the Clatronic IRL233 heat lamp had an optical red filter to reduce the light component of its radiation
Clatronic IRL2333 heat lamp
The temperature of the filament of an incandescent
lamp for lighting purposes was about 2800K. For
therapeutic heat lamps a shift of the radiation peak
level towards the infrared area was desirable and
one might expect the temperature of the filament of
a heat lamp to be lower than that of an incandescent
lamp. For a great number of heat lamps, as used for
instance in drying kilns or in incubators, this indeed
was the case. These lamps operated at a filament
temperature of about 2200K and their light
production was lower than that of incandescent
lamps for lighting purposes while their lifetime could
be much longer. From medical experiments it was
know however that infrared radiation of about 1200
nanometers was the most effective in pain relief.
The corresponding temperature for the filament then
should be about 2400K, which was still lower than
that of an incandescent lamp for lighting purposes.
Since the total emission of energy happened to be proportional to the fourth power of
the temperature however, it turned out to be effective to increase the temperature of
the filament as much as possible and filter out the surplus of visible light. The total
quantity of radiation of a filament of 2900K for instance, was about two times as high
as that of a filament of 2400K and also the relative contribution of short wave infrared
radiation increased with temperature. Therefore, as a result of the increased filament
temperature, the absolute quantity of infrared radiation in the desired wavelength
area increased significantly. Of course there was an increase in the light component
too, but an optical red filter could easily reduce this, as with the Clatronic IRL233
heat lamp on display here.
The maximum temperature of the filament was limited by the properties of the material
it was made of. With increasing energy, not only the intensity of the vibrational
transitions increased but also the energy transitions of individual electrons. At
sufficiently high levels of energy, a part of these electrons would leave the filament
and finally even entire atoms would escape from the grid and react with the
surrounding gasses. The presence of a vacuum around the filament or an
environment with inert gasses without any possibility of electric conduction, would
permit the temperature of the filament to climb just below the melting point of the
material. For pure tungsten this melting point was around 3700K, sufficient to
withstand the high temperatures required for therapeutic heat lamps. The price for the
higher temperature and the increase in infrared radiation was a shortening of the
lifetime of such a lamp. The construction of therapeutic heat lamps was therefore
always a compromise between a long lifetime and a high operating temperature and
in practice the lifetime was limited to about 300 to 500 burning hours while the lifetime
for an ordinary incandescent lamp for lighting purposes was in the order of 1000
hours. For comparison: the earlier described infrared drying lamps operating at
2200K, could have a lifetime of 3- to even 5000 burning hours.