ILLUMINATION
-is the deliberate use of light to achieve a practical or aesthetic effect.
Lighting includes the use of both artificial light sources like lamps and light
fixtures, as well as natural illumination by capturing daylight.
Luminous Flux
-is the measure of the perceived power of light. It differs from radiant flux,
the measure of the total power of electromagnetic radiation. Luminous flux is
measured in lumen which is defined as the total "amount" of visible light
emitted by a source.
Luminous Intensity
-is a measure of the wavelength-weighted power emitted by a light source in
a particular direction per unit solid angle, based on the luminosity function, a
standardized model of the sensitivity of the human eye.
Candela
-is the unit of luminous intensity. One candela is equal to 1/60 th of the
luminous intensity per sq. cm of a black body radiator maintained at the
temperature of solidification of platinum.
ω=
ϕ=4 πI
A
r2
I=
ϕ
ω
Mean Spherical Candle Power
-is the rating measured at the design voltage and is the total amount of light
given off from a light source in all directions. One MSCP is equivalent to all
the light emitted from all directions of one standard spermaceti candle.
MSCP=
ϕ
4π
mhscp=
ϕ
2π
Luminous Efficacy
-is a measure of how well a light source produces visible light. It is the ratio
of luminous flux to power.
Illuminance
-is the total luminous flux incident on a surface, per unit area.
1
F=
ϕ
Prating
E=
ϕ
A
E=
I cos θ
2
r
Note: If area is given in square meters, unit of illuminance is in lux.
If area is given in square centimeters, unit of illuminance is in phot.
If area is given in square feet, unit of illuminance is in footcandles.
Conversionof Units ∈Illumination
1 fc=10.75 lux
1 phot=10000 lux
ϕ=luminous flux
E=illuminance
ω=solid angle∈steradians
I =luminousintensity
ILLUMINATION CALCULATIONS
-are the methods used to determine how many lights and where to place
them to get the correct level of illumination for a given activity.
Accent Lighting
-is the directional lighting to emphasize a particular object or draw attention
to a display item. It is also a type of lighting used to accentuate a given
locality or object in the home or office, for example; a doorway or a painting.
Task Lighting
-refers to office lighting, where the task light is used to increase illuminance
on the reading area. Other instances of task lighting are in machinery, where
a specific work area needs illumination, and in workshops, where a task light
may illuminate the actual working area.
Lumen Method∨Zonal Cavity Method
-is a simplified method to calculate the light level in a room. The method is a
series of calculations that uses horizontal illuminance criteria to establish a
uniform luminaire layout in a space. In its simplest form, the lumen method
is merely the total number of lumens available in a room divided by the area
of the room.
Room Cavity Ratio
-is a measure of room proportion as determined by dimensions of length,
width, and height.
2
Space Height Ratio
-is the ratio of the distance between adjacent luminaires (centre to centre),
to their height above the working plane.
RCR=
5 h ( l +w )
lw
SHR =
1
hm
√
A
n
Utilization Factor∨Coefficient of Utilization
-is the proportion of the downward luminous flux that reaches the work area
directly without reflection. It depends on the shape of the room and the light
distribution from the luminaires.
Maintenance Factor∨Depreciation Factor
-is the ratio of illumination on a given area after a period of time to the initial
illumination on the same area; used in lighting calculations to account for the
depreciation of lamps or reflective surfaces.
ϕ=
A ×E
UF × MF
n=
A× E
ϕ ×UF × MF
h=height of the room
hm =mounting height of luminaire
l=length of the room
n=number of luminaires
w=width of theroom
A=area of the room
Ex .1 . An area of 0.3 square meters is to be illuminated by a lamp at a level
of 600 lux. What is the total luminous flux emitted by the lamp?
let A=0.3 m
2
ϕ=EA
let E=600 lux
ϕ=600 lux ( 0.3 m2 )
solving for ϕ
ϕ=180 lm
E=
ϕ
A
3
Ex .2 . A light source located 2.75 m from a surface produces an illumination
of 528 lux on that surface. Find the illumination if the distance is changed to
1.55 m.
let r 1=2.75 m
E1 r 12=E2 r 22
let E 1=528 lux
E r
E2= 1 21
r2
let r 2=1.55 m
let I 1 cos θ1=I 2 cos θ2
E 2=
solving for E2
2
528 lux (2.75 m )2
( 1.55 m)2
E2=1662 lux
Ex .3 . At what distance from a wall will a 40 cd lamp provide the same
illumination as an 80 cd lamp located 4 m from the wall?
let I 1=40 cd
I1
r
let I 2=80 cd
let r 2=4 m
2
1
=
I2
r2
r 1=r 2
let E 1=E2
2
√
r 1=4 m
let cos θ1=cos θ 2
solving for r 1
I1
I2
(√ 4080 cdcd )
r 1=2.83 m
Ex .4 . A piece of paper lies on a table 2 m away from a point directly below
a bulb of 100 cd and is 4 m above the table. Calculate the illumination on the
center of the paper in lux.
let I=100 cd
r= √ ( 4 m ) + (2 m )
let a=4 m
r=2 √ 5 m
let b=2 m
solving for r ∧θ
r= √ a 2+ b2
2
2
solving for E
E=
θ=tan −1 b/a
−1
θ=tan ( 2m/4 m )
θ=26.565°
E=
I cos θ
2
r
100 cd ( cos 26.565 ° )
( 2 √5 m )
2
E=4.47 lux
4