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A Radiometric and Photometric Quantities
Photometric quantities consider only the visible part of the electromagnetic spec-
trum and are weighted by the visibility factor V(λ), which is the photopic sens-
itivity curve of the human eye. The quantities of the radiometric measurements
correspond to photometric quantities, but consider the total amount of radiation
with the watt replacing the lumen. Table 15 lists radiometric and corresponding
photometric quantities.
Radiometric Quantities Photometric Quantities
Radiant Flux is the total radiant
power that is either emitted by a source
or received by a surface. The unit is
watt [W ].
Radiant Intensity is the radiant power
of a source emitted in a certain direction.
The unit is watts per steradian
[W · sr −1 ].
Irradiance is a measure of the total radiant
flux incident on a surface. The
unit is watts per square meter [W ·
m −2 ].
Radiance is the total radiant intensity
emitted or reflected from a certain location
on an emitting or reflecting surface
in a particular direction. The unit
is watts per steradian per square meter
[W · sr −1 · m −2 ].
Luminous Flux is a measure of the
energy emitted by a light source or received
by a surface, weighted by the
visibility factor V(λ). The SI unit of
Luminous flux is the lumen [lm] or
candela · steradian [cd · sr]. One lumen
is defined as the amount of light
that falls on a unit spherical area at
unit distance from a light source of one
candela.
Luminous intensity is a measure of
luminous flux emitted by a light source
in a particular direction. The SI unit of
luminous intensity is candela [cd].
Illuminance is the total luminous flux
incident per unit area. It refers to the
amount of incident light. The unit is
lux [lx] or lumen per square meter [lm·
m −2 ].
Luminance is the measure of luminous
flux emitted from, or reflected by a
surface. The unit is candela per square
meter [cd · m −2 ].
Table 15: Radiometric and photometric quantities.
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