Handbook of Electrical Installation Practice - BeKnowledge

Lighting 479
Fig. 18.3 A typical discharge lamp, in this case an MBF lamp. All modern lamps operate in
a translucent enclosure containing the appropriate metals or metal halides.
dichroic-coated mirror (diameter is only 50mm for the 50W rating) gives great
optical benefits, particularly for the display world. The reflector produces a precise,
uniform beam resulting in intense, efficient light of high colour rendering and good
appearance. The dichroic mirror allows most of the heat to pass backward while
reflecting a relatively cool beam of light forward. This resulting ‘cool-lightstream’ is
most important in displays as a wide range of merchandise may suffer from heat.
Owing to the efficiency, a 50W lamp compares with a 150W PAR 38 spot and thus
leading retailers are now utilising the benefits. Low-voltage tungsten-halogen lamps
require a transformer to operate.
Discharge lamps
When an arc is struck in a gas or metallic vapour it radiates energy in characteristic
wavebands. For example, neon gives red light, sodium yellow and mercury vapour
four distinct lines in the visible and two in the ultraviolet region of the spectrum.
All modern discharge lamps operate in a translucent enclosure (Fig. 18.3) containing
the appropriate metals or metal halides; the initial discharge is usually struck
in argon or neon. As the metal or metal halide evaporates, it takes over the discharge
from the starter gas and emits light at its characteristic wavelengths.
Because more light and less heat is radiated by these lamps, they are more
efficient in terms of lumens per watt than filament lamps, but where a line spectrum
is emitted there is a marked distortion of colours seen under their light.
Low-pressure sodium lamps
These are the most efficient lamps in terms of lumens per watt, because the
monochromatic yellow light they produce is in the area near the peak of the eye