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Lighting Calculations
How Many Fittings are Required for a Room The utilisation factor table and a few simple formulas allow us to calculate the number of fittings required for any room.
Step1: Understand the utilisation factor table that is available on most fittings.
UTILISATION FACTOR TABLE (for2x36 bare batten)
LOR:88.1%
SHR NOM1.75:1.0
Disregard SHR MAX.
Reflectance
Room Index (K)
Ceiling
Wall
Floor
0.75
1.00
1.25
1.5
2.0
2.50
3.00
4.00
5.00
0.50
0.30
0.20
0.30
0.37
0.42
0.46
0.52
0.56
0.59
0.63
0.66
Across the Top of the Table
Name
Description
LOR
Light output ratio for this fitting. Not required in a simple calculation.
SHR NOM
Nominal Spacing to Height ratio. For a given mounting height we can see the nominal spacing between fittings. Eg.1.75 to1 means that for every1 metre of mounting height (above work plane) we should have a maximum1.75 metres between fittings.
SHR MAX
Used by lighting design software and not needed for manual calculations.
Reflectance
Select a horizontal line for the reflectance values that best describe the room.
Typical Reflectance Values are:
Ceiling
Walls
Floor
Air Conditioned Office
0.7
0.5
0.2
Industrial
0.5
0.3
0.2
Utilisation Factor (body of the table) This is a value between0 and1 that represents the percentage of total lamp lumens in the room that fall on the work plane. It takes into account the room reflectances, room shape, polar distribution and light output ratio of the fitting.
Step2: Calculate Room Index (K)
Room Index: The room index is a number that describes the ratios of the rooms length, width and height.
Formula:
K = L x W
Hm (L+W)
Where:
L = Room Length
W = Room Width
Hm = Mounting Height of Fitting (from working plane)
Work Plane = Desk or Bench Height
The result of this calculation will be a number usually between0.75 and5.
Note: This formula for K is only valid when room length is less than4 times the width or when the K value is greater than0.75.
Step3: Using the room index and reflectance values in the utilisation factor table
For the horizontal row select the reflectance that best describes the room.
For the vertical column select the room index value K as calculated above.
The utilisation factor for this fitting in this room is where the row and column intersect.
Step4: To calculate the number of fittings required use the following formula:
Formula:
N =E x A
F x uF x LLF
Where:
N = Number of Fittings
E = Lux Level Required on Working Plane
A = Area of Room (L x W)
F = Total Flux (Lumens) from all the Lamps in one Fitting
UF = Utilisation Factor from the Table for the Fitting to be Used
LLF = Light Loss Factor. This takes account of the depreciation over time of lamp output and dirt accumulation on the fitting and walls of the building.
Typical LLF Values
Air Conditioned Office
0.8
Clean Industrial
0.7
Dirty Industrial
0.6
Standard
Triphos
Lumen outputs of fluorescent lamps
18 Watt /1150 Lumens
1300 Lumens
36 Watt /3000 Lumens
3350 Lumens
Lumen output of PL lamps
18 Watt /1200 Lumens
36 Watt /2900 Lumens
Step5: Space the number of fittings uniformly around the room drawing and check the SHR nominal for the fitting has not been exceeded. If it has been exceeded re space the fittings to get back to SHR nom.
Step6: Work out the number of fittings required in each axis of the room:
Number in Length =
Number in Width =
Example:
Consider the below fig as small office room length is12 m, width is8 m & height of the room is3.5 m. Lux required for the work area is150 lux & consider the36 watts lamb.
Lamp mounting in fall ceiling at height of3m. Calculate the no of lamp required ?
Formula: N= (E X A)/ (F X UF X LLF)
K= (L X W) / Hm ( L + W)
= (12 X8) /3 X (12 +8)
K =1.6
CHECK THE FIRST TABLE OF UTILISATION FACCTOR TABLE
FIND THE VALUE OF U.F. FOR THE CORRESPONDING K VALUE
UF = 0.52 (FOR k=1.6) X O.7 (REFLECTANCE VALUE FOR CEILING)
UF =0.364
LIGHT LOSS FACTOR VALUE ( LLF)= 0.8 (For Air conditioned office)
N= (E X A)/ (F X UF X LLF)
= (150 X96) / (3000 X0.364 X0.8)
N =16.48=16 Nos.
