INDOOR LIGHTING DESIGN.doc

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INDOOR LIGHTING DESIGN

1. Introduction to the Lumen Method

The lumen method is applicable to design of a uniform (general) lighting scheme ina space where flexibility of working locations or other activities is required.

Figure 1 Working PlaneThe lumen method is applied only to square or rectangular rooms with a regulararrayluminaires as shown in Figure 2.

Figure 2 Spacing of Luminaires in Lumen Design Method

2. Lumen Method CalculationsThe lumen method is based on fundamental lighting calculations. The lumen methodformula is easiest to appreciate in the following form.

(1)where E = average illuminance over the horizontal working plane

n = number of lamps in each luminaireN = number of luminaire

F = lighting design lumens per lamp, i.e. initial bare lamp luminousflux

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UF = utilisation factor for the horizontal working planeLLF = light loss factor

A = area of the horizontal working plane2.1 Light Loss Factor

Light loss factor (LLF) is the ratio of the illuminance produced by the lightinginstallation at the some specified time to the illuminance produced by the sameinstallation when new. It allows for effects such as decrease in light output caused by(a) the fall in lamp luminous flux with hours of use,(b) the deposition of dirt on luminaire, and(c) reflectances of room surfaces over time.In fact, light loss factor is the product of three other factors:

(2)where LLMF = lamp lumen maintenance factor

LMF = luminaire maintenance factorRSMF = room surface maintenance factor2.1.1 Lamp Lumen Maintenance Factor

Lamp lumen maintenance factor(LLMF) is the proportion of the initial light outputof a lamp produced after a set time to those produced when new. It allows for thedecline in lumen output from a lamp with age. Its value can be determined in twoways:(a) by consulting a lamp manufacturer's catalog for a lumen depreciation chart, and

(b) by dividing the maintained lumens by the initial lamps.2.1.2 Luminaire Maintenance FactorLuminaire maintenance factor(LMF) is the proportion of the initial light output froma luminaire after a set time to the initial light output from a lamp after a set time. Itconstitutes the greatest loss in light output and is mainly due to the accumulation ofatmospheric dirt on luminaire. Three factors must be considered in its determination:(a) the type of luminaire,(b) atmospheric conditions, and(c) maintenance interval.

2.1.3 Room Surface Maintenance Factor

Room surface maintenance factor (RSMF) is the proportion of the illuminanceprovided by a lighting installation in a room after a set time compared with thatoccurred when the room was clean. It takes into account that dirt accumulates onroom surfaces and reduces surface reflectance. Figure 4 shows the typical changesin the illuminance from an installation that occur with time due to dirt deposition onthe room surfaces.

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2.2 Utilisation Factor

Utilisation factor (UF) is the proportion of the luminous flux emitted by the lampswhich reaches the working plane. It is a measure of the effectiveness of the lightingscheme. Factors that affect the value of UF are as follows:

(a) light output ratio of luminaire(b) flux distribution of luminaire(c) room proportions(d) room reflectances(e) spacing/mounting height ratio2.2.1 Light Output Ratio of LuminaireLight output ratio of luminaire (LOR) takes into account for the loss of light energyboth inside and by transmission through light fittings. It is given by the followingexpression.

(3)Example 1The total, upward and downward lamp output from a lamp are 1000 lm, 300 lm and500 lm respectively. Calculate upward light output ratio (ULOR), downward lightoutput ratio (DLOR), light output ratio (LOR) of luminaire and percentage of lightenergy absorbed in luminaire.

Amount of light energy absorbed in luminaire = 100 - 80 = 20 %A greater DLOR usually means a higher UF.

A simple classification of luminaires according to their distribution is based on fluxfractions, as shown in Figure 5. Upward flux fraction (UFF) and downward fluxfraction (DFF) are used as a basis of comparison.

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Example 2

For data given in Example 1 determine upward flux fraction (UFF), downward fluxfraction (DFF) and flux fraction ratio (FRR).

Figure 5 Flux Fraction of Various Luminaires2.2.2 Flux Distribution of Luminaire

Direct ratio is the proportion of the total downward luminous flux from a conventionalinstallation of luminaires which his directly incident on the working plane. It is used toassess the flux distribution of luminaire. Since the intensity distribution pattern of thelight radiated from a luminaire in the lower hemisphere will affect:

(a) the quantity of the downward flux falls directly on the working plane and(b) the quantity of flux available for reflection from the walls in a given room,Direct ratio depends on both the room proportions and the luminaires. Direct ratiohas a low value with a narrow room (small room index) and a luminaire which emitsmost of its light sideways (BZ 10), and on the contrary, a high value with a wide room(large room index) and a luminaire which emits most of its light downwards (BZ 1).2.2.3 Room ProportionRoom index (RI) is the ratio of room plan area to half the wall area between theworking and luminaire planes.

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(4)where L = length of roomW = width of room

Hm = mounting height, i.e. the vertical distance between the working plane and theluminaire.2.2.4 Room ReflectancesThe room is considered to consist of three main surfaces:(a) the ceiling cavity,(b) the walls, and(c) the floor cavity (or the horizontal working plane).The effective reflectances of the above three surfaces affect the quantity ofreflected light received by the working plane.

2.2.5 Spacing to Height Ratio

Spacing to Height ratio (SHR or S/Hm) is defined as the ratio of the distancebetween adjacent luminaires (centre to centre), to their height above the workingplane. For a rectangular arrangement of luminaires and by approximation,

(5)

where A = total floor areaN = number of luminairesHm = mounting height

Under a regular array of luminaires the illuminance on the working plane is notuniform. The closer spaced the luminaires for a given mounting height, the higher theuniformity; or the greater the mounting height for a given spacing, the greater theuniformity. If uniformity of illuminance is to be acceptable for general lighting,(a) SHR should not exceed maximum spacing to height ratio (SHR MAX) of thegiven luminaire as quoted by the manufacturer, and(b) geometric mean spacing to height ratio of the luminaire layout should be withinthe range ofnominal spacing to height ratio (SHR NOM) of the given luminaire as

quoted by the manufacturer, i.e.

(6)

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3. Summary of Procedures for Lumen Design Method

(a) Calculate the room index.(b) Determine the effective reflectances of the ceiling cavity, walls and floor cavity.(c) Determine the utilisation factor from the manufacturer's data sheet, using the

room index and effective surface reflectances as found in (a) and (b) above.(d) Determine the light loss factor.(e) Inert the appropriate variables into the lumen method formula to obtain thenumber of luminaires required.(f) Determine a suitable layout.(g) Check that the geometric mean spacing to height ratio of the layout is within theSHR NOM range:

(h) Check that the proposed layout does not exceed the maximum spacing to height

ratios (SHR MAX).(i) Calculate the illuminance that will be achieved by the final layout and checkagainst the standard.

Example 3

Design a lighting installation for a college seminar room so that the averageilluminance is 500 lux on the horizontal working plane, using the data listed below.Suggest the layout and check appropriate spacing to mounting height.

Room dimensions: 12 m long x 8 m wide x 3.2 m highWorking plane at 0.7 m above floorReflection factors: Ceiling 70 %Walls 50 %Working plane 20 %Light Loss factor: 0.779Luminaires: 1800 mm twin tube with opal diffuserCeiling mountedDownward light output ratio 36 %SHR MAX 1.60 : 1SHR NOM 1.50 : 1

Dimensions : 1800 mm long x 200 mm wideLamps: 1800 mm 75 W plus white5800 average initial lumens per lamp2 lamps per luminaire

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Solution

(a) Initial calculation

From manufacturer's photometric data sheet (Table 3), utilisation factor (UF) is0.5336 by interpolation.

Therefore, the number of luminairs is 10.Initial check on S/Hm ratio gives:

From the manufacture's photometric data, maximum S/Hm is 1.6 : 1. Therefore, itshould be possible to use 10 luminaires.(b) Proposed layoutA 5 x 2 array is proposed fro the lighting installation. (A 10 x 1 array is an alternative.)(c) Checking the proposed layoutSince 2 x 1.8 m = 3.6 m < 8 m (width of room), the proposed layout will fit.(Usually checking only the linear dimension of the fitting for space is enoughas the other dimension (i.e. 200 mm in this case) is much smaller.)For long axis,

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For short axis,

Note that if the checks had worked out to be unsatisfactory, the number of luminairesshould be reconsidered and the calculations on the illuminance should be repeated.

For example, a 3x3 array for lower lux level or a 4x4 array for higher lux level.

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