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11.02
Glossary of the most
important lighting terms
As with any technical or scientific discipline, lighting
technology has its own special terms and concepts
for defining the characteristics of lamps and fixtures
and for standardizing the units of measurement.
The most important terms are described here.
Luminous flux Φ
Unit of measurement: lumen [lm]
Luminous flux Φ is all the radiated power emitted by a
light source evaluated with the spectral sensitivity of the
eye and the photometric radiation equivalent k
m
.
Luminous intensity
Ι
Unit of measurement: candela [cd]
Generally speaking, a light source emits its luminous flux
Φ in different directions and at different intensities.
Luminous intensity is the luminous flux radiated in a
particular direction (solid angle Ω).
Illuminance E
Unit of measurement: lux [lx]
Illuminance E is the ratio between the luminous flux and
the area being illuminated.
An illuminance of 1 lx occurs when a luminous flux of 1 lm
is evenly distributed over an area of 1 m
2
.
Luminance L
Unit of measurement: candela per square meter [cd/m
2
]
The luminance L of a light source or an illuminated area is
a measure of the impression of brightness.
Luminous efficacy η
Unit of measurement: lumen per watt [lm/W]
Luminous efficacy η indicates the efficiency with which
the electrical power consumed is converted into light.
Light and radiation
Light is taken to mean the electromagnetic radiation that
the human eye perceives as brightness, in other words
that part of the spectrum that can be seen. This is the
radiation between 380 and 780 nm, a tiny fraction of the
known spectrum of electromagnetic radiation.
ΦΩ
Luminous
Ι
intensity is a
measure of the luminous flux
Φ
emitted in solid angle
Ω
.
Illuminance E
0
°
4
0
0
0
c
d
6
0
0
0
c
d
8
0
0
0
c
d
1
0
00
0
c
d
0
°
70
°
50
°
30
°
0
°
10
°
10
°
70
°
50
°
30
°
Luminance L
Observer
Viewed surface
Luminous intensity
Illuminated
area
Polar diagram
The most important photometric formulae:
Luminous intensity
Ι
[cd]
Illuminance
E
[lx]
Luminous flux [lm]
Solid angle Ω [sr]
Luminous flux falling on area [lm]
Illuminated area [m
2
]
Luminance
L
[cd/m
2
]
Luminous efficacy
η
[lm/W]
Luminous intensity [cd]
Viewed luminous area [m
2
]
Generated luminous flux [lm]
Electrical power consumed [W]