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Steve Mackin
Solartech Inc.
26101 Harbour Pointe Dr. N.
Harrison Township, MI 48045 USA
Editor's Note: Steve Mackin (Fig. 1) is a classic
example of an engineer who entered a field outside his
specialty and developed a successful business. Exactly
nine years ago as this is written (9 July 1999), I began
making regular solar noon measurements using one of
Mackin's UV radiometers. The device compared so well
with my own homemade filter UV-B radiometer and a commercial
Solar Light 501 Biometer that Mackin loaned me more
instruments over the years. I have recommended his instruments
for various UV-B monitoring programs, and I plan to
write a paper that shows the excellent stability of
these relatively inexpensive UV radiometers. Full disclosure
requires that I state that I have no business connections
of any kind with Steve Mackin. We have never even met
one another. I just happen to be very impressed by what
he has done and the many important roles that have been
found for his UV radiometers. Forrest M. Mims III.
Handheld ultraviolet (UV) meters have
found many unexpected uses in recent years. Originally
analog needle gauges like
this, they have evolved into convenient digital
units like this
and this
and the Solartech Solarmeter® units such as the
UV Index model shown in Fig.
2.
My name is Steve Mackin, president of Solartech, and
this is a snapshot of how the UV meters we manufacture
developed from a spontaneous idea out-of-the blue.
Back in 1990, while on vacation in Florida with my family,
we were sitting on a Ft. Lauderdale beach soaking up
(too many) rays from the noon sun. Feeling the familiar
burn, I looked down at a newspaper brought down to the
seaside from the restaurant where we had lunch. A headline
read something like, "Hole in Ozone Layer: More
ultraviolet radiation reaching earth!" I looked
up toward the sky and wondered: well how much UV is
really hitting our skin. I wanted to create a way to
detect the intensity of these invisible wavelengths.
While driving home several days later, I noticed a small,
domed sensor on the dashboard that regulated the automatic
air conditioning relative to the infrared radiation
through the windshield. I wondered if a sensor like
that could detect ultraviolet and decided to find out
when we got home and I returned to my job as an engineer
at an automotive company.
After consulting with several air conditioning and electrical
engineering friends, it became apparent that making
a UV meter would not be too difficult if a suitable
sensor that detected only UV wavelengths could be obtained.
This was important, because standard silicon detectors
respond from the UV to the near infrared and would not
provide an appropriate UV sensor without very expensive
filters that transmitted only the UV wavelengths. Eventually
I located a new kind of commercially available UV photodiode/filter
combination. This devices employed a GaAsP photodiode
provided a good method for detecting the 280-400 nm
bandwidth.
A few Radio Shack parts and homemade printed circuit
boards later, a working prototype was ready. It was
the size of a pint of milk. And it responded proportionately
to UV sunlight and a UV lamp. At this time there was
no internet search engine, so as far as I knew this
"invention" was a first. Little did I know
that several companies already offered digital UV meters!
A decent library or trade publication search might have
alerted me to that. In retrospect, I am very glad I
didn't know, because I might have abandoned the project.
After downsizing the design to a small, shirt-pocket
size package, I sent a unit to a solar light company
for NIST traceable calibration. That unit became a "master"
meter for subsequent transfer calibrations to saleable
units. The price was arbitrarily set at about $150,
and several companies that sold beach accessories were
approached about selling the device. Their replies amounted
to, "No way! All our customers want is sun and
fun. They don't care how much UV is coming from the
sun."
I put the idea on a shelf and forgot about it for several
months. Then a friend showed me a tanning trade magazine
and suggested I check with tanning bed distributors
to see if they had any interest for a UV radiometer
to test their UV lamps. Eureka! Every one I talked to
wanted lots of meters immediately. So was born the Solartech
Model 5.0 total UV (A+B) meter. To this very day the
demand remains strong, as many more distributors are
carrying it.
As time went by, the National Weather Service/Environmental
Protection Agency (NWS/EPA) UV Index was created in
1994-5. This became popular, for it provided an easily
understood scale of UV, with 1-3 being on the low end
and 9 and higher being on the high end. Since the UV
Index is based primarily on UV-B that causes sunburn,
and the Model 5.0 was detecting mainly the longer wavelength
UV-A, a search was begun to find a detector material
that would not respond to sunlight above a wavelength
of about 320 nm, a so-called solar-blind detector.
The best photodiode that could be
found was composed of silicon carbide, but it responded
to radiation as high 380 nm in the UV-A. So a special
interference filter would be needed to cut off irradiance
above 320 nm. That was accomplished by using a sputter
coated, vacuum metalized, thin-film deposited on a UV
transmitting glass window cap mounted over the photodiode.
Unfortunately, spectral radiometer tests showed the
sensor response to be somewhat "flat" compared
to the real UV Index. This was confirmed by a visit
to the National Institute
of Standards and Technology (NIST) in Maryland for
an intercomparison with their rooftop UV instrumentation.
Another setback.
Since the new detector was responsive to broadband UV-B,
the Solartech Model 6.0 series meters were introduced
for measuring UV-B from lamps and determining the UV-B
relative to Model 5.0 meters. This created a whole new
demand in the tanning industry, and lately in the reptile
lighting industry. The latter industry is concerned
that lamps provide sufficient UV-B to stimulate the
production of vitamin D in pet reptiles. Again, an accident
of sorts.
Not wanting to give up on an accurate UV Index meter,
several attempts were made and several NIST traceable
calibrations were commissioned until, finally, an interference
filter coating was designed that successfully duplicated
the Diffey erythemal (sunburn) action spectrum response.
This led to the introduction of the Solartech Model
6.5 UVI (UV Index) in 1996 and later the Model 7.0 MED/hr.
The key to accuracy in this case was rejecting irradiance
above the erythemal action spectrum curve, another solar
blind challenge spanning several orders of magnitude.
Figure 3 shows the spectral response of these instruments
and the erythemal action spectrum curve.
(Figure 4 shows the excellent
long-term stability and correlation of a pair of Solartech
Model 6.5 radiometers used to measure sunlight at or
near solar noon for exactly six years at Geronimo Creek
Observatory in South-Central Texas. These
instruments were handheld. A plot of the results for
clear days only provides an even higher correlation.
Editor)
The only UV challenge left was to design a radiometer
that detects UV-C. I tried every commercially available
UV-C sensor and found they ALL detected irradiance well
above the UV-C range, including harmonics of 254 nm,
a standard UV-V wavelength because it is generated by
mercury-vapor lamps. This meant that all these instruments
gave false readings when used to measure sources emitting
UV-A, UV-B, and even visible and near-infrared wavelengths.
So, once again, a unique detector had to be created
that detects only UV-C and does not respond above about
262 nm. This unit, the Solartech Model 8.0, is becoming
popular for checking the output of germicidal lamps
used in Hong Kong to control of SARS and furnace air
purifiers in the United States. Unlike other UV-C radiometers,
it does not provide any reading (the output stays at
000) when illuminated by sunlight, flood lamps, and
any light source that does not emit UV-C wavelengths.
Once again, not wanting to stand still in the UV radiometer
field, a new Solartech instrument has been introduced
that monitors the wavelengths of UV-B that stimulate
the production of vitamin D3 in human skin. This is
the Model
6.4, and its digital output corresponds to the production
of vitamin D3 in International Units per minute of exposure.
This unit responds to the vitamin D effective irradiance
of natural sunlight and artificial UV sources. It helps
quantify the beneficial effects of UV-B in less-than-erythemal
(adjusted by skin type) dose levels. Full details and
specs can be seen here.
Some unexpected uses of the various
Solartech UV meters include window film transmission
testing, monitoring of the activation of dental teeth
whitening gel, and XP patient UV avoidance. In summary,
a newspaper on a beach has indirectly created a full
line of UV instrumentation "out of the blue"
so to speak! 
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