posted Oct 14, 2008, 7:30 PM by Rock Mineral   [ updated Oct 16, 2008, 9:21 AM ]

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.

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.

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%.

** more information http://www.hoyaoptics.com/color_filter/uv_transmitting.htm

** Corning Glass Works (now Kopp) with their #9863 filter.

** Hoya Optics with their U-325C filter

** Raytech Industries with their Color Blaze filter.

** Schott Glass Technology Inc. with their UG 5 filter.

** UVP Inc. with their UV G filter.

Hoya Optics Inc. showed the overall lowest rate of solarization in the experiment. The average of the six U-325C filters also had the highest short-wave ultraviolet transmission at 253.7 nanometers during the entire duration of the test.

Search uv visible absorbing filters

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

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.
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.

More details:

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.

More details:

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.

More details:

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.

More details:

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.

More Details:

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.

More Details:

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.

More details:

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.

More details:

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.

More details:

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.

More details:

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.

More details:

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).

More details:

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.

BL in the tube designation (e.g., F40T12BL) means "blacklight", which is a fluorescent lamp with a phosphor that emits the longest largely invisible UV wavelengths that are both efficiently and fairly cheaply possible. This phosphor seems to emit a band of UV mainly from 350 to 370 nanometers, in the UV-A range.

BLB means "blacklight-blue", which differs from "blacklight" only in that the glass tube of this lamp is darkly tinted with something with a dark violet-blue color to absorb most visible light. Most UV gets through this, along with much of the dimly visible deep-violet 404.7 nanometer line of mercury. Most of the violetish-blue 435.8 nanometer line is absorbed, but enough of this wavelength gets through to largely dominate the color of the visible light from this lamp. Longer visible light wavelengths do not significantly penetrate the BLB's very deep violet-blue glass, which is known as 'Wood's glass'. The UV is the same as that of the BL lamp, being mostly between 350 and 370 nanometers.

There is a 350BL blacklight lamp, using a different phosphor that emits a band of slightly shorter UV wavelengths in the UV-A range. The reasoning for this lamp is that it is supposedly optimized for attracting insects. These lamps are one variety of UV lamps used in electric bug killers.

There are other UV fluorescent lamps. There are at least two different UV/deep violet emitting fluorescent lamps used mainly in the graphic arts industry, emitting mainly wavelengths between 360 and 420 nanometers. Possibly one of these is also used in bug killers. I have noticed one kind of UV fluorescent lamp for bug killers with a broadish band phosphor with significant output from the 360 nanometer range (maybe also shorter) into visible wavelengths around 410-420 nanometers or so.

There is an even shorter UV-A lamp used for suntanning purposes. I would guess the phosphor emits mainly within the 315 to 345 nanometer range. One brand of such lamps is "Uvalux".

There is even a UV-B emitting fluorescent lamp. Its phosphor emits mostly at UV-B wavelengths (286 to 315 nanometers). It is used mainly for special medicinal purposes. Exposing skin to UV-B causes erythema, which is to some extent a burn reaction of the skin to a slightly destructive irritant. Use of UV-B largely limits this to outer layers of the skin (perhaps mainly the epidermis) and to parts of the body where skin is thinner. UV-A wavelengths just over 315 nanometers can also cause sunburn, but they are more penetrating and can affect the dermis. Please note that the deadliest varieties of skin cancer usually originate in the epidermis and are usually most easily caused by UV-B rays.

There are clear UV-emitting lamps made of a special glass that lets through the main shortwave UV (UV-C) mercury radiation at 253.7 nanometers. These lamps are marketed as germicidal lamps, and ones in standard fluorescent lamp sizes have part numbers that start with G instead of F. These lamps will work in standard fluorescent lamp fixtures.

Cold-cathode germicidal lamps are also in use; these somewhat resemble "neon" tubing.

Be warned that the shortwave UV emitted by germicidal lamps is intended to be dangerous to living cells and is hazardous, especially to the conjunctiva of eyes. Signs of injury by the UV are often delayed, often first becoming apparent several minutes after exposure and peaking out a half hour to several hours afterwards.

Please note that non-fluorescent (high pressure mercury vapor discharge) sunlamps generally emit more UV-B rays rather than the tanning-range UV-A rays. These lamps do have substantial UV-A output, but mainly at a small cluster of wavelengths around 365 nanometers. Tanning is most effectively accomplished by wavelengths in the 315-345 nanometer range. In addition, no UV suntanning is completely safe.

It differs from light only that it's wavelengths are too short to be seen by the human eye. UV-A , or long-wave radiation is 315 nanometers and above. UV-B, or medium-range radiation, is 280 nanometers to 315 nanometers. UV-C, or short wave radiation, is 280 nanometers and below.

The phenomenon known as fluorescence occurs at the subatomic level by a process called electron excitation. Electrons are subatomic particles that orbit the nucleus of an atom at specific distances known as electron shells. These shells are arranged in layers around the nucleus, the exact number of electrons and their shells depending on the type of atom (element). The electrons contained in the shells nearest the nucleus carry less energy than the electrons in the outer shells.

When certain atoms are exposed to ultraviolet (UV) light, a photon (particle of light energy) of UV will cause an electron residing in a lower-energy inner electron shell to be temporarily boosted to a higher-energy outer shell. In this condition, the electron is said to be excited. It will then drop back to its original inner electron shell, releasing its extra energy in the form of a photon of visible light. This visible light is the fluorescent color that our eyes perceive. The exact color depends on the wavelength of the visible light emitted, with the wavelength itself being dependent on the type of atom undergoing the electron excitation.

The specific atoms which undergo the fluorescence are known as activators. They are usually present as impurities in the normal molecular structure of the mineral, but sometimes are an intrinsic part of the mineral's composition. In fluorescent minerals, very often the activators are cations, which are atoms or molecules which carry a net positive charge (due to the loss of one or more electrons, each of which display a negative charge). For example, the activator which causes the bright red fluorescence of calcite is the manganese cation, Mn+2. The "Mn" is the chemical symbol for the element manganese, and the "+2" indicates a manganese atom which has lost two electrons and therefore has a net positive charge. A cation which has lost two electrons is also referred to as divalent; three electrons, trivalent; four, quadrivalent, etc. Activators can also sometimes be anions (containing a net negative charge).

Ozone is generated naturally by short-wave solar ultraviolet radiation, and appears in our upper atmosphere (ozonosphere) in the form of a gas. Ozone also may be produced naturally by passing an electrical discharge - such as lightning - through oxygen molecules. Lightning is a perfect example of making an abundance of O3 to purify the earth's atmosphere Nature's way. Most of us have noticed the clean, fresh smell in the outdoor air after a thunderstorm, or the way clothing smells after it's been dried outside on a clothesline in the sun.

Oxygen, as we know, has two atoms. High voltage, as from lightning, breaks these two atoms apart. Quickly, these atoms hop back together in threes {O3}. Confused, these atoms do not like this arrangement and want desperately to dissolve this uncomfortable trio. So as this O3 molecule floats in the air, when one of the atoms spots a contaminant molecule to attach itself to, it breaks away from the other two atoms. To its surprise, this attachment is actually an attack on the contaminant and creates a microscopic explosion. Both the contaminant and the atom are destroyed. This leaves the other two atoms behind as pure oxygen {O2} without the presence of the contaminant. The explosion changes the contaminant into carbon dioxide and hydrogen, which we can breathe.

Should the O3 molecule not find a contaminant in its environment, it will attack itself to change its configuration of O3 back to O2 (normal oxygen) in 20 to 30 minutes at room temperature and normal humidity.

Short-wave solar ultraviolet radiation - ultraviolet light - is another method used by many air purifier manufacturers. When ultraviolet light rays collide with a contaminant such as carbon monoxide (CO) and nitrogen oxides (NO2 and N2O) in the presence of oxygen (O2), ozone is produced.

Ozone reacts with and oxidizes pollutants it encounters, rendering them harmless, while also removing odors. O3 loses one of its oxygen molecules in this oxidation process, causing it to revert back to oxygen, leaving behind pure, fresh air. Ozone can be effective against chemical sources, bacteria, mold, odors, etc. Once a pollutant is oxidized by ozone, it is no longer toxic, allergenic, or odor causing. As a result, even if an oxidized contaminant remains in the air and is inhaled, it has no negative effect. Microorganisms (such as mold spores or bacteria) that have been exposed to ozone are no longer able to reproduce, which causes their numbers to quickly diminish.