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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.
Step 1: Understand the utilisation factor table
that is available on most fittings.
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UTILISATION FACTOR TABLE (for 2x36 bare
batten)
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LOR: 88.1%
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SHR NOM 1.75: 1.0
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Disregard SHR MAX.
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Reflectance
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Room Index (K)
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Ceiling
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Wall
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Floor
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0.75
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1.00
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1.25
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1.5
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2.0
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2.50
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3.00
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4.00
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5.00
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0.50
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0.30
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0.20
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0.30
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0.37
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0.42
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0.46
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0.52
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0.56
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0.59
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0.63
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0.66
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Across the Top of the Table
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Name
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Description
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LOR
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Light output ratio for this fitting. Not
required in a simple calculation.
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SHR NOM
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Nominal Spacing to Height ratio. For a
given mounting height we can see the nominal spacing between
fittings. Eg. 1.75 to 1 means that for every 1 metre of mounting
height (above work plane) we should have a maximum 1.75 metres
between fittings.
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SHR MAX
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Used by lighting design software and not
needed for manual calculations.
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Reflectance
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Select a horizontal line for the reflectance
values that best describe the room.
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Typical Reflectance Values
are:
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Ceiling
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Walls
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Floor
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Air Conditioned Office
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0.7
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0.5
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0.2
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Industrial
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0.5
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0.3
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0.2
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Utilisation Factor (body of the table)
This is a value between 0 and 1 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.
Step 2: 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 |
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Hm (L+W) |
| Where: |
L = Room Length |
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W = Room Width |
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Hm = Mounting Height of Fitting (from working plane) |
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Work Plane = Desk or Bench Height |
The result of this calculation will be a number
usually between 0.75 and 5.
Note: This formula for K is only valid when
room length is less than 4 times the width or when the K value is
greater than 0.75.
Step 3: Using the room index and reflectance
values in the utilisation factor table
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For the horizontal row select the reflectance that best describes
the room. |
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For the vertical column select the room index value K as calculated
above. |
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The utilisation factor for this fitting in this room is where
the row and column intersect. |
Step 4: To calculate the number of fittings
required use the following formula:
| Formula: |
N =E
x A |
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F
x uF x LLF |
| Where: |
N = Number of Fittings |
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E = Lux Level Required on Working Plane |
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A = Area of Room (L x W) |
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F = Total Flux (Lumens) from all the Lamps in one Fitting |
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UF = Utilisation Factor from the Table for the Fitting to
be Used |
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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. |
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Typical LLF Values
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| Air Conditioned Office |
0.8 |
| Clean Industrial |
0.7 |
| Dirty Industrial |
0.6 |
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Standard
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Triphos
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Lumen outputs of fluorescent lamps
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18 Watt / 1150 Lumens
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1300 Lumens
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36 Watt / 3000 Lumens
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3350 Lumens
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Lumen output of PL lamps
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18 Watt / 1200 Lumens
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36 Watt / 2900 Lumens
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Step 5: 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.
Step 6: Work out the number of fittings required
in each axis of the room:
| Number in Length = |
 |
| Number in Width = |
 |
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