PAUL HAY Capital Projects
Electric Lighting
Author: Paul Hay
e-mail: paul.hay@phcjam.com
profile: www.linkedin.com/in/phcjam
1.0 1.0 SPECTRAL PROPERTIES OF LAMPS VARY
1.1 Chromaticity, Colour Temperature or Correlated Colour Temperature (CCT) indicates the apparent colour of light emitted from a lamp:
1.1.1 Incandescent lamps emit "warm" reddish-white light [CCT = 2,700K];
1.1.2 Some fluorescent lamps emit "neutral" white light [CCT = 3,500K];
1.1.3 Other fluorescent lamps is a "cool" bluish-white [CCT = 4,100K].
1.2 Colour Rendering Index (CRI) indicates the ability of a light source to accurately represent the colour of an object [0 < CRI <100]:
1.2.1 CRI = 0 indicates a light source without colour (eg. black and white television);
1.2.2 Good colour rendering exists when 70 < CRI < 80.
1.3 High Intensity Discharge (HID) lamps include Mercury Vapour (MV), Metal Halide (MH), and High Pressure Sodium (HPS):
1.3.1 Mercury Vapour lamps emit a bluish-white light:
1.3.1.1 Colour rendition is generally worse than fluorescent lighting;
1.3.1.2 It is used outdoors because of its low cost and long life.
1.3.2 Metal Halide lamps can have twice the efficacy of MV lamps:
1.3.2.1 They have good colour rendition; but
1.3.2.2 They are expensive and have a shorter life than MV lamps.
1.3.3 High Pressure Sodium lamps emit yellowish light:
1.3.3.1 They have poor colour rendition, and
1.3.3.2 They are used to illuminate streets and parking lots.
2.0 LUMINAIRE EFFICIENCY VARIES
2.1 Luminaire type refers to the construction of the light fixture: bulbs enclosed, ballasts, volume of the casing, materials used and ventilation conditions.
2.2 The efficiency with which a light source converts electricity to light is called efficacy.
2.3 The efficacy of different types of luminaires vary from low to high are as follows: Incandescent [8 - 20 lm/W], Mercury Vapour [30 - 60 lm/W], Fluorescent [32 - 102 lm/W], Metal Halide [70 - 100 lm/W], High-pressure Sodium [50 - 130 lm/W], ballast losses being included for all but incandescent luminaires.
2.4 Incandescent lamps emit a reddish-yellow light.
2.5 Incandescent lamps with higher efficacies generally have special reflectors designated as follows: "R" - reflector, "PAR" - parabolic aluminized reflector, "ER" - elliptical reflector.
2.6 Fluorescent lamps last 20 times longer than incandescent lamps but are more expensive.
3.0 ELECTRIC LIGHTING IS TRANSFORMED INTO HEAT
3.1 All electrical power used to produce light is eventually converted into heat:
3.1.1 In many instances, electric lighting is the most significant contributor to a room=s cooling load;
3.1.2 Where fixtures are recessed into ceilings, cooling loads increase by two means:
2.1.2.1 Electric light directly heats the room enclosure; and
2.1.2.2 Heat passed into ceiling space is extracted by the return air.
3.1.3 In multi-storey buildings, all heat produced by electric lighting increases the building's cooling load, if lights are on long enough.
3.2 There are instances when heat produced by electric lighting does not contribute to a room=s cooling load:
3.2.1 If air-returns are ducted through the ceiling space;
3.2.2 When air-conditioning systems do not run continuously; and
3.2.3 In rooms adjacent to unconditioned spaces.
3.3 Convective components become instantaneous cooling loads.
3.4 Radiative components contribute to cooling loads at a later time:
3.4.1 Building structure stores the radiative component, which only becomes a cooling load when surface temperatures exceed the air temperature:
3.4.1.1 Materials with greater heat capacities produce longer delays;
3.4.1.2 Increasing thickness of materials increase their heat capacities.
3.4.2 Furniture and floor finishes can reduce thermal storage within rooms:
3.4.2.1 Simple heavy-weight furniture without carpets are least effective;
3.4.2.2 Ordinary furniture with carpets are most effective.
3.4.3 Transmission of heat is also a function of the internal surface coefficients and the room's ventilation rate:
3.4.3.1 Carpeting reduces heat transfer from the floor; and
3.4.3.2 Rooms with higher ventilation rates have greater heat transfers.
3.5 Transfer of thermal energy from luminaires (i.e. radiation, convection, and conduction) depends on the type of luminaire, method of mounting, and ceiling construction.
4.0 METHODS OF MOUNTING AFFECT DISTRIBUTION OF HEAT
4.1 Suspended luminaires distribute thermal energy predominantly by convection.
4.2 Recessed luminaires distribute thermal energy as suspended luminaires.
4.3 Surface-mounted luminaires have greater conduction of thermal energy.
5.0 CEILING CONSTRUCTION AFFECTS COOLING LOADS
5.1 Heat passing into ceiling spaces can either enter room enclosures through cracks between suspended ceiling tiles and framing:
5.1.1 If ceiling space is used as a plenum, recessed luminaires transfer heat as if the luminaires were suspended without a ceiling;
5.1.2 Where return-air is ducted, heat from the ceiling space is convected into the conditioned space and increases the cooling load.
5.2 Heat passing into ceiling without gaps do not contribute to cooling load if return-air is ducted.
6.0 QUANTITY OF LUMINAIRES HAVE TO BE CALCULATED
6.1 The quantity of luminaires required can be calculated using the Lumen method as follows:
No. Luminaires = Desired Illumination (lux) x Room Area (m2 ) [6.1]
M x UF x LLF x Lamps/luminaire x lm/lamp
where,
M = Multiplier (as per manufacturer=s specifications)
UF = Utilization Factor (as per manufacturer=s specifications)
LLF = Light-Loss Factor
6.2 Utilization Factor (UF) is the ratio of Luminous Flux on a reference surface to the total flux of the installation:
6.2.1 UF can be found in manufacturer=s photometric tables or Lighting Handbook of Illumination Engineering Society (IES);
6.2.2 UF depends on (a) room geometry, (b) reflectivity of surfaces, and (c) photometric characteristics of the luminaires; and
6.2.3 Room Cavity Ratio (RCR) is a lumped parameter used to express the room characteristics:
RCR = 5h x [L + W]/[(L x W)] [6.2]
where,
L = Length of room (m)
W = Width of room (m)
h = Mounting height above a reference plane (approx. 0.85 m above the finished floor level)
h = H - e - p [6.3]
where,
H = Height of room (m)
e = Height of reference plane above floor (m)
p = Distance Luminaire is suspended from ceiling (m)
6.3 Light Loss Factor (LLF) is the ratio of mean luminous flux from a lamp to its rated output, after aging of the lamp, accumulation of dirt luminaire and room surfaces, and other contributing loss factors.
Figure 1: Light Dirt Depreciation
6.4 The ratio of the minimum to mean illumination on a task area should equal or exceed 0.8:
6.4.1 This is normally achieved within the central region if the ratio of the minimum to maximum direct illuminance is greater than 0.7;
6.4.2 The maximum spacing-to-height ratio (SHR MAX) is defined for a square arrangement of luminaires which satisfies condition 5.5.1 as follows:
S MAX = SHR MAX x h [6.4]
where,
S MAX = Maximum spacing between rows of luminaires
7.0 CALCULATION OF REQUIRED LUMINAIRES
Example 7.1
Calculate required quantity of luminaires given the following:-
Room dimensions (W x L x H), m = 2.5 x 4.0 x 2.5
Reflectances (Clg. & Wall) = 0.7 & 0.5
Desired Illumination = 500 lux
Lamp/Luminaire = 2 No.
Lumen/Lampa = 2,800 lm
RCR = 5 h [L + W]/[L x W)] [6.2]
= 5(2.5 - 0.85)[4.0 +2.5]/[4.0 x 2.5]
= 5 x 1.65 x 6.5/10 = 5.36
Using RCR = 6,
UFb = 0.41
No. of Luminaires = Desired Illumination x Room Area [6.1]
M x UF x LLF x Lamp/Lumin. x Lumen/Lamp
= 500 x (2.5 x 4)
1.0 x 0.41 x 0.85 x 2.0 x 2800
= 5000 = 2.562 (use 3 No.)
1951.6
Smax = h x SHRmax
= 1.65 x 1.6 = 2.64 m
a. Value of Lumen/Lamp complies with Phillips F40T12/CW/IS lamp;
b. UF is manufacturer's specifications for 2 lamp strip luminaire & listed surface reflectances.
Figure 2: Utilization Factors
FURTHER READING
Construction Materials & Processes, Don G. Watson, McGrawHill Book Co., USA, 1978;
Lighting Handbook, Philips Lighting Company, USA, 1984
