Mercury lamps

Mercury lamps are discharge lamps which operate on the same principle as fluorescent
lamps. At sufficiently high pressure the radiation emitted by the mercury is in the visible
spectrum so that such a lamp can be used without a fluorescent coating. The original
mercury lamps had no coating, but modern lamps are provided with one in order to
improve the colour rendering. It also increases the lamp efficiency.
The basic mercury lamp consists of an arc tube with an electrode at each end and a
starting electrode near one of the main electrodes, the electrodes themselves being similar
to those in the fluorescent lamp. The starting electrode is connected to the opposite main  electrode through a high resistance. The arc tube is fitted inside an outer glass envelope.
In the UK the main types are designated MBF. The construction of an ordinary MBF
lamp is shown in Figure 7.6. The two main electrodes are sealed inside the arc tube with
a starting electrode near the lower one connected to the upper one through a high
resistance outside the tube. The arc tube is made of fused silica and the operating pressure
inside it is between two and ten atmospheres. It is held inside an outer envelope of heatresisting
glass, and the resistance between the starting and main electrode is also within
this envelope. The connections are brought out to an end cap at the base of the outer
envelope.
The radiation from the arc tube is greenish-white in the visible spectrum with some
ultraviolet. The latter is converted into a visible red by a fluorescent coating on the inside
of the outer envelope and the combined light from the are tube and the coating has a
colour which is considered acceptable for street lighting and for some factory and
warehouse applications. In order to prevent oxidation and internal arcing the outer
envelope is filled with an inert gas at a pressure between 0.04 and 0.9 atmosphere.
When the lamp is switched on the voltage is sufficient only to start a small discharge
between the starting electrode and the nearby main electrode. The small discharge makes
the argon in the tube conductive and this enables a discharge to start between the main
electrodes. This discharge bypasses the starting electrode, warms the tube and evaporates
the mercury. As it evaporates the mercury takes over the conduction of the main
discharge.
Figure
After two or three minutes, steady conditions are reached and the mercury emits its
radiation. The current through the discharge is limited by a choke.

A capacitor is also included in the circuit for power factor correction, and the capacitor
and choke must be contained either in the luminaire or in a casing conveniently near it.
When these lamps are used for street lighting the control gear can be accommodated
inside the column on which the lantern is supported. If it is intended to do this, the
column manufacturer’s specification must be checked to make sure that the column is
large enough to hold the gear.
When the lamp is switched off, it will not restart until it has cooled and the pressure
has dropped again. This takes two to three minutes, but the lamp will not come to any
harm if it is left switched on while cooling down. In other words, if the lamp is switched
off and switched on again immediately, it will not re-light immediately but will not be
damaged.
Both the light output and the power consumed increase linearly with an increase of
supply voltage. The relationship is shown in Figure 7.7.
A variation on this lamp is the MBTF which has an additional tungsten filament inside
the outer envelope. The tungsten filament is connected in series with the discharge tube
and acts as a ballast. Consequently there is no need for a choke and, because the filament
is not inductive, no power factor correction capacitor is required either. The filament has
to be specially designed to fulfil its primary function of controlling the lamp current
whilst also having as long a life as the arc tube and being capable of withstanding the
starting conditions; at starting the voltage drop across the are tube is low with the result
that the voltage across the tungsten filament is higher than during operation. The light
from the filament provides additional colour

correction. This and the absence of control gear are advantages, but the lamp has a lower
efficiency than the MBF.
Another version is the MBFR. This has an outer envelope with a parabolic reflector
shape. It has an internal coating of fine titanium dioxide powder which has a high
reflectance in the visible region. The fluorescent coating is applied on top of this, but the
front surface of the bulb is left clear.
The ME is a lamp which operates at a working pressure above 30 atmospheres and a
correspondingly high temperature. This necessitates a special construction of the
electrodes and the use of quartz instead of glass for the outer envelope. At powers above
1000W, a.c. operation leads to pitting of the electrode surfaces and d.c. operation is
preferable. ME lamps are used for optical projection, for which they are suitable because
of the small physical dimensions in relation to the amount of light produced.
The MD lamp operates at pressures of 50 to 200 atmospheres. This makes it so hot
that it has to be water cooled. Consequently it can only be used in optical equipment
which incorporates a water pumping system; even then it has a very short life.