Lighting calculations

The Illuminance: (E) شدةاإلضاءة

=Luminous Flux / area of the sphere surface

E=Φ/AE= I/ r2

= lumen/ m2 = lux

The luminance (Luminous Intensity) (I) شدة االستضاءة

I= Φ/ω cd Where: ω = solid angle

 = (Φ/A)* r2

= E* r2 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 EFFICACY Is 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 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 

The Luminous Flux (Φ)= EA/MU

Calculate the utilization factor Calculate the Room Factor:

K = 0.8W + 0.2Lhe

Where: W= width of the room

L = length of the room

From the table find “η”

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.