coefficient of uti;ization calculation is explained in the presentation

1. 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)

2. 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.

6. 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).

7. CU Determination
Step 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

8. 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

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

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

11. 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.

12. Example 1
Classroom 20’ x 27’ x 12’ E=50 fc
WP= 2’-6” AFF
ρc= 80% hcc= 0.0’
ρw= 50% hrc= 9.5’
ρf= 20% hfc= 2.5’
fixture: fluorescent (#38)
maintenance: yearly
replacement: on burnout
voltages & ballast: normal
environment: medium clean

13. Example 1
Confirm fixture data
S: T.15.1 p. 641

14. Example 1
Complete #1-6

15. 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

16. Lumen Flux
Method
8. Record
dimensional
data
20’
27’
0’
9.5’
2.5’
ρc= 80%
ρw= 50%
ρf= 20%

17. Coefficient of
Utilization
Factor(CU)
Calculation
9. Calculate
Cavity
Ratios

18. 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

19. Coefficient of
Utilization
Factor(CU)
Calculation
10. Calculate
Effective
Ceiling
Reflectance

20. 3. Obtain effective ceiling reflectance:
Example 1: Coefficient of Utilization (CU)
S: T.15.2 p. 667

21. Example 1
11. Calculate
Effective
Floor
Reflectance
Stein: T.15.2 P. 666

22. 3. Obtain effective ceiling reflectance:
Example 1: Coefficient of Utilization (CU)
S: T.15.2 p. 667
CU= 0.19  0.20

23. Example 1
12. Select CU
from mfr’s
data or see

24. CU=0.32
Example 1: Coefficient of Utilization (CU)
RCR CU
4.0 0.39
4.1 X
5.0 0.35
CU= 0.386
S: T.15.1 p. 641