properties of light sources and the effects that are produced.
2.2.1 Luminous flux
Luminous flux describes the total amount of light emitted by a light source. This radiation could basically be measured or expressed inwatt. This does not, however, describe the optical effect of a light source adequately, since the varying spectral sensitivity of the eye is not taken into account.
To include the spectral sensitivity of the eye the luminous flux is measured in lumen. Radiant flux of 1 W emitted at the peak of the spectral sensitivity (in the photopic range at 555 nm) produces a luminous flux of 683lm. Due to the shape of the V (l) curve the same radiant flux will produce correspondingly less luminous flux at different frequency points.
2.2.2 Luminous efficacy
Luminous efficacy describes the luminous flux of a lamp in relation to its power consumption and is therefore expressed in lumen per watt (lm/W). The maximum value theoretically attainable when the total radiant power is transformed into visible light is 683 lm/W. Luminous efficacy varies from light source to light source, but always remains well below this optimum value.
2.2.3 Quantity of light
The quantity of light, or luminous energy (US), is a product of the luminous flux emitted multiplied by time; luminous energy is generally expressed in klm · h.
2.2.4 Luminous intensity
An ideal point-source lamp radiates lumi- nous flux uniformly into the space in all directions; its luminous intensity is the same in all directions. In practice, how- ever, luminous flux is not distributed uni- formly. This results partly from the design of the light source, and partly on the way the light is intentionally directed.
It makes sense, therefore, to have a way of presenting the spatial distribution of luminous flux, i.e. the luminous intensity distribution of the light source.
The unit for measuring luminous in- tensity is candela (cd). The candela is the primary basic unit in lighting technology from which all others are derived.
The candela was originally defined by the luminous intensity of a standardised candle.
Later thorium powder at the temperature of the solidification of platinum was de-
2.2
Terms and units
The amount of light emitted by a light source is the luminous fluxÏ.
Luminous intensity I is the luminous flux Ïradiating in a given direction per solid angle Ø.
[I] = cd [I'] = cd/kIm [Ï] = kIm
I = I' . Ï
C 90/270˚
C 0/180˚
C 0/180˚
C 90/270˚
-40˚ -20˚ 0˚ 20˚ 40˚ I'2
I'
G
å ß å
©
0˚ 30˚
60˚
90˚
-30˚
-60˚
-90˚
I' I'2
G
å
ß
© 0˚ I
90˚
90˚
I 0˚
fined as the standard; since 1979 the candela has been defined by a source of radiation that radiates 1/683 W per steradian at a frequency of 540 · 1012Hz.
The distribution of the luminous in- tensity of a light source throughout a space produces a three-dimensional graph.
A section through this graph results in a luminous intensity distribution curve, which describes the luminous intensity on one plane. The lumious intensity is usually indicated in a polar coordinate system as the function of the beam angle. To allow comparison between different light sources to be made, the light distribution curves are based on an output of 1000 lm. In the case of symmetrical luminaires one light distribution curve is sufficient to describe one luminaire, axially symmetrical luminaires require two curves, which are usually depicted in one diagram. The polar coordinate diagram is not sufficiently accurate for narrow-beam luminaires, e.g. stage projectors. In this case it is usual to provide a Cartesian coordinate system.
Luminous intensity distribution body and diagram (for planes C 0/180° and C 90/270°) of an axially symmetri- cal luminaire.
Luminous intensity distribution of a light source having rotational symmetry. A section through the C plane produces the luminous intensity distribution curve.
Conversion of 1000 lm-related luminous intensity I’ to effective luminous intensity l.
Luminous intensity distribution curve standardised to 1000 lm, represented in polar coordinates and Cartesian coordinates.
The angle within which the maximum luminous intensity l' is reduced to l'/2, identifies the beam spread∫. The cut- off angle åis the limiting angle of the luminous intensity dis- tribution curve.
Ï E A
Eh Ev
Ï
mE A
I Ep
a
L
I Ap
Eh Ev
®1
®2 L1
L2
2.2.5 Illuminance
Illuminance is the means of evaluating the density of luminous flux. It indicates the amount of luminous flux from a light source falling on a given area. Illuminance need not necessarily be related to a real surface. It can be measured at any point within a space. Illuminance can be deter- mined from the luminous intensity of the light source.Illuminance decreases with the square of the distance from the light source (inverse square law).
2.2.6 Exposure
Exposure is described as the product of the illuminance and the exposure time.
Exposure is an important issue, for example, regarding the calculating of light exposure on exhibits in museums.
2.2.7 Luminance
Whereas illuminance indicates the amount of luminous flux falling on a given surface, luminance describes the brightness of an illuminated or luminous surface. Luminance is defined as the ratio of luminous intensity of a surface (cd) to the projected area of this surface (m2).
In the case of illumination the light can be reflected by the surface or transmitted through the surface. In the case of diffuse reflecting (matt) and diffuse transmitting (opaque) materials luminance can be calculated from the illuminance and the reflectance or transmittance.
Luminance is the basis for describing perceived brightness; the actual brightness is, however, still influenced by the state of adaptation of the eye, the surrounding contrast ratios and the information content of the perceived surface.
Illuminance E indicates the amount of lumi- nous flux from a light source falling on a given surface A.
Horizontal illuminance Ehand vertical illumin- nance Evin interior spaces.
Average illuminance Em is calculated from the luminous flux Ïfalling on the given surface A.
The luminance of a lu- minous surface is the ratio of luminous intensity l and the pro- jected surface area Ap. The illuminance at a point Epis calculated from the luminous intensity l and the dis- tance a between the light source and the given point.
The luminance of an illuminated surface with diffuse reflectance is proportional to the illuminance and the reflectance of the surface.
Em= Ï A
Ep= I a2
L = I Ap
L1= Eh . ®1
¿ L2= Ev . ®2
¿ [L] = cdm2
[L] = cdm2 [Ep] = Ix [I] = cd [a] = m
[E] = Ix
1,0 0,8 0,6 0,4 0,2
500 1000 1500 2000 2500
¬ (nm) Se (%)
1016 1014 1012 1010 10 10 10 10 100 10 10 10-6
-4 -2 2 4 6 8
Medium wave
Cosmic radiation
Long wave Audio frequencies
Short wave Ultra short wave
Microwaves Radar IR radiation
Centimetre wavesDecimetre waves
UV radiation X rays Gamma rays
¬ (nm) ¬ (nm)
300 350 400 450 500 550 600 650 700 750 800 850
IR radiation
UV radiation
2.3 Light and light sources
2.3
Light, the basis for all vision, is an element of our lives that we take for granted.
We are so familiar with brightness, darkness and the spectrum of visible colours that another form of perception in a different frequency range and with different colour sensitivity is difficult for us to imagine.
Visible light is in fact just a small part of an essentially broader spectrum of electro- magnetic waves, which range from cosmic rays to radio waves.
It is not just by chance that the 380 to 780 nm range forms the basis for our vision, i.e. “visible light”. It is this very range that we have at our disposal as solar radiation on earth in relatively uniform amounts and can therefore serve as a re- liable basis for our perception.
The human eye therefore utilises the part of the spectrum of electromagnetic waves available to gather information about the world around us. It perceives the amount and distribution of the light that is radiated or reflected from objects to gain information about their existence or their quality;it also perceives the colour of this light to acquire additional infor- mation about these objects.
The human eye is adjusted to the only light source that has been available for millions of years – the sun. The eye is therefore at its most sensitive in the area in which we experience maximum solar radiation.
Our perception of colour is therefore also attuned to the continuous spectrum of sunlight.
The first artificial light source was the flame of fire, in which glowing particles of carbon produce light that, like sunlight, has a continuous spectrum. For a long time the production of light was based on this principle, which exploited flaming torches and kindling, then the candle and the oil lamp and gas light to an increasingly effective degree.
With the development of the incan- descent mantle for gas lighting in the second half of the 19th century the principle of the self luminous flame became outdated; in its place we find a material that can be made to glow by heating – the flame was now only needed to produce the required temperature. Incandescent gas light was accompanied practically simultaneously by the development of electric arc and incandescent lamps, which were joined at the end of the 19th century by discharge lamps.
In the 1930s gas light had practically been completely replaced by a whole range of electric light sources,whose operation provides the bases for all modern light