orbitec - Palissy Galvani

Discharge lamps
The performance of
the metal halide lamp
The performance of the metal halide lamp is
remarkable:
• Extremely good lighting power (60 to 120 lm/W).
• An average lifespan of 10,000 hours.
• A light spectrum close to real light (daylight).
The range of metal halide lamps is very extensive
according to the following three parameters:
• Colour temperature (2000°K to 6000°K).
• Lighting power (50 à 150 lm/W).
• Colour quality (20 < Ra < 95).
Considering the range of possibilities provided by
their features, it is necessary to establish a compromise
tailored to the use requirements. The user’s choice
will take into account three parameters to the
detriment of the others.
Low and high pressure
discharge lamps with
sodium gas
• Constitution
The principle of sodium lamps is that of metal halide
lamps and mercury lamps. The burner contains
sodium.
The light produced by these lamps is orangey-yellow
and deforms colours perceived by the human eye. To
make up for the poor colour quality, the solution is to
increase pressure of the gas. However, when under
high pressure, sodium gas attacks the quartz on the
burner and the lamp does not resist these attacks.
The first generation sodium gas discharge lamp was
therefore “low pressure”, to avoid the sodium rapidly
destroying the lamp.
Technological progress has enabled manufacturers to
produce a “high pressure” lamp with an alumina
burner resistant to very high temperatures and
attacks by sodium gas. The second generation of
discharge lamps produces better colour quality than
“low pressure” lamps.
The sodium lamp is recognised for its very good lighting
power. We can have a good light, even with fog.
That’s why it is used for road and street lamps.
High pressure discharge
lamps with mercury gas
• Constitution and operation
The operating principle is the same as metal halide
lamps.
For mercury lamps, lighting power and homogeneity
of the light are favoured with regard to colour quality
(mediocre). The light given off is very white.
• A variant: the combined lamp
Combined lamps are mercury lamps containing a
filament. This filament allows preheating of the burner
and gives the light given off by the lamp a slightly
warmer shade. When the lamp is connected, the
filament positioned around the burner becomes
incandescent and heats the quartz envelope until the
mercury evaporates. The mercury gas favours the
passage of electrons between the two electrodes and
the lamp is thus lit. The orangey-red colour of the
incandescent filament allows gives the lamp a better
colour quality.
Combined lamps have the advantage of being used
directly without a lighting system, but the presence
of a filament makes them fragile.
What you need to know
The use of mercury lamps requires a lighting
system (except for combined lamps).