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.

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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.

If the electron is slow in returning to its normal energy state it can give off light for several seconds or even days. The slow release of light is called phosphorescence or after-glow. Note, all minerals that are phosphorescent will fluoresce, but not all minerals that are fluorescent will show phosphorescence.

Part of what makes fluorescing minerals so interesting is that the color that you see is emitted light rather than reflected light. That is, the light is generated at the mineral itself. Most of the colors that we see in our daily lives are reflected light, but the "glow" character of fluorescent minerals is different, and therefore fascinating.

What is ultraviolet (UV) light?

UV light is energy (actually electromagnetic energy) below the visible wavelengths [infrared is light energy above the visible wavelengths].

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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 different divisions.

What are UV-A, UV-B, and UV-C?

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.

More details:

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 divisions.

What are the primary wavelengths for ultraviolet lights?

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.

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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 ozone.

Sometimes I see Angstroms, nanometers, and microns to describe UV wavelengths, what is the difference?

One Angstroms () is 10-8 centimeters long (0.00000001), one nanometer (nm) is 10-7 centimeters long (0.0000001), and one micron () is 10-4 centimeters long (0.0001). The nm is now the standard unit used to measure wavelength.

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The nanometer (nm) is 10-7 cm and is the accepted unit to measure wavelength. The micron (), which is 10-4 cm, and the millimicron (m) [sometimes abbreviated m], which is 10-7 cm, are usually not used anymore. The Angstroms () 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 cm-1.

Where on the spectrum are the ultraviolet and infrared wavelengths compared to visible light? And what are some of those wavelengths?

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.

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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.

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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.

What are the primary ultraviolet light sources?

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.

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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.

Why can't I use an incandescent (regular, halogen or krypton filled) lamp to produce ultraviolet?

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.

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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.

Some novelty stores sell an incandescent LW "Blacklight" lamp with a filter coating over the outside of the glass envelope or as part of the glass envelope. Those lamps are very inefficient, get very hot, have a very short life, and produce too much visible light. There are not recommended for any fluorescent application.

What is the difference between how the screw-in lamps I use at home (incandescent) operate and fluorescent tubes works?

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.

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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.

The fluorescent lamp is composed of a hollow tube with a small amount of argon gas and mercury (Hg). On the inside of the tube, the walls are coated with a powdered phosphor that will fluoresce under 254 nm UV. When the current is flowing in the fluorescent lamp the Hg arc produces 254 nm UV, which in turn causes the phosphor to fluoresce. The color of the light is primarily determined by the fluorescent color of the phosphor. The invisible 254 nm UV is not transmitted by typical glass tubing so a fluorescent lamp makes a very safe and efficient light. For a fluorescent UV light the phosphor fluoresces in the UV instead of the visible light spectrum. For a fluorescent SW UV light there is no phosphor on the lamp and the bulb wall is made of a very special glass that will transmit the 254 nm wavelength.

Why do I need a ballast, and what is it actually?

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.

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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.

Some electromagnetic or solid-state ballasts also have step-up transformers in them. Some cold-cathode lamps (similar to neon signs) require a high-voltage to start the lamp and therefore have a high-voltage transformer as part of the light assembly. Those high-voltage transformers also limit the current in the lamp, so they act like ballasts. Sometimes those high-voltages transformers are just called transformers instead of ballasts (even if they function as ballasts).

A starter for a UV lamp allows the lamp to start. Most starters are a "glow plug" type with a neon gas and a thermal switch inside a glass capsule. When the voltage is first applied to the light fixture the neon gas in the capsule will conduct, causing the thermal switch to close and apply voltage to the filaments of the fluorescent type lamp. Then, because the circuit in the capsule is shorted, the gas stops conducting and the thermal switch cools and opens and the lamp then starts. Some older UV or fluorescent type lights have mechanical push buttons that do the same function.

Why is an ultraviolet filter required for a UV light?

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.

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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 visible-absorbing filters.

Special LW filters can be made so opaque that you cannot see any visible light coming through the filter when you look at the light. However, those more expensive LW filters are usually only used in special scientific UV lights. SW filters can never be made that opaque and some small amount of visible light can always be seen if you look at the UV light. Of course you should never look directly into a SW UV light with out protective goggles to block the SW UV from your eyes.

What filters are required for MW UV light assemblies?

SW UV filters are required for Medium Wave UV lights.

More details:

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%.

Note that a SW filter can be used for LW, MW, and SW since the filter transmits UV in the 230 to 400 nm band. A LW filter is much less expensive than a SW filter, but a LW filter does not transmit SW or MW and so ONLY works with LW lamps.

UV SYSTEMS has just completed some long term scientific tests that determine that MW UV light will not solarize SW filters. That means the SW filter in a MW UV light should last for a very long time if the filter is kept from moisture (which can harm the filter, see question #18).

Why do SW filters need to be replaced? And why do LW filters not need to be replaced?

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.

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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.

Although the SW filter absorbs visible light, a small amount of visible light does get through. This small amount is constant, neither increasing nor decreasing over the life of the filter. Therefore you can not look at a SW filter and tell if it is solarized or not. A UV radiometer or other specialized equipment is needed to determine the SW transmission of a filter.

Another process can affect SW filters negatively. They can absorb moisture from the air and form a chemical compound coating on their surface. This white film coating will block some of the SW UV. The coating can be easily scrubbed off with common household cleaners like "Comet". However, some glass technologists believe that once that white film coating has formed on the filter surface, then the inside of the glass has also been affected (thereby reducing the SW transmission). Therefore cleaning the coating off does not restore the transmission to previous levels. SW UV filters or UV lights with SW filters should always be stored in dry environments and especially away from high humidity air.

The LW UV light does not have the active wavelengths necessary to chemically change the transmission of LW filters. Therefore LW filters will not solarize and never have to be changed and they are not affected by moisture.

How long will a SW filter last? Is there such a thing as a "life time" SW filter?

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.

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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 anyone will.

To determine the expected life of a SW filter you have to decide on an arbitrary cut-off point. In other words, at what point do you consider that a SW filters needs to be replaced? A new filter usually has a 57 to 65% transmission and UV SYSTEMS considers that when a filter gets down to 25% transmission it should be replaced. [Therefore a filter with a 25% transmission would be only about 44 to 38% of its original transmission]. So the filter that needs to be replaced would be transmitting much less than half of the SW UV that it originally transmitted.

UV SYSTEMS has just completed scientific tests on the solarization rate of the SW, FS-60 filters used in the SW TripleBright II (models FSLS and 2FSLS) light with its LS-60-254 lamp. The results show that the FS-60 filters will last about 7,000 hours (to the 25% transmission point), providing they are in a dry environment. It is not clear why the SW filters will last so much longer in a TripleBright II then in a SuperBright II, it could be the higher ambient temperature of the TripleBright II, but that is just a unproven theory at this time.

What is the percent transmission of typical SW and LW filters?

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.

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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).

The typical poured or rolled LW filter for use with fluorescent type UV lamps has a peak transmission of about 79% at 365 nm. Those LW filters are used in the SuperBright II models 3352 or 3368 and the LW TripleBright II model FLL52 or FLL68. Other types of LW filters could have lower transmissions at 365 nm (not good), but they would block more of the visible light (good) that is transmitted, especially the red wavelengths. The transmission of the integral filters on the BLB lamps would usually have a higher transmission (and pass more of the visible light).

What is solarization?

Solarization is a chemical chance in glass that causes it to decreases its UV transmission when exposed to SW UV.

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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.

UV SYSTEMS has just completed scientific tests that determine that MW UV light will not solarize SW filters.

How does the wattage of the UV lamps compare to the UV output of the light?

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.

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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.

Another factor is the arc-power of a lamp. There is a very close relationship between arc-power and UV output. Arc-power is basically the current in the lamp's arc, and the longer the arc the more efficient the lamp. However, arc-power is usually difficult for the average used to measure without very specialized equipment.

Why is glass not recommended as my display case window?

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.

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Special UV absorbing plastic is recommended for windows in display cases. Plastic such as Cyro Industries, OP-2 or OP-3 is recommended for display windows.

All SW UV lights produce a small amount of LW UV, and of course all LW lights produce a large amount of LW UV. LW UV will pass through almost all glass windows. If the public is looking at a fluorescent mineral display in the dark soon they (especially kids) will notice that their shoe strings, paints, blouses, or other clothes will be fluorescing and now you have lost them. They are more interested in their clothes fluorescing than in minerals! OP-2 or OP-3 not only does not fluoresce like glass does, but it blocks all UV not just the harmful UV-C radiation. And by blocking the LW UV none of the public's clothes will fluoresce.

Other plastics made by other companies also have UV absorbing plastic; however, I have not tested them to determine if they are non-fluorescent in the dark. Cyro calls their OP-2 or OP-3 sheets Acrylite, they also make a scratch resistance version called GAR OP-2 or AR OP-3. All of the sheets can be purchased from plastic sheet companies and they will cut them to your size, usually at no extra cost. In the FAQ section, Spectral Data, Filter transmissions, is a transmission curve called "Typical Cyro Industries OP-3 window plastic".

Do I have to worry about LW UV exposure to my eyes or skin?

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.

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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.

How does a UV light work?

A lamp inside the light produces UV, and the reflector focuses the UV through the filter.

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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 output.

However, there are a lot of variations in UV lights. Some lights have more than one lamp per housing, and some have more than one drive current for the lamp. Some have fans to maintain the lamp(s) at an optimum operating temperature. Some do not have any cover or filter (usually for irradiation applications).

What are the primary applications for ultraviolet light?

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.

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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.

All of these make up the majority of UV applications, even though there are hundreds of other applications for the use of UV energy.

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.
Irradiation applications shine the UV on the object directly without any UV filter being used. The irradiation can be done in the dark or in daylight.

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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.

Irradiation applications do not require a UV filter to cure the ink, glue, coating, or disinfect air or water. High power lamps are usually required for disinfection uses. For irradiation, tubular fluorescent type lamps are usually used. They are either the low-pressure mercury (Hg) arc lamps (similar to typical germicidal fluorescent lamps); or special high current, low pressure Hg arc lamps with a bulb wall that has a high transmission. Often quartz is used in those high current lamps. An example is the custom-made UV SYSTEMS LS-60-254 lamp for the TripleBright II light. It is a high current lamp that is made with high transmission quartz instead of UV erythemal glass.

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.

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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 problem with the BLB lamps made by other lamp vendors is that the true fluorescent colors of some minerals are not seen, for example, if you are looking at an orange fluorescent mineral it would look pink to you. The reason is the blue light coming through the filter would reflect off of the specimen and mix with the orange fluorescence and you would see a pink color instead of the orange. That is not a problem with the Philips Lighting BLB lamps such as the LL-20-368BLB or the LL-15-368BLB. The Philips BLB lamps have much denser LW filters and therefore transmit less of the visible light. The Philips BLB lamps only come with the LW370 phosphor.

Why do LW UV fluorescent type lamps (also called tubes or bulbs) lose efficiency with use (have reduced UV output)?

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.

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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.

LW phosphor lamps used in most hand-held UV lights (4W, 6W, and 8W) have been given "life" rating by the lighting industry. This rating is called "average rated life" and is about 6,000 hours for those 4W, 6W and 8W lamps. Note the lighting industry "life rating" is based on burning the lamp for 3 hours "on" and 30 min. "off". The actual life of most UV lamps depends on how many times it is turned "on" and "off". The more "on/off" cycles the shorter the life of the lamp. However, most people turn their lamp "off" in much less than 3 hours, and so the actual life could be less than 1/2 the standard life rating.

When white fluorescent lamps were first introduced commercially in about 1935 the average life was about 5,000 hours and the lumen depreciation was greater than about 60% at about 2,500 hours. Now the typical white 4 ft. fluorescent lamp has an average life greater than 24,000 hours and a lumen depreciation of only 4% at about 9,600 hours. While the lamp and phosphor scientists have made great strides in improving the lighting industry standard 4 ft. white fluorescent lamp very little progress have been made for the two LW Blacklight phosphors (one with a peak at 352 nm, called LW350, and the other with a peak at 368 nm, called LW370). Of the approximately 456 types of fluorescent lamps made all of the UV or Blacklight lamps make up less than 1% of the total lighting industries lamp production. Therefore there is not much economic incentive to do research to develop low UV depreciation LW phosphors and lamps. The typical UV depreciation of a 4W, 6W, or 8W LW lamp might be as much as 40% to 50% in about 2,400 hours.

UV SYSTEMS has just completed a scientific UV depreciation test using one TripleBright II LW LL-60-352 lamp. From this test it was determined that the UV output of that lamp depreciated to about 80% of initial output (a 20% reduction) after about 7,000 hours of use. While this was only a sample of one, it could be typical of all LL-60-352 lamps.

When should LW UV fluorescent type lamps be replaced if they are not burned out?

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).

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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 "poisoning".

Unless you have access to a UV radiometer or integrating sphere and can measure the UV output of your LW lamps it is hard to tell when they have depreciated significantly. As a rule of thumb I suggest to museums with LW displays with heavy usage that they replace their LW lamps after an estimated 7,000 to 8,000 hours of use.

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.

More details:

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.

The 185 nm mercury (Hg) arc emission line produces a lot of ozone gas since it is very efficient in turning most of the oxygen near the lamp into ozone. Fortunately the erythemal glass (which is in most germicidal lamps) does not transmit that 185 nm Hg emission line. Most quartz SW lamps have an additive added to the quartz when they are making the tubing that absorbs that 185 nm Hg arc emission line. That quartz is called "ozone free" (even if the 253.7 nm line produces a small amount of ozone). The UV SYSTEMS LS-16X and the LS-60-254 lamps are made from that "ozone free" quartz, while the LS-60-185 lamp is designed to produce ozone and it transmits the 185 nm emission line. The LS-60-185 lamp is used in applications where either the ozone itself or the 185 nm emission line is needed.

What is required for UV to kill microorganisms?

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.

More details:

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.

What wavelength is used to cure ultraviolet adhesives, glues, coatings, or inks?

LW UV is used to cure UV adhesives. Usually LW370 is the most efficient wavelength.

More details:

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.

What wavelengths are used in non-destructive testing?

Usually LW365 or LW370 are the wavelengths most often used.

More details:

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.

A similar non-destructive technique is used to find excess solder paste on a printed circuit board (PCB) or other electronic parts. Excess solder paste could lead to corrosion on a PCB or potentially cause electrical shorts. There are many other applications of using UV-A for non-destructive testing.

What wavelengths are used in forensic science applications?

All UV wavelengths are used in forensic science. And both fluorescent and irradiation applications are used.

More details:

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.