Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 115

Fig. 10.4 Examples of
different types of fluorescent
powder as seen under white
light ( left ) and under
ultraviolet radiation ( right )

All three types of interactions are essential and need to be controlled in a proper
way. Elastic collisions determine the gas temperature, ionization the lamp's current
and, of course, excitation the emission of light. The wavelength of the electromag-
netic radiation depends on the type of gas or vapour and on the pressure of the gas
(Meyer and Nienhuis 1988 ; Coaton and Marsden 1997 ). Controlling the temperature
sometimes requires a secondary bulb around the discharge tube that acts as a thermal
isolator. To avoid an unlimited increase in free-moving electrons and thus in the
lamp current, a current-limiting device called a ballast is needed. Most gas discharge
lamps need an ignition device that temporarily produces a high peak voltage to help it
start viz. emitting electrons from the electrodes when the discharge tube is still cold.
Some gas discharges emit a relatively large part of their electromagnetic radiation
not in the visible but in the ultraviolet part of the spectrum. In that case fluores-
cent powder is used to convert the ultraviolet radiation into visible light. Fluorescent
powders are commonly referred to as phosphors. The physical phenomenon of con-
verting short-wave electromagnetic radiation into longer-wave visible radiation is
called photo-luminescence. High energy short wavelength electromagnetic radiation
temporarily excites an electron of a molecule of the fluorescent material, sending it
into a larger orbit. When that electron falls back into its original orbit electromagnetic
radiation of lower energy and thus longer wavelength is emitted 2 . The fluorescent
powders used today most often contain rare-earth metals that belong to a group of
17 elements with properties also widely used in energy technology devices. By mix-
ing different fluorescent powders in different proportions, lamps emitting different
tints of white, i.e. with different colour temperatures and colour rendering, can be
produced. Figure 10.4 shows three different types of fluorescent powders that under
white light all look whitish-yellow, while under ultraviolet radiation the resulting,
converted, visible light is yellow, red and blue respectively.

2 According to Max Planck's quantum theory the energy E of a photon is proportional to the
reciprocal of the wavelength: E = hxf = hxc/ ʻ . With h = Planck's constant, f = frequency, c =

speed of light and wavelength. Photons with a longer wavelength thus have less energy than

photons with a shorter wavelength.

ʻ =