this Helitron sunlamp was equipped with an adjustable UV-C filter
Helitron sunlamp
More effective than carbon arc lamps were
ultraviolet radiators based on the principle of
mercury vapour arc discharge. Atoms from
evaporated metals like mercury and sodium emit a
significant amount of electromagnetic radiation when
they return to their normal state after being exited
by an electric current. For mercury the wavelength
of this radiation is within the range of the ultraviolet
area. The yield of high-pressure mercury vapour arc
discharge lamps was about 20% ultraviolet rays,
20% light and 60% infrared rays. The life span of
the lamps was limited by the decreasing
transparency of the glass over time. To prevent
sunlamps from emitting too much of the undesirable
UV-C, often an additional protection was added that
was opaque for UV-C. In many cases this filter
consisted of an additional glass bulb that surrounded
the discharge lamp itself. Some discharge lamps
were equipped with a removable or adjustable UV-C filter to regulate the effect of the
treatment. An adjustable filter in its half-way position can be seen with the Helitron
sunlamp on display here.