Ultraviolet (UV) Measurement
for Formulators: Part I
By Paul Mills and
Jim Raymont
or those involved in UV processes,
UV measurement and process
control is an important topic.
Measurement answers critical
questions, such as “How do we know
whether a process is running properly?”
“How do we troubleshoot problems
with cure?” “How do we set up our
production process in the first place?”
or “How do we maintain a process we
have set up?” These are the kind of
problems that an understanding of UV
measurement helps solve. And these
are the kind of issues that much of
the existing information about UV
measurement has addressed.
Photo provided by EIT, Inc.
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Ultraviolet (UV)
Lab measurement of irradiance and energy density values with a radiometer.
But these concerns are different for
formulators and raw material providers
as they also need to consider how to
establish a UV specification for their
customers to follow, how to optimize &
communicate their curing specification
and how their specifications can be
reproduced and applied in the field.
While end-users are often concerned
with relative UV measurement (“Are
my UV levels today different than
yesterday?” “Can I run production?”),
suppliers often need to express UV
measurement in absolute terms.
For a specific product and application,
these are the conditions and ranges that
we have identified as starting points
to get an “adequate cure.” And while
it may be sufficient for manufacturers
to develop a language that is unique
to their own production environment,
suppliers need to speak in a more
universal language so that any customer
can clearly understand and implement
the information and also communicate
with their entire supply chain.
Clear, meaningful and useful
communication is the goal. Often
the release of clear, meaningful and
useful information is at odds with the
proprietary nature of formulations.
We hope that the formulator, perhaps
on an individual basis, can work
with their customers to get them the
information needed to succeed. The
formulator must provide his customer
with enough starting information to
enable the customer to reproduce
Typical medium-pressure mercury discharge lamp
special distribution
“colors” by letters (UVA, UVB, UVC)
per ISO-DIS-21348. Individual spectral
peaks are sometimes referred to in
nanometers. Common UV bands and
their ranges include:
UVA 400-315 nm
UVB 315-280 nm
UVC 280-200 nm
UV wavelengths shorter than
200 nm are sometimes described as
Vacuum UV (VUV) while wavelengths
that are longer and on the UV-Visible
border are sometimes referred to UVV.
Do not confuse the two terms.
From a practical standpoint, UV
the laboratory cure conditions in his
plant. But, in order to do so, both
should understand what information is
necessary to fully describe and repeat
the process. It’s also helpful when
making these measurements to be
aware of subtle factors that can distort
or confuse these measurements so that
appropriate care is taken.
This paper is aimed at suppliers
of UV equipment and chemicals,
though it will certainly be useful
to manufacturers as well. In fact,
as the ads for a well-known men’s
clothing manufacturer used to say,
“an educated consumer is our best
customer.” For suppliers, the more
everybody in the chain understands
and speaks the same language—the
smoother things will probably go. As
a benefit to clear communication, a
formulator may also notice a reduction
in the dreaded, “your product is not
curing” phone calls.
The Basics of UV Measurement
We begin with a review of the three
fundamental parameters at the top of
the UV measurement list—wavelength,
irradiance and energy density.
sources, especially broad-spectra
lamps like medium-pressure mercury
lamps and those with additives such
as iron or gallium have peaks that
fall into these various regions. The
chart in Figure 1 is a typical spectra
of a UV lamp. Formulators ordinarily
choose lamp sources that match the
UV absorption properties of the photo
package (photoinitiator, sensitizers,
stabilizers and other materials). These
materials have their own absorption
spectra, such as in Figure 2, which
must coincide with the planned UV
source to provide a workable system.
Radiation is nothing more than
a specific type of electromagnetic
energy. Visible radiation is present when
a light bulb is on. Infrared radiation is
present when an oven broiler is turned
on. Ultraviolet radiation is present when
a UV source is on.
UV is characterized by the
emission of energy in a portion of the
electromagnetic spectrum that is, as
the name implies, “ultra” (Latin for
beyond) violet. Violet light, at
the extreme end of visible light,
has the shortest wavelength,
Typical UV absorption spectra
near 400 nanometers (nm).
Ultraviolet picks up where
violet ends, with wavelengths
running from 10nm to 400nm.
UV below approximately 200
nm exists primarily in a vacuum
and is not particularly useful
for industrial applications.
We think of “industrial” UV as
covering a range from 200nm
to 400nm. This region of the
electromagnetic spectrum has
no color equivalent “names”
such as “green” or “orange,”
and we often refer to the UV
Figure 2
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Figure 1
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In an ideal world, the UV source
would be chosen based on what the
formulator requests. In the real world,
the formulator often needs to work
with an existing UV source that the
customer already has or plans to use
with a particular formula.
Measuring the individual
wavelengths of a few nanometers
from a UV source requires a
sophisticated instrument called a
spectrometer or spectral radiometer.
Spectral radiometers, because they
can measure individual peaks, can
be useful tools for R&D of optical
components or new bulb types. For
formulators and end-users, the output
spectra of a UV source is well known.
In a production environment, it’s
not often practical to make detailed
spectral measurements with a spectral
radiometer. Simpler devices which
measure UV emissions over a broader
“band” are more common, affordable,
practical and easier to use.
UV Intensity and Irradiance
In simple terms, intensity is the
output energy of the UV source.
Imagine a UV source as though it
were the dining room chandelier.
More power is applied as the light
dimmer is turned up and the visible
light output increases. We can also
increase the UV output by turning
up the “dimmer.” Like a visible bulb,
some UV sources have the capability
to produce higher intensities than
other UV sources. When more power is
applied, UV sources generally behave
the same way and produce more UV.
Doubling the power does not mean
that the UV output will double. UV
sources are categorized by the amount
of applied electrical power to the
source. A 300 watt/inch bulb has 300
watts of electrical energy applied for
each inch of the bulb. A 300 watt/inch
system with a 10” bulb would have
3,000 watts of applied power and a
300 watt/system with a 20” bulb would
have 6,000 watts of applied power.
The applied power does not indicate
the amount and type of UV, if the UV
is matched to a particular formulation
and how much UV is arriving at the
cure surface. The amount of UV
arriving at the cure surface is called
the irradiance. A square centimeter is
the area that is used to track arriving
UV from all arriving angles and it
is generally determined by direct
measurement. In theory, we always like
to measure at the cure surface. In the
lab, this is generally easier than the
realities of measurement in the real
world on process equipment.
In the real world, the terms
“intensity” and “irradiance” are
sometimes used interchangeably but,
from a scientific standpoint, irradiance
is the UV arriving at a particular (cure)
surface based on a specified area—in
our case, a square centimeter (cm2).
How and where to measure
irradiance is important. Many UV
lamps have reflectors to either focus
or diffuse their UV energy. UV output
may vary depending on the geometry
of these reflectors and whether the
lamp’s energy is concentrated to one
area (focused) or whether it is diffused
(non-focused), and will also typically
fall off significantly at the ends of an
electrode lamp. If we imagine moving a
“light meter” around the space near a
UV lamp, the needle will jump around
as we move the meter. Because of this
variation, it might be most informative
to record the maximum value we
measure; this “peak irradiance” is a
value that conveys some meaningful
information that is otherwise hard to
communicate. Peak irradiance (usually
specified as having been measured
at a specified distance from the light
source) is, therefore, a common
expression of lamp output.
As you might imagine, thinking back
to the chandelier analogy, a number
of factors affect peak intensity and
irradiance—the output of the lamp
(how high is the dimmer control
“turned up”), the distance to the lamp,
and the angle that the incident light
strikes the meter.
In a simple scenario, in which
the lamp output is maintained at
some constant level and the meter
is moved from position 1 to position
2, then 3 and 4, the meter responds
appropriately (Figure 3). Rising as the
meter moves into the direct path of
the light source, and then falling off
again as it moves to the other side. It
Figure 3
With consistant lamp output, the meter is moved
to position 1, 2, 3 and 4 and the meter responds
UV Energy Density
Energy density factors in the time
element of the UV exposure. One
watt for one second equals one Joule.
Energy density is expressed in terms
of joules (or mJ) per cm2. In an ideal
world, the product being cured would
be exposed to the UV in a ‘square’ or
constant irradiance level. An exposure
in which the UV source is on, the
product is static and a shutter opens
and closes for a set time approximates
a ‘square’ exposure profile. The energy
density could be approximated if the
irradiance is known as well as the
time of exposure. A surface exposed
to a ‘square’ UV source with a peak
irradiance of 750mW/cm2 for three
seconds receives an energy density
reading of 3 x 750 or 2,250 mJ/cm2
(2.25J/cm2). The majority of exposures
increased irradiance. As we know from
in the real world are not ‘square’
exposures. Either the product moves
under the lamp or the lamp moves over
the product. With changing irradiance
levels, we must rely on a radiometer
to measure the exposure and then
calculate the total energy density.
Energy density is important for
the total cure of the material and,
historically, it has been the most
common “UV value” shared with
end-users by formulators. It does not
always tell the entire picture.
A note on the “D” (Dose) word: In
the real world, the word “dose” is often
used in the place of the term energy
density or radiant energy density.
Think of ‘dose’ in the medical sense
as in the administration, absorption
or addition of a drug and most people
would agree that we are exposing the
cure surface. In the UV curing world,
energy density is a better term but
be aware of customers who may use
either term. Decide as a company on
what you will use and be consistent
with your terminology. I have tried to
stick with energy density in this article.
To understand how irradiance
and energy density differ and their
respective roles in UV curing,
consider the example of cooking some
microwave popcorn. The directions on
the bag of our microwaveable popcorn
may recommend that we cook it for
three minutes at a high setting on the
microwave. If we use the microwave at
its lowest power setting, how long until
the popcorn is done? Maybe never.
Why? For popping corn, the microwave
intensity (irradiance) needs to exceed
a certain threshold. This is common
for popcorn (as well as baking cakes or
roasting Thanksgiving turkeys). When
roasting a turkey, turning the oven up
to 1,500º F degrees instead of 375º F
our popcorn experience, however, time
does not ensure a golden brown bird
in a short time either. So too there
is a limit to the beneficial effect of
also plays a critical role in the process.
Turn off the microwave too early
and there are likely to be a cluster of
unpopped kernels on the bottom of
the bag. We cannot assume that while
there is a direct and linear relationship
between the mathematics of irradiance
and energy density, that this means
the materials will cure proportionately.
While this may hold true for a narrow
process window, it is likely to break
down at extremes.
Determine how many joules are
needed to cure a particular formulation
and application under a UV source
in the lab. Take a second sample
and leave it outside in the sun for an
equivalent joule exposure. Chances
are the properties of the sample left in
the sun are different because the UV
irradiance was much lower.
In summary, because wavelength
is determined by the type of lamp
selected (and not a common
measurement requirement), specifying
the optimum peak irradiance (W/cm2)
and energy density (J/cm2) remain the
two critical recipe variables needed
to produce the perfect UV turkey
or popcorn. That said, there are a
number of factors that can affect these
measurements and introduce potential
sources for errors as well.
Irradiance Measurement—
Understand Your Instrument
Understanding your UV instruments
and their proper use and limits will
help both you and your customers
to better understand their readings.
What factors exist that can cause
potential confusion and measurement
errors? Are the factors a result of
the instrument limits, misuse by the
customers or unrealistic expectations?
Some of these factors are caused
by the limits of the UV measuring
device and some by the measurement
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also falls off as the distance increases
from position 2 to 3. We know that as
we move away from the light source,
the measured irradiance decreases.
The irradiance will decrease as the
square of the distance—move the
measurement point twice as far away
and we would expect the irradiance
to decrease by 1/4. (22= 4, power
decreases by 1/4)
Irradiance measurements and
more commonly peak irradiance are
customarily expressed in the units
of Watts/cm2 or mW/cm2. Typical
industrial UV sources output less than
100 mW/cm2 of UVA to over 5,000 mW/
cm2 (5 W/cm2) of UVA.
Irradiance is often viewed as the
“punching power” behind UV curing.
Irradiance (assuming the proper
wavelength) provides the needed
energy to penetrate coatings and
films. This “punching power” provides
the power to achieve depth of cure
and substrate adhesion. And, while
inadequate irradiance will prohibit
complete curing for some applications,
irradiance and energy density work
together to provide an optimum curing
profile for many applications.
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procedure. In any case, being aware
of these issues may avoid recording or
reporting incorrect or misleading data.
and to maximize the sensitivity of
measurements by band—UVA, UVB,
these circuits. An everyday example
Band-Pass Filtering and Attenuation
of telephones and other audio devices.
UVC and visible UVV. This division
makes it possible to get more precise
measurements within each band. But
the problem is that each “channel” uses
a band-pass filter which can attenuate
measurements at extreme ends of
the band. The advantage of individual
filters is that within each band,
sensitivity is improved. The downside
is not being able to steer clear of the
edges of the band-pass envelope.
Some actual band-pass filter
response curves are illustrated in
Figures 4 and 5. The curves in Figure 4
are for a single, broadband instrument,
while those in Figure 5 are for a fourchannel radiometer.
Problems related to filtering do
arise. For instance, the development
of UV-LED light sources with narrow,
single wavelenghth output right near
395nm is not ideally suited to some of
the exisiting filters since the spectral
response of the current filters is low in
this area where LED output is strong.
New filters are now being developed to
address this situation.
In engineering detector circuits,
it’s a common practice for engineers
to use band-pass filters to prevent
extraneous signals from interfering
with measurement in the desired band,
of band-pass filtering is in the circuitry
For optimal human hearing, telephones
reproduce sound from 200 cycles to
20,000 cycles (Hertz). How does this
relate to UV? For the wide spectrum
of UV, it is convenient to subdivide
Figure 4
Band-pass filter response curve for a single,
broadband instrument
Figure courtesy of International Light
Figure 5
Band-pass filter response curves for separate bandwidths (UVA, UVB, UVC, UVV)
Figure courtesy of EIT, Inc.
Typical “optical stack” of components used by
raiometers to collect, filter and diffuse incoming light
Optical window/filter
Aperature opening(s)
Optical filter
Silicon Photodiode
or other type detector
There are differences in the
bandwidths between different
manufacturers. Each manufacturer
selects the optical response
components based on their product/
instrument design. For the user, this
means that readings taken with one
brand of radiometer will be different
from another even under the exact
same conditions. For example, while
one EIT radiometer measures UVA
with a filter ranging from 320 to 390
nm, another brand uses a broad
250-415 nm filter. Readings taken
with these two devices will differ. (In
fact, slight variations from model to
model and even device to device are
common as manufacturers need to
balance the performance of the optics
with the cost.) Manufacturing of
optical components has gotten much
better but it is still normal to have
slight variations between optics and/
or batches. Eliminating these slight
variations would drive the cost of the
instrument up significantly.
Most radiometers use a photo
detector (such as a photo diode),
preceded by an array of optical
components used to collect, filter
and diffuse incoming light. A typical
“optical stack” of these components is
shown in Figure 6. The materials and
approach vary by manufacturer.
Most radiometers try to replicate a
cosine response. Why? It is thought/
assumed that UV-cured coatings
behave in a cosine manner and that
the UV arriving at 90º has the potential
to provide more ‘cure’ power than UV
arriving at 45º or some other angle. In
theory, the ‘cure’ power decreases by a
component proportional to the cosine
of the angle on incidence. For example,
as shown in Figure 7, at a 45° angle
the reading is decreased by Cos(45) or
0.707 times the maximum reading. This
cosine component is an ideal condition,
and the optics of the radiometer try to
match this response.
A second type of error is related to
the geometry of taking measurements.
When the measuring tool is located
directly in line with the light source,
we obtain a maximum reading. As
described in the figure above, we have
already seen that as the radiometer
is moved away from this axis, the
reading decreases. The accompanying
Figure 8 shows a real-world example of
how optics can distort the reading from
Figure 7
Cure power decreases by a component proportional
to the cosine of the angle on incidence
In this example,
at a 45° angle
the reading
is decreased
by Cos(45) or
0.707 times the
maximum reading
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Cosine Error
Figure 6
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Figure 8
Real-world example of how optics can distort the reading from the expected response
the expected response. The innermost
curve shows an original optical design
for a probe-type instrument where
the cosine response was significantly
distorted from the expected values
shown by the out-most cosine curve.
After re-engineering the design, the
actual response (shown by the middle
curve) is a much better fit and provides
much more accurate data. Cosine
response is an important characteristic
in which profiling of a source will
require making measurements that are
not directly in line with the maximum
irradiance of the lamp. While it’s possible
to check the cosine response of a unit by
moving the radiometer to known angles
from a fixed source and comparing to an
ideal cosine curve, it may be easier to
purchase equipment from manufacturers
who can attest to the quality of the
optical stack in their radiometers.
Dynamic Range/Solarization of Optics
Weighing a newborn baby on a
truck scale would not result in a
very accurate measurement because
the truck scale is set up for much
higher weights. Be sure to also select
a radiometer with the proper scale
for your measurements. Using an
instrument geared for high- (power)
intensity measurements on a lowintensity (power) source will produce
the same results as the truck scale
example above. Using an instrument
that was designed for low- (power)
intensity sources on a high- (power)
intensity source will, more than likely,
damage the instrument.
Radiometers rely on small
electronic detectors for measurement
and a number of optical components
to condition the incoming UV energy.
The instrument needs to balance the
amount of UV reaching the detector.
Enough UV needs to reach the
detector to generate a proper signal.
Too much UV reaching the detector
has the potential to damage the
instrument through the process of
solarization. Some components, when
exposed continuously to high levels
of UV, can deteriorate (or ‘solarize’)
with age. Solarization typically
changes the transmission properties
of the materials and affects accurate
measurement by attenuating
(or decreasing) UV measurements
over time. While the solarization
process can be minimized by choosing
superior materials, it is unfortunately
often unavoidable. Periodic calibration
can compensate for minor change
and, in the long term, some optical
components may need to be replaced.
Process heat is the unwanted,
but often unavoidable by-product of
UV curing. Many UV lamps radiate
more energy in the long wavelength
infrared and convection portion of
the spectrum than they do in the UV
region. And while UV radiometers may
not be measuring this energy, they can
be affected by it and the heat in the
process may unknowingly introduce
UV measurement error, especially on
long exposures of very high-power
sources. Check with your supplier
to see how the detector on their
instrument responds. The response of
Writing a UV Specification
One of the most common and
important uses of good measurement
practices for suppliers is developing
curing specifications. Hopefully, the
preceding discussion sheds some
light on what is important in a good
specification. Wavelength, irradiance
and energy density are all important
to the process and should be part of a
well-crafted specification.
While there is frequently a need to
protect proprietary information, the
curing specification should be viewed
as a communications tool and as a
starting point for your customers to
optimize their process.
It’s not enough to describe the UV
source since source output can (and
does) change over time, and because
not all customers will choose to use
the specified source. So, while testing
may be done with a 300W/in Fusion H
lamp or a 200 W/in iron additive lamp,
this description is not a substitute for a
Here are some examples of
UV specifications:
“Fusion 600W/in Lamp”
A poor description. There’s too little
information to do much of anything
here. 600 W/in is a measure of the
power that goes into this lamp and does
not give any information about the UV
at the part. Which wavelength? H, V, D
lamp? What about energy density?
“400W/in mercury arc lamp for
5 seconds”
A little better, but there is still more
information about the light source than
about UV reaching the part. What we
want to see is some irradiance or dose
measurement information.
“600 mJ/cm2”
Better again, at least there’s now some
measurement information. But a good
deal of important data is still left out.
For example, at what wavelength?
“600 mJ/cm2 UVA”
A good specification. There’s enough
here to replicate this specification if we
only knew what measuring tool to use.
“600 mJ/cm2 UVA (EIT 320-390)”
A better specification. We can tell
what type of radiometer was used to
any specification process, clear
communication is essential to ensure
that the recipient of the information
clearly understands what the provider
intends. To that extent, a specification
should include the basic elements
needed to replicate the process.
We have seen that UV measurement
includes a description related to
wavelength, irradiance (typically
peak irradiance) and energy density
(dose). In Part I, we looked at some
factors that can influence irradiance
measurements such as filtering,
solarization and cosine error. Part
II will examine some factors that
can influence energy density, how
reflectors influence measurements
and how to select a radiometer for
laboratory work. w
measure this data.
—Paul Mills is a consultant and
Jim Raymont is director of sales,
EIT Incorporated, Sterling, Va.
“300 mW/cm2 600 mJ/cm2 UVA
(EIT 320-390)”
Best. There’s a complete set of
information to help ensure proper cure
and how to replicate the laboratory
conditions in the field.
For some coating processes,
two lamps are required to achieve
complete cure of both the surface and
through cure. This should be described
with appropriate data and curing
information. In all cases, additional
information such as the UV bulb
type as well as coating thickness and
application data will help customers
understand your intentions, and
provide a more complete guideline
to help them establish and maintain
a proper process. This up-front
communication helps to eliminate
costly finger-pointing mistakes down
the road.
End-users rely on UV measurement
to monitor their process, and to
troubleshoot when things go wrong.
But suppliers often need to help
establish these specifications. Like
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many detectors will cause the readings
to drop slightly.
Extreme temperatures can even
damage some instruments. In general,
“if it’s too hot to touch—it’s too hot
to measure.” Some radiometers
provide a convenient display of
internal temperature so that this can
be recorded in the lab notebook, and
an internal alarm that will warn if
the internal temperature exceeds a
recommended operating temperature
(say, 65° C).
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