LIGHTING UNITS

Quantity Symbol Unit

Luminous Flux P or Φ lumen

Luminous Intensity I Candela

Illuminance E Lux

Luminance L cd/m2
 Luminous Flux (Transmission)
• Total amount of light emitted by the source

• Radiation – measured or expressed in ‘watt’

• luminous flux - measured in ‘Lumens’ - To

include the spectral sensitivity of the eye.

• One watt of radiant power with a wave length of

555nm equals 683 lumens
Luminous Flux (transmission)
• Luminous flux is the concept for
the total quantity of light
emitted per second by a light
source.
• It is designated by the symbol
Φ. The unit is the lumen (lm).
• One lumen (lm) is defined as
the amount of light emitted by
a 1cd point source within one
unit solid angle.
• Picture courtesy: Philips lighting
 Luminous Intensity (production)

• Luminous intensity is the

concept for the
concentration of light in a
specific direction, radiated
per second.
• Denoted by I and the unit
is ‘candela’

• Picture courtesy: Philips lighting

 Solid Angle / Luminous Intensity (production)
• Three dimensional angle – Steradian
• Steradian (sr) is defined as “the solid
angle subtended at the centre of a
sphere by an area on its surface
numerically equal to the square of the
radius.”
• The concentration of luminous flux
within this narrow cone can now be
defined as the luminous flux in this
cone divided by the opening of the
cone expressed in terms of the solid
angle of the cone.
• The result is called the luminous
intensity (I), measured in candelas
(cd), in the direction of the centre-line
of the cone.
 Illuminance (Incidence)
• Illuminance is
defined as the
quantity of light
(flux) falling on a
unit area of surface.
• Denoted by E
• Measurement with
lux-meter
(illumance-meter)
Courtesy: Philips lighting
 LUMINANCE
• Luminance can be defined as
the ratio of the luminous
intensity from a surface in a
given direction to the apparent
area of that surface
Or L = I / Aa
• It is designated by the symbol L
• The unit is the candela per
square meter.
Courtesy: Philips lighting
Courtesy: Philips lighting
 Brightness
• The luminous intensity radiated by a light source or
an illuminated surface per unit of apparent area (i.e.
the luminance) evokes a sensation of brightness.

• Luminance - objective measure

Brightness - subjective evaluation made by the
observer.

• Brightness - largely dependent on the luminance of

the surface, and the overall luminance distribution in
the field of view
 Brightness

• What we really 'see' in life are luminances, or

rather luminance variations in the field of view

• It is therefore the most important quantity in

lighting engineering, although the other three
- luminous flux, luminous intensity and
illuminance - are generally easier to work with
when performing calculations or
measurements.
 Relation between luminous flux (f)
and luminous intensity (l)
• The luminous intensity in any direction of light
source whose light distribution is uniform in
all directions, is equal to the luminous flux
divided by

• Luminous flux (lm) = × luminous intensity

(cd)
 Relation between luminous intensity (I)
and illuminance (E)
• The inverse square law
• The illuminance on a point in a
plane perpendicular to the
direction of light incidence is equal
to the luminous intensity in the
direction of the point, divided by
the square of the distance
between the (point) light source
and the point in question.
• If we call the distance ‘d’, the
following formula applies:
Ep = I / d2
 The cosine law
• The illuminance at a point in a
plane not perpendicular to the
direction of the luminous
intensity is equal to the luminous
intensity in the direction of the
point, divided by the square of
the distance between the light
source and the point in question,
multiplied by the cosine of the
angle gamma that the direction
of light incidence makes with the
normal (perpendicular) to the
plane

• Ep = I cos γ / d2
 Relation between luminous intensity (I) and
luminance (L)

• The surface luminance of a light source or of a

light-reflecting surface (secondary
light source) is equal to the luminous intensity
divided by the apparent area (Aa) of the
surface.
• L = I / Aa
 Relation between illuminance (E) and
luminance (L)
• In the case of a light-reflecting surface, the luminous intensity
of the surface is usually not known, whereas the illuminance
on the surface is known.
• For perfectly diffusing surfaces a relationship exists between
the illuminance (E) on the surface, the surface reflectance (ρ),
and the luminance (L) of the surface
• L=ρE/π
• This equation is valid for perfectly diffuse (matt) surfaces.
These display an equal luminance in all directions, no matter
what the direction of view.
• The formula is not valid for specular surfaces and for surfaces
exhibiting compound reflection - such as road surfaces - when
viewed in the direction of the specular component
 Lambert's cosine law
• The reflection characteristics of perfectly diffusing
surfaces are laid down in Lambert's Cosine Law
• The luminous intensity reflected by a diffusing
surface in any direction is proportional to the
cosine of the angle which that direction makes with
the normal to the surface
• Iα = I0 cos α
where l0 = luminous intensity perpendicular to the
surface,
Iα = luminous intensity in the given direction
α = angle between l0 and Iα
• This law and its derivations are very important in
practical lighting technology.
 Law of Transmission
• For any material of uniform transmission
characteristics, transmittance decreases
exponentially with the thickness of the material.

• I t = II x τt
where: lt = intensity of transmitted light
Il = Intensity of Incident light
τ = transmittance per millimeters
t = thickness in millimeters
 Snell’s law of refraction
• shows the relationship
between the incident
angle and the refractive
index: n1sin θ1 = n2sin θ2
• Where
n1 = the refractive index of medium 1
n2 = the refractive index of medium 2
θ1 = the incident angle of the light ray
(with respect to the normal)
θ´1 = the reflected angle (with respect to
the normal)
θ2 = the refracted angle (with respect to
the normal)