Illumination Examples Sides 1 out of 8

1. Vijay Balu Raskar (BE Electrical)

2. Methods for calculating illumination
There are three method for Lighting
calculations:-
1)Watts per square meter method
2)Lumen or Light flux method
3)Point to Point OR Inverse – Square law
method Vijay Balu Raskar (BE Electrical)

3. Watts per square meter
method
Principal based on “Rule of thumb”.
Very handy for rough calculation or
checking.
Illumination based on assumption
and consists in making an allowance
of watts per square meter of area.
Vijay Balu Raskar (BE Electrical)

4. The problem of the lighting designer
at the functional level
• To determine how many lights and
• Where to place them to get the correct level of
illumination for a given activity.
Vijay Balu Raskar (BE Electrical)

5. Calculations can be divided into two
types
• Accent or task lighting.
• To determine the illumination at a small
specific location in the room from a point light
source.
• Simply get polar plot for luminaire and work
out the illumination on surface for a given
lamp.
• Called the point illumination method.
Vijay Balu Raskar (BE Electrical)

6. The lumen method
• To estimate the illumination pattern from a set of
diffuse lighting sources over a broad area in a room.
• This method gives rough and reasonable estimates
of the lamps / luminaires needed.
• Is used in areas where a uniform light intensity is
required for the work area.
• Used for rectangular rooms with gridded luminaire
pattern.
• Also called as Light Flux Method
Vijay Balu Raskar (BE Electrical)

7. The lumen method
• Illumination level required at the work surface is obtained
by, rec
E=
A
• E = illumination level required at the work surface
• A = total area of the plane where the work is done.
• Φrec = flux of light received on the working surface.
• It is to determine how much flux needs to be installed, i.e.
Φinst, to get the required amount of received flux Φrec.
Vijay Balu Raskar (BE Electrical)

8. The lumen method
• First determine how much flux is to be received.
• Multiply the illumination required by the surface area.
• Φrec = E A
• The received flux is related to the installed flux by a
formula
rec = MF UF inst
• where
• MF = Maintenance factor (Light Loss Factor LLF)
• UF = Utilization factor
• Φ = Total Lumen
Vijay Balu Raskar (BE Electrical)

9. Maintenance Factor
• Gives an estimate of how lighting conditions
will deteriorate through use.
• Some factors are dust and dirt inside luminaire
surfaces.
• Aging of light bulbs emitting less light
• Cleaning of room surfaces, e.g. ceiling
• Without detailed knowledge of a maintenance
plan, MF is assumed to be = 0.80.
Vijay Balu Raskar (BE Electrical)

10. Depreciation factor
• Light emitted by source reduces due to dust or dust
decomposition on light.
• So, quantity of light reflected from ceiling also get
reduced.
• This reduction of light can be minimized by cleaning
the light fittings or white washing.
• But absolute cleanliness is not possible hence we
need to consider depreciation factor.
• D. F. = 1/ M. F.
Illumination under ideally clean condition
D.F. =
Illumination under normal working condition
Vijay Balu Raskar (BE Electrical)

11. Utilization Factor (η)
• Is the ratio of effective luminous flux to the
total luminous flux of light sources.
• Always less than one
Fn Effective luminous flux
U.F.= =
F Total luminous flux of all the sources
Vijay Balu Raskar (BE Electrical)

12. Effective Luminous Flux (Fn)
• The luminous flux incident on the working
area.
• Rest luminous flux is used for lighting the
walls and ceilings and is partly absorbed by the
fittings.
3
2 2
1
Working plane
Vijay Balu Raskar (BE Electrical)

13. U. F. Depend on the following
• The luminaire properties.
• The light output ratio (LOR).
• How much of the light emitted by the lamps actually
leaves the luminaire?
• The reflection factors of ceiling and walls.
• Reflectance are available from manufacturers of
paints and furniture finishing.
Colour Reflectance
White, Off-white, light shades of gray, 75-90%
brown, blue
Medium green, yellow, brown or gray 30-60%
Dark gray, medium blue 10-20%
Dark blue, green, wood Balu Raskar (BE Electrical)
Vijay
paneling 5-10%

14. U. F. Depend on the following
• Distribution of light sources in the room
• It is usual to make the reflectance of the ceiling
highest, walls slightly lower and the floor darker.
• Typically recommends in offices
Light
C eiling 70-90%
Wall 50-70%
Floor 20-50%
• Do not choose very dark wood, or bright surfaces.
Vijay Balu Raskar (BE Electrical)

15. U. F. Depend on the following
• The geometric shape of the room i.e. length,
breadth and height of a room will affect the
UF.
• A factor called the room index (RI) is defined
from the horizontal vertical areas of the room.
Vijay Balu Raskar (BE Electrical)

16. Room index
• The horizontal areas are
Area H = 2 Length Width
=2 L W
• The vertical areas are
Area L = 2 Length Width Height Lum
=2 L W Hm
• Hm = mounting height = Vertical distance from the
work place to the luminaire.
• The Room index is
Area H L×W
RI = =
Area V(BE Electrical)
Vijay Balu Raskar (L+W) × H m

17. For uniform illumination
• As a rule of thumb, to achieve uniform lighting
spacing between the luminaries should be less
than 1.5 times the mounting height.
• Lumnaire Spacing < 1.5 Hm
Vijay Balu Raskar (BE Electrical)

18. No. of lamps required
rec = MF UF
inst
rec E×A
inst MF UF MF UF
Φ
So, No. of lamps required = inst
Lumen o/p
So, No. of lamps required = E×A
Lumen o/p MF UF
Vijay Balu Raskar (BE Electrical)