind conditions.

Water spray facilities (F) Resistance to water penetration is evaluated in this closed cycle water spray system. Luminaires can be tested for standard UL wet-location and outdoor marine suitability; also, a special 100 gallon per minute, 100 psi capacity can be used to test such severe conditions as those found in tunnels.

CAD system (G) A Computer Aided Design system is used for the precise design of optical and fixture components to assure precise light control and manufacturing tolerances from all the elements which make up the luminaire assembly.

Electronics laboratory A complete facility for the design, development and testing of electronic components of a luminaire. All designs are

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thoroughly life tested to assure full service life and performance.

Light and Vision institute (H) A facility for teaching principles of lighting design and calculation as well as a center for the consideration of lighting problems in consultation with recognized experts in the field.

Seminars on energy conservation, lighting for retail and roadway lighting are conducted together with schools for electrical consultants, distributors and utility personnel. Contact your local Holophane representative for schedule.

Reasearch & Development

Lighting demonstration center (I) In this laboratory, complete luminaires and systems are installed for measurement and visual evaluation of performance. The room is highly flexible and mounting heights can be altered to duplicate various lighting conditions.

Outdoor lighting laboratory (J/K) A street and parking lot area arranged for the measure-ment and visual evaluation of a variety of lighting systems including signage.

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Outdoor architectural, historical and municipal luminaires may also be examined in an adjacent park-like setting

Technical Support Group (L) A computer equipped department, staffed with professional lighting designers and engineers, to aid consultants and users in reaching their lighting decisions. The department uses the CALAPro lighting analysis program for all of their lighting designs.

Optical laboratory (M) A visual evaluation facility to aid in the optical design of high quality light control elements of Holophane luminaires.

Materials laboratory (N) A facility for the testing of materials for strength, corrosion resistance and other properties related to luminaires.

Model shop (O) A complete wood and metal working shop for the preparation of models - and working prototypes of luminaires under design.

Lighting Basics

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

Luminous flux Luminous 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 for measurement of total lamp output.

Luminous intensity The candela is the unit of intensity (I) and is analogous to pressure in a hydraulic system. it is sometimes called "candlepower" and describes the amount of light (lumens) in a unit of solid angle. This unit of solid angle is called the steradian. It will be seen from figure 1 that while the light travels away from the source the solid angle covers a larger and larger area; but the angle itself remains the same, as does the amount of light it contains. Intensity therefore, in a given direction is constant regardless of distance.

Illuminance (E) Illuminance is the quantity of light reaching a unit area of surface and is measured in footcandles or lux. It is defined by intensity (Î), in candelas, directed toward 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 the surface decreases, therefore, as the inverse square of the distance. This forniula holds only if the receiving surface is perpendicular to the source direction. If light is incident at 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 I "brightness", is the name given to what we see. "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 direction divided by a projected area as seen by the observer. Luminance is usually referred to in one 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 major factor in the visual comfort evaluation of an installation using those luminaires. In general, it is desirable to minimize the brightness of ceiling mounted luminaires at the high vertical angles, 60°-90°. When the intensity is in candelas, and the projected area is 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 room surfaces. Many room surfaces are diffuse in nature and as a result the correct term to use 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 system As the U.S.A. moves toward conversion to the metric system to conform with the scientific fields and the rest of the world, our illumination engineering, will convert to the International System of Units (SI). Only the terms involving length or area, illuminance and luminance, are affected. Illuminance (E) is stated in lux in the metric System. lfc= 10.76 lux. Luminance (L) is stated in nits in the metric system.

Light Sources Lamp Characteristics

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One of the first decisions in the design of a good lighting system is the choice of a light source. A number of light sources are available, each with its own unique combination of operating characteristics. A few of the lamp characteristics that a lighting designer should consider when choosing a light source include efficacy, or lumens per watt; color; lamp life; and lamp lumen depreciation, or the percent of output 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 four major classes: high pressure sodium, metal halide, mercury and low pressure sodium.

Incandescent An incandescent filament lamp is the light source most commonly used in residential lighting. Light is produced in this source by a wire or filament being heated to incandescence by a flow of current through it. The short life and low efficacy (lumens per watt) of this source limits its use mostly to residential and decorative commercial lighting. Efficacy varies with wattage and filament type, but generally ranges from 15 to 25 lumens per watt for general service lamps.

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

Fluorescent The fluorescent lamp produces light by activating selected phosphors on the inner surface of the bulb with ultraviolet energy which is generated by a mercury, arc. Because of the characteristics of a gaseous arc, a ballast is needed to start and operate fluorescent lamps.

The advantages of the fluorescent light source include improved efficacy and longer life than incandescent lamps. Efficiencies for fluorescent lamps range anywhere from 45 to 90 lumens per watt. Their low surface brightness and heat generation make them 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 light produced. This makes light control more difficult which results in a diffuse shadowless environment. Their use in outdoor areas becomes less economical because 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, higher lumens per watt often can be achieved by high pressure sodium or metal halide lamps.

High Intensity Discharge (HID) High intensity discharge sources include mercury, 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 of elements. Each HID lamp consists of an arc tube which contains certain elements or mixtures of elements which when an arc is created between the electrodes at each end, gasify and generate visible radiation.

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

Light Sources Lamp Characteristics

Mercury (MV) The mercury source was the first HID lamp developed, filling the need for a more efficient, yet compact, high output lamp. When first developed, the major disadvantage of this lamp was its poor color rendition. The color of the deluxe white 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 wattage lamps. However, because the output diminishes so greatly over time, economic operational 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 is short, though, taking 4-7 minutes to achieve maximum output depending upon the ambient 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 major benefits of this change are an increase in efficacy to 60 to 100 lumens per watt and an improvement in color rendition to the degree that this source is suitable for commercial areas. Light control of a metal halide lamp is also more precise than that of a deluxe mercury lamp since light emanates from the small arc tube, not the total outer bulb of the coated lamp.

A disadvantage of the metal halide lamp is its shorter life (7,500 to 20,000 hrs) as compared to mercury and high pressure sodium lamps. Starting time of the metal halide 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, ranging from 4 to 12 minutes depending on the time required for the lamp to cool.

High pressure sodium (HPS) In the 1970's, as increasing energy costs placed more emphasis on the efficiency of lighting, high pressure sodium lamps (developed in the 1960's) gained widespread usage. With efficacies ranging from 80 to 140 lumens per watt, these lamps provide about 7 times as much light per watt as incandescent and about twice as much as some mercury or fluorescent. The efficacy of this source is not its only, advantage. An HPS lamp also offers the longest life (24,000 hrs.) and the best lumen maintenance characteristics of all HID sources.

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

Low pressure sodium (LPS) Low pressure sodium offers the highest initial efficacy 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 visible spectrum, it produces extremely poor and unattractive color rendition. Control of this source is more difficult than other HID sources because of the large size of the arc tube. 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 lamp watts reducing the efficiency of this lamp type with use.

Photometry

The term "Photometry" is used to define any test data which describes the characteristics of a luminaire's light output. The most common type of photometric data includes candlepower distribution curves, spacing criteria, luminaire efficiency, isofootcandle charts, coefficient of utilization and luminance data. The purpose of photometry is to accurately describe the performance of a luminaire to enable the designer to select

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

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

Candlepower distribution curve (Figure 1) The photometric distribution curve is one of the lighting designers most valuable tools. It is a cross sectional "map" of intensity (candelas) measured at many different angles. It is a two dimensional representation and therefore shows data for one plane only. If the distribution of the unit is symmetric, the curve in one plane is sufficient for all calculations. If asymmetric, such as with street lighting and fluorescent units, three or more planes are required. In general, incandescent and HID reflector units are described by a single vertical plane of photometry. Fluorescent luminaires require a minimum of one plane along the lamp axis, one across the lamp axis and one at a 45° angle. The greater the departure from symmetry, the more planes are needed for accurate calculations.

Coefficient of utilization (Figure 2) A coefficient of utilization refers to the ratio of lumens which ultimately reach the work plane to the total lumens generated by the lamp. CU figures are necessary for calculating average illuminance levels, and are provided in one of two ways: a CU table or a utilization curve. A utilization curve is usually provided for units intended for outdoor use or units with a distribution radically asymmetric. A CU table is provided for units which are used primarily indoors, where the zonal cavity method of calculation applies. Use of CU data will be discussed in the section covering calculation methods.

Isofootcandle chart (Figure 3) Isofootcandle charts are often used to describe the light pattern when a fixture produces a distribution other than symmetric. These charts are derived from the candlepower data and show exact plots or lines of equal footcandle levels on the work plane when the fixture is at a designated mounting height. Use of isofootcandle charts in determining illuminance at designated points will be discussed in the point calculations section.

Spacing criteria Spacing criteria provides the designer with information regarding how far apart to space luminaires and maintain acceptable illumination uniformity on the work plane. Criteria for spacing is generally conservative i.e., it takes into account the direct component of illumination only

Candlepower curve Figure 1

Figure 2

Figure 3

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

Calculations

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Methods of calculating illuminance

In order to design a luminaire layout which best meets the illuminance and uniformity requirements of the job, two types of information are generally needed; average illuminance levels and illuminance levels at a given point. Calculation of illuminance at specific points is often done to help the designer evaluate the lighting uniformity especially when using luminaires where maximum spacing recommendations are not supplied or where task lighting levels 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 data from a coefficient of utilization table.

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

The following two methods can be used if calculations are to be done to determine 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 be calculated from this data using the point to point method.

The following sections describe these methods of calculation.

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Zonal Cavity Method

The zonal cavity method is the currently accepted method for calculating average illuminance levels for indoor areas unless the light distribution is radically asymmetric. It is an accurate hand method for indoor applications because it takes into consideration the effect that interreflectance has on the level of illuminance. Although it takes into account several variables, the basic 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 spaces or cavities. The space between the ceiling and the fixtures, if they are suspended, is defined as the "ceiling cavity"; the space between the work plane and the floor, the " floor cavity"; and the space between the fixtures and the work plane, the "room cavity".

Once the concept of these cavities is understood, it is possible to calculate numerical relationships called "cavity ratios", which can be used to determine effective reflectance of the ceiling and floor and then to find the coefficient of utilization.

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 following formulas:

Ceiling Cavity Ratio (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 cavity and for the floor cavity. These are located in Table A (pg. 12) under the applicable combination of cavity ratio and actual reflectance of ceiling, walls and floor. Note that if the luminaire is recessed or surface mounted, or if the floor is the work plane, the CCR or FCR will be 0 and then the actual reflectance of the ceiling or floor will also be the effective reflectance. The effective reflectance values found will then be pcc (effective ceiling cavity reflectance) and pfc (effective floor cavity reflectance) .

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

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

Cavity Ratio (RCR) =

5 hrc (L+W) L x W

Floor Cavity Ratio (FCR) =

5 hfc (L+W) L x W

Cavity Ratio =

2.5 x height of cavity x cavity 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% or 30% then interpolate or extrapolate and multiply by this factor.

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

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

Zonal Cavity Method

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

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

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

# of luminaires =

maintained footcandles desired

x area in sq. ft.

lamp/fixture x lumen/lamp x CU x LLF

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Zonal 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 plane is 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 of utilization for the Prismawrap luminaire in a room having an RCR of 2.0 and effective reflectances as follows:

pcc = 62%; pw = 30%; pfc = 20%. By interpolation between boxed numbers in the table this CU is .55. Note that this CU is for an effective reflectance of 20% while the actual effective reflectance of the floor pfc is 10%. To correct for this, locate the appropriate 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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is found by interpolating between the boxed number in Table B for 70% pcc, 30% pw, and 50% pcc, 30% pw at an RCR of 2.0.

Then: CU final = .55 x .962 = .53

Note that all interpolations only need to be of the approximate "eyeball" type giving a credible degree of accuracy to the calculation.

(4) Illuminance level can now be calculated if we know the number of units to be used and the lamp lumen rating.

A possible arrangement for these fixtures is three columns of six fixtures spaced ten feet on center in each direction. The Spacing Criterion is 1.4, making the maximum allowable spacing 11 .2-feet. The actual spacing is less than the maximum allowable spacing, therefore the illumination on the work plane should be

uniform.

FC initial = # of fixtures x lamps/fixturex

lumens/lamp x CU area

FC initial = 18 x 4 x 3150 x .53

60 x 30

FC initial = 67

Check spacing of luminaires.

Lumen Method and Example

Calculating average illuminance levels using a utilization curve

The standard lumen method formula is also used to calculate average illuminance

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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, calculates how far apart the fixtures must be spaced to produce the necessary average illuminance.

A utilization curve shows the percent of light which falls onto an area having a designated width and an infinite length. This width is expressed on the utilization curve 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 CU line 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 of the fixture and may be used to find illumination on the roadway or sidewalk areas or added 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 of 1.0 fc. Holophane Mongoose® MV400HPNC6 is to be used. They will be mounted on 30 ft. poles which are set back 36 ft. from the road. Find the spacing required.

Footcanales = (maintained)

lumens/lamp x lamps/

luminaire x # luminaires x CU x LLF

area in square feet

# of luminaires=

maintained footcandles desired x area in sq. ft. lumens/lamp x lamps/ luminaire x CU x LLF

Spacing =lamp lumens x CU x LLF Avg. MTD FC x width of

Road

Spacing =

lamp lumens x CU x LLF

Avg. MTD FC x width of Road

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

Chart 1 The CU for the roadway area only is determined by subtracting the CU of the setback area from the CU of the total area of both roadway and setback. The width of the total 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 the standard lumen method formula results in a spacing of 371 feet.

Spacing = 50,000 x .22 x .81

1.0 x 24 = 371 ft.

Point Calculations and Example

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 most frequently used in heavy industrial and design where interreflections are not a consideration.

The point-by-point method accurately computes the illuminance level at any given point in an installation by summing up the illumination contributions to that point from every luminaire individually. It does not account for contributions from other sources such as reflection from walls, ceiling, etc. For accuracy the calculation distance 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 we may 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 is desired to find the initial horizontal illumination at a point 15' to one side of the luminaire. See figure 2.

Solution:

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

Since D2 = a2 + h2 D2 = (15)2 + (26)2 D = 30'

Then we can determine the candlepower of this luminaire from the cp curve, figure 3, to be, 18936 (cp).

fc = candlepower x Cosq

D2

Since fc =candlepower x Cosq

D2

and tangent g =

a h

y = arc tangent

a h

y = 30°

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The illumination then is:

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

This version of the formula lets us deal with only the net mounting height of the fixtures and candlepower angles and eliminates the necessity to calculate each separate distance "D".Point calculations using the isofootcandle chart The isofootcandle chart can also be used to find the illumination at a specific point. It is found by defining the horizontal distance from the fixture to that point in terms of a ratio of distance to mounting height, then looking up that ratio on the chart. If the actual mounting height of the fixture is different than the isofootcandle chart mounting 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 determine the illuminance level, between the two units, on the outside edge of the road using Chart 1.

Solution: From either fixture, point A is 60 feet to the street side (2.0 M.H.) and 143 feet down the 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 MH2 actual MH2

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

Computer programs Point by point calculations can be time consuming. Several computer programs are available that perform such calculations for many analysis points and luminaires in a fraction of the time necessary to do the same calculations by hand.

Lighting Quality

Achieving the required illuminance level does not necessarily ensure good lighting quality. The quality as well as the quantity of illuminance is important to produce a comfortable, productive, aesthetically pleasing lighting environment. The quality of the lighting system refers, but is not limited to, aspects of lighting such as proper color, good uniformity, proper room surface luminances, adequate brightness control 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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visual clarity, spaciousness and pleasantness. These feelings occur in spaces that are uniformly lighted with emphasis on higher luminances on room surfaces.

The improved user satisfaction from such spaces may or may not have any effect on worker performance. However, given two lighting systems with equal lifetime costs, lighting systems which provide improved worker satisfaction should be considered.

User satisfaction is often considered in the design of offices and commercial spaces, but ignored in industrial spaces. However, the industrial environment should be designed to provide a high quality visual environment, yielding improved worker satisfaction. This can be accomplished by using lighting systems which produce the proper luminance on ceilings and walls.

The photo below illustrates two lighting systems in the same industrial environment. Both lighting systems provide the same quantity of horizontal illuminance on the work plane. The system on the right provides little uplight, resulting in the typical "cavern" effect associated with industrial spaces. The system at left provides uplight and improves the luminance of the ceiling and vertical surfaces. This system can provide workers with a feeling of increased spaciousness. The uplight component also tends to improve work plane illuminance uniformity, possibly yielding improved feelings of visual clarity.

Any lighting design should consider the impressions of the user of the space. The photograph below indicates that even an industrial environment can be improved with the hope of providing better working conditions and improved satisfaction for the worker.

Selection of level of Illuminance

The following procedure is the currently accepted method of the Illuminating Engineering Society of North America for determining the level of maintained illuminance needed to perform a given task. This method takes into consideration the factors which most commonly contribute to the "seeability" of the task. It provides a range of illuminance levels for a given task, then defines a target illuminance level from within that range by the use of weighting factors which have been determined through research of lighting performance needs.

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The following conditions are factored into this method:1. The task to be performed 2. The details of the object to be viewed 3. The age of the observer 4. The importance of speed and/or accuracy for visual performance 5. The reflectance of the background material

This method, then, allows the designer to use his own evaluation of the environmental 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 following methods:

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

B. If a specific task cannot be established, the illuminance category may be determined 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 levels are established on Table D by matching the appropriate user, room, and task characteristics with the previously determined illuminance category.

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

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

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Selection of level of Illuminance

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Selection of level of Illuminance

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Selection of level of Illuminance

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Selection of level of Illuminance

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Selection of level of Illuminance

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Selection of level of Illuminance

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Holophane Corporation, 214 Oakwood Ave., Newark, OH 43055 / Holophane Canada, Inc. 294 Walker Drive, Unit #3, Brampton, ON Canada L6T 4Z2 / Holophane Europe Limited, Bond Ave., 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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Naucalpan de Juarez, 53000 Edo. de Mexico

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

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

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

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