UV Curing Lamp FAQ
Ultraviolet curing for the new and experienced
designer offers many challenges. We have compiled various facts, tips
and definitions to assist the designer and user in understanding and
applying these products to your own UV curing situation. If you have
questions or comments we have not addressed, please send those to us.
What is an arc lamp and how does it work?
arc lamp consists of pressurized, sealed envelope containing gasses
and/or metals with two electrodes,
one at either end. A pulse of high voltage arcs the gap between the
electrodes, (in early arc lamps the light emitting plasma was noted as
bowing upwards in a curve or an arc, hence the name arc lamp). Heat from
the arc vaporizes gases and/or metals in the envelope forming a plasma. This
plasma generates light and UV energy.
How do you calculate input current required by the ballast?
Input current to ballast = [(NOMINAL WATTS / Power Factor) * (Power supply efficiency)] / Ballast input voltage
NOMINAL WATTS = 10,000
Power Factor = 0.92
Power supply efficiency) = 90%
Ballast input voltage = 220VAC
Input current to ballast = [( 10,000 / 0.92) * ( 0.90)] / 220 = 44.47 Amps
How to measure lamp voltage?
Since lamp voltages can exceed the safe measuring
range of most meters a potential transformer (PT) is required. The PT
is placed in parallel with the lamp. A typical 100:1 ratio is normally
used. Measuring lamp voltage is one of the best ways to control lamp
temperature in order to properly cool the lamp (link to cooling).
How do I know at what power level I am operating?
A current transformer is placed around the lamp
lead wire allowing safe measurement of lamp current. Typical ratios
used at 50:5A, 75:5A, 100:5 and 150:5A. (Place link to 2560 data sheet)
How can I tell if a lamp is an additive lamp?
If the lamp is not operating you will see a yellow
brown deposit inside the lamp in addition to a bead of mercury for
Gallium lamps. For iron additives you need to look very closely but
metal like "filings" are visible on the inside of the lamp. You can
also tell lamp type by looking at the color of the light when
operational. NOTE you should never look directly or indirectly at
reflected lamp light, as it is DANGEROUS. If you look at the
"surrounding" area of the curing system it is possible to distinguish
the color of different additives:
White with a slight hint of green
Slight hint of purple
Slight hint of blue
What is curling?
Normally the arc plasma that occurs between the two
electrodes of a lamp is approximately a straight line. When curling
occurs the arc plasma instead of being a straight line forms a helix
around the inside surface of the lamp causing the lamp to curl. The
resonance that typically causes this curling effect occurs at arc
lengths of 42", 55" and 77" for 50Hz and between 38" and 42"for
additive lamps. Systems with these arc lengths should be avoided if
possible. Ballast and lamp combinations can be configured to prevent
Should I use an additive lamp?
You need to ask your supplier of the UV material you
are trying to cure. Ask them if additional UV energy in the 390nm or
420nm range will provide any benefit.
Is Super UV right for me?
Super UV effect only increases UV when operating lamp
at above 400 WPI. Unfortunately there are no hard and fast rules
on what works with different photo-initiators so empirical testing is
Why does UV output fall off with lamp life?
UV solarizes impurities in the lamp; Bromide is one
example of an impurity. This solarization causes an output reduction of
15 to 20% in the first 20 hours of lamp operation. An additional
reduction of 10% occurs in the next 100 hours. After 120 hours these
transmission losses remain constant until lamp failure.
Why do some lamps have a Gold coating near the electrodes?
This gold plating in some cases helps to allow
mercury trapped behind the electrode to vaporize; in most cases this
coating is not required.
Is there tolerance to lamp operating voltages?
Typical lamp voltages can range from 3 to 6%, this range can sometimes vary as much as 10%.
What causes this operating voltage range?
Lamp voltage range is given because volume inside the
quartz tube varies from lamp to lamp from when the quartz is drawn.
This affects the concentration of gases in the lamp, which in turn
affects lamp voltage.
Wattage and current increasing and no change to lamp voltage seems counter intuitive to Ohms law, what is going on?
Initially there is no voltage drop across lamp, but
as the pressure builds in the lamp voltage increases. This voltage then
remains roughly constant through the whole wattage range. A ballast
power supply is designed to be current limiting; the addition of
capacitive reactance to this circuit increases lamp current and total
wattage since lamp voltage is constant.
What is the typical time required for a lamp to come up to full power?
Lamp start up time is typically 1 to 5 minutes Note that lamp must cool down to allow mercury to re-condense before restart.
What is the silver colored bead inside the lamp?
This is mercury. When high voltage from a ballast is
applied to the lamp mercury inside the lamp is energized into plasma
where it generates specific wavelengths of ultraviolet energy, which
are used to initiate the polymerization of UV curable inks and
Can a higher-powered lamp be used in our existing curing system?
Power supplies and UV lamps are usually designed and
matched to provide peak performance. The lamp is cooled in a specially
designed lamp irradiator assembly, which should provide an appropriate
cooled environment for reliable lamp operation. Usually, upgrading
would require both the lamp and power supply to be replaced, and
additional modifications to the lamp irradiator assembly to provide an
Can a standard lamp be replaced with an additive lamp to enhance UV output?
A metal halide lamp will enhance output in specific
spectrums. If the material you are using is responsive to those
additional spectrums you will see a benefit. Note that the power supply
must be capable of supplying the correct operating voltage to initiate
the arc and have the correct waveform to maintain the arc.
What would cause the ballast capacitor to catastrophically fail?
Over cooling a lamp can sometime have this effect. In
most cases when you over-cool a lamp it will extinguish but under
certain circumstances the lamp will hover just above extinguishing
level noticeably oscillating or pulsing. If you were to use an
oscilloscope you will notice voltage spikes, these spikes over stress
the capacitors eventually resulting in capacitor failure.
What if my lamp will not light when power is applied?
- Confirm that all of the terminations are tight.
the lamp and confirm that the mercury is distributed between the
electrodes. If the lamp was stored on end, it is possible that the
mercury has deposited behind the electrode and will not enter the
plasma stream. Simply shake the Lamp from end to end to remove the
mercury from behind the electrodes.
- Try to strike the lamp again.
- Confirm that the power supplies are operating properly.
What if my substrate is not properly curing?
- Confirm that the dryer reflectors are focused properly and that they are clean.
- Check for external contamination of the lamp by items like: spray, powder, reflector material or other particles.
sure that coating and ink material is fully mixed before application.
This allows the photo initiator to be evenly distributed in the coating
- Verify the number of
operating hours that the lamp has run. Different applications result in
different lamp lifetimes. Lamps generally have an energy output of
about 80% of their original specification after 1000 hours, provided
that the lamp is operated in an appropriate environment. If the lamp
has over 1000 hours of use, it may not generate enough ultraviolet
energy for curing your application.
What if my lamp is bowed?
A bowed lamp should be replaced immediately
along with evaluation of the lamp cooling system. This condition is a
direct result of improper lamp cooling. The lamp must be controlled
where the surface temperature is between 600 - 800 degrees C. If the
air around the lamp is not properly controlled, this temperature will
rise causing the quartz tube to soften and lose its rigidity. Adjust
the cooling and airflow around the lamp to reduce the temperature of
the lamp body. However, ensure that the lamp body is not cooled below
600 degrees C, as below this temperature could lead to mercury
condensing out of the plasma, which will effect lamp power and
What if my lamp is discolored?
Discoloration can occur for several reasons.
There is natural "blackening" of the quartz tube at each end during the
life of the lamp. This is the result of the electrode material
depositing on the inside of the tube during its use. Discoloration of
the quartz will occur over time due to natural solarization or
"clouding" of the quartz. This occurs as the quartz reverts to its
natural crystalline structure, which is opaque to ultraviolet energy.
Discoloration may also occur if the lamp is overcooled. This
action results in mercury being deposited on the inside of the lamp
giving a mirror-coated affect.
What if the outside edges of my substrate are not curing?
If this is occurring in existing system where
this problem has not occurring before, check for darkening at the lamps
ends which could reduce light output. Note that there is a natural
optical roll off of UV output at the end of the lamp in a refectorized
What if my lamp initially lights, flickers and then extinguishes?
There are many possibilities, a few to consider:
- UV lamps need time to warm up and come
up to power. Depending on the system, cooling fans should not started
for 30 seconds to several minutes to allow the lamp stabilize.
could be operating at to low of a lower level. It is not recommended to
operate a medium pressure linear lamp below 125 watts per inch and 150
watts per inch is recommended.
- Incorrect ballast being used with lamp.
- Bad or incorrect lamp.
Do metal halide lamps have special cooling requirements?
Gallium and multi-spectrum lamps have cooling
requirements fairly equivalent to mercury lamps. Special care is
required for iron lamps. If an iron lamp overheats, the iron will
irreversibly become part of the quartz tube. This will not cause lamp
failure but you will lose the iron portion of the UV spectrum.
Is the ballast connected properly?
A simple test to see whether the ballast is
connected up correctly is to directly connect the capacitor lead to the
lamp lead and measure the current through this wire. The ballast is
current limiting, if it is wired correctly on the primary side this
current should be 1.5 times lamp current.
Different manufacturers have names for the spectrum their lamps produce, this is confusing.
How to cross this to actual UV spectrum?
You can look at the spectrums listed on each web site for comparison; below we have done this for you.
Iron Cobalt Spectrum
What can cause lamp-operating voltage to change?
Lamps running too hot can expand causing a drop in the running voltage and a reduction in output.
[ return to top ]
- How much longer is the LIA than the arc of the lamp?
- How do I install the LIA?
- Are these available with shutters?
- Do the LIA's require any maintenance?
- How are lamps replaced?
- Do I need UV shielding for my LIA?
How much longer is the LIA than the arc of the lamp?
* Overall length is approximately 10 to 11 inches longer than the lamp arc length.
Lamp Arc Length
* Above chart lists a few of our LIA's.
How do I install the LIA?
- The irradiator can be mounted by using 8-32
screws in the pre-drilled holes on the irradiator making sure that all
of the vents have unrestricted airflow. Failure to provide free airflow
will result in overheating of the irradiator and/or lamps.
- The focal point is located 2.5 inches from the face of the irradiator.
qualified electrician must make electrical connections. The two lamp
leads are made of silicon insulated wire and are to be connected to the
ballast according to the manufacturer's specifications
- Fan leads need to be connected to specified voltages.
- Fans may require a time delay of 1 to 1.5 minutes to allow for lamp startup.
the interlock leads as a safety switch to the power supply. They will
indicate open or closed state of the irradiator (shuttered version
Are these available with shutters?
Yes, an air cylinder powered shutter is a
standard option. Air pressure that you supply is required for
operation. Air pressure requirements are different for each size. This
information will be included with your order.
Do the LIA's require any maintenance?
Yes, the irradiator will require routine
cleaning due to the large amount of air being circulated. Accumulations
of dust on the cooling fins and around the vents should be removed
routinely. Failure to do so will result in overheating and component
failure. Accumulations of dust around the electrical connections will
result in arcing and can be extremely hazardous.
How are lamps replaced?
Lamp Replacement procedure:
- A qualified person following proper lockout procedures must perform lamp replacement.
- Turn power off. Be sure that the irradiator and lamp have cooled before removing the lamp.
- Disconnect all power and high voltage cable from the LIA.
- Remove irradiator cover by removing both knobs at each end of the LIA and lifting out the assembly by its handles.
- Place the LIA upside down on its handles
- Remove one metal diffuser. Loosen the screws on the lamp holder to release the top bracket.
- Disconnect the lamp leads and pull lamp out of the LIA.
the replacement UV lamp and reflector liners with a lint free cloth and
denatured isopropyl alcohol. Use latex or vinyl gloves during this step
to keep the lamp free of any fingerprints.
one lamp lead and end fitting through the mounted metal air diffuser
and on top of the lamp holder. Place the loose metal air
diffuser around the other lamp end and place the lamp end onto the lamp
- Remount the metal air diffuser and place the top bracket over the lamp end and tighten the top bracket.
- Connect lamp leads to ceramic standoffs.
the irradiator back in place and tighten the four knobs and reconnect
the high voltage cable by matching index to index and screwing the
connector cable down completely.
Do I need UV shielding for my LIA?
LIA's emit ultraviolet radiation, which is
harmful to the eyes and skin. Precautions should be taken to protect
personnel from exposure of direct or reflected radiation. Personnel
should use suitable eye and skin protection. We offer a light gauge
metal shielding which is capable of blocking ultraviolet radiation.
Remember that one third of the energy emitted by the lamp is in the
infrared range and fireproof material should always be used.
Definitions of UV terms
Halide - A binary compound of a Halogen with a more electropositive element or radical i.e. iron, cobalt, gallium, iodide.
Halogen - Consists of one of these five elements: Fluorine, Chlorine, Bromine, Iodine, Astatine.
Arc Length - This
is measured in full inches. For example, lamps 15.4 inches and below
are priced as 15-inch lamps; 15.5 inches and above are priced as 16
HD - Lamps
designated as HD (heavy duty) are made from thicker-walled quartz than
standard lamps. Ideally, the thinnest wall quartz should be used to
provide maximum UV output. An added benefit is that the lamp is less
expensive. However, there is always a compromise, which in this case is
physical strength and heat. By their nature, very long lamps and lamps
of 500 watts per inch (wpi) or higher must be made from HD material,
and are automatically handled as such. In addition, some systems are
prone to overheating, and it is not always possible to treat the cause
of the problem by reducing the lamp environment temperature. So in some
cases, HD material is selected for its ability to operate acceptably
with hot equipment. There is no hard and fast rule; experience will
determine which applications require HD lamps
O/F - Ozone-Free lamps are made with special quartz.
Additive - This category includes any additive lamp, including iron
Quartz - Basic
silicon diode that has a specific gravity of 265 and hardness of 7.
Natural quartz has a melting point of 783 degrees C. When quartz is
processed it is changed to a form called tridymite quartz and the
melting temperature increases to 1470 degrees C.
Ampere - (A)
The constant current that, if maintained in two straight parallel
conductors of infinite length and negligible cross section and
separated from each other by a distance of 1 meter in a vacuum, will
produce in these conductors a force equal to 2 X 10 -7 Newton/meter.
Angstrom - A unit of length equal to 10 - 10 meter. Usually the measurement of wavelengths or frequencies of the electromagnetic spectrum.
Atmosphere - A unit of pressure. One standard atmosphere equals 101,325 newtons per square meter.
Devitrification - When
the quartz envelope of a lamp is subject to high temperatures it will
become porous and eventually allow lamp atmosphere to fail.
Mercury Lamp - Lamp
in which light is generated through presence of mercury vapor. Most UV
lamps are mercury vapor lamps.
Nanometer - A unit of distance commonly used in measuring wavelength in the electromagnetic spectrum-one billionth of a meter.
Nominal Watts - Arc length * Watts per inch (WPI)
Photoinitiator - A
molecule which when expressed to a specific wavelength of energy forms
a reactive species which starts the chain reaction to cause formation.
Photosensitizer - A chemical which will transfer energy and form free radicals by interacting with another chemical.
Photopolymer - A
composition which will either crosslink or depolymerize on exposure to
light, forming a physical differentiation between the exposed and
Plasma - A vapor in which there are energetic free radicals, ions, or molecules. These are usually formed by radio frequency discharge.
Polymer - A
macromolecule consisting of an indefinite number of monomer units. The
molecular weights may range from about 20,000 into the millions.
Quartz Tube - A
lamp made from a silicate material called quartz which is fitted
with electrical connections to form an irradiator. It may be made into
an infrared emitter or it may be filled with mercury vapor to produce
Substrate - The
upon which a finishing (e.g., coating, ink, or adhesive) is placed.
Stabilizers - Additives to coatings, inks, or adhesives formulations which help extend shelf life, resistance to heat or other degradation.
Ultraviolet Light UV - That light emitted in the 200-400nm wavelength range.
Normal lamp life is 1000 to 3000 hours and
under optimal conditions can be up to 4000 hours. Our lamps typically
have longer life because of the superior materials used and
construction. We use quartz capillaries to attach to the quartz
envelope. A lamp that uses glass capillaries to attach to the quartz
envelope can have a short life because the dissimilar materials have
slightly different CTE (coefficient of thermal expansion). These joints
are a weak link in lamp construction.
Three main items affect Lamp Life
- Number of Starts
- Maintaining correct cooling
- Proper lamp installation.
Number of Starts
Starts refer to the number of times a lamp is turned
completely on and off. The pressure in lamp atmosphere changes when the
lamp is under full power, which mechanically stresses the lamp. There
are other mechanical stresses caused by Thermal cycling of the lamp
components. Another reason number of starts affects lamp life is the
introduction of tungsten and small amounts of other materials from the
electrodes each time the lamp is ignited. This contamination when
introduced into the lamp atmosphere can effectively increase lamp open
Circuit voltage, making it more difficult to operate the lamp. If
possible start lamps in high power to reduce the length of time the
lamp is in the starting mode. Note: Excessive starts cause premature
darkening of the ends and a drop in output.
Maintaining correct cooling
See Lamp cooling section
for details on monitoring lamp characteristics for maintaining the
correct cooling. Periodic maintenance of the cooling system will also
help to extend lamp life, this includes:
- Keeping the reflectors clean
- Making sure reflectors are free of distortion from caused by heat from the lamp
- Cleaning dirt on the blower blades, this blocks air flow and reduces the cooling efficiency.
Proper lamp installation.
Proper lamp installation includes the following:
- Avoid finger oil on the lamp body by using a
paper towel or glove. Using an alcohol wipe is recommended to assure
- Maintain good electrical connections.
flexibility when mounting. The lamp should not be held tightly in
place. This can restrict movement as the reflectors expand, and
contract during operation.
For medium pressure UV lamps to operate properly the
envelope must be at a rather high temperature 700 to 850C and at the
same time the end seals/fittings must be kept cool (250 to 300C). This
can be physically measured by using a temperature probe. A K-type
probe, with Kromel-Alumel wire rated for 1250C. are available from
several sources such as Omega Engineering .
One of the best ways to check for over-cooling or under-cooling of a
lamp is to monitor lamp voltage. Lamp voltage should stay with in
+/-10% of nominal operating voltage. If the voltage drops below 10%
then you are over cooling the lamp, if it exceeds 10% you are
under-cooling the lamp. A potential transformer (PT) is used for this
measurement (link). Most power supplies are current limiting devices so
an increase or decrease in the operating voltage will not necessarily
change lamp current. Lamps that are over cooled will show a telltale
blackening of the inside of the quartz near the ends of the lamp. Lamp
temperature ranges for optimal life
Part of Lamp
Effect of Improper Cooling
250 to 350 C
Ceramic paste on the end fitting will crack, Molybdenum foil oxidizes and causes the seals to fracture.
650 to 800 C
Quartz will devitrify allowing the lamp atmosphere to escape. Devitrification can also occur if lamps are improperly handled.
Quartz softens lamp bowing and deformation.
For lamps that use stainless steel they discolors.
The light that is perceptible to the human
eye is in the range of 400 to 750nm. Red is roughly 700nm, green 550nm
and blue is 450nm. For an overview perspective below you will see a
wavelength spectrum from X-ray to Far Infrared.
Ultraviolet Spectrum Ultraviolet falls in the
spectrum below blue and refers to all electromagnetic radiation with
wavelengths in the range of 10-400 nanometers. The Ultraviolet spectrum
is further broken up in to UVA, UVB, UVC, VUV and EUV.
UVA range is wavelengths from 315-400 nanometers.
Part of the UVA spectrum from approximately 340-380nm is used for
"Blacklight" (this causes fluorescent objects to glow) and actually is
slightly visible to the human eye if isolated from more visible
wavelengths. The other portion of the UVA spectrum wavelengths from
320-380nm are used for standard UV curing, NDT (non-destructive testing
and inspection. UVB range is wavelengths from 280-315 nanometers and is
typically used in sun tanning. These wavelengths are actually more
dangerous to the skin and eyes than UVA and are mainly responsible for
sunburn. UVC range is wavelengths from 200-280 nanometers and has
applications in Dermatology. VUV wavelengths range from 100-200nm
"Vacuum" UV these wavelengths are absorbed by air are used for various
space, and scientific applications and are often even used for germ
killing. EUV is from 10 to 100nu and is the least explored of all UV
UV Curing is a photochemical process by which
monomers harden or cure (polymerize or cross-ink) upon exposure to
ultraviolet radiation. A specifically formulated monomer will
polymerize when exposed to ultraviolet light. This UV "curable" monomer
includes a sensitizer, which absorbs UV energy and initiates a
polymerizing reaction in the monomer
Five main components UV Curing System
- UV source, lamp
- ILIA (irradiator) or reflector assembly
- Ballast (Power Supply)
- Electrical controls including current and potential transformers
- Safety equipment for shielding