What actually causes fluorescence?
Some minerals fluoresce (give off light) when exposed to invisible
ultraviolet light. This phenomenon is called fluorescence. Some other
objects such as paints, clothes, dentures, urine, scorpions, sagebrush,
and greases will also fluoresce. However, for this discussion, we will
only refer to fluorescent minerals, but the principal of why they or any
other objects fluoresce is the same. When the electrons in fluorescent
minerals absorb ultraviolet light energy those electrons go to another
energy state, and then when they return to their normal energy state
they give off energy in the form of visible light (glow); for example,
the green light (color) that we see from fluorescent willemite and red
light from calcite when exposed to SW UV.
The primary causes of fluorescence have to do with the atomic structure
of the fluorescent mineral or other fluorescent object. You know that
all matter is composed of atoms; while atoms are composed of protons and
neutrons in the nucleus, or center, and electrons on the outside. The
electrons outside the nucleus are in different energy states; that is
they can be thought of as being in different orbits around the nucleus
(even though not technically true, that illustration is a good one).
Thinking of it that way, the greater the orbit around the nucleus, the
less energy (or energy state) that electron has. When fluorescent
minerals are exposed to ultraviolet (UV) light either short wave (SW),
medium wave (MW), long wave (LW), the electrons move to a different
energy state for only a fraction of a second (about 1 millionth of a
second) and when they return to their normal energy state they give off
energy in the form of light. That colored light is what we see when a
mineral (or object) is fluorescing.
UV light is energy (actually electromagnetic energy) below the visible wavelengths [infrared is light energy above the visible wavelengths].
UV light is electromagnetic energy that is from about 400 nm to about
160 nm or so [technically it is 380 nm to the where the Vacuum UV starts
at about 100 nm]. It is usually divided into three divisions, (1) UV-A
from 400 nm to 315 nm, (2) UV-B from 315 nm to 280 nm, and (3) UV-C from
280 nm to about 160 nm. Some European scientific agencies have slightly
UV-A, UV-B, and UV-C are simply different wavelengths of UV energy. UV-A is more commonly known as Long Wave (LW) or Blacklight, UV-B can be called Midwave (MW) or Medium Wave, and UV-C refers to called Short Wave (SW) or Germicidal.
Just as visible light is divided into different colors (wavelengths):
red, orange, yellow, etc. so is UV energy. The UV-A wavelengths are from
400 nm to 315 nm, UV-B is from 315 nm to 280 nm, and UV-C is from 280 nm
to about 160 nm. Some European agencies have slightly different
There are five primary wavelengths in use. They are (1) 368 nm from a fluorescent type UV LW light, also called LW370 (and spoken as "long wave three seventy"); (2) 365 nm from a high pressure mercury vapor LW UV light, also called LW365; (3) 351 nm from a fluorescent type UV lamp, also called LW350; (4) 312 nm from a fluorescent type MW UV light; and (5) 254 nm from a SW UV light.
There are three LW UV wavelengths in use: (a) about 368 nm from a
fluorescent type (BL or BLB) type UV light [also called LW370], (b) 351
nm (also listed as 352 nm or 350 nm) from a LW BL or BLB type UV light
[also called LW350], and (c) 365 nm from a high-pressure mercury (Hg)
arc light (also called LW365). MW UV wavelengths are 312 nm and 306 nm
and each of these is from a different type of fluorescent light. The
only SW UV wavelength is 253.7 nm and is from a fluorescent type light
without any phosphor in the lamp. There is one exception: some SW UV
lamps are made to transmit the 185 nm Hg line, which is used to produce
One Angstroms (Å) is 10-8 centimeters long (0.00000001), one nanometer (nm) is 10-7 centimeters long (0.0000001), and one micron (µ) is 10-4 centimeters long (0.0001). The nm is now the standard unit used to measure wavelength.
The nanometer (nm) is 10-7 cm and is the accepted unit to measure
wavelength. The micron (µ), which is 10-4 cm, and the millimicron (mµ)
[sometimes abbreviated µm], which is 10-7 cm, are usually not used
anymore. The Angstroms (Å) is 10-8 cm and for the most part is not used
anymore. However, "Angstrom" is an old unit not used by the scientific
community although still used in some medical or biotechnology areas.
Wavenumber is another unit used in the biotechnology area, it is equal
to the inverse of the wavelength in nm times 108 and the units are in
Infrared (IR) are wavelengths longer than visible light and are longer (larger) than 750 nm. Ultraviolet (IR) are wavelengths shorter than visible light and are shorter (smaller) than 400 nm. Visible light is usually defined as between 750 nm and 400 nm. 555 nm is visible green light, 850 nm is invisible IR light, while 368 nm is invisible LW UV light.
Infrared (IR) are wavelengths from 750 nm (technically from 770 nm) to
about 1,000,000 nm. Ultraviolet (UV) is from 400 nm (technically 380 nm)
to about 100 nm. While visible is in-between at 380 to 770 nm. Some of
the visible light wavelengths are 650 nm, which is red; 580 nm, which is
yellow; 555 nm, which is green (and the wavelength that our eyes are
most sensitive to); and 440 nm, which is blue. Some of the UV
wavelengths are 368 nm, which is LW370; 351 nm, which is also LW350; 312
nm, which is MW; and 254 nm, which is SW.
[What is a UV lamp or light?] What is the difference between an ultraviolet lamp, bulb, and light?
A lamp is often called a bulb or tube; it is the part inside a UV light that generates the UV. The bulb is the glass or quartz wall of the lamp. A light is the complete light assembly.
I use the engineering term, "lamp," while some people call them bulbs or
tubes. But the bulb is just the glass or quartz wall of the lamp or
tube. A light is the complete light fixture with the lamp and UV filters
(if used). Sometimes people use the term "lamp" to mean the bulb and in
the same sentence they use lamp to mean the complete light assembly.
This can cause confusion therefore, except for only a few locations on
this web site, I call a lamp the part that you need to replace if the
light stops working. And I call a light the complete light assembly. A
lamp is NOT a UV light fixture.
For most fluorescent applications, the tube type fluorescent lamps (bulbs), and the high-pressure mercury arc lamps are used. For irradiation applications beside the above, high current low-pressure mercury arc tube type lamps are also used.
Most fluorescent applications use the fluorescent-type lamps: long wave
(LW) with two different lamps with peaks at 368 nm or 352 nm, medium
wave (MW) with two or more lamps with peaks at either 312 nm or 306 nm,
or short wave (SW) which peaks at 254 nm. Usually those lamps come in
sizes from 6 in. long at 4 W to 48 in. long at 40 W. Custom made "U"
shaped lamps like the UV SYSTEMS LL-16-351 and LL-16-368 for LW and
LS-16X for SW are also used. Also, for LW, screw-in high-pressure Hg arc
lamps are sometimes used, these lamps are usually rated at 100 W or 150
W or more.
The spectrum of a incandescent lamp has very little UV to begin with and the UV filter that you would need transmits some of the red light that is generate by the incandescent lamp and therefore the output would be red light with almost no UV.
An incandescent spectrum starts at about 320 nm (in the UV-A) and rises
until it reaches a peak at about 850 nm (in the IR). The only UV is from
about 320 to 400 nm, while most of the lamp energy is actually in the
deep red (about 600 nm) to the IR. Therefore, there is very little UV to
start with. Then for a fluorescent application you would need a LW
filter over the lamp to filter out as much of the visible light as you
could but still transmit most of the small amount of UV. All LW UV
filters (and SW filters) transmit a significant amount of red light
(from about 650 to 750 nm), and at those wavelengths, the incandescent
lamp produces the most energy. And so the net result would be red light
coming through the LW filter. The red transmission of the UV filters is
normally not significant because the SW and LW fluorescent type lamps do
not produce any red wavelengths.
Incandescent lamps product light because of a bright incandescent filament. Fluorescent lamps produce light because the arc inside the tube produces UV which causes the phosphor on the inside of the tube to fluoresce.
The tungsten filament of an incandescent lamp is heated up by electric
current the heat causes it to glow or incandesce, similar to the way
that coals in a camp fire glow.
A fluorescent UV lamp (bulb) needs an electrical device to control the current in the arc inside the lamp. That device is called a ballast. Without ballast the lamp would take too much current, and the wires would melt, or the lamp would fail, or something else would fail. Some lamps also require a starter so the lamp can start.
A ballast is an electrical device that is designed to limit the amount
of current inside any arc lamp (low pressure or high pressure). Since an
electrical arc is a negative resistance phenomena, without a ballast
wired in series with the fluorescent lamp the arc would draw so much
current that the lamp would fail in seconds! The ballast limits the
current to the lamp so it would operate correctly. There is no such
thing as a UV lamp without a ballast; it is always a lamp and ballast
combination. A lamp will not work without a ballast. A ballast can be an
electromagnetic device, or a solid-state electronic device.
All UV lamps (bulbs) produce visible light in addition to the UV that we want. The visible light from the lamp will wash out the fluorescence if a filter is not used.
The UV lamp will produce significant amounts of visible light, usually a
blue color (depending on the lamp). That visible light is usually more
intense than the faint fluorescence and will wash out or dilute the
fluorescence. The UV filter [SW, MW, or LW] will absorb most of the
visible light generated by the lamp and will transmit primarily the
invisible UV light so you can see the fluorescence of the object you are
looking at. That is partly why we usually look at fluorescent minerals
in the dark, so the ambient lighting does not wash out the fluorescence.
These filters are technically called ultraviolet-transmitting
SW UV filters are required for Medium Wave UV lights.
Because SW UV filters transmit from about 230 to 400 nm they are used
for MW UV lights. The UV transmission of new Hoya Optics U-325C filters
at 312 nm in the MW is about 84%.
The SW filters wear out and transmit less and less UV with exposure time. That wear out phenomena is called solarization. LW filters do not solarize.
With exposure to SW UV, the SW filters undergo a chemical process that
decreases their SW UV transmission. This chemical process is called
solarization, and is the greatest at the beginning of its use. As the
filter gets more and more exposure to SW UV, the rate of decrease (rate
of solarization) decreases. Solarization never stops but after about 100
hours of exposure, the rate of further solarization slows.
The number of hours of use you can get from a SW filter depend on many factors, and so no one answer will apply to all situations. Contrary to what one manufacturer claims however, there is no such thing as a "life time" SW filter. All will solarize, (meaning decrease in SW transmission) with use.
The rate of solarization of a SW filter, will vary depending on several
factors. First is the new filter itself. Presently one manufacturer
(Hoya Optics) makes their U-325C SW filter that has a superior
solarization rate compared to the other two manufacturers (Schott Glass
Technologies, and Kopp Glass). Other factors are: intensity of the SW UV
that it is used with, the duration of exposure to the UV, the amount of
moisture or humidity that the filter is exposed to, and other lesser
factors such as the temperature of the filter. No one has been able to
make a SW filter that will not solarize, and it is not expected that
A brand new non-polished SW filter such as the Hoya Optics U-325C filter will have a SW transmission of about 57% to 65% at 254 nm. A typical non-polished LW filter will have a transmission of about 79% at 365 nm.
A brand new molded or poured SW filter such as the U-325C made by Hoya
Optics that is 5 mm thick will have a transmission of about 57.5 to 65%
at 253.7 nm. If that same filter is polished thinner then the
transmission would be higher. The reason for the variation [57.5 to 65%]
is because the transmission curve is very steep at 253.7 nm and
therefore there can be slight differences between one filter batch and
another. That same Hoya filter has almost a flat 84% transmission from
about 290 nm to about 345 nm, and therefore it works very well with MW
lamps (that produce UV with a peak at 306 to 312 nm).
Solarization is a chemical chance in glass that causes it to decreases its UV transmission when exposed to SW UV.
SW UV filters go through a chemical change that decreases their ability
to the transmit SW UV energy. This decrease is called solarization, and
is primarily a function of the amount of SW UV that the filter is
exposed to. The longer the exposure time or the higher the SW UV
intensity (or both) the more the solarization. In most germicidal SW UV
lamps made with erythemal glass solarization also affects the glass of
the bulb wall. Quartz lamps like those made for the SuperBright II model
3254 (LS-16X) or the SW TripleBright II (LS-60-254) have the least
amount of solarization, much less than the erythemal glass. Also the
quartz lamp solarization is much less that what occurs in SW UV filters.
Wattage of a UV lamp is only one factor in the UV output of a light, and therefore cannot be used as a measure of how powerful a light is.
The electrical watts powering a UV light or lamp does not indicate the
UV output. For example, the erythemal glass used by other manufacturers
in the lamps in their SW UV lights transmits less than 80% of the UV
generated. But a quartz lamp such as the UV SYSTEMS LS-16X, which is
used in the SuperBright II model 3254, transmits more than 90% of the
253.7 nm UV wavelength. If two lamps were made physically identical,
with one made from quartz and one with the more common erythemal glass,
and if the electrical watts used by both lamps were the same, the quartz
lamp would produce more SW UV (because of higher transmission). Also the
ballast (driving circuit) affects the efficacy of a lamp. The LS-16X in
the SuperBright II model 3254 is driven by a 23 KHz inverter-ballast
which is more efficient than the typical 60 Hz household powered
ballasts that other manufacturers use.
Glass can be used, but it is not recommended beside it will fluorescent under SW, one side will fluoresce much brighter than the other side. Glass will block the harmful SW UV, but it also transmits all LW UV.
Special UV absorbing plastic is recommended for windows in display
cases. Plastic such as Cyro Industries, OP-2 or OP-3 is recommended for
No. LW UV (any wavelength) is not harmful to your eyes or skin. LW will cause the lens of your eyes to fluoresce during exposure which would interfere with viewing of the fluorescent minerals, but that is not harmful to your eyes. The GB Goggles will block the LW to keep your eyes from fluorescing.
The LW350, LW365, or LW370 wavelengths are classified as Risk Group I
per the ANSI/IESNA RP-27.3-96 (1997 Recommended Practice for
Photobiological Safety for Lamps and Lamp Systems: General
Requirements). This category is referred to as "low risk" where "the
lamp does not pose any photobiological hazard due to normal behavioral
limitations on exposure." LW lamps are safe.
A lamp inside the light produces UV, and the reflector focuses the UV through the filter.
Most UV lights are made up of housing, reflector, electrical ballast,
lamp socket, and cover with an attached filter. The UV lamp is inside
the housing, and its output is controlled by the ballast and the
reflector. A good design directs the most UV thought the UV filter while
still maintaining the optimum lamp bulb wall temperature for maximum UV
The majority of applications for UV light can be listed in two categories: (1) fluorescent uses and (2) irradiation. Fluorescent applications include displaying fluorescent minerals using UV lights like the ones sold here, theatrical, and disco lighting. Other fluorescent uses are in forensic science, biotechnology, non-destructive testing, identifying sagebrush, medical diagnostics like finding "ringworm", and even for finding scorpions. Irradiation application include curing substances (inks, glues, coatings), cross linking polymers in chemistry, disinfecting air or water, and killing microorganisms.
Fluorescent applications. Both private collectors and museums use
ultraviolet lights, such as those shown here, to display the beauty of
fluorescent minerals. Most use SW as vs. LW350 and LW370, but some also
use MW. Most other fluorescent applications use only LW UV. These are
for special effects in theatrical shows or discos, for signs, or for
non-destructive testing. In biotechnology UV is used to visualize DNA
that has been stained with ethidium bromide, or to see cells that have
absorbed special fluorescent stains. In biochemistry TLC plates with DNA
or RNA will appear blue under UV light. Irradiation applications.
Irradiation applications include curing inks, coatings, glues and
adhesives, and for water and air disinfections. For irradiation
applications involving curing glues, coatings and inks usually only
LW365 or LW370 in the UV-A range are used. For water or air
disinfections only SW (UV-C) at 253.7 nm is used.
What is the difference in how fluorescent applications are done and how irradiation applications are done?
Fluorescent applications are almost always done in the dark or in very
low ambient lighting conditions. Also the UV light must have a UV filter
(usually looking black in daylight) over the lamp so that only the
invisible UV comes through the filter on to the object being fluoresced.
Fluorescent applications usually require that the object be in a dark
environment or dark room. The exception might be the use of the
TripleBright II display light with bright fluorescent minerals. The UV
light should have a visible-absorbing ultraviolet-transmitting filter
(UV filter) over the lamp so that the visible light generated by the
lamp will be absorbed and only the invisible UV will get through the
filter. Without a UV filter the visible light generated by the lamp
would override (or wash out) the fluorescence emitted by the object.
Should BLB lamps be used for LW fluorescent mineral displays?
For years I have advised against using the BLB (Blacklight Blue) lamps for fluorescent mineral displays because the integral filter in the bulb wall let through too much visible light. But now Philips Lighting is making their BLB lamps with a much denser filter glass; which produces less visible light. These BLB lamps can be used for LW fluorescent mineral displays.
The Philips Lights LW BLB lamps transmit much less visible light and
therefore can be used for LW fluorescent mineral displays. This is not
true for lamps from the other vendors such as General Electric, Sylvania
- Osram, Sankyo Denki, or other lamp manufacturers. Only BLB lamps from
Philips Lighting are acceptable for LW fluorescent mineral displays.
The lamp phosphors deteriorate with use. All the exact reasons are not known, but one of the reasons is that the mercury vapor in the lamp penetrates the phosphor with use, which reduces the efficiency of the phosphor to produce LW UV.
There are no LW, MW or even white phosphor fluorescent lamps that are
immune to lumen depreciation (reduction in efficiency with use). [For UV
lamps it is called UV depreciation]. The Illuminating Engineering
Society of North America in their 1981 IES Lighting Handbook, Reference
Volume, says, "The lumen output of fluorescent lamps decreases with
accumulated burning time. Although the exact nature of the change in the
phosphor which causes the phenomenon is not fully understood, it is
known that at least during the first 4000 hours of operation the
reduction in efficacy is related to arc-power to phosphor-area ratios."
What that means is the harder the lamp is driven (the more current
through the lamp) the greater the lumen or UV depreciation. The 4W, 6W
and 8W lamps used in most hand-held UV lights are not driven hard
compared to the lighting industry standard 4 ft. fluorescent lamp. But
those 4, 6, and 8 W lamps still have UV depreciation.
No specific answer applies to every situation, but maybe the best suggestion is to replace the lamps when your fluorescent minerals (or what ever your application is) appear significantly less bright than earlier. With "normal use" a rule of thumb might be to replace them at least every two to three years (or maybe every 9 to 12 months if the lamps are used several hours per day).
When I was working at Boeing Commercial Airplane Group in the Flat Panel
Display Group, we learned from a lamp manufacturer that they had
determined empirically that mercury (Hg) was one of the culprits in
reducing the lumen output of the phosphor. Apparently the Hg works its
way into the phosphor to effectively "poison" the phosphor with use.
There now is a UV transparent coating that can be applied over the
phosphor to protect the phosphor some. While the coating is not 100%
effective, it reduces the UV depreciation in LW phosphors by maybe 25%
to 35%. However, it requires another step or two in the manufacturing
process so very few commercial lamp manufacturers use this protective
coating -it is just not economical. Without the coating the lamps have
to be replaced more often. Custom-made lamps like the LL-16-352 and
LL-16-368 lamps that are used in the UV SYSTEMS SuperBright II models
3352 or 3368 are coated with the special coating to reduce the Hg
What is that odor I smell when I turn on my SW UV light?
All SW UV lamps produce a small amount of ozone gas which is what you smell.
The SW 253.7 nm UV energy turns some of the oxygen molecules (O2) to
ozone (O3). The ozone is very unstable and two ozone molecules quickly
turn into three oxygen molecules.
A minimum of four things are involved; the right wavelength (SW UV), the type of microorganism, the intensity of the UV, and the SW exposure duration to the microorganism.
SW UV at 253.7 nm is also called the germicidal wavelength and it is the
wavelength that will kill most microorganisms. However, some
microorganisms are more resistant to UV than others. For example most
molds are more resistant to UV than bacteria. Some microorganisms
require a higher intensity or longer exposure time for the same kill
rate. Temperature and humidity can also affect the kill rate. Generally
the longer the exposure time or the higher the UV intensity (or both)
the higher the kill rate. Usually the kill rate is expressed as a
percentage of microorganisms killed, a 99% kill rate is usually the
highest rate listed, and an 80% or 90% kill rate is often more commonly
used. Note that only microorganisms that have direct exposure to the SW
UV will be killed. For exact kill rates for a specific microorganism, a
bacteriologist should be consulted.
LW UV is used to cure UV adhesives. Usually LW370 is the most efficient wavelength.
Usually LW370 (with a peak at about 368 nm) is the most efficient
wavelength to cure UV adhesives, glues, coatings, cements or inks.
However, in some cases both LW350 (with a peak at about 352 nm) and
LW370 were equally effective in curing a specific brand of UV adhesive.
Since UV curing is an irradiation process the UV lights used do not need
covers or UV filters.
Usually LW365 or LW370 are the wavelengths most often used.
Non-destructive testing is a fluorescent application where a fluorescent
dye is added to some solution or liquid. Parts (often metal castings)
are dipped in the solution and then removed and exposed to the UV light.
The dye will get in microscopic cracks in the casting and when exposed
to LW365, LW370 or LW350 will fluoresce brightly in the dark.
Non-destructive testing is used to find if a part has any potential
cracks that could lead to failure of that part.
All UV wavelengths are used in forensic science. And both fluorescent and irradiation applications are used.
All UV wavelengths are used for fluorescent applications in forensic science. For irradiation applications, usually LW350, LW370, or MW wavelengths will be used for UV photography applications; however, there might be some applications for SW UV photography.