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Germicidal Ultraviolet Radiation

This page defines germicidal UV radiation, describes the technology used to produce it, lists companies that make reliable sources of it, shows a simple test to detect its presence, and provides a list of ways to spot a fake UVC lamp.

Ultraviolet Radiation Spectrum

Ultraviolet radiation is broken down into three wavelength ranges called UVA, UVB, and UVC. The ranges in terms of nanometers are:

The UVA range is the only one that contains visible light. The shortest wavelength visible to the human eye is around 380 nm. It has a deep violet color. Radiation in the UVA range is relatively safe. It causes no direct biological damage.

The UVB range is invisible to the human eye and is capable of causing biological damage. In humans it can cause sun burns, skin aging and DNA damage. It can also have germicidal effects. Some microorganisms can be deactivated by exposure to UVB. Most UVB radiation from the sun is blocked by the ozone layer of the upper atmosphere.

The UVC range is invisible to the human eye and is the most strongly germicidal. It can produce DNA damage in most microorganisms. The strongest germicidal effects are seen at wavelengths between 260 and 265 nanometers. Radiation at wavelengths below 200nm will produce ozone which is also germicidal.

Generating Germicidal UV

The most efficient and cost effective way to generate germicidal UVC radiation is with a low pressure mercury vapor lamp. These lamps are very similar to fluorescent lamps. Both generate light by creating an electric arc, a flow of electrons, through a gas of mostly argon with a small amount of mercury. The energy of the electrons excites the mercury atoms which then emit light mostly in the ultraviolet range. With a fluorescent lamp the UVC is converted into visible light by a white phosphor coating on the inside of the lamp. The tube of a fluorescent lamp is also made of regular glass which blocks UVC radiation.

The tube of a UVC lamp is made of fused quartz. It has no phosphor coating and is clear. There two types of UVC lamps, those that produce ozone and those that do not. The non-ozone producing lamps are usually made with specially doped fused quartz that blocks the ozone producing wavelength of mercury at 184.95 nm. You generally cannot tell the difference between an ozone and non-ozone producing lamp just by looking at them. Ozone however does have a strong smell and can cause lung, throat and sinus irritation so you can tell them apart that way. Note that a non-ozone producing lamp does produce very small amounts of ozone but usually not enough to cause irritation.

The picture below shows a regular fluorescent lamp on the top and a germicidal UVC lamp on the bottom.

Regular fluorescent lamp (top) and a germicidal UVC lamp (bottom)

There is also something called a fluorescent black light which has a dark blue fluorescent coating. These lamps emit light in the UVA range and have little or no germicidal effect. A true UVC lamp will have a clear tube where you can easily see the heating filaments at the ends of the tube.

The major UVC lamp manufacturers are:

The second most common way to generate germicidal UVC is with light emitting diodes or LED's. The major drawback of UVC LED's is their inefficiency and cost. These LED's consume a lot of power, require large heatsinks, don't last as long as UVC lamps and cost more per UVC power output than lamps. On the plus side you can get LED's with wavelengths in the ideal germicidal range of 260 to 265 nanometers. The fact that a UVC lamp has most of its output at 253.7 nanometers which is slightly outside this range is compensated by the much greater output power of the lamp.

There are LED's being sold that produce blue and violet light. These are UVA LED's that have no germicidal effect. Always remember that germicidal UVC cannot be seen by the human eye. If you see an LED lamp being sold that puts out strong blue and violet light it is almost certainly a UVA lamp and has no germicidal effect. We have seen cheap lamps with dozens of LED's that purport to put out germicidal UVC. If true they would consume a large amount of power, would require heatsinks and would be very expensive.

A Simple Test for Germicidal UV

Here's a simple test to check if your lamp is producing germicidal UVC. Take a white piece of paper and draw lines across it with a pink or blue Sharpie highlighter. Cover half the lines with aluminum foil, and expose the other half to your purported germicidal UVC source for an hour, with the paper 2 to 3 inches from the lamp. When the hour is up, turn the lamp off, remove the aluminum foil, and check if the exposed part of the lines has faded significantly compared to the part covered up by the foil. If it has, then your lamp is producing germicidal UVC. Below is the result of such an experiment using a 9 watt germicidal UVC bulb made by Philips (#32512-6).

Pink and blue Sharpie highlighter on paper after exposing the right side with a 9W germicidal UVC lamp a few inches away for one hour

Referring to the image above, the left side of the paper was covered by aluminum foil, and the right side was exposed to the lamp. We can see that the right side has significantly faded. This lamp is therefore producing germicidal UVC.

If you need a lamp, like that used in the experiment above, we have some available, as part of a kit that includes a socket and ballast. Experience in AC house wiring is required.

If you do the same experiment as described above, but using sunshine as the UV source, you'll find that only the pink fades, while the blue is relatively unchanged. Below is the result of such an experiment using sunshine in Longmont, Colorado on August 9, 2020 for 2 hours, 33 minutes.

Pink and blue Sharpie highlighter on paper after exposing the right side with sunshine in Longmont, Colorado on August 9, 2020 for 2 hours, 33 minutes

Note that sunshine, after having gone through the atmosphere to reach the Earth's surface, contains no UVC, only UVB and UVA. Referring to the previous two images, we see that UVC fades both pink and blue Sharpie highlighter ink, whereas UVB fades only the pink. UVA can safely be removed from suspicion of playing a major part of the pink fading, since that is produced by office lighting, and there is no highlighter fading problem indoors.

In summary, below are ways to spot a fake UVC lamp.

4 Ways to Spot a Fake UVC Lamp

  1. Strong blue or violet light.
  2. Many LED's with low power consumption.
  3. No smell in the surrounding air after it's been on a few minutes.
  4. Pink and blue Sharpie highlighter on paper does not fade after an hour or two within a few inches of the lamp.

Our UVC Germicidal Lamp Kits

Note that these kits require experience in AC house wiring to assemble.


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