Standard UV Curing Lamps
These lamps, also called mercury arc lamps, are the workhorse
of the light curing industry. They combine cure efficiency
with design versatility and moderate cost. An electrode
type medium pressure lamp consists of three major components:
- A quartz glass sleeve
- Fill material, typically argon and precisely dosed
amount of liquid mercury
- Electrodes sealed into the end of the quartz sleeve
A high voltage is applied to the lamp across the electrode
terminals. The voltage field ionizes the argon and produces
heat to vaporize the mercury, which creates a pressure
of about two atmospheres. This pressure is needed to maintain
a balance between the required high intensity while maintaining
the spectral output essentially in the UV region. The
mercury vapor arc, which is created, exhibits the unique
property of high emission of light in the ultraviolet
region of the spectrum. In particular, intense emission
occurs in the 240-270 nm and 350-380 nm areas, which is
where typical UV photo initiators absorb. This intense
light beyond the violet region of the visible spectrum
has sufficient energy to interact with photo initiators
and cause their fragmentation, which initiates polymerization.
Some visible light and infrared radiation are also generated.
Figure: A presents the spectral output of a typical medium
pressure mercury vapor lamp. American Arc manufacture
electrode type mercury vapor lamps in sizes ranging from
couple of inches to 135 inches in length and various power
levels ranging from 100 Watts/inch to about 800 Watts/inch.
The most popular lamps are in the 200-400 Watt/inch range.
A disadvantage of mercury vapor lamp, which has electrodes,
is that a warm up and cool down time is required. They
cannot be turned on and off instantly. This problem is
overcome somewhat by switching to 1/2 or 1/3 of power
during down times. If the lamp is held at reduced power,
it can be instantly raised to full power. A shutter is
often used with the low power setting to block light from
the cure area during the partial power time.
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Metal Halide Lamps
Metal halide lamps are type of medium pressure mercury
arc lamp (with electrodes) in which the spectral output
is modified by addition of metal halides to the bulb contents.
The most common dopant is an iron halide, which enhances
output in the UV region and gallium and indium where output
has been shifted strongly into the visible region.
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