UVC Production by Germicidal UV Lamps
There are many types of lamps that artificially produce UV. There are UV
lamps for tanning, for counterfeit money detection, blacklight stage lamps and
lamps for mineral displays, lamps that produce Ozone and germicidal UV lamps.
The focus of this document are the germicidal UV lamps, which emit shortwave UV
light in the ultraviolet section of the specter also known as UVC or germicidal
UV. There is more information on the
nature of UV in the UV
Science Facts section. Here we will discuss the artificial UV production by
the different UV lamps and the specs of the different types of UV lamps. Many
times people refer to the UV lamps as UV bulbs as in regular light bulb. Even
though bulb is not the correct term, replacement bulb, UV bulb or bulbs are
widely accepted in the industry as a reference to the UV lamps.
UV Lamps – History and Development
UV is artificially produced by mercury vapor low and medium pressure lamps.
The low pressure lamps are most effective, because they emit most of the radiant
energy in the germicidal wavelength of 253.7nm also known as the UVC part of the
specter. This is the reason low pressure lamps are mostly used in germicidal UV
applications. These lamps are sometimes called “amalgam” lamps because they
contain solid amalgam “spots” (an amalgam is an alloy of mercury with another
element, such as indium or gallium) that controls the mercury vapor pressure.
All lamps have secondary emissions, including small amounts of UVA, UVB, visible
light (above 400nm wavelength) and heat. The blue glow of the germicidal UV
lamps is not indicative to the effective germicidal output they produce – that
could be only determined with a properly calibrated UV sensor and monitor.
As with all gas discharge lamps, the UV output of germicidal lamps is reduced
when the temperature of the lamp surface deviates from the optimum. The
performance data of the various lamp types and the influences of air or water
cooling play an important part in an effective and reliable UV disinfection. If
this is neglected it may lead to an inadequate UV installation.
For effective UV disinfection not only the temperature but the transparency of
the medium for UVC (253.7nm wavelength) is of great importance. The greater the
energy lost through absorption, the less energy remains to kill microbes. Tests
have shown that there is a reduction in the UV lamps disinfecting performance if
there is a high humidity level. For the effectiveness of UV water purification
systems transmittance of the water is very important.
It is important to take the reducing factors under consideration when sizing the
UV lamps for an effective UV disinfecting process.
In air stream UV irradiation reflective materials with high UVC reflection
properties should be used as these materials will multiply the UV efficiency of
the germicidal lamps.
Development of UV disinfection lamps started in the early forties when
Westinghouse began the development of the Cold Cathode lamps in an economical
production. After that UVC lamps were tried out for disinfection everywhere –
surfaces, goods, water and air. The early extensive testing still applies today
as a basic knowledge, underlining the UV technology.
Types UV Lamps
Cold Cathode Germicidal UV Lamps
The Cold Cathode Germicidal UV Lamps are instant-start lamps with a
cylindrical cold cathode type of electrode. These lamps are available in
different sizes and may be operated either from single lamp transformers or in
series trough the medium of high voltage transformers.
The combination of Vycor glass tubing, used in most Cold Cathode lamps, and
sturdy electrode construction make lamp life considerably longer than other
types of lamps. Good ultraviolet maintenance is provided at lower temperatures
and lamp life is unaffected by frequent starts.
Although the amount of radiant energy at 253.7nm wavelength emitted is the same
for both high and low ozone lamps, the high ozone lamps use a special Vycor
glass which transmits a controlled amount of radiation at 184.9nm, which
wavelength produces ozone. Ozone has deodorizing properties and is in itself a
bactericidal and fungicidal agent. However tests have shown that the Ozone has a
negative health effect if used indoors so the use of Ozone producing lamps is
not recommended for most applications.
The Vycor glass Cold Cathode ultraviolet germicidal lamp is the most economical
type for the majority of general germicidal applications, because of its long
life electrode and good glass maintenance.
Hot Cathode Germicidal UV Lamps
The Hot Cathode Germicidal UV Lamps are similar in their operation to the
standard fluorescent lamps. The Hot Cathode lamp operates from a ballast or
transformer and requires a device such as the glow switch starter to preheat the
electrodes in order to start the lamp. The electrodes, located at the ends of
the lamp, are tungsten filaments coated with emission material and, under normal
operation, govern the life of the lamp. In view of the fact that the life of the
electrodes is shortened by frequent starts, the lamp life is rated according to
the number of times the lamp is started. Operation at refrigerator temperatures
may result in excessive lamp blackening and rapid depreciation in ultraviolet
output. Starting of the Hot Cathode lamps at low temperature is sometimes
unreliable and may require special equipment.
Slimline Germicidal Ultraviolet Lamps
The Slimline Germicidal Ultraviolet Lamp is an instant-start lamp, similar to
the Slimline fluorescent lamp. The Slimline lamps are available in low, high and
very high ozone types. The lamp life is governed by the electrode life and
number of starts.
Because of their high initial ultraviolet emission and good maintenance,
Slimline Germicidal ultraviolet lamps are well adapted for applications such as
indirect air irradiation, conveyor lines, surface sterilization and other
applications which require high intensity lamps.
High Output Germicidal UV Lamps
The most recent addition to the UV lamp line of products is the type of High
Output germicidal UV lamps. The HO lamps are the consequent result of constantly
applied know-how and the latest lamp manufacturing processes. High UV output
over a great temperature spectrum, a long life and good behavioral patterns are
the pointers for the High Output UV lamps. Only high quality raw materials are
used in the lamp production. A fine tuning with the automatic electronic
ballasts guarantees the lowest tolerance and a maximum UV stability.
With life duration of 12,000 hours and almost linear performance degradation the
high output UV lamps are setting the standards for the development of high
performance UV technology.
The most important factor in using germicidal UV lamp technology is the
knowledge about their behavior under real working conditions (e.g. the effect of
air stream cooling). It is definitely not just the lamp performance under
laboratory conditions that count. Only in gaining this knowledge high quality
disinfecting technology can be achieved.
Taking the example of air stream cooling the High Output lamps do show their
real advantage. While classic UV lamps heavily depreciate under real working
conditions inside an air duct, this is not the case with the High Output UV
Light Emitting Diodes (LED) UV Lamps
Recently a completely new technology for UV production started to emerge.
This is the UV Light Emitting Diodes or UV LED lamps. The UV LEDs are the next
generation UV producing devises that will compete with the established UV lamps.
Some researchers claim that the UV LEDs have better characteristics surpassing
the regular UV lamps. However at this time there is no UV LED equipment that can
compete with the high output UV lamps in real world production conditions.
UV Lamps Aging
The decrease in UV lamp output over the typical lifespan of 8,000 - 12,000
hours can vary between 15-40%. The manufacturer should be consulted for
information on the end-of-life output of UV lamps. The decrease in UV output
should be accounted for in the design phase such that the lamp output does not
decrease to a point where the air treatment system becomes ineffective. The most
conservative approach is to size the system based on the end-of-life of the lamp
UV output. Selecting lamps based on end-of-life UV output will avoid the lamps
The lamps should be kept clean and free of dust at all times. If dust
accumulates on the lamp it will absorb the UV and convert it to heat, therefore
lowering the effectiveness of the UV lamp. Appropriate filtration of the air
prior to the UV lamps is recommended.