[Review] Sofirn SP31 UV - a throwy, dual-mode 18650 UV light

Hi all! This is the first review I've written on this site before, so if you have any tips I'd be happy to read them!

Sofirn offered me the opportunity to review their newest ultraviolet (UV) light, the SP31 UV. They supplied me with this light for free, though they were clear that they just wanted a detailed review (and not necessarily a positive one). You can find more information about it on their product page: https://sofirnlight.com/sp31-uv-flashlight-p0151.html.

This light has two modes and a throwy beam profile, which sets it apart from other popular UV lights I’ve seen. It’s got a couple limitations that prevent me from recommending it as a general-use UV light, but I still quite like what Sofirn has produced here.

The Sofirn SP31 UV is another light from Sofirn’s SP31 line, this time sporting a 365nm LG UVA35W01RL00 emitter.

The flashlight has dual switches, two modes (low and high), and no other special functions.

What was included

The flashlight came with the usual accessories – a lanyard, two spare O-rings, a generic Li-ion USB charger, and a contact card.

No manual was included, but because the light only has two modes, it’s not hard to figure out the UI. Some estimated runtimes would have been nice, though.

The included cell is a 3000mAh Sofirn 18650, with a paper disk to prevent activation during transport. I measured the cell to be about 3.7V on arrival, which is a bit above the 30% state-of-charge mandated for shipping.

A discharge test indicated a capacity of 3069mAh, so its capacity is as advertised.


The flashlight is well-constructed and has no visible defects, as I expect from Sofirn these days.

Black anodizing coats the body, including the threads which allows for mechanical lockout. The forward-clicky tailswitch turns the light on/off, and the electronic side-switch controls the mode. An indicator in the button shows the battery level, and the clip is made of the usual strong spring steel.

The threads are nicely trapezoidal and come with enough grease to operate smoothly.

If you’ve ever owned an SP31 V2.0, then you’ll know exactly what to expect from this light – the hosts are identical.

The key difference is the emitter – here you can see the LG UV LED, with a much smaller surface area than the XP-L HI on the left. It also lacks a dome, which will no doubt make for a throwier beam profile.

Sofirn’s listing says this is a 365nm LG UVA35W01RL00 LED...but I have no idea what that is. I couldn’t find any official documentation, and it looks nothing like the emitter in my Convoy S2 LG UV.

Speaking of the Convoy S2 LG UV, the Sofirn is more than a centimetre longer than it on account of the Sofirn’s control mechanism and deeper reflector.

Another key difference is that the Convoy S2 LG UV has a ZWB2 filter that cuts down on visible light, whereas the Sofirn has clear glass. This will be quite noticeable later.


Comparing the beam patterns of the Convoy S2 and the Sofirn SP31, we see some familiar traits that are reminiscent of their non-UV counterparts; the Convoy S2 has a wide hotspot and more usable spill, and the Sofirn SP31 has a smaller, intense hotspot with weaker spill.

I measured a tailcap current for the Sofirn SP31 of 0.16A on Low and 0.51A on High. In comparison, the Convoy S2 drew 1.04A.

(My measurements of the higher currents may be a bit low, because I don’t have the best setup. But the relative power draw should still be valid.)

Against a non-fluorescent surface, the lack of a visible-light filter is apparent. The hotspot of the Convoy S2 is about as bright as the Sofirn SP31 on Low, despite the Convoy S2 emitting much more UV light.

The effect of the visible light can also be seen when looking for fluorescent marks on currency; in this comparison, the smaller fluorescent marks are harder to spot due to the Sofirn SP31’s visible light.

However, the tight hotspot does mean that the Sofirn SP31 UV is great for curing spots of UV-activated glue, without wasting light in the spill.

In such an application, the additional visible light really doesn’t have an impact on usability.



  • Well-constructed, and I always like a good forward-clicky switch
  • Having two modes is a nice feature – no need to go full blast when you’re just checking a bill


  • Enough visible light to be distracting – including a ZWB filter or selling a compatible one separately would be a nice improvement


  • The tight beam is useful when targeting a specific area (e.g. Curing UV-activated glue), but is less suitable for scanning an area (e.g. Looking for fluorescent rocks or fluid residues)

Ohhh, now I want one… :laughing:

Hi, thanks for your review. It’s aswsome and if any one interested in the UV light, please PM me for a 50%off discount code.

I’ve also tried the Sofirn SP31UV, and my initial observation is that I think Sofirn should offer a ZWB2 UV (or better: pre-install one). The visible light is quite noticeable and distracts from the UV light.

Note that the usual 20mm or 20.5mm ZWB2 UV filter (eg. the one that fits the Convoy S2 / S2+) will not fit the SP31UV since the head diameter is just slightly smaller (slightly smaller than 20mm).
I had a few spare 20.5mm ZWB2 UV filters and tried to fit it inside the SP31UV and found it doesn’t fit — I sanded the ZWB2 UV filter slightly with a sandpaper and then it fits inside the SP31UV. This improves the UV light a lot (less visible white light).

I’ve asked Sofirn some time ago if they have the ZWB2 UV filter (19.5mm) and they have already posted this item on some of their stores — I suggest to get them when purchasing the SP31UV, to lessen the visible white light.

I’m not so familiar with how much current is ideal for the LG UV365 LED — Sofirn’s uses around 0.5A on max, whereas the Convoy S2 uses around 1.05A (7135x3). There is another Convoy S2 model that uses 7135x5 (1.75A), it uses a different LG UV365 LED than the Convoy S2 with 7153x3. (although I notice that the description for Convoy S2 (7153x3 variant) now indicates it uses “Seoul UV365 LED” instead of the previous LG UV365 LED… The other Convoy S2+ (Nichia UV365 LED) uses 0.7A (7135x2).

Will the LG UV365 LED used on the SP31UV, just using 0.5A have noticeably less UV output than the (for instance, Convoy S2+ / Convoy S2 variants that uses 0.7A, 1.05A, 1.75A)?

Thank you Serlite for your trustworthy review of the Sofirn SP31UV which I am now interested in and I also thank Sofirn for posting in this thread to offer a 50% discount code.

This Sofirn listing for the SP31UV on AmazonUS presents some additional information:


Runtimes as listed on AmazonUS by Sofirn are 8 hours on High and 21 hours on Low.

The lens as described by Sofirn on AmazonUS indicates it is something more than clear glass:

Received a SP31UV with accessories from AmazonUS for $18.49 (before sales tax) using the generous 50% discount code offered by Sofirn earlier in this thread and the build quality fit and finish are excellent.

Forum member Serlite's review was spot on and I received exactly what I was expecting which is a light that I do not really need, but the price was right for adding it to my collection for showing others and making comparisons. It's throwy bright UV hotspot will be useful if I ever need to cure UV-activated glue and in the meantime it will compliment the floody soft hotspot of my Wurkkos WK30 which also has a 365 nm UV LED along with both a white and red LED.

Sofirn's AmazonUS listing indicates the SP31UV is "Equipped with black filter optical lens which makes the ultraviolet light purer." I am not sure if the "black filter optical lens" is supposed to be noticeable or have a visually different appearance to the naked eye (as viewed when the light is OFF to avoid damaging your eyes from UV light), however the lens looks like plain crystal clear glass to me and I do not believe I am seeing any beneficial effect from this "black filter" since when the light is ON at either brightness level, the non-UV visible light almost seems to overwhelm the UV light portion of the LED.

On plain white printer paper the tint of the two lights appear virtually identical to me but sometimes there is an obvious UV light spectrum difference between the 365 nm UV LEDs of the SP31UV and Wurkkos WK30 which could be caused by the "black filter optical lens" of the SP31UV or more likely from physical differences between the two LEDs and this is most noticeable when examining white chicken eggs. The white eggs fluoresce and appear to be a "golden yellow" color using the SP31UV but are a very different "brownish orange" color when using the UV LED of the WK30.

Besides the color difference when examining eggs, every object I tested that fluoresced with the UV LED on the WK30 could also be noticed with the SP31UV, sometimes standing out less and sometimes standing out more, but I could not identify any patterns or scientifically understand exactly why these differences occurred.

The visual patterns on the two LEDs look different using 10x magnification. The 365 nm UV LED in the SP31UV looks sort of like a "pin cushion checker board" with 23 pin prick holes (four rows of five holes then one row of three holes where this particular row does not have a hole at each end of the row and is also one of the four sides of the LED and there appear to be thin wire electrical connections to the two corners of the LED where the holes are missing).

The 365 nm UV LED in the WK30 looks sort of like a "crown" or in ASCII representation as a |V| except the sides slope out like this \ / and the V is shorter like this v but wide enough to connect the top ends of the two sides and the bottom of the v does not touch the bottom line. All the lines are connected at their corners but there are two small gaps, one gap is at the point on the bottom of the V and the other gap is in the middle of the bottom line. The electrical connections to the LED appear to be at the two top corners of the V.

If I could have only one of these two UV lights, for me it would be the WK30 because it functions more suitably for my casual playful use. Neither of the two brightness levels of the WK30 UV LED are as bright as the corresponding brightness levels of the SP31UV but the UV tint of the WK30 UV LED contains proportionately less visible white light and being more floody with a less defined hotspot just makes for a more satisfying experience to psychologically (not physiologically) "blow unsuspecting people's eyeballs out" when using the UV light to examine the normally invisible security features on money, driver licenses, passports, various minerals and the incredibly pervasive normally invisible stained interiors of their homes.

Hmmm. Now I’m really curious about this…

Just got mine this morning, popped in a full cell while the come-with cell is charging, and there seems to be quite a lot of “greenish-white light” leakage. It could definitely use a ZWB2 filter, but the front glass is clear. No hint of any coating or anything that would alter the light.

Even with sunlight streaming in the front window, glowy stuff across the room lights up like it’s on fire :laughing: , but stuff that doesn’t fluoresce still bounces back visible light.

I could get, or in fact I think I have, some ZWB2 filters, at least 20mm ones for S2+es that should fit, but I suspect opening up the critter to swap glass might be more than a bit of a chore.

Anyone else confirm that it’s just plain clear glass in there, and/or have suggestions how to crack it open without destroying anything? :laughing:

Btw, this would be a hoot if anyone does urbex stuff. Glowy paint way off in the distance that probably wouldn’t show up at all with crappy all-flood 405nm 3×AAA “UV lights”, but would light up from a distance with this critter. :laughing:

Hmm, I can’t verify what exactly the lens is (looks like pretty normal glass to me).

But I checked my light and the bezel is unglued - so swapping the lens should be as simple as unscrewing it and popping out the old one.
This is a welcome surprise, since all of my SP31/SP32/SC31-series lights have had glued bezels. No need for a heat gun and strap wrenches for this one!
(Hopefully all the SP31 UV lights are like this, since I reckon a lot of people may want to swap the lens.)

Interesting, just tried and the bezel on my SP321UV unscrewed easily by hand. The threads and O-ring even appeared to have been lubricated prior to assembly.

Checked my SP31 V2.0 and I could not unscrew the bezel by hand.

Ha! Just tried it, and yep, takes a little doing (and grip) by hand, but it unscrewed nicely.

Now all I gotta do is find those filters… :confounded:

UV sure to let us know how those ZWB2 filters perform on the SP31UV, Thanks!

20.5mm ZWB2 filters won’t fit.

You need something like 19.7-19.8mm ZWB2 filter (use a 19.5mm ZWB2 filter).

I used a (20mm or 20.5mm = the one from Convoy’s store), and had to sand the circumference of this ZWB2 filter to make it fit inside the SP31UV head.

On the other hand, I notice there are 19.5mm ZWB2 filters on Sofirn’s website (I believe they put these accessories up for sale, AFTER I had inquired whether they have ZWB2 filters for the SP31UV) and also on other stores like AliExpress (unfortunately, I’m unable to order from Amazon since I’m on the other side of the US and shipping would be too expensive if available… besides AliExpress and some other online stores nearer to my country have these 19.5mm filters).

Again, I didn’t get my SP31UV from Amazon, but from other official Sofirn stores and these came with a plain glass lens only. (not AR coated nor ‘black lens’).

Mmm, that figures. But I could sand it down if I had to, I guess.

In fact, given the choice between sanding down one immediately vs waiting ’til June to have one shipped…

The factory lens on my SP31UV measured 0.784" (19.91 mm) diameter x 0.059" (1.50 mm) thick.

The inside diameter of the bezel (where the lens fits into) measured 0.800" (20.32 mm).

The inside diameter of the bezel's front ridge (which stops the lens from falling out) measured 0.728" (18.49 mm).

Depending on the exact size of the finished lens, a nominally specified 19.5 or 20 mm diameter x 1.5 mm thick ZWB2 filter lens would probably be suitable (with a 20mm diameter lens preferred if it fits), but I could not find any on sofirnlight.com and would appreciate a web link recommendation for a ZWB2 lens from a seller that delivers to the US.

The factory installed lens was initially stuck in the bezel and I should have dislodged the lens with the bezel's base placed down on a flat surface, but I popped the lens out into my hand and three pieces (the lens, an O-ring and a flat white plastic washer about the same size as the O-ring) landed mixed up in my hand, so now I am not exactly sure about the correct order to re-install them, especially the white plastic washer.

I guessed and installed the white plastic washer (which was the same size as the O-ring) into the bezel first, then the lens, and finally the O-ring followed by the reflector.

There was a recess on the reflector which fit the O-ring but was just a little too large in diameter for the plastic washer to fit over, so I guessed (and maybe incorrectly) that the O-ring went next to the reflector and that the plastic washer was supposed to be next to the lens, but I was not sure on which side of the lens it should be.

I'm hoping that this was the correct orientation for the O-ring and flat white plastic washer but if anyone knows for sure I would appreciate knowing if this is the order that they are installed in by Sofirn.

Hate when that happens…

I just popped open mine to check, and yep - it looks like you guessed right.
The order I saw was: bezel => plastic ring => lens => O-ring => reflector

Link to ZWB2 UV filter 19mm in Sofirn’s AliExpress Store:

Had a bit of trouble searching for this item, since it appears that AliExpress filters out items that cannot ship to the destination country.
(The above item used to be able to ship to my country, so I recall I saw the above item before; but can’t find the item, until I tried changing the country to another country, and the item appeared again, but listed as ‘cannot deliver to my country’).

Fortunately it was not one of those unexpected compressed springs that without warning surprisingly zings across the room disappearing forever when you disassemble something you are not familiar with.

Thanks for confirming the order of assembly for those parts. I appreciate the help from everyone and have enjoyed my "flashlight journey" on this forum, which is approaching it's one year anniversary.

That flat white plastic washer is only 0.014" (0.356 mm) thick. If it had been 0.5 mm thick, I would have guessed it was so a 2.0 mm thick lens could be used in place of a 1.5 mm thick lens when the plastic washer was removed. But since it is not that thick, I'm guessing the purpose of that plastic washer is to either cushion, avoiding scratching or reduce twisting friction between the glass lens and the aluminum bezel.

The white plastic cone washer around the LED and under the reflector also fell out when I tilted the body of the light after the bezel and reflector were removed, but I was expecting that and the correct orientation (which centers the washer on the LED) is easy to determine by matching the one beveled side of that washer to fit the reflector's tapered base.

Thank you very much for the link. That size of 19.0 mm diameter x 2.0 mm thick will not be ideal but should work.

Was there a dramatic improvement in the reduction of visible light and a better UV experience from the SP31UV after you installed the ZWB2 filter lens?

Yes, there is a difference.

I feel that the SP31UV should have shipped with the ZWB2 UV filter pre-installed, much like the Convoy S2 that uses LG UV365 LED and comes pre-installed with ZWB2 filter.

(The Convoy S2+ model that uses Nichia UV365 LED doesn’t come pre-installed with ZWB2 UV filter, but that’s acceptable since the Nichia has little visible white light).

The LG UV365 on the Convoy S2, assuming the pre-installed ZWB2 filter is removed, will also have produce visible white light (which is likely why Convoy opted to have the ZWB2 filter pre-installed on their models that use the LG UV LED, but not the Nichia UV LED).