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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.
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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.
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| 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.
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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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TCS
Technologies UV Lamps
American
manufacturer of UV curing lamps & bulbs,Ultraviolet equipment,CD
DVD curing,UV inkjet,UV reflectors and more.
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