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Ultraviolet
lamp design
Introduction
Ultraviolet
curing lamps belong to the general group of electric discharge lamps.
This lighting technology replaces the filament of the "light bulb" with
a capsule of gas. UV energy is emitted from an arc discharge between
two electrodes hermetically sealed inside a quartz glass tubular
envelope. Unlike filament lamps, electric discharge lamps have three
great virtues: they are efficient UV energy converters; they last a
long time; and they have excellent maintenance of UV output. They also
have disadvantages: lamps and control gear are relatively expensive;
lamps do not function well in short term service; full light output
does not occur immediately when power is applied; once lamps have
started, a power interruption of 1/4 cycle (1/240th of a second) or
more may cause the lamps to extinguish. Once extinguished, it could
take up to several minutes before an arc can be re-established and full
output attained.
| All Ultraviolet
curing lamps convert electrical energy into "UV light" by transforming
electrical energy into kinetic energy of moving electrons, in turn
converted into radiation resulting from electron collision. Light is
produced by passing a current through a metal vapor. Free electrons
colliding with an atom in the vapor momentarily knock an electron into
a higher orbit of the atom. When the displaced electron falls back to
its former level, a quantum of radiation is emitted. The wavelength of
radiation depends on the energy state of the disturbed electron and on
the type of metal vapor used in the arc tube. |
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The basic process is
comprised of three steps: free electrons are accelerated by an applied
potential difference (UV power supply); the motion of electrons being
the electric current in the device (lamp current); the kinetic energy
of the electrons is transformed and radiation produced as energy states
return from high (excited) to lower state. |
Lamp
construction
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UV curing
lamps are made from quartz which can be safely operated at temperatures
of 1000 degrees Celsius and is highly transparent to UV radiation.
There are two tungsten electrodes from which the arc is sustained. The
distance between electrodes is known as the lamp arc length. With lamp
arc temperatures approaching 3000 degrees Celsius; the entire electrode
design process is extremely complex. It is not possible to bond
tungsten directly with the quartz glass. Most UV lamps use a special
molybdenum seal foil to provide a hermetic and thermally stable seal.
At the outer end of the foil is a high voltage Teflon wire which
provides the electrical connection. The lamp end cap which can be metal
or ceramic is cemented over the assembly. The end cap provides
mechanical support and mounting area.
The
principal limitation in the manufacture of UV curing lamps is the
transition of the lead in connector (lamp wire or end cap) to the
electrode. This is called the lamp seal. There are two types of
lamp seals used: pressed (sometimes called pinched) and vacuum
(sometimes called shrink or bonded seal).
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Pressed
sealed lamps are machine made; are economical to produce and have a
filling tip somewhere on the body of the lamp. The flat seal is
fragile and extremely easy to snap. Care must be used when
installing press sealed lamps. This technique is limited to short arc
length lamps where cost and lamp size are main issues.
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Vacuum sealed
lamps are hand made; extremely strong and normally do not require a
filling tip. Seal shape is round and can be made any length. In
general, the longer the seal the less chance of seal failure. These
are by far the best choice for UV curing lamps. Another advantage
of vacuum sealed lamps is they allow the lamp to be rotated in any
position for maintenance purposes. This extends lamp life especially in
long arc length lamps. |
| Filling tips (the
little bump on the lamp body) pose another problem. They must be
always pointed up or to the side, never downward! Sometimes the tip
poses an installation problem as it tends to get snagged. See
illustration. Often times the fill tip is a weak spot on the lamp
and limits lamp positioning. Care must be taken not to strike the fill
tip as lamp will immediately fail. |
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Lamp
life
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Lamp life depends on
many factors including number of starts, thermal operating conditions,
burning position, quartz diameter, power rating and proper handling.
Under normal conditions, the vast majority of lamps will provide at
least 1000 hours of useful life. Some equipment manufacturers utilize
power supplies that employ low voltage high current lamps. Lamps
operating in excess of 13 amps have greater electrode blackening and
generally shorter lamp life. Keeping lamp current between 6 and 11 amps
will significantly increase lamp life. Lamps must be kept clean. All
types of dust, powder, grease, smoke and misting ink must be cleaned
from lamp. Overheating from a dirty condition will cause warping and
short life. |
Ozone
production
Another
health concern regarding the ultraviolet lamp is the generation of
ozone. The interaction of short-wavelength UV light with oxygen causes
the generation of ozone. Although it is possible to use an "ozone free"
lamp, the negative impact on curing is significant enough that very few
people use these lamps. Most suppliers handle the ozone concern by
ducting the ozone away from the work environment. Due to the high
reactivity of the ozone, the molecule will typically decompose back to
oxygen while traveling through the exhaust system.
Ultraviolet
safety
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Ultraviolet
curing lamps produce intense UV light. Shielding is absolutely
mandatory. UV lamps produce harmful UV radiation that can cause serious
burns to skin and eyes. While thermal burns are felt immediately, UV
burns are not felt for several hours. Short exposure to lamp radiation
can cause severe burning to eyes and skin. Fortunately, UV light does
not reflect significantly from most surfaces. If one does not have a
direct line of sight to the lamp or reflector, there typically is not a
significant amount of UV energy to worry about. The fact that visible
light can be seen does not mean that significant UV energy is present.
Normally, even escaping visible light is minimal with a well-engineered
UV lamp system. If a large amount of light is escaping, the system
supplier should be notified to determine if a problem exists.
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Cleaning
of ultraviolet curing lamps
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Use a lint free
cloth with Windex or Simple Green to clean. Don't waste your money on
special UV lamp cleaners as they have dubious value! If solvents are
permitted, use isopropyl alcohol. For extreme cases use a mild abrasive
such as Soft Scrub to clean the UV Lamp. Be sure to rinse any residue
off the glass before reinstalling lamp. Always allow lamp to cool
and disconnect all power prior to any cleaning. |
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