AWP x EWP

No. of = luminaires (lumens/lamp) x (lamps/luminaires)

x CU x LLFTOTAL

The coefficient of utilization is a factor used to determine the efficiency of a lighting fixture in delivering light for a specific application

The coefficient of utilization is determined as a ratio of light output from the luminaire that reaches the workplane to the light output of the lamps alone

Coefficient of Utilization (CU)

Coefficient of Utilization (CU)

Factors influencing coefficient of utilization:

The efficiency of the luminaire

The luminaire distribution

The geometry of the space

The reflectance's of the room surface

Each luminaire has its own CU table specific to that luminaire’s light distribution and efficiency. CU values are listed in tables for different room geometries and room surface reflectance.

For a given room, the cavity ratios are in direct proportion to their respective cavity heights. For the case where the luminaires are mounted on the surface of the ceiling or are recessed into the ceiling, the ceiling cavity ratio is zero.

It is necessary to convert the actual ceiling reflectance into an effective ceiling cavity reflectance (pCC) and the actual floor reflectance must be converted to an effective floor cavity reflectance (pFC).

CU DeterminationStep 1. Determine the room cavity ratio using the equation below

Room cavity height (hRC) = Luminaire height – Workplane height

RCR = 5 x Room cavity height (Length + Width)

Length x Width

Step 2. Since the Lumen Method considers what occurs only within the room cavity, the ceiling and floor reflection factor are replaced with their effective reflectances.

Ceiling reflection factor:

70% For very light colours and white

50% For light colours

30% for medium shades

Wall reflection factor:

50% For light colours and white

30% For medium shades

10% For dark shades

To find the effective reflectance of a floor or ceiling cavity, find the floor cavity ratio and ceiling cavity ratio using the equations below

• Step 3. Find the effective cavity reflectances using cavity surface reflectances.

Step 4. Once all room cavity reflectances and the room cavity ratio are known, the CU value can be determined by selecting the appropriate value from the luminaire’s CU table.

Example 1Classroom 20’ x 27’ x 12’ E=50 fcWP= 2’-6” AFFρc= 80% hcc= 0.0’

ρw= 50% hrc= 9.5’

ρf= 20% hfc= 2.5’

fixture: fluorescent (#38)maintenance: yearlyreplacement: on burnout voltages & ballast: normalenvironment: medium clean

Example 1

Confirm fixture data

S: T.15.1 p. 641

Example 1

Complete #1-6

Example 1

7. Determine lumens per luminaire

Obtain lamp lumens from manufacturer’s data (or see Stein: Chapter 12)

S: T. 12.5 p. 546

Lumen Flux Method

8. Record dimensional data

20’

27’

0’

9.5’

2.5’

ρc= 80%ρw= 50%ρf= 20%

Coefficient of Utilization Factor(CU) Calculation9. Calculate Cavity Ratios

Example 1: Cavity Ratios

CR = 5 H x (L+W)/(L x W)

RCR = 5 Hrc x (L+W)/(LxW) = 4.1

CCR = 5 Hcc x (L+W)/(LxW) = 0

FCR = 5 Hfc x (L+W)/(LxW) = 1.1

Coefficient of Utilization Factor(CU) Calculation10. Calculate EffectiveCeiling Reflectance

3. Obtain effective ceiling reflectance:

Example 1: Coefficient of Utilization (CU)

S: T.15.2 p. 667

Example 1

11. Calculate EffectiveFloor Reflectance

Stein: T.15.2 P. 666

3. Obtain effective ceiling reflectance:

Example 1: Coefficient of Utilization (CU)

S: T.15.2 p. 667

CU= 0.19 0.20

Example 1

12. Select CU from mfr’s data or see

CU=0.32

Example 1: Coefficient of Utilization (CU)

RCR CU 4.0 0.394.1 X5.0 0.35

CU= 0.386

S: T.15.1 p. 641