the Penetray Model PH Pyrex heat lamp had a bulb with an integrated red filter
Penetray Model PH Pyrex heat lamp
An incandescent lamp consisted of a
metal wire in a glass bulb without oxygen.
An electric current heated the metal wire
and made it glow white-hot while the lack
of oxygen prevented the wire from
burning. Relative to the amount of light
they produced, incandescent lamps
produced an even greater amount of
infrared radiation. The yield of infrared
incandescent lamps was about 45% IR-A,
45% IR-B and 10% IR-C making them
more suitable for therapeutic applications
than for instance metal- and quartz
tubulars. It was the near infrared radiation
between 700 and 1400 nm (IR-A) that was supposed to have therapeutic qualities by
stimulating tissues below the upper part of the skin. The light from an incandescent
lamp for therapeutic infrared applications was therefore no more than an
(unavoidable) by-product that should be minimised as much as possible. At first the
lamps were commonly fabricated as clear, blown glass bulbs and eye protectors were
necessary to cover the eyes from the intense light of the filament. To overcome this
disadvantage the lamps were soon equipped with integrated red filters that stopped
light with wavelengths below 650 nm, as with the Penetray Model PH Pyrex heat lamp
on display here.