UV torches, seven say 365, seven say 395, and five never say at all

Twenty ultraviolet torches, the kind sold for finding pet stains, checking a banknote, hunting scorpions or making things glow at a party. Every one of them was taken from a single search and none has been handled here.

This shelf breaks the pattern of every other one on this site. Usually the number that decides the purchase is the number no seller prints. Here fifteen of the twenty print it, and the interesting part is what the other five do, and what happens on the one listing that prints both.

The whole decision is two numbers, and they are closer than they look

Both numbers are ultraviolet. The difference is not how much UV comes out, it is how much visible light comes out alongside it.

An eye stops seeing at somewhere around 400 nanometres, and it stops gradually rather than suddenly. A 395nm LED is sitting right on that edge, so a good deal of what it emits is violet light you can see perfectly well. The beam looks bright and purple, and that purple lands on the carpet, the wall and the stain together.

A 365nm LED sits further out. Much less visible light escapes, so the beam looks weak, almost disappointing, and the thing you are hunting glows against something closer to darkness.

So you are not choosing brightness. You are choosing contrast, and the lamp that looks better in the shop is the one that shows you less.

That is also why 365nm costs more. The emitter is harder to make and less efficient, so at the same price a 365nm torch is smaller, dimmer or both.

The sellers make this argument better than I can

This is the part I did not expect. While opening the images to choose photographs for the pins, two listings turned out to be explaining the wavelength question themselves.

One prints the same rocks photographed three times: under ordinary light, under 395nm, and under 365nm. The 365nm frame is the one where the minerals light up and the background does not. It is the clearest demonstration of the whole argument on the page and it is sitting in a seller’s gallery.

The other is the listing whose title says 365/395nm. Its fifth image is a panel headed “Difference between 365nm UV and 395nm UV”, and its own text says that 365nm is hard to see and that 395nm is, in its words, especially cost effective. A seller telling you the cheaper option is chosen for being cheap is more useful than most of what is written above it.

A filter is what the dearer lamps are really selling. Two listings show a dark, almost black lens and explain that it filters out the leftover visible light so the purple is purer. One puts a number on it and claims 99 per cent of stray light removed. That is the contrast argument again, in glass, and it is why a good 365nm torch looks dim and finds more.

One LED cannot be two wavelengths

An LED emits a narrow band around one wavelength. It cannot be 365nm and 395nm at the same time, so a title saying both is not describing a product, it is describing a menu.

Sometimes that is harmless, as above, where the listing explains the options and you simply pick. But the price you see belongs to one of those options, and on a shelf where 365nm costs more to make, it is not hard to guess which.

The same habit shows up in the buyers’ list this site is built from. Of its seven ultraviolet torches, three give both numbers. So this is how the category is sold, not a quirk of one search.

One listing says 365nm in words and 395nm in its own photograph

Worth a section on its own, because it took about four seconds to find once the images were open.

A torch whose title begins “USB Rechargeable 365nm UV Flashlight” has, as its first image, a row of specification icons. The first icon reads 395nm.

Nothing about that is subtle. The title and the picture disagree about the one property that decides whether the thing is worth buying, and only the title is searchable. It is not picked below, and it is the single best argument on this page for scrolling through every image before you spend anything.

Three wattage claims, and a pattern that does not repeat

Three titles give a wattage: 120W, 90W with a 30W white laser beside it, and 60W.

A torch of this size runs on one 18650 cell, which holds somewhere around ten to twelve watt hours. A genuine 120 watt draw empties that in about five minutes, and a 120 watt LED needs a heatsink bigger than the torch it is in. What these numbers usually describe is the sum of what the LEDs are rated at, which is not the same thing at all. The 60W listing is refreshingly clear about it: its own first image reads 6X365NM, so the sixty is six emitters.

The pattern named on this site yesterday does not hold here, and that is worth saying. On the head torch shelf the impossible numbers sat on the cheapest listings with nobody’s name on them. Here the wattage claims are on the dearest things on the page, both of them several times the price of the ordinary torches around them. A pattern that fails to repeat is still information: on this shelf a big number is a sign of a bigger lamp, not of a bolder lie.

What does repeat is the variant trap. The 120W listing shows 60W, 80W and 120W across the bottom of its own first picture, so the wattage is a choice you make at the checkout, not a fact about the thing whose price drew you in.

What people are actually buying these for

Counted across the twenty titles: twelve name pet urine, five name scorpions, three name banknotes or documents, two name minerals or jade, one names invisible ink, one names resin curing, and one is sold for magnetic particle inspection, which is a metal testing job rather than a household one.

Match that to the wavelength and it comes out simply.

• Faint things, where contrast decides it. Pet stains, leaks, scorpions, security marks on a document. 365nm, and a filtered one if you can.
• Things that glow hard anyway. Highlighter ink, party paint, washing powder residue, tonic water. 395nm is fine and costs less.
• Curing resin. 395nm, because that is what the resin is made for.

One more thing about the pet listings. Several sell these lamps for finding things on animals, and one stages a vet in a white coat holding a cat under the beam. A lamp makes some things fluoresce and that is all it does. If you think an animal has a skin problem, that is a vet’s job, not a torch’s.

A note on the most expensive thing here, which is not a pick

One listing is a YUSHI 365nm inspection lamp for magnetic particle testing, and it costs roughly two hundred times the cheapest torch on this page. It is not a household product and it is not recommended below.

It is worth knowing it is there, though, because it is the only listing on the shelf that names its maker, gives its model, and says exactly what professional job it is certified for. That is what the top of this market looks like, and it is a useful yardstick for what three pounds is buying: something that emits ultraviolet light, sold on the strength of a number in a title.

Do not look into the beam

It should not need saying and it is said on almost none of these listings, so it is said here. Do not stare into a UV torch and do not shine one at anybody’s eyes, your own or a pet’s. That is true of every torch on this page regardless of which number is on it.