ARTIFICIAL LIGHTING
. Lumen Method Calculations
The lumen method is based on fundamental lighting calculations. The lumen method
formula is easiest to appreciate in the following form.
(1)
where E = average illuminance over the horizontal working plane
n = number of lamps in each luminaire
N = number of luminaire
F = lighting design lumens per lamp, i.e. initial bare lamp luminous flux
UF = utilisation factor for the horizontal working plane
LLF = light loss factor
A = area of the horizontal working plane
Room Proportion
Room index (RI) is the ratio of room plan area to half the wall area between the working
and luminaire planes.
(2)
where L = length of room
W = width of room
Hm = mounting height, i.e. the vertical distance between the working plane and the
luminaire.
1
Spacing to Height Ratio
Spacing to Height ratio (SHR or S/Hm) is defined as the ratio of the distance between
adjacent luminaires (centre to centre), to their height above the working plane. For a
rectangular arrangement of luminaires and by approximation,
(3)
where A = total floor area
N = number of luminaires
Hm = mounting height
(a) SHR should not exceed maximum spacing to height ratio (SHR MAX) of the given
luminaire as quoted by the manufacturer, and
(b) geometric mean spacing to height ratio of the luminaire layout should be within the
range of nominal spacing to height ratio (SHR NOM) of the given luminaire as quoted
by the manufacturer, i.e.
2
NATURAL LIGHTING
. Daylight factor
The daylight factor is defined as :
(5)
where
Ei = illumiance due to daylight at a point on the indoors working plane
Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed
hemisphere of overcast sky (5000 lux for CIE overcast sky)
(6)
Figure 6 Components of the Daylight Factor
3
BOTH SC (SKY COMPONANT) AND ERC (EXTERNALLY REFLECTED
COMPONANT) CAN BE FOUND BY USING BRE PROTRACTORS
Figure 7 BRE Sky Component Protractor for Vertical Glazing
(CIE Overcast Sky)
4
The internally reflected component (IRC)
(A) Formula
The average IRC can be determined quite precisely from the BRS inter-reflection
formula. The simplified form of this is:
(7)
where,
0.85 = transmittance of window glazing assumed,
W = window area (m2),
A = total area (ceiling + floor + walls including windows),
r = average reflectance of area A,
r fw = average reflectance of floor and the three walls below the plane at the level mid-
height of the window (excluding the window wall),
r cw = average reflectance of ceiling and the upper (remaining) part of the above three
walls,
C = a coefficient depending on external obstructions, as given below:
Angle of 0o 10o 20o 30o 40o 50o 60o 70o 80o
obstructions
Coefficient C 39 35 31 25 20 14 10 7 5
5
APPENDIX
The following table shows the reflection of light on the surfaces of
various colors factor:
Color reflection Color reflection
coefficient % R coefficient % R
White, oily 80-85 Yellow 61-75
paint
White (new) 82-89 Tawny 30-40
White( Old ) 75-85 Light 48-75
green
Cardinal 49-66 Dark 11-25
green
Ivory 73-78 Light 34-61
Blue
Gray 17-63 Light red 36-61
6
The table shows the approved lighting together with the International
Committee of the lighting:
The building Lighting ability
LX
offices 500
Reading rooms, libraries 500
General halls, banks, and 1000
showcases
Conference rooms 300
Dinning areas in restaurants 200
Building Stairs 150
Kitchens in hotels and restaurants 500
Residential spaces Lighting ability
LX
The dining hall 200
7
Kitchens 500
Reception halls 100
Bedroom 50
Children's rooms 150
Baths 100
Sitting and reading rooms 500
WATTS TO LUMENS
You can calculate lumens from watts and luminous efficacy.
Watt and lumen units represent different quantities, so you can't convert watts to lumens.
Watts to lumens calculation formula
The luminous flux ΦV in lumens (lm) is equal to the power P in watts (W), times the
luminous efficacy η in lumens per watt (lm/W):
ΦV(lm) = P(W) × η(lm/W)
8
So
lumens = watts × (lumens per watt)
or
lm = W × (lm/W)
Example
What is the luminous flux of a lamp that has power consumption of 60 watts and
luminous efficacy of 15 lumens per watt?
ΦV = 60 W × 15 lm/W = 900 lm
Luminous efficacy table
Typical
Light type luminous efficacy
(lumens/watt)
Tungsten incandescent light bulb 12.5-17.5 lm/W
Halogen lamp 16-24 lm/W
Fluorescent lamp 45-75 lm/W
LED lamp 30-90 lm/W
Metal halide lamp 75-100 lm/W
High pressure sodium vapor lamp 85-150 lm/W
Low pressure sodium vapor lamp 100-200 lm/W
Mercury vapor lamp 35-65 lm/W
Energy saving lamps have high luminous efficacy (more lumens per watt).
