Light can be generated in various ways. In the field of signalling technology LEDs are used in the majority of applications.
Light emitting diodes are constructed using certain semiconductors. Foreign atoms are built into the semiconductor with the purpose of optimising the conductibility. Half of the semiconductor (n-region) is doped with foreign atoms that contain one bonding electron more than the semiconductor atom. This surplus atom can move freely and increases conductibility.
The other half (p-region) is doped with foreign atoms containing one electron less than the semiconductor. When the LED is switched on, these faults (“holes”) fill up with free electrons (recombination). Energy in the form of radiant photons is hereby released. The energy and therefore the colour of the light emitted is determined by the material the semiconductor is made of; e.g. GaAsP (Gallium Arsenic Phosphide) results in red light.
A tungsten filament is heated up to a high temperature, so radiating energy over a wide wavelength. This is perceived as light similar to sunlight. The tungsten filament evaporates with time. When the tungsten content falls below a certain level, the maximum life duration of the bulb is reached. As tungsten oxidises quickly and is destroyed when it comes into contact with air, the filament must be kept in a non-oxidising atmosphere such as vacuum. This leads us to the familiar light bulb with its sealed glass body.
These are bulbs wherein the tungsten filament is enclosed by a small amount of halogen. The resulting chemical reaction has the effect of lengthening the life of the tungsten and stabilising the light output throughout the entire life duration of the bulb.
Xenon flash tubes are widely used in signalling technology. They consist of a glass tube filled with the inert gas xenon. A sufficiently high voltage leads to a discharge of energy with a spark gap and a flash of high intensity.
The fields of lighting and signalling technology differentiate between fundamental units to define light itself.
The most important of these are the units Lumen, Candela and Lux.
Light current is measured in Lumen; this is the unit for the entire visible light output of a light-emitting source.
The light current is defined by the following formula known as the brightness characteristic:
Light current φ [in lm] = radiation capacity x brightness characteristic V(λ)
The brightness impression upon the human eye is based on a sensitivity curve V(λ) which reproduces the sensation felt by the eye in relation to the wavelength. The maximum point on this curve is at about 555 nm; we see best at this wavelength; V(555 nm) = 1.
In signalling technology only the part of the light current that is emitted in a certain direction is of importance. This light intensity is measured in Candela. It is defined by the light current of a lamp and the steradian measure
Light intensity [in cd] = Light current φ/ Steration measure Ω
A complete sphere has a dihedral angle of Ω = 4 π sr. sr stands for the steradian and is the unit for the dihedral angle.
Example: a household candle emitting a light intensity of 12,566 Lumen has a light intensity in relation to the steridian measure 12,566 lm / 4π sr ≈1cd.
This explains the name: candela is the Latin word for candle.
Illumination density is an important unit in lighting installations. It is the measure of the brightness with which an area is illuminated. Whereas light intensity (in cd) is a property of a light source, illumination density is calculated in regard to the area to be illuminated. Where the light current emitted is constant, the following formula is applicable:
Light density E [in Lux] = Light current φ / Surface A