CU Determination CU Determination
In this example, the luminaires are recessed in the ceiling so the Step 2. Since the Lumen Method considers what occurs only within
luminaire height is the as the ceiling height. Computing the room the room cavity, the ceiling and floor cavities are replaced with their
cavity ratio, we have: effective reflectances.
To find the effective reflectance of a floor or ceiling cavity, find the
RCR = 5 x Room cavity height (Length + Width) floor cavity ratio and ceiling cavity ratio using the equations below
Length x Width
RCR = 5 x 1.83m (3.05m + 4.57m)
3.05m x 4.57m
RCR = 5
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CU Determination CU Determination
Step 3. Find the effective cavity reflectances using cavity surface
reflectances. The surface that is opposite the opening to the cavity is
called the base cavity. The base reflectance, the wall reflectances, and
the cavity ratio determine the effective cavity reflectance. Using the
IESNA Lighting Handbook, look for the cavity reflectances and cavity
ratios.
For the ceiling cavity, the base reflectance is the actual ceiling surface
reflectance while the floor cavity, the base reflectance is the actual floor
surface reflectance.
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CU Determination CU Determination
Step 4. Once all room cavity reflectances and the room CU = 0.50, which means that 50% of the lumens given off by the
cavity ratio are known, the CU value can be determined lamps reach the workplane and the other 50% are absorbed by the
by selecting the appropriate value from the luminaire’s luminaire or the room surfaces and never reach the workplane.
CU table.
Continuing with Example 1, the following assumptions
are made after consulting the IES Lighting Handbook
Table on Effective Reflectances:
Effective Ceiling Cavity Reflectance, ρCC = 0.70
Wall Reflectance, ρW = 0.50
Effective Floor Cavity Reflectance, ρFC = 0.20
RCR = 5 (calculated in Step 1)
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3
Coefficients of Utilization for Some Luminaire Light Loss Factor
Two types of Light Loss Factor (LLF)
Recoverable
Non-recoverable
Total Light Loss Factor (LLF) is the product of
the individual light loss factors, recoverable and
non
- recoverable
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Light Loss Factor Lamp Lumen Depreciation
Recoverable LLF The lamp lumen depreciation factor is the
Lamp Lumen Depreciation (LLD) fraction of initial lumens at a specific time during
Lamp Burnout Factor (LBO) the life of the lamp
Luminaire Dirt Depreciation Factor (LDD) Lamp lumen depreciation comes from aging and
Room Surface Dirt Depreciation Factor (RSDD) dirt accumulation on lamps, reflectors, lenses
Area of workplane (AWP) and room surfaces.
Most lighting designs base calculations on
“maintained” as opposed to “initial” lamp lumens
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Lamp Burnout Factor Room Surface Dirt Depreciation
If lamps are not replaced immediately after Room Surface Dirt Depreciation Factor (RSDD)
burnout, a lamp burnout factor should be applied is influenced by:
to any analysis of the system. The amount of dirt in the environment
Unreplaced burned - out lamps will vary in The room cavity ratio (proportions of the room)
quantity, depending on the kind of lamps and the Type of lighting equipment used
relamping program used.
This factor is simply the ratio of the number of
lamps that would be burning o the total number
of lamps in the system.
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