Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 129

Burning Position The majority of the metal halide lamps employed in road lighting
have a free burning position. Many of the types developed for floodlighting purposes
have restrictions as to their burning position.

10.3 Solid-State Light Sources

In 1907, the radio engineer Round reported that light was emitted from a silicon
carbide crystal, intended to be used in radio receivers (Round 1907 ). This was in fact
the first solid-state light source (SSL): in this case, a small light emitting diode or
LED. It took until 1962 for the first practical light-emitting diode to be demonstrated
(Holonyak and Bevacqua 1962 ). Since then LEDs have been widely used as signal
lamps in all kind of electrical appliances. In the 1990s high power LEDs were
introduced. When, in 1995, Nakamura also developed a high power blue LED, it
became possible to produce white light LEDs, based on such blue LEDs covered
with fluorescent powders (Nakamura et al. 1995 ). It was then clear that solid state
lighting would become a possibility for many lighting applications. Today, in most
application fields, especially in road lighting, solid-state light sources are often used.
With further improvements to be expected, they surely will become the dominant
light source of the future.

10.3.1 Principle

10.3.1.1 Light Emission

In solid-state light sources the light is created inside solid-state material consisting
of a sandwich of two different kinds of semiconductor material, so-called p and n
materials (Schubert 2006 ; van Driel and Fan 2013 ). The physical phenomenon of
creating light in this way is called electric luminescence. Semiconductors are made
of a material that is a poor conductor of electricity. By adding specific impurities
(atoms of another material) to the material (a process called doping), the atoms of the
material either acquire extra electrons or become deficient in electrons. The doping
process makes the material more conductive, hence the name semi-conductor. The
material with extra electrons is an n-type of semiconductor (negatively charged)
while the material with a deficiency of electrons viz. positively-charged “holes”, is
a p-type of semiconductor. In the n-type of material the extra electron of an atom
moves in an outer orbit with a correspondingly-higher energy level. In the p-type
of material the missing electron of an atom was moving in a lower orbit with lower
energy level. Connecting the n-type material to a negative charge and the p-type to a
positive charge, pushes the n- and p-atoms towards the junction of the p-n sandwich
layer (Fig. 10.16 ).