Lighting calculations

The Illuminance: (E)

Luminous Flux / area of the sphere surface

E=Φ/A

E= I/ r2

= lumen/ m2 = lux

The luminance (Luminous Intensity) (I)

I= Φ/ω cd

Where: ω = solid angle

= (Φ/A)* r2

= E* r

2 Candela

Inverse square law: Double the Radius:•

Surface area bounded by a solid angle of 1 sr now increases to

4m2

• Luminous flux remains Φ

= 1lm• Illuminance E drops from

1 lux to ¼ lux

What can we do with this?

It’s the basic building lightingWe can find the illuminance on a desk from a incandescent light

bulb overhead:

Bulb: 60W * 20 lm/W = 1200 lm

1200/4π

100 cd if point source

Desk : E = 100/22

= 25 lux

LUMINOUS EFFICACYIs the ratio of the luminous flux emitted by a lamp to the power taken

by it. (unit: LUMENS PER WATT, lm/W)

Calculation of point illuminance level:Ep

Which are generated by individual light source:

} E = I/r2

But, r = h/ cos (

)

So,

} Where,} Luminous Intensity(I)

} Hieght (h)

} Light Incidence Angle (

)

E p = I /h2 cos3

Factors affected on illuminance calculations

1. Room Reflectance

} The room is considered to consist of three main surfaces:

(a) the ceiling cavity,

(b) the walls, and

(c) the floor cavity (or the horizontal working plane).

} The effective reflectances of the above three surfaces affect the quantity of reflected light received by

the working plane.

2. Luminaire maintenance factor (LMF) is the proportion

of the initial light output from a luminaire after a set time to the initial light output from a lamp after a set time.

} Three factors must be considered in its determination:

(a) the type of luminaire,

(b) atmospheric conditions, and

(c) maintenance interval.

3. Room surface maintenance factor (RSMF) is the proportion of the illuminance provided by a lighting installation in a room after a set time compared with that occurred when the room was clean.

4. Utilization factor (UF) is the proportion of the luminous flux emitted by the lamps which reaches the working plane. Factors that affect the value of UF are as follows:

(a) light output ratio of luminaire

(b) flux distribution of luminaire

(c) room proportions

(d) room reflectance

(e) spacing/mounting height ratio

Factors affected on illuminance calcuations

Lumen Method Calculations

Where: E = average illuminance over the horizontal working plane

n = number of lamps in each luminaireN = number of luminaireF = lighting design lumens per lamp,

i.e. initial bare lamp luminous fluxUF = utilisation factor for the horizontal working planeLLF = light loss factor

A = area of the horizontal working plane

The Luminous Flux (Φ)= EA/MU

Ø Calculate the utilization factor

Ø Calculate the Room Factor:

K = 0.8W + 0.2L

he

Where: W= width of the room

L = length of the room

Ø From the table find “η”

Ø The Luminous Flux (Φ)= EA

η

Room Factor

} It depends on room dimension

is the proportion of the initial light output from a luminaire after a set time to the initial light output from a lamp after a set time.