The inside arc tube of quartz allows more ultraviolet light to be
transmitted towards the inner phosphor coating of the outer bulb. This
has the effect of increasing the light output and improving colour
rendering properties. The outer bulb is filled with nitrogen, or
nitrogen/argon gas mixture, and maintains an even lamp temperature.
Figure 1 shows the circuit for the high pressure mercury lamp.
To commence the discharge, a starting electrode is located close to the
main lamp electrode. When the supply is first switched on a small
discharge takes place in the argon gas between these two electrodes, the
current being limited by the high resistance (10 Ωk to 30 Ωk) in series
with starting electrode. This discharge is now transmitted in the argon
gas between the two main electrodes, causing an increase in temperature
and mercury vaporization. Lamp pressure builds up and maximum lamp
brilliance is attained when all the mercury is vaporized. The chock
limits the current flowing though the lamp and the capacitor is
incorporated to improve the power factor.
High pressure mercury lamp circuit data
. lamp efficacy 34-60 lumens per watt
. adequate colour rendering
. lamp takes time to reach its full brilliance
. after switching off, it will not restart until the pressure inside the lamp has fallen
. average lamp life 22000 hours
. applications
– used where colour rendering is not of major importance, for example
street lighting, car parks, floodlighting of buildings, general outdoor
commercial and industrial uses.
. lamp designation MBF