Ultraviolet curing is a process in which UV energy
produced by a mercury discharge lamp is absorbed
by a sensitizer, causing a reaction in the monomer
which makes it hard and dry. The rate of the curing
process depends on the following:
1. Chemical compound - Each monomer cures at
a different rate, depending on the amounts and
compositions of sensitizer, pigment, and chemical
2. Thickness of Coating - The thickness of a
specific coating is not directly proportional
to exposure time. The amount of UV energy inside
a layer of coating decreases exponentially with
depth. If 70% of the UV energy is absorbed in
the top .001" of coating, then 70% of the
remainder or 7% of the initial amount will be
absorbed in the second .001" of coating.
Thus, a two-fold increase in the thickness requires
a ten-fold increase in UV intensity.
3. Amount of UV per Unit Surface - Normally,
the curing speed will increase with the amount
of UV energy per unit surface at the non-linear
rate. If a 200 watt per inch mercury lamp was
increased to 400 watt, the curing speed could
increase ten fold. In additional, special metal
halide lamps can trigger the receptor at an even
faster rate with a standard 200 watt per inch
lamp. The chart on Figure 1 demonstrates the flexible
control of Innovative's lamps. We can pinpoint
the UV energy produced by our curing lamps to
your exact specifications.
The sensitizer should absorb UV in the range
which is not absorbed by the monomer or pigment.
The wavelength produced by a medium pressure mercury
lamp or metal halide lamp should coincide with
the wavelength absorbed by the sensitizer.
The types of radiation emitted from our mercury
lamps depend on many factors. In a gas discharge
lamp, the output is a function of the atomic structure
of the gas molecules, their temperature, and the
pressure of the gas vapor.
If you require the absence of ozone gases, our
engineering department can respond with a special
grade of quartz that prevents the 250 nanometer
line from reaching oxygen in the atmosphere. Please
bear in mind that all effective lines below 254
nanometers are blocked, which may interfere with
the curing of some inks. Figure 2 shows the basic
area of interest for industrial curing.
The U V Spectrum
Ultraviolet refers to all electromagnetic radiation
with wavelengths in the range of 10 to 400 nanometers,
or frequencies from 7.5E14 to 3E16 Hz.
The UVA range is wavelengths from 315 to 400 nanometers.
Wavelengths from about 345 to 400 nm are used
for "Blacklight" effects (causing many
fluorescent objects to glow) and are usually very
slightly visible if isolated from more visible
wavelengths. Shorter UVA wavelengths from 315
to 345 nM are used for suntanning.
UVB refers to wavelengths from 280 to 315 nanometers.
These wavelengths are more hazardous than UVA
wavelengths, and are largely responsible for sunburn.
The ozone layer partially blocks these wavelengths.
Strangely, UVB lasers are considered less hazardous
than UVA lasers, since UVB is more easily absorbed
by various fluids and tissues in the eye and cannot
reach the retina in significant amounts. UVB also
does not penetrate as deeply in the skin as UVA.
However, the deadliest types of skin cancer (malignant
melanomas) start in the epidermis, an upper layer
of the skin. UVB is largely blamed for these cancers,
although shorter UVA wavelengths are considered
UVC refers to shorter UV wavelengths, usually
200 to 280 nM. Even shorter wavelengths from 10
to 200 nM are usually considered separately as
"Vacuum Ultraviolet" since they are
absorbed by air, although these wavelengths are
also considered a shorter range of UVC. Wavelengths
in the UVC range, especially from the low 200's
to about 275 nM, are especially damaging to exposed
cells. Such shortwave UV is often used for germ
How a UV Lamp Works
UV-light is generated in a mercury discharge lamp:
a quartz tube with mercury vapor, an inert gas,
two electrodes and insulators for suspension.
The mercury generates radiation between 200 and
400 nm with peaks at 254 nm, 310 nm and 366 nm.
The lower wavelengths are cut off by the quartz,
which does not transmit any radiation below 230
Each atom consists of a nucleus, around which
a number of electrons float in fixed orbits. By
adding energy (electricity) the electron is brought
in a higher orbit. Each element shows a tendency
to go back to its original condition. The electron
will fall back in its former orbit: the excess
energy is emitted as a photon.
The most commonly used UV-lamp is the medium-pressure
mercury-arc lamp or MPMA-lamp. It can be manufactured
in lengths of some mm to more than 2 meter. The
life span of these lamps varies from 1000 to 2500
This lamp is made of quartz because this is the
only material that transmits UV-light and at the
same time endures high temperatures of 6 to 800¡ÆC.
This lamp will expand little and does have a high
melting temperature (from 1100¡ÆC).
The electrodes are made from tungsten: the process
to manufacture them is extremely complex. Tungsten
is used because the temperature of the curve may
rise up to more than 3000¡ÆC.
The connection between the electrode and the
wire is accomplished with a molybdenum plate that
can expand together with the quartz when warming
up and still does endure the high voltages.
The lamp finally is hung up to ceramic insulators.
As the supply current is often insufficient to
power a MPMA-lamp, our lamps employ transformers.
Two types are used: inductive and fixed-power
transformers. Standard ballast is also used up
to 5 kW capacity.
Medium pressure UV lamps
In a high pressure lamp, the average kinetic energy
of free electrons (also known as electron temperature)
is only slightly higher than the temperature of
the gas in the discharge. Both temperatures are
similar, and one can refer to a general temperature
of the discharge. The electron temperature is
higher in order for the electrons to have the
net effect of transferring energy to the gas.
It is quite fair to think of the gas or vapor
as a thermal radiator, which is usually spectrally
selective and radiating mainly in specific spectral
Typically, the discharge diameter is over 50 times
the mean free path of an electron. There is generally
no non-thermal, non-optical, non-mechanical interaction
between the discharge and any container it is
in, except at the ends of the discharge.
In order to efficiently produce light, the power
input must be much greater than the heat conduction
from the discharge, which is usually near or over
10 watts per centimeter of discharge length. Therefore,
power input to a high pressure lamp is usually
near or over 20 watts per centimeter of discharge
The above can all be satisfied even if the pressure
is less than 1 atmosphere. It can even be satisfied
in a nearly practical lamp with a pressure of
High pressure lamps are often referred to as
High Intensity Discharge lamps, or HID lamps.
In a low pressure lamp, the electron and gas
temperatures are very different, and the pressure
is generally below .05 atmosphere. Power input
is generally near or less than 1 watt per centimeter.
Low pressure mercury UV lamp
In a low pressure lamp with mercury vapor as an
active ingredient, the mercury vapor is mixed
with an inert gas, often neon or argon. The mercury
vapor's pressure is usually well under 1/1000
atmosphere, or a fraction of a mm. of Hg. The
mixture is generally 1 percent or even as little
as .1 percent metal vapor, 99 to 99.99 percent
The desired spectral output generally results
from atomic transitions that terminate in the
atom's "ground" or unexcited state.
This means that most of the metal vapor atoms,
since they are not excited, can easily absorb
this radiation. Therefore, you don't want too
much mercury vapor. The inert gas largely determines
electrical characteristics, mainly by controlling
the mean path traveled by electrons between collisions.
The gas also reduces collisions of electrons,
ions, and excited mercury vapor atoms into the