Illumination - Method of calculation

Vijay Balu Raskar (BE Electrical)

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)


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.

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.


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.


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


• Illumination level required at the work surface is obtained by,

• 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.

recE =

A

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

• where• MF = Maintenance factor (Light Loss Factor LLF)• UF = Utilization factor• Φ = Total Lumen

= MF UF rec inst

The lumen method


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)

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 conditionD.F. =

Illumination under normal working conditionVijay Balu Raskar (BE Electrical)

Utilization Factor (η)

• Is the ratio of effective luminous flux to the total luminous flux of light sources.

• Always less than one

nF Effective luminous fluxU.F.= =

F Total luminous flux of all the sources


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.

12 2

3

Working planeVijay Balu Raskar (BE Electrical)

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, brown, blue

75-90%

Medium green, yellow, brown or gray 30-60%

Dark gray, medium blue 10-20%

Dark blue, green, wood paneling 5-10%Vijay Balu Raskar (BE Electrical)

• 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

• Do not choose very dark wood, or bright surfaces.

LightC eiling 70-90%

Wall 50-70%

Floor 20-50%

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.

U. F. Depend on the following


Room index• The horizontal areas are

• The vertical areas are

• Hm = mounting height = Vertical distance from the work place to the luminaire.

• The Room index is

Area = 2 Length Width

= 2 L WH

L Lum

m

Area = 2 Length Width Height

= 2 L W H

H

V m

Area L×WRI = =

Area (L+W) × HVijay Balu Raskar (BE Electrical)

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


No. of lamps required

= MF UF

E×AMF UF MF UF

Φ

instSo, No. of lamps required = Lumen o/p

E×ASo, No. of lamps required = Lumen o/p MF UF

rec inst

recinst


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Illumination - Method of calculation