All About Ultra Violet Testers

Ultra violet (UV) light. How it works.  What you can do with it. Several products  reviewed.

New - shortwave UV for decontamination of viruses incl. COVID
Also, UV jewellers loupe





Ultra violet light (known is the U.S.A. as 'black light') is a very intense blue light, it is not visible to the human eye. Fluorescent-tube UV torches give off a very dull purple or violet glow. LED UV torches give off a dull lilac-blue. But the actual UV light is invisible, so the only way you can tell how effective it is - is to observe its effects.

If porcelain has been repaired, the missing part will have been 'rebuilt' or the crack glued, and then carefully repainted so that the colour matches. The colour will match perfectly in white light (e.g. daylight) but under UV light the new paint shows up a totally different shade. This applies to many decorative items, watch dials, dolls heads, faint signatures on letters, modern touch-up paint on old paintings. You have probably noticed a similar effect when looking at a car parked under a yellow street light, new paint on the car can show up quite a different shade, whereas in daylight it matches perfectly.

Other examples in the world of antiques and art: oil paintings, art on paper, marble, jade and ivory, all show characteristic patterns under UV light:





Dark bluish-violet

Chartreuse glaze

Very small blue dots

Dark blotches

Bright yellow

Lining compound

Picture putty

Old varnish


Repairs, new signatures & overpainting

Recent touch-up with white lead or Naples yellow



Bright areas

Smudged areas

Very faint writing



New patches of paper


Erased signature


Strong purple

Mottled white


Fresh cut

Old marble


Intense colour



Fresh carved

Old jade



Yellow tone


Newly carved

Old ivory


Some amber does fluoresce under UV light (notably blue amber) but so too will many substances so this test is not diagnostic.

The one reliable test is destructive, which is never recommended in gemmology. Touch the tip of a red-hot needle to it. If the burning smells acrid (like plastic) then it's plastic; if the burning smells sweet, it's amber.

Failing the burn-test, the best test is to drop it into heavily-salted water, amber will float, GENERALLY plastics sink. But there are many thousands of different plastics and I doubt that anyone has compared them all with amber, there might be some that float in salt water.

Uranium glass
(Vaseline Glass)

Glows spectacularly.

How does it work? It works by reflecting off the surface and showing up the difference in the chemical composition of the paint, or by showing characteristics of the item's basic chemical make-up.


Small lights (typically used for gemstones and minerals) are encased in a 'viewing box' for safety. Large lights (for 'cleaning' an area of viruses) must have safety features such as a cut-out if someone approaches, a remote control to operated it at a distance, a timer, safety glasses and audio warnings- we do sell one of these (ref. UV-sterilization) - scroll down for full details.

It is well-established that shortwave UV light (254nm) kills bacteria and viruses, they are used:

- in the food packaging industry (UV-treated foods last longer)
- in pipes supplying water to tropical fish tanks and garden ponds
- in air-conditioners
- as a small light positioned very close to switches (e.g. the buttons in a lift)
- carefully placed on ceilings (you need to know which way the air flows through the room)
- in cabinets for sterilising anything (anything that will fit in he cabinet)
- as automatic ceiling lights that come on when nobody is in the room (e.g. washrooms and toilet cubicles)

Most UV torches are longwave (scroll down for the technical explanation) and they do have dozens of uses (see the rest of this article) but they are bad at killing viruses and should not be used for this purpose. For killing viruses you need a shortwave UV light, and if you want to 'sterilise' a wide area (e.g. a room, as opposed to a small object) then you need a large floor-standing light such as our UV-sterilization.

A good measure of whether you are buying a shortwave or longwave light is the price. For a shortwave light, £50.00 gets you a tiny lights (such as our UV-SW) which is designed for viewing gemstones. Larger viewing boxes (with shortwave lights) in which you can place larger mineral samples cost £200.00 to £400.00 (we don't sell these); large cabinets used in industry can treat large objects coming of a production line, they cost £2000.00 to £3000.00 (we don't sell these). 

Here is a summary of seven "virus-killing" hand-held UV wands bought on the internet:

We've been looking at kill rates of 200Jm2, enough energy to ill 99% of known germs. A lot of devices didn't deliver any UV-C [shortwave] at all. Plenty of them were just based on 405mn LEDs [longwave] so you get a sort of indigo glow. For some wands that did deliver UV-C [shortwave] the dose was so weak that it would have taken ten minutes to reach the specified kill rate. These things tend to be marketed as something you can sweep across the surface for two minutes or so. The wands should also have gravity-sensing switches that turn off the light when it's turned towards a user, to protect their eyes. However, only one of the seven wands had this switch and delivered enough UV-C [shortwave] to kill viruses quickly.
SOURCE: The International Conference on UV Disinfection for Air and Surfaces, talk by Gareth John of the Lighting Industry Association.

I carried our my own I investigation into a cheap (£50.00) UV box advertised as, 8 UVC LEDs. 260-280nm, bleeps after 3mns to say the item is clear of bacteria

I asked the supplier for clarification and they replied, "We cannot guarantee that the device is killing viruses it only remove bacteria's. We don't think we can be sure at 100% that if the item is contaminated it will be clean after the process. I'm sorry but the theme is very complicate, nobody wants to take responsibility for these 'medical' devices." In other words, cheap UV 'sterilisation boxes' might kill some bacteria sometimes and even less viruses even less of the time...but probably not.

You could use a cheap shortwave UV light to kill viruses, for instance our UV-SW designed for testing gemstones.
INSTRUCTIONS: place a ring under the light (the box is just about large enough to hold one ring) and switch to 'shortwave'. We don't know how long the exposure should be, give it 10mns to be on the safe side; then turn the ring  over and expose the other side for 10mns, then prop it up and expose the underneath for 10mns, it will then be clean of viruses, providing all parts have been exposed to the UV light. ALTERNATIVELY, give it a quick wipe with an alcohol-based sanitiser - that will take five seconds and will be just as effective.
SUGGESTION: if you already use one of these for gemstones, clean the ring with sanitiser, then (in front of the customer) put it in the UV box for a few seconds, just to inspire confidence.

For treating an entire room go for our large shortwave floor-standing UV light (ref. UV-sterilization) specifically designed for killing viruses.


Many everyday substances glow under UV light: chlorophyll, teeth and antifreeze; olive oil, honey and ketchup; some cosmetics, some drugs, some postage stamps and some flowers.

There is starch in ordinary paper that makes it glow a vivid blue when lit with ultra violet light, but old paper banknotes (which were made of a linen-type material, not paper) merely look 'slightly blue', hence UV lights are used as 'forged banknote detectors'; some paper banknotes (and all plastic banknotes) are printed with 'secret' marks, using UV-ink, which can only be seen under UV light - see some examples at the bottom of this page.

Additives in soap powders glow under ultra violet light, they are added for that very reason: UV light in daylight makes your white washing glow blue-white, which is why advertisers once used the catch line, 'washes whiter than white'.

Quinine is an ingredient of some drinks (e.g. tonic water), it gives them a distinctive bitter taste. Quinine glows under UV light. So the next time you are in a dimly-lit bar drinking a gin and tonic, shine your UV torch on it and see it glow.


Objects painted with UV paint (most of which are completely invisible under ordinary light) will glow under UV light.

- security-marking: e.g. with your postcode
- finding things: gold balls, tortoises, keys
- fun: makeup / face painting, spooky decorations
- theatre and dance: dim the lights, light the stage with UV light, people and objects 'float' or appear headless
- leak detection: in car radiators and central heating systems, slow leaks (which don't appear wet) leave behind a residue that glows under UV light

Specialist Uses

- cross linkers and light boxes for viewing gels in biological laboratories
- germicidal UV lights for treating food before it goes on sale in the supermarkets
- welding operations
- curing uv-sensitive adhesives or uv-sensitive nail varnish ("gel-varnish")
- for inserting into pipes carrying water to fish tanks or fish ponds to kill algae and bacteria
- identification of some minerals and gemstones

Medical and food

- checking major skin wounds before operating (to show up bacteria). The presence of bacteria (porphyrin molecules in the bacteria fluoresce red or blue-green under 405nm light) does not necessarily indicate an infection, the area must then be swabbed for further analysis.
- checking laboratory equipment after cleaning, UV can show up bacteria (we sold several of the UV-1 to a pharmaceutical company for this purpose)
- treating minor skin wounds, though there is a balance, here, between the benefit of stimulating healing and the risk of causing cancer, because they use very powerful short wave UV LIGHTS (which we don't sell).
- a diary bought a UV-1 for checking their steel containers when empty, the UV makes any remaining organic matter, such as milk or cream, fluoresce.

Animals and plants

Humans need UV light (from sunlight) to produce vitamin D in the skin (nearly everyone in Northern Europe is vitamin-D-deficient in the winter due to lack of sunshine); in the summer it's popular to take advantage of prolonged UV light from the sun to burn the skin (it's called a suntan) - and if there's no sun there are tanning parlours in which you can be gently grilled under giant UV lights. Over-exposure to UV light can cause cancer but some exposure is essential, the solution is to go out in the sun (without sun cream) whenever possible but don't stay out long enough to get a tan.

In humans, the lens of the eye filters out UV light so that we cannot see it, but some people who have had eye operations (e.g. following a cataract operation) can see UV, it appears as a bluish or purplish glow. An example of this is in the paintings of water lilies by Monet following his cataract operation.

For most humans, the only way to see the effect of UV light is to use a UV torch. For instance, to detect lichen, shade it from daylight, shine a UV light on it, and see if it fluoresces white, bluish or orange - if so, it's probably lichen (try it out on Cladonia portentosa).

The same applies to freshly-spilt blood and urine, the human eye can't see it fluoresce in daylight, but you can see it glow under a UV light in the dark (used by the the forensic services at crime scenes).

Until recently it was thought that just a handful of animals could see UV light. It now seems that a great many can: many fish, reptiles and birds, and most mammals (though not primates). For instance, Kestrels can detect the urine trails of prey. For reindeer living in the Arctic urine appears black against the ice and snow as it absorbs UV light (urine is bad, it indicates nearby predators) whilst lichens appear white (lichens are good, reindeers eat them).


Here are four questions we get asked about UV light.

Question: so why do some UV lights appear really bright and others appear really dim?

Answer: many popular UV torches are 'popular' because they appear 'really bright' to the human eye, but the amount of actual UV light (which is not visible to the human eye) is very small, and its effect is very the point of being useless. By contrast, some UV torches appear dim to the human eye, but the amount of actual UV light is large and they work well. So when you look at all our UV lights, that is the main difference between one that costs £5.00 and one that costs £39.00.

Question: do I have to use a UV torch in the dark?

Answer: the effect is certainly spectacular if you go into a completely dark room, but usually it is sufficient to use it in dim light. Use a square of dark cloth, or the underneath of a table in a dim corner or even the inside of your jacket (you may have seen antiques dealers lifting their jacket and, it would seem, examining their armpit with a blue light). UV lights used in shops for testing banknotes include a shade. This is no different from using an ordinary torch, turn it on outside in bright sunlight and you won't see any effect.

Our two best models (UV-1 and UV-100) give out a huge amount of UV light and can be used in daylight, though not in direct sunlight. All UV lights work best in dim light, the dimmer the better.

Question: can't I just use a brighter UV torch, so that I don't have to use it in 'dim' light?

Answer: To an extent - yes. But it depends on the type (wavelength) of the light, if it is wrong (as with all the cheap UV torches) then it won't work, no matter how bright the UV light. Providing the type (wavelength) is right, then yes, a larger brighter UV torch will light up a wider area and work in brighter light, just the same as an ordinary torch. Scroll down or click here for recommendations.

Question: if I use it by shining the light on the surface of an antique, does that mean that it won't see below the surface?

Answer: Exactly so! If the item has been repaired and then re-glazed, a UV torch will be of no use, there will be no 'new' paint to see, the entire surface will be new. Similarly, you cannot see through layers of paint to see 'hidden paintings', UV light is not the same as x-rays. But you can see differences in surface paint, and that can indicate that a painting has been 'touched up'. Similarly, if a signature reacts differently to the surrounding work, it's clearly made of a different paint and may have been added.

Question: how do I see below the surface, e.g. to see an original drawing underneath a painting?

Answer: there are now a few ways of doing this, but none involve a simple UV light. For instance, by examining the fluorescence caused by x-rays rather than UV rays (XRF, x-ray fluorescence), or by using very specific wavelengths (colours) and analysing how it is reflected.


The electromagnetic spectrum ranges from AM at one end (which includes Medium Wave and Long Wave radio) to Gamma Rays at the other. The measurements on the right of the chart, below, are the length of each wave section (from peak to trough) in nanometres (nm). 1nm = 0.000001mm.

As you see from the chart, "visible light" falls between microwaves and x-rays.

UV light, although close to x-rays, cannot penetrate the human body, its effect (for shortwave UV) is limited to burning the surface of skin (though it will cause damage to the eyes). Having your skin burnt by shortwave UV light is very popular, it's called 'getting a suntan', and if there is no sun, you can get yourself grilled under giant UV lights on a sun bed.

The tiny band we call 'light' ranges from infrared (the human eye can't detect far-infrared) to ultraviolet (the human eye can't detect ultraviolet):

And within this, the tiny bit we call 'ultraviolet light' looks like this:

365nm - 302mn - 254nm

UV light is divided into three types depending on the wavelength (these classifications are approximate, since there's nothing to distinguish the boundary from one to another):

"Longwave" UV, also known as UV-A, 400 to 300nm

"Midrange" UV, also known as UV-B, 315 to 280nm

"Shortwave" UV, also known as UV-C, 280 to 250nm

All UV light is invisible to the human eye, the UV torches you buy also give out some visible light and it is this you can see as a dull glow. Manufacturers of the cheaper torches ensure that the 'dull' glow isn't too dull, so that the user says, "Wow, this UV torch is so powerful!". Giving out some visible light is, of course, useful, otherwise you wouldn't know if your UV torch was switched on or off.

All the UV torches we sell are UV-A / longwave, they are not shortwave (shortwave burns skin and can damage eyes) but we do have one shortwave light encased in a 'safe' viewing box (UV-SW). Here is a list of how the various types (wavelengths) of UV-A / long wave lights compare. My standard test is on a British five pound note.

(see examples below)

ON AN OLD (PAPER) TWENTY POUND NOTE - it has no effect on a twenty pound note, it does not show up any of the security markings.
ON A NEW (PLASTIC) FIVE POUND NOTE, scroll down to see pictures of how the UV lights react.

ON AN OLD (PAPER) TWENTY POUND NOTE - the short lines are clearly visible but the "20" is not - not even if you use the it in the dark.
ON A NEW (PLASTIC) FIVE POUND NOTE, scroll down to see pictures of how the UV lights react.


UVA, SUMMARY. All our UV torches are UVA (see above). It is quite safe to point these at the skin, there is no way this type of UV can burn skin. However, please do not stare directly into the light.

UVB and UVC (but mostly UVC) can burn the eyes or skin. When used in sun beds they are specifically designed to burn (tan) the skin, but the user must wear dark glasses to protect the eyes. Prolonged exposure, such as over-sunbathing, can lead to skin cancer. We do not sell UVB or UVC torches.

Susceptibility to skin being burnt by UV light depends on skin colour. Brown-skinned people are less sensitive to being burnt by UV light, melanin in the skin provides protection; completely black-skinned people have a variation, eumelanin, which gives even greater protection. This dates from the time when most humans lived in Africa. In populations that spread northwards, to lands where there wasn't much sunshine, fair skin evolved; it evolved to be more sensitive to UV light, because UV light is necessary for producing vitamin D in the skin.


I have illustrated each UV light by shining it on a British banknote and also two bottles of liquid, and you will see how effective each model is. If you just want to see the effect of UV light and really have no interest in buying anything, then there is no need to scroll down, just click on these two examples: 

This is what the two bottles of liquid look like without any UV light, one contains tap water, the other contains tonic water from Tesco. Tonic water contains a tiny amount of quinine, which fluoresces (glows) under UV light (click on the picture to see them glow).



The products in this section are torches, except for these two items.

ref. UV-Decontaminator


Click on the picture to see an example.
Click here to buy one of these.

Shortwave (254nm), will kill bacteria and viruses that are exposed to it, powerful enough to 'clean' a medium-size room (about 4m radius), see full details.


ref. UV-SW

UV (ultra violet) SW / shortwave LEDs, 254nm
UV (ultra violet) LW / longwave LEDs, 365nm
and ordinary white light

Generally, you would use this for viewing gemstones or small mineral samples, because some react to LW UV light and SW UV light.

The shortwave UV can also be used for killing viruses, to sterIlise small items of jewellery (scroll to the top of this article for details).


ref. UV-1


Rechargeable, charger supplied (it plugs directly into the torch) - but the battery is user-changeable should it ever wear out. 

One charge lasts up to 8 hours of use.

Best wavelength combined with the most powerful spot light, so powerful that you can use it outside in broad daylight (though not direct sunlight) or indoors under bright lights, you won't have to build a shade or cover the item with a cloth). It is, of course, most spectacular in the dark.

The torch was several inches away. Close-up (and in dim lighting) there really isn't any difference between the two models. But if we move the torch

Supposing you are examining a huge painting or an entire collection of porcelain in a dimly-lit gallery (or a workshop where you can dim the lights) - this is with the torch held six feet away, it's certainly not as effective as holding it a few inches away but, amazingly, it still works well.

The torch was several inches away, it was aimed between the bottles, you can see how much more powerful than the 3W model the spot of light is, the area covered is almost bright enough to fluoresce the tonic water in addition to lighting up the tap water.

The torch was several inches away., it was aimed at the left bottle (tap water), the visible light illuminates the bottles (and the bottle appears blue because the visible light is blue); the light (visible light) is far brighter than the 3W model, though the UV light does nothing.

The torch was several inches away., it was aimed at the right bottle (tonic water), it glows a vibrant blue (more dramatic in real life, I couldn't get the 'glow' to show on the photo); compare this with the effect on the 3W model (see previous item, above).

ref. UV-100
£24.50 incl. VAT


The wavelength ("blue-ness) isn't as good as the UV-1 models, above, as you can see from the pictures, but for brightness it's every bit as powerful as the very best model (UV1-5w) and it costs dramatically less.

The torch was several inches away.

The torch was six feet away, compare this with the UV-1 models above.

Now let's make the conditions difficult, this is with the torch several inches away and the floodlights turned on in the studio, simulating a shop or exhibition stand under bright lights or a bright day outside (but not in direct sunlight). I thought, at first, that with a bit of imagination you could almost see the '5', but on closer examination - you can't. Despite this being incredibly wide angle (i.e. it lights up a huge area) the power combined with the wavelength isn't good enough for use in bright conditions. Compare this with the UV-1 models above. However, there is one huge difference between this uv-100 and the UV-1 models: price!

The torch was several inches away, it was aimed between the bottles.
You will notice that
- there is no 'spot' of light, this is a wide-angle UV torch, the light illuminates both bottles at once (and a large area around them!)
- unlike the uv-1 torches, above, there is a lot of visible light illuminating the left hand (tap water) bottle
- the effect on the right (tonic water) bottles is clear, but not nearly as spectacular as with the UV-1 torches, above.

The advantage of this torch is that it's wide-angle, you can 'sweep' a large area with it, you can use it for UV-effects (to light up an entire showcase, market stall); or 'in the field' hunting for plant, animal or mineral specimens (though you've have to go out at dusk or at night); or use two or three torch to light up a dance floor. It's by far the brightest of all the UV torches though, as you see, this does not mean it's the most effective, I'd recommend the UV-1 models for use in daylight because the wavelength (blue-ness) is better.



The banknote rests under the shade, click on the picture.


All the other models on this page are battery-powered, this one is mains-only,
for testing banknotes, includes a shade.


ref. UV-41

41 LEDs with a 380nm output.

Banknote at 3 inches in the dark:

Banknote at 6 feet in the dark:

Banknote at 3 inches with studio floodlights:

Tonic water at 3 inches in the dark:



There are many dozens of items that fluoresce (glow) under UV light, scroll to the top of this page to read about them. There are even items that fluoresce 'accidentally', i.e. the manufacturers just happened to use a material that fluoresces, as you can see from my last photo: Lesley's shoe laces.



Finally, if you want a magnifier with a UV light, be extra careful which one you buy. Below are two sets of photographs showing a ruby, one with an unbranded 10X loupe with UV light (the size happens to be 10X21), one with QUICKTEST 10X20 loupe with UV light.  These loupes also have a rim of ordinary (white) light, you push the switch one way for white light, the other way for ultra violet light.

For a closer view, click on each of the two images.

Secondly, below, the  QUICKTEST 10X20 loupe with UV light (all four pictures are of the same loupe).
Top left: the loupe.
Bottom left: note how there is only one ultraviolet light, not nearly as pretty as the rim of lights.
Top right: a ruby with the white light switched on, the ruby is dark, the light reflects off the surface.
Bottom right: the same ruby with the white light off and the ultra violet light switched on, it glows spectacularly. That's because there is a huge amount of ultraviolet light, even though the human eye cannot see it.

Please note that using ultraviolet light is not a 'test' for ruby, because some ruby (especially Burma ruby) fluoresces brilliantly, some ruby does not fluoresce at all; the above is just to show you the difference between the quality of ultraviolet lights (scroll up to read all about wavelengths).