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Glossary
of 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 luminaires
and for standardizing the units of measurement. The
most important of these are described here.
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 360 and 830 nm, a tiny fraction
of the known spectrum of electromagnetic radiation.
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Luminous
flux 
Unit of measurement: lumen [lm]
All the radiated power emitted by a light source and perceived
by the eye is called luminous flux .
The unit of measurement is the lumen (lm). The visible
radiated power of a light source is not expressed in watts
but in lumens since the sensitivity of the human eye differs
according to the particular wavelength. In US practice,
total luminous flux is defined by the mean spherical candle
power I0. The unit is the mscp. The conversion
formula is:
= I0 · 4 ·  . |
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Luminous intensity
is a measure of the luminous flux
emitted in solid angle  . |
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Fig. 2 Luminous Flux click to enlarge image
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Luminous
Intensity
Unit of measurement: candela [cd]
Generally speaking, a light source emits its luminous
flux in different directions (solid angle
; the unit of measurement is the steradian sr) and at
different intensities. The visible radiant intensity in
a particular direction is called luminous intensity .
The unit of measurement is the candela (cd). |
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Fig. 3 Polar Diagram graph Click to enlarge image
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Illuminance E
Unit of measurement: lux [lx]
Illuminance E indicates the degree to which an area is
illuminated. It is the ratio between luminous flux and
the area to be illuminated. The unit of measurement is
the lux (lx). An illuminance of 1 lx occurs when a luminous
flux of 1 lm is evenly distributed over an area of 1 square
meter. In practice, however, it is unlikely that the luminous
flux will be so evenly distributed over the illuminated
area that all the points in this area will have the same
illuminance value.
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Fig. 4 Illuminance E click to enlarge image
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Luminance L
Unit of measurement: candelas per square meter [cd/m2]
The luminance L of a light source or an illuminated area
is a measure of how much the eye is stimulated and therefore
of how great an impression of brightness is created in
the brain. Let’s assume we are looking at an illuminated
(or self-luminous) area from a particular direction. The
luminous intensity of this area divided by its size apparent
to our eyes is its luminance L. It is measured in candelas
per square meter (cd/m2).
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Fig. 5 Luminance L click to enlarge image
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Luminous efficacy 
Unit of measurement: lumens per watt [lm/W]
Luminous efficacy
indicates the efficiency with which the electrical power
consumed is converted into light. It is measured in lumens
per watt (lm/W). The luminous efficacy of conventional
incandescent lamps (such as R5 W) is typically 10 lm/W,
that of tungsten-halogen lamps (such as H7) 26 lm/W, and
that of gas discharge lamp (such as D2S) 91 lm/W.
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Color
temperature
Unit of measurement: Kelvin [K]
The color temperature of a light source is defined in
comparison with a “black body radiator” and
plotted on what is known as the “Planckian curve”
as seen in Figure 6. The higher the temperature of this
“black body radiator” the greater the blue
component in the spectrum and the smaller the red component.
An incandescent lamp with a warm white light, for example,
has a color temperature of 2700K, whereas a daylight fluorescent
lamp has a color temperature of 6000 K.
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Fig.
6 Chromaticity Diagram click to enlarge image
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Light color
The light color of a lamp can be neatly defined in terms
of color temperature. There are three main categories
here:
Warm White < 3300 K
Cool White 3300 to 5000 K
Daylight > 5000 K
Despite having the same light color, lamps may have very
different color rendering properties owing to the spectral
composition of their light.
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Color rendering
As a rule, artificial light should enable the human eye
to perceive colors correctly, as it would in natural daylight.
Obviously, this depends to some extent on the location
and purpose for which light is required. The criterion
here is the color rendering property of a light source.
This is expressed as a “general color rendering
index” (Ra). The color rendering index
is a measure of the correspondence between the color of
an object (its “self-luminous color”) and
its appearance under a reference light source. To determine
the Ra values, eight test colors defined in
accordance with DIN 6169 are illuminated with the reference
light source and the light source under test. The smaller
the discrepancy, the better the color rendering property
of the lamp being tested. A light source with an Ra
value of 100 displays all colors exactly as they appear
under the reference light source. The lower the Ra
value, the worse the colors are rendered.
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Weibull data for lamp life
Weibull distribution has become established as the standard
method for analyzing the life of technical products. Endurance
tests on products without technical faults produce failure
curves which represent the proportion of products that
fail in the course of use. Because of their special properties,
these curves can be normalized for the purposes of comparison.
This involves approximating the distributions to a straight
line. To show this, a special coordinate system is needed
(named after Dr. E. H. Wallodi Weibull, 1887 to 1979).
Service life distribution can now be accurately defined
by specifying two points on the straight line, without
having to refer to a failure curve.
The following failure rates have been defined as standard
failure rates. The B3 life value represents
the premature failure rate of lamps. It means that 3%
of all the tested lamps failed after this number of
hours burned. The Tc life value represents
the failure rate of 63.2% of the lamps. B3
and Tc are of particular interest in industry.
Another important point is the B50 value.
This indicates the average life of the lamps (50%).
Fig.
7 click to enlarge graph
To show this approximation to a straight line, a special
graphic matrix is needed. The failure rates are indicated
as a percentage plotted against the number of hours
burned. The advantage here is that the distribution
of the life of, say, a headlight lamp can be uniquely
defined by specifying particular failure rates
Point 1: The B3
life value means that 3% of the tested lamps failed
after this number of hours burned.
Point 2: The B10
life value means that 10% of the tested lamps failed
after this number of hours burned.
Point 3: The B50
life value indicates the average life of the lamps.
The failure rate is 50%.
Point 4: The Tc
life value represents the characteristic failure rate
of 63.2% of the lamps.
Rule of thumb:
5% overvoltage ->
- Half the life
- 15% higher luminous flux
- 8% higher power consumption
- 3% higher current
5% undervoltage ->
- Twice
the life
- 15%
lower luminous flux
- 8%
lower power consumption
- 3%
lower current
All life data presented in this
catalog are given as B50 @ 14.0
volts for halogen products and @ bulb rated
voltage for all others unless noted.
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