Seeing Is Believing, But You Can't See Micro-Organisms!


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.


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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 (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.


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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 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.

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.

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 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.


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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.


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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.






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