lighting calculations. the illuminance: (e) شدة الإضاءة =luminous flux / area of the...
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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.