Average Estimated Illumination
The Average Estimated
Illumination (AEI) for each space is calculated by determining the
Lumens contributed by each fixture at the lighting calculation workplane,
and dividing by the space's area.
Space Illumination Example
In this example:
- The lighting equipment is a Troffer Corner
Insert - 2x4 3Lamp using the 483T8_S.ies Photometric Web File.
- The dimensions for the space are 20'
0" x 30' 0".
- The lighting fixture is placed at 9'
The additional formulas
used to calculate the average estimated illumination for the space
are as follows:
Lumens at Workplane
The Lumens at the workplane
contributed by each fixture are computed as follows: WL = II * LLF
- WL = Lumens at workplane
- II = Initial Intensity
- LLF = Light Loss Factor
- CU = Coefficient of Utilization
The Initial Intensity
and Light Loss Factor are both type properties of each lighting
fixture type. You access these properties in the fixture’s Type Properties
For this example, the
fixture's Initial Intensity is 7560 lumens, and with the Light Loss
Factors indicated here, the resulting LLF is 0.71478.
Determining the CU for
each fixture is more complicated. The lighting fixture determines
what space it is in. If the lighting fixture is not in a space,
the CU cannot be determined, and the fixture does not contribute
to a space's AEI.
The CU is computed based
on the Space reflectance properties (ceiling, wall, and floor),
as well as the space's Room Cavity Ratio (RCR).
The RCR is calculated
based on the Room Cavity Height (RCH) and the Lighting Calculation
Workplane, which is a property of each space instance.
Room Cavity Height
The Room Cavity Height
is computed as: RCH = LFH - LCW
- RCH = Room Cavity Height
- LFH = Lighting Fixture Height
- LCW = Lighting Calculation Workplane
In this example: RCH
= 9' 6" - 3' 0" = 6' 6" (If there are multiple fixtures at varying
heights, the average fixture height is used).
Room Cavity Ratio
Room Cavity Ratio is
calculated as: RCR = 2.5 * RCH * P / A
- RCR = Room Cavity Ratio
- P = Room Perimeter
- A = Room Area
In this example: RCR
= 2.5 * 6.5 * 97.3333 / 567.11 = 2.788989
To determine the Coefficient
of Utilization manually, you would refer to a luminaire datasheet.
A portion of the datasheet for the fixture described by the 483T8_S.ies
Photometric Web File is shown here.
The floor, wall, and
ceiling reflectances are all properties of the space instance, and
the RCR is computed as described above. By interpolation, you can
estimate the CU as approximately 38, which corresponds to 0.382762,
the value computed by Revit based on the Photometric Web File.
Coefficients of Utilization
Applying the Lumens at Workplane formula
for this example: WL = 7560 * 0.71478 * 0.382762 = 2068.345 lm
The WL for this fixture
represents the total lumens for the space. For this single fixture
in the space, the resulting foot candle level is determined using
the AEI formula :
2068.345 lm / 567.11 sf = 3.65 fc.
If there are multiple
fixtures in the space, the RCR is averaged based on the height of
each fixture, the CU is computed for each fixture instance, and
the WL for each fixture are summed, then divided by the space's area.
For example, assuming
another fixture is added at an 8' 0" elevation (subscripts indicate
- RCH2 = 8' 0"
- 3' 0" = 5' 0"
- RCR2 = 2.5
* 5 * 97.3333 / 567.11 = 2.145376
- Average RCR = (2.145376 + 2.788989)
/ 2 = 2.467183
For the second fixture,
the CU is computed using the new RCR:
- WL2 = 7560
* 0.71478 * 0.411155 = 2221.773 lm
Thus, the total AEI =
(WL1 + WL2) / Area= (2068.345 + 2221.773) / 567.11 = 7.56 fc
space instance reports the average RCR as its Room Cavity Ratio
value. This average value is not used to determine the CU for each