Outdoor Lighting Design

Outdoor lighting is the lighting of open spaces such as roads, streets, car parks, sports grounds, building fronts, monuments and industrial yards after dark.

Major purposes of outdoor lighting

1. Safety of traffic: drivers and pedestrians can see the road, vehicles and obstacles, reducing accidents.
2. Security: discourages theft and crime around industrial plants, roads and public places.
3. Working at night: loading yards, ports, construction sites and switchyards can operate after sunset.
4. Sports and recreation: stadium and playground lighting for night games.
5. Advertising and display: shop fronts, hoardings and signs.
6. Decoration: floodlighting of monuments, temples and building facades for beauty and tourism.

Methods of street lighting

1. Diffusion (direct-light) method

• Uses luminaires with diffusing reflectors that send most light downward onto the road surface.
• The road is seen by the light reflected from the road surface in all directions.
• Illumination at a point is found by the inverse square and cosine law: E = I cos³θ / h².
• Gives good results on rough (matt) road surfaces.

2. Specular-reflection method

• Uses the mirror-like reflection of a smooth or wet road surface.
• Lamps are mounted so that light reflected from the road towards the driver forms a bright background; objects on the road appear as dark silhouettes against it.
• Needs fewer lamps but is suitable only for smooth roads and straight stretches; it can cause glare.

Arrangements of poles: single side, staggered (both sides alternately), opposite (both sides facing), and central (twin-arm) arrangement on the median.

 Single side      Staggered        Opposite
 o   o   o        o       o        o   o   o
 ========         ========         ========
                      o       o    o   o   o

Factors considered in designing street lighting

1. Class of road and traffic density – decides the required average luminance/illuminance (main roads ~ 15–30 lux; minor roads 4–8 lux).
2. Uniformity: minimum/average ratio of about 0.4 or more; avoid dark patches.
3. Mounting height: usually 7.5–12 m; higher mounting gives better uniformity and less glare.
4. Spacing of poles: spacing/height ratio around 3–4.
5. Type of lamp: HPSV, LED or metal halide – efficacy, life, colour and cost.
6. Luminaire light distribution (cut-off, semi cut-off) to control glare.
7. Road width and arrangement of poles (overhang, outreach of bracket).
8. Road surface reflection (dry/wet, smooth/rough).
9. Junctions, curves, bridges and pedestrian crossings need extra lamps.
10. Maintenance factor, cost of energy, cabling and control (photocells, timers, dimming).

Floodlighting and street lighting are both outdoor schemes, but floodlighting lights vertical surfaces or large areas with concentrated beams from projectors, while street lighting lights a long horizontal road surface from poles.

Factors for floodlighting design

1. Illumination level required – depends on the brightness of surroundings and reflectance of the surface (light surfaces in dark surroundings need 50–100 lux; dark surfaces in bright city areas 200–400 lux).
2. Area to be lit (A = length × height of the face).
3. Type of projector and beam spread – narrow (< 25°), medium (25–40°) or wide (> 40°) depending on distance from the surface.
4. Location and distance of projectors – usually 0.7–1 times the height of the building; avoid glare to the public.
5. Coefficient of utilisation (beam factor) – fraction of lamp lumens in the beam (0.3–0.6).
6. Waste light factor – light spilling past edges (1.2 for regular surfaces, 1.5 for irregular objects).
7. Depreciation / maintenance factor – dirt on glass and lamp ageing.
8. Lamp type, wattage and efficacy, colour of light for good appearance.

Example (floodlighting): A 40 m × 12 m building front needs 80 lux; CU = 0.5, DF = 1.3, WLF = 1.2; 400 W metal-halide projectors of 80 lm/W are used.

Area       = 40 × 12              = 480 m²
Total lm   = E·A·DF·WLF / CU
           = 80 × 480 × 1.3 × 1.2 / 0.5
           = 119 808 lm
Lamp lm    = 400 × 80             = 32 000 lm
N          = 119 808 / 32 000     = 3.74 → 4

So 4 projectors of 400 W are used.

Factors for street lighting design

1. Road class and traffic – required average illuminance and uniformity.
2. Road width – decides single-side, staggered or opposite arrangement.
3. Mounting height (h) and spacing (S) – S/h ratio about 3–4.
4. Type of lamp – HPSV, LED; efficacy and life.
5. Luminaire distribution – cut-off type to reduce glare.
6. Coefficient of utilisation and maintenance factor.
7. Road surface reflection and weather conditions.
8. Special locations – bends, junctions, bridges.

Example (street lighting): A road 10 m wide needs 10 lux average. LED lanterns of 9000 lm, CU = 0.4, MF = 0.8 are used.

S = F·CU·MF / (E·W)
  = 9000 × 0.4 × 0.8 / (10 × 10)
  = 28.8 m  → take 28 m

With 9 m poles, S/h = 28/9 ≈ 3.1, which is acceptable for good uniformity.

Given: wall 25 m × 10 m, distance d = 30 m, E = 120 lux, beam factor (BF) = 0.5, maintenance factor (MF) = 0.625, waste light factor (WLF) = 1.2, lamp efficacy = 116 lm/W, vertical beam spread = 12.5°.

Step 1: Total lamp lumens required

Area A   = 25 × 10 = 250 m²
Total lm = E × A × WLF / (BF × MF)
         = 120 × 250 × 1.2 / (0.5 × 0.625)
         = 36 000 / 0.3125
         = 115 200 lm

Step 2: Coverage of one beam (decides arrangement)

At 30 m, a beam of total spread 12.5° covers a length of

D = 2 × d × tan(12.5°/2)
  = 2 × 30 × tan 6.25°
  = 6.57 m

• Vertically: the wall is 10 m high; the wall subtends tan⁻¹(10/30) = 18.4° at ground-level fittings, which is more than 12.5°. One beam (6.57 m) cannot cover 10 m, so 2 tiers of aiming are needed (one row aimed at the lower half, one at the upper half).
• Horizontally: 25 / 6.57 = 3.8 → 4 beams across the wall (assuming a symmetrical beam).

So number of fittings = 2 × 4 = 8.

Step 3: Lamp wattage

Lumens per lamp = 115 200 / 8 = 14 400 lm
Wattage         = 14 400 / 116 = 124.1 W

Choose the next standard size: 150 W lamps (e.g. 150 W metal-halide or LED floodlight). This gives a little extra margin (8 × 150 × 116 = 139 200 lm).

Arrangement

          25 m wall (10 m high)
 ┌──────┬──────┬──────┬──────┐
 │  U1  │  U2  │  U3  │  U4  │ upper aim
 ├──────┼──────┼──────┼──────┤
 │  L1  │  L2  │  L3  │  L4  │ lower aim
 └──────┴──────┴──────┴──────┘
   ↑ 4 ground positions, 6.25 m apart,
   30 m in front; 2 fittings per position

The fittings are placed at 4 points about 6.25 m apart, 30 m in front of the wall, with two fittings at each point – one aimed at the upper half and one at the lower half. Beams overlap slightly for uniformity.

Answer: 8 fittings (2 rows × 4), each about 124 W → use 150 W lamps; total lamp lumens required 115 200 lm.

Street lighting works on two principles: the road is seen either by diffused light from a rough road or by specular (mirror-like) reflection from a smooth road.

Diffusion principle

• Luminaires with diffusing reflectors send light downward on the road; about 30–40% of light falls on the road.
• The rough road surface reflects light equally in all directions, so it appears uniformly bright from every viewing angle.
• Illumination at a point P at horizontal distance x from a pole of height h: E = (I / d²) cos θ = I h / d³, with d = √(h² + x²).
• Lamps are placed so the dip in illuminance midway between poles is small.
• Suitable for rough, dry roads, pedestrian areas and residential streets.

Specular reflection principle

• On a smooth or wet road, light is reflected like a mirror. The lamps are placed so that light reflected towards the motorist forms a bright streak/background on the road ahead.
• Objects on the road (vehicle, person) appear as dark silhouettes against this bright background, so they are easily seen.
• Luminaires direct light at high angles (near 75–80° from the vertical) along the road.
• Needs fewer, more widely spaced lamps; but works well only on straight, smooth roads and may cause glare.

 Lamp                      Driver's eye
  ●                           ◉
   \  incident          reflected /
    \                          /
 ════\════ bright patch ══/════ road

Design procedure of a floodlighting system

1. Study the object: size, shape, colour and reflection factor of the surface; surroundings (bright/dark).
2. Select illumination level (E): from tables, based on surface reflectance and surrounding brightness.
3. Find the area (A) to be lit: A = length × height.
4. Choose location and distance of projectors: generally a distance of 0.7–1 times the height of the building, outside the line of sight of viewers.
5. Select the beam spread: narrow beam for long throw, wide beam for short throw. Beam spread needed = 2 tan⁻¹(half-size / distance).
6. Decide factors: coefficient of utilisation or beam factor (0.3–0.6), depreciation or maintenance factor, waste light factor (1.2 regular / 1.5 irregular surfaces).
7. Total lumens required: F = E × A × DF × WLF / CU (or E × A × WLF / (BF × MF)).
8. Select lamp size and type: lumens per lamp = W × efficacy.
9. Number of projectors: N = F / lumens per lamp; round up.
10. Arrange and aim the projectors so beams overlap and give uniform brightness; check glare and spill light.
11. Design supply, control and cabling (timers, switches) and check cost.

Each lamp has I = 4750 cd in all directions below the horizontal, mounting height h = 6 m, spacing 16 m.

For a point at horizontal distance x from the base:

d = √(h² + x²)
cos θ = h / d
E = I cos θ / d² = I h / d³

(a)(i) Directly underneath (x = 0)

E = I / h² = 4750 / 6² = 4750 / 36
  = 131.94 lux

(a)(ii) 16 m from the base (x = 16 m)

d² = 6² + 16² = 36 + 256 = 292
d  = 17.09 m
cos θ = 6 / 17.09 = 0.351
E = 4750 × 6 / 292^1.5
  = 28 500 / 4989.8
  = 5.71 lux

(b) Total illuminance midway between two lamp posts (x = 8 m)

Each of the two adjacent lamps is 8 m away horizontally:

d² = 6² + 8² = 100,  d = 10 m
cos θ = 6 / 10 = 0.6
E (one lamp) = 4750 × 0.6 / 100 = 28.5 lux
E (total)    = 2 × 28.5         = 57.0 lux

(The next lamps on either side, 24 m away, add only about 1.88 lux each, so they are normally neglected.)

   L1                    L2
   ●──────── 16 m ───────●
   │6 m                  │
 ──┴──────────┬──────────┴──
   P1       Pmid(8 m)    P2

Answer: (a)(i) 131.94 lux, (a)(ii) 5.71 lux per lamp; (b) total at midway = 57.0 lux (28.5 lux from each lamp).

Given: building face 50 m × 16 m, projectors 25 m away, E = 100 lm/m² (lux), CU = 0.5, depreciation factor DF = 1.5, waste light factor WLF = 1.2.

Total lumens required

Area A   = 50 × 16 = 800 m²
Total lm = E × A × DF × WLF / CU
         = 100 × 800 × 1.5 × 1.2 / 0.5
         = 288 000 lm

Number and size of projectors

Size is not given, so assume 1000 W tungsten-halogen floodlight projectors with efficacy about 20 lm/W (standard textbook assumption):

Lumens per lamp = 1000 × 20 = 20 000 lm
N = 288 000 / 20 000 = 14.4 → 15 projectors

(If 400 W metal-halide lamps of 80 lm/W = 32 000 lm are used instead, N = 9.)

Angle of spread

With projectors 25 m away, aimed at the middle of the face (8 m high), the vertical beam must cover 16 m:

θ = 2 tan⁻¹(8 / 25) = 35.5°

(If projectors are at ground level aimed upward, the face subtends tan⁻¹(16/25) = 32.6°.) So projectors with a medium beam of about 35° are chosen. Horizontally the 15 projectors are spread along the 50 m frontage so that their beams overlap.

Answer: total 288 000 lm; 15 projectors of 1000 W (20 lm/W assumed); beam spread ≈ 35°.

Given: road length 2 km, width W = 7 m, 125 W SOX/HPSV lamps, efficacy 85 lm/W, pole height h = 9 m, span S = 30 m, CU = 0.5, minimum illumination 1.5 lux, feeder: 3.5-core underground cable.

Step 1: Number of lamps

Spans = 2000 / 30 = 66.7 → 67 spans
Actual spacing = 2000 / 67 = 29.85 m
Poles (one side) = 67 + 1 = 68 lamps

For a 7 m road a single-side arrangement is enough.

Step 2: Check average illumination

Lamp lumens F = 125 × 85 = 10 625 lm
E_av = F × CU / (S × W)
     = 10 625 × 0.5 / (30 × 7)
     = 25.3 lux

Step 3: Check minimum illumination (midway between poles)

Assume the lamp radiates uniformly: I = F / 4π = 10 625 / 4π = 845.5 cd. Midway point is 15 m from each pole:

d² = 9² + 15² = 306
E (one lamp) = I h / d³ = 845.5 × 9 / 306^1.5
             = 1.42 lux
E_min (two lamps) = 2 × 1.42 = 2.84 lux > 1.5 lux  ✓

So the scheme meets the minimum of 1.5 lux. (Directly below a lamp: E = 845.5/81 = 10.4 lux.)

Step 4: Electrical load and cable

Assume 15 W ballast loss per lamp, supply 400/230 V, 3-phase, pf 0.9 (with capacitor).

Total load P = 68 × (125 + 15) = 9520 W
Line current I = 9520 / (√3 × 400 × 0.9) = 15.3 A

• Lamps are connected phase-to-neutral, taken in turn on R, Y, B phases (≈ 23 lamps per phase) so the load is balanced and the 0.5 (half-size) core carries the neutral.
• Feed from the middle of the road, so each side carries about 7.6 A over 1 km.
• Uniformly distributed load: voltage drop = I × R × L/2. For 16 mm² Al (R ≈ 1.77 Ω/km):

ΔV = 7.6 × 1.77 × 0.5 = 6.8 V
   = 6.8 / 230 × 100 = 2.9 %  (< 5 % allowed)

• Choose 3.5-core, 16 mm² aluminium, PVC/XLPE armoured cable laid underground, with a feeder pillar containing MCB/fuses, contactor and photocell/timer for automatic switching.

 Feeder pillar (middle, 3φ+N)
 ◄───── 1 km ─────┼───── 1 km ─────►
 o  o  o  o  o  o │ o  o  o  o  o  o
 R  Y  B  R  Y  B   R  Y  B  R  Y  B

Answer: 68 lamps of 125 W on 9 m poles at ~30 m spacing on one side; E_av ≈ 25.3 lux, E_min ≈ 2.84 lux (> 1.5 lux); 3.5-core 16 mm² Al cable fed from the centre.

Design procedure of floodlighting

1. Survey the object: size, shape, colour and reflection factor of the surface and the brightness of surroundings.
2. Choose the illumination level (E) from tables (higher for dark surfaces and bright surroundings).
3. Find the area to be illuminated: A = length × height.
4. Fix projector location – distance about 0.7–1 times the building height, out of the viewers' sight line.
5. Select beam spread to suit the throw: θ = 2 tan⁻¹(half-height / distance).
6. Choose factors: coefficient of utilisation (CU), depreciation factor (DF), waste light factor (WLF).
7. Total lumens: F = E × A × DF × WLF / CU.
8. Lamp lumens = wattage × efficacy; number N = F / lamp lumens (round up).
9. Arrange and aim projectors for uniform, glare-free lighting; design cabling and control.

Numerical

Given: 50 m × 15 m, distance 25 m, E = 100 lux, CU = 0.5, DF = 1.5, WLF = 1.2, 200 W lamps of 90 lm/W.

Area A    = 50 × 15            = 750 m²
Total lm  = E·A·DF·WLF / CU
          = 100 × 750 × 1.5 × 1.2 / 0.5
          = 270 000 lm
Lamp lm   = 200 × 90           = 18 000 lm
N         = 270 000 / 18 000   = 15

Vertical beam spread needed (aimed at mid-height): 2 tan⁻¹(7.5/25) = 33.4°, so medium-beam projectors are suitable.

Answer: 15 projectors of 200 W each.

Waste light factor

Waste light factor (WLF) is the factor by which the required lumens are increased to allow for light from the projectors that spills beyond the edges of the surface (or passes between parts of it) and so does no useful work. Usual values: 1.2 for regular, flat surfaces and 1.5 for irregular objects such as statues and towers.

Why it must be considered

• Beams are circular or oval, while buildings are rectangular; overlapping beams at the edges always spill some light.
• Without WLF, the actual illumination on the surface would be about 15–35% lower than designed, making the facade look dull and patchy.
• It gives a realistic number and wattage of projectors and avoids redesign after installation.

Floodlight calculation process

1. Decide illumination level E (lux) from the surface colour and surroundings.
2. Find area A of the surface.
3. Choose CU (or beam factor), depreciation factor DF and WLF.
4. Total lumens: F = E × A × DF × WLF / CU.
5. Choose lamp wattage and efficacy; lamp lumens = W × η.
6. Number of projectors N = F / lamp lumens.
7. Choose beam spread from distance: θ = 2 tan⁻¹(half-height / distance), then arrange and aim.

Example: A building front 30 m × 12 m, viewed in a medium-bright area, needs 120 lux. Projectors at 15 m; CU = 0.4, DF = 1.3, WLF = 1.2; 250 W metal-halide lamps of 80 lm/W.

A        = 30 × 12              = 360 m²
F        = 120 × 360 × 1.3 × 1.2 / 0.4
         = 168 480 lm
Lamp lm  = 250 × 80             = 20 000 lm
N        = 168 480 / 20 000     = 8.4 → 9
θ        = 2 tan⁻¹(6 / 15)      = 43.6°  (wide beam)

So 9 wide-beam 250 W projectors are used. Without WLF (=1.0), only 7 would be calculated and the surface would be under-lit.

Outdoor lighting is artificial lighting of open areas – roads, building facades, monuments, sports grounds, yards and parking areas – for safety, security, night work, advertising and decoration. The two main calculations are for floodlighting and street lighting.

Floodlighting calculation

Floodlighting means lighting a large surface with projectors giving concentrated beams.

1. Select illumination E (lux) based on the surface reflectance and surroundings.
2. Area A = length × height of the surface.
3. Total lumens required:

F = E × A × DF × WLF / CU

where CU = coefficient of utilisation (beam lumens/lamp lumens, 0.3–0.6), DF = depreciation factor (1.3–1.5), WLF = waste light factor (1.2–1.5). 4. Number of projectors N = F / (lamp watts × efficacy). 5. Beam spread θ = 2 tan⁻¹(half-height / distance).

Example: 20 m × 10 m face, E = 150 lux, CU = 0.5, DF = 1.3, WLF = 1.2, 150 W LED floodlights of 110 lm/W.

F = 150 × 200 × 1.3 × 1.2 / 0.5 = 93 600 lm
Lamp lm = 150 × 110 = 16 500 lm
N = 93 600 / 16 500 = 5.7 → 6 floodlights

Street lighting calculation

(a) Lumen method (average illumination):

E_av = F × CU × MF / (S × W)
S    = F × CU × MF / (E_av × W)

F = lamp lumens, S = pole spacing, W = road width, MF = maintenance factor.

(b) Point-by-point method (minimum illumination): for a lamp of intensity I at height h, a point at horizontal distance x gets

E = I cos θ / d² = I h / (h² + x²)^1.5

Add contributions of adjacent lamps (usually the two nearest) midway between poles to check minimum and uniformity.

Example: Road 8 m wide, 150 W HPSV lamps (15 000 lm), CU = 0.4, MF = 0.8, E_av = 15 lux, h = 9 m.

S = 15 000 × 0.4 × 0.8 / (15 × 8) = 40 m
S/h = 40/9 = 4.4  → too high; take S = 35 m
E_av = 15 000×0.4×0.8/(35×8) = 17.1 lux

Floodlighting is the lighting of large surfaces (building facades, monuments, yards, stadiums) by projectors that concentrate the light into a controlled beam. Its design consists of finding the lumens, choosing suitable luminaires (projectors) and arranging them.

Design procedure

1. Study the object and site: dimensions, shape, material, colour and reflection factor; brightness of surroundings; viewing direction.
2. Choose illumination level (E): e.g. light-coloured stone in dark area 50–100 lux; dark brick in bright city area 200–300 lux.
3. Area to be illuminated: A = length × height.
4. Location of projectors:
   • On the ground in front, at a distance of about 0.7–1 × height of building;
   • Or on nearby poles/roofs; or on the building itself (for tall buildings).
   • They must be hidden from viewers and must not dazzle traffic.
5. Beam spread: found from the distance and the size to be covered, θ = 2 tan⁻¹(half-size / distance).
6. Select factors: CU or beam factor, depreciation factor, waste light factor.
7. Total lumens: F = E × A × DF × WLF / CU.
8. Lamps: select type and wattage; N = F / (W × efficacy).
9. Aim and arrange: overlap beams by about 10–20% for uniformity; trial aiming at night.
10. Electrical design: circuits, cables, control by timer/photocell, protection.

   Building face
 ┌───────────────┐
 │  overlapping  │
 │    beams      │
 └───────────────┘
     \  |  /
      \ | /   distance ≈ 0.7–1 × H
       [P P P]  projectors

Selection of luminaires (projectors)

1. Beam spread:

2. Lamp type: LED, metal-halide (good colour), HPSV (high efficacy, yellow), halogen (low efficacy, being phased out).
3. Efficacy and life – lower running and replacement cost.
4. Colour rendering and colour temperature to suit the surface (warm light for brick, cool light for stone/glass).
5. Beam factor – proportion of lamp lumens inside the beam.
6. Ingress protection (IP65 or more), corrosion-resistant housing, toughened glass.
7. Mounting and aiming facility (adjustable brackets, aiming scale).
8. Glare control – visors and louvres.
9. Cost and maintenance – ease of lamp replacement and cleaning.

Street lighting design means choosing lamps, mounting height, spacing and arrangement so that a road gets the required average illumination with good uniformity and low glare at minimum cost.

Design procedure

1. Classify the road (main, secondary, residential) and traffic; select required average illumination E_av and uniformity (E_min/E_av ≥ 0.4).
2. Note road data: width W, length, number of lanes, median, surface type.
3. Select lamp and luminaire: HPSV or LED; cut-off/semi-cut-off luminaire; lumen output F.
4. Choose mounting height h: 7.5–9 m for minor roads, 10–12 m for major roads.
5. Choose arrangement:
   • Single side: W ≤ h
   • Staggered: W up to 1.5 h
   • Opposite / central twin-arm: wider roads or dual carriageways
6. Find spacing (lumen method): S = F × CU × MF / (E_av × W); keep S/h ≈ 3–4.
7. Check minimum and uniformity by point method midway between poles: E = I h / d³.
8. Number of poles: N = L/S + 1.
9. Electrical design: balance lamps on three phases, size cables for ≤ 5% voltage drop, provide feeder pillar, MCBs, photocell/timer control.
10. Cost estimate and maintenance plan.

Example

A 1 km road, 9 m wide, needs E_av = 12 lux. Use 100 W LED lanterns of 12 000 lm, CU = 0.45, MF = 0.8, h = 9 m, single-side.

S = F·CU·MF / (E·W)
  = 12 000 × 0.45 × 0.8 / (12 × 9)
  = 40 m
S/h = 40/9 = 4.4  > 4  → take S = 35 m
E_av = 12 000×0.45×0.8 / (35×9) = 13.7 lux
Poles = 1000/35 = 28.6 → 29 spans, 30 poles

Check midway point (assume I = 1500 cd towards it, x = 17.5 m):

d² = 9² + 17.5² = 387.25
E (one lamp) = 1500 × 9 / 387.25^1.5 = 1.77 lux
E_min (2 lamps) ≈ 3.5 lux → uniformity ≈ 0.26

Uniformity is low, so use a staggered arrangement or a luminaire with wider (batwing) distribution, then re-check. Load: 30 × 100 W = 3 kW, about 10 lamps per phase.

a) Factors in designing a floodlighting system

1. Size and shape of the surface to be lit (area A).
2. Colour and reflection factor of the surface.
3. Brightness of the surroundings.
4. Required illumination level (lux).
5. Distance and location of projectors.
6. Beam spread of projectors (narrow, medium, wide).
7. Coefficient of utilisation / beam factor.
8. Depreciation (maintenance) factor.
9. Waste light factor (spill light).
10. Lamp type, wattage, efficacy and colour; glare to viewers and traffic; cost.

b) Factors in designing a street lighting system

1. Class of road and density of traffic (vehicles and pedestrians).
2. Required average illumination/luminance.
3. Uniformity ratio (E_min/E_av).
4. Road width and number of lanes.
5. Mounting height of lamps.
6. Spacing between poles (S/h ratio).
7. Arrangement: single-side, staggered, opposite, central.
8. Road surface reflection (rough/smooth, dry/wet).
9. Glare control and luminaire distribution (cut-off type).
10. Junctions, curves, bridges; maintenance factor; energy cost and control (photocell, timer, dimming).

c) Factors in selecting luminaires

1. Light distribution (direct, indirect, cut-off, beam spread).
2. Lamp type and efficacy (LED, HPSV, metal halide).
3. Light output ratio / efficiency of the fitting.
4. Glare control (shielding angle, diffusers, louvres).
5. Colour rendering and colour temperature.
6. Environment: IP rating against dust and water; flame-proof or corrosion-proof where needed.
7. Mechanical strength and heat dissipation.
8. Mounting method and aiming facility.
9. Ease of maintenance and cleaning.
10. Initial cost, life and running cost; appearance.