Coefficient of Utilization (CU) Lookup

    Coefficient of Utilization (CU)

    The coefficient of utilization is the percent of the lumens from the lamp that finally find their way to the work plane. This number depends on the particular fixture type, the number of lamps in it, the lens used, its beam pattern, the shape of the room (Room Cavity Ratio, RCR) and the reflectances of the ceiling (Rc), walls (Rw) and floor (Rf). Generally a Dirt Depreciation Factor and other Light Loss Factors are used to multiply CU.

    Typical Luminarie Dirt Depreciation (LCC)
      Environment (Dirt Level)
      Light Medium Heavy
    Enclosed Fixtures 0.94 0.86 0.77
    Enclosed & Filtered 0.97 0.93 0.88
    Open & Ventilated 0.94 0.84 0.74

    Use for Estimating Purposes Only.

    • The numbers are taken from generic fixture data from the IES handbook for an RCR = 1 and from representative data from fixture manufacturers.
    • Contact fixture manufacturer for correct values.

Industrials with Reflectances

    Ceiling: Rc = 50%, Walls: Rw = 50%, Floor: Rf = 20%, LDD = 85%

    High Bay Spun Aluminum - CU ∼ 0.80 High Bay Spun Aluminum
    CU ∼ 0.80
    Fluorescent Industrial - CU ∼ 0.85 Fluorescent Industrial
    CU ∼ 0.85
    High Bay Acrylic - CU ∼ 0.75 High Bay Acrylic
    CU ∼ 0.75
    T5 or T8 Industrial with - Lens CU ∼ 0.75 T5 or T8 Industrial with Lens
    CU ∼ 0.75
    High Bay Acrylic - CU ∼ 0.85 High Bay Acrylic
    CU ∼ 0.85
    T5 or T8 Industrial Open - CU ∼ 0.88 T5 or T8 Industrial Open
    CU ∼ 0.88
    Low Bay Lensed - CU ∼ 0.70 Low Bay Lensed
    CU ∼ 0.70
       
    Low Bay Acrylic - CU ∼ 0.75 Low Bay Acrylic
    CU ∼ 0.75
       

Offices with Reflectances

    Ceiling: Rc = 80%, Walls: Rw = 50%, Floor: Rf = 20%, LDD = 94%

    Acrylic Lensed Troffer - CU ∼ 0.81 Acrylic Lensed Troffer
    CU ∼ 0.81
    Basic Fluorescent Strip - CU ∼ 0.84 Basic Fluorescent Strip
    CU ∼ 0.84
    Deep-Cell Parabolic Louver - CU ∼ 0.72 Deep-Cell Parabolic Louver
    CU ∼ 0.72
    Indirect Lighting - Rc = 80% use CU ∼ 0.59 - Rc = 50% use CU ∼ 0.37 Indirect Lighting
    Rc = 80% use CU ∼ 0.59
    Rc = 50% use CU ∼ 0.37
    Small-Cell Parabolic Louver - CU ∼ 0.55 Small-Cell Parabolic Louver
    CU ∼ 0.55
    Incandescent / CFL Downlight - CU ∼ 0.71 Incandescent / CFL Downlight
    CU ∼ 0.71
    Small-Cell Plastic Grid 1/2" - CU ∼ 0.78 Small-Cell Plastic Grid 1/2″
    CU ∼ 0.78