Lighting Calculations

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 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 * CU

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 dialog.

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

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

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 fixture number):

For the second fixture, the CU is computed using the new RCR:

Thus, the total AEI = (WL1 + WL2) / Area= (2068.345 + 2221.773) / 567.11 = 7.56 fc

NoteThe space instance reports the average RCR as its Room Cavity Ratio value. This average value is not used to determine the CU for each fixture instance.