Here’s another light from Sofirn. This time their SP31UV (or SP31 UV). This is their SP31 light with an ultra violet LED.

About the light

You may have already seen Sofirn’s white LED SP31 flashlight. It’s a dual switch flashlight with an 18650 cell and a CREE XP-L HI LED. Sofirn have reused the host from their SP31 and replaced the XP-L HI with a 365nm UV LED.

Warning: this is a UV light! Don’t shine it at anyone’s eyes!


You can pick up the SP31UV direct from Sofirn or from AliExpress. It’s not currently on Sofirn’s Amazon store (affiliate link) but may be by the time you’re reading this.


It looks like there’s only one SP31 UV but Sofirn sell it in kit form and by itself. The kit version comes with a 18650 cell, USB charger and USB cable.

Sofirn SP31 specs

It looks like some store listings are showing half the specs from the white LED SP31. The UV version is not 1200 lumens!

  • LED: LG UV 365nm (UV-A) UVA35W01RL00
  • Material: Aircraft-grade aluminum alloy and Premium type Ⅲ hard-anodized anti-abrasive finish
  • Working Voltage: 2.6-4.4 Volt
  • Battery support: single 18650 battery
  • Dimension: 134mm(length) 23.4mm(head)
  • Net Weight: 70g (no battery included)
  • The water-resistant level IPX8
  • Low voltage warning: It is lower than 3.0 volts if the LED Red indicator (on the side switch) flashes rapidly when you turn on. Then please recharge the battery.
  • Low Voltage Protection: If the voltage is lower than 2.6 volt automatic turn off.

The physical light

What’s in the box

The kit version of the SP31 UV comes with:

  • UV flashlight
  • 18650 cell
  • Micro USB charger
  • Micro USB cable
  • 2 spare O-rings
  • Manual
  • Lanyard

My sample from Sofirn didn’t come with a manual and the listing on Sofirn’s store doesn’t include the lanyard.

Appearance and quality

I have a few “SC” lights from Sofirn but this is my first from the “SP” series. There’s not a huge amount of difference, other that the tail switch. The threads are triangle cut but are still smooth. The anodising is Sofirn’s usual good quality, so nothing to complain about there.

The springs aren’t especially thick but for a UV light they shouldn’t need to be.

Physical comparison

Here’s the SP31UV next to the slightly shorter SC31 Pro. The SP31 UV is a fairly standard flashlight, taking 18650 cells and with a 22.4mm width.

Carrying and everyday use

Being an UV light, the SP31 UV isn’t really an every day light for most people. Still, just like the white SP31 version, the SP31UV is a good size for EDC.

The clip is strong. Very strong. It’s not reversible but the whole body tube is, so you can use it either way round.

The light will tail stand, even with the lanyard.

Interface and switch

The SP31 UV (and SP31) have 2 switches: a tail switch and a side switch. The tail switch is a forward clicky (explanation) used to turn it on and off. The side switch is supposedly to change modes between low and high.

The side switch also has a charge indicator:

  • Steady Green: enough power
  • Flash Green:50-75%
  • Steady Red: 25-50%
  • Flashing Red: running out of power

At 3.8V (flashing green) the 2 modes are almost identical, so I thought the side switch wasn’t working. A cell at 4.2V (steady green) shows a much bigger difference between the modes.

There don’t seem to be any special or hidden modes.

Cell and charging

The SP31 comes with a standard 3000mAh 18650 cell. The charger is single cell and with a micro-USB port. It’s the same 1A charger that comes with the Sofirn SD05 and Sofirn C8F and it takes 26650, 21700 and smaller cells, as well as the 18650 provided.

Both flat top and button top cells seemed to work fine in the light. There’s a spring at each end which means almost any 18650 cell should fit.

Light output

I don’t have a way to measure UV light, so there’s no lux readings or run times here. You also can’t see UV, so photos of the beam itself aren’t that useful. UV makes some things fluoresce though, so here’s the SP31UV shining on a white board.


The SP31 UV has 2 modes. I measured the high at 0.61A and the low at 0.14A. With the included cell, that should be around 5 hours on high and 21 hours on low.


The LED is listed as a LG UV 365nm UVA35W01RL00. That means it’s in the middle of the UV-A range, only giving out a bit of visible light (above 400nm). Some UV LEDs are 395nm and produce much more visible light. This article on UV explains some more.

I couldn’t detect any PWM in the light.

Beam comparison

I only have 2 other UV lights at the moment. One is a AA zoomie with a single LED with terrible PWM. The other is a 3 AA light with about 50 LEDs. These photos are all on pure white card, which fluoresces violet under UV.

Here’s the SP31 UV (left) compared to the other lights. The SP31 UV is significantly brighter than the other 2.

Against an off-white wall, the SP31UV looks white to the eye. Here it is compared to a Zebralight H53w on 12 lumen mode. These are all on fixed exposure, with the lights both constantly on, moving a Rubiks cube in front of either light.


UV is useful for making things fluoresce. That’s when materials absorb UV light and reflect it back at different wavelengths. Some things you can use the Sofirn SP31 UV to flouresce:

  • Scorpions
  • White paper
  • Fluorescent pink, orange and green items, such as pens and some book covers
  • Dust, dirt and various biological liquids
  • Some gems
  • Money
  • Some plants fluoresce under UV
  • Fluorescent dye, such as in radiators. Useful for spotting leaks
  • Some paint
  • Glow in the dark things (UV recharges them quickly)

Here’s my bright orange digital thermometer in the dark, lit with a white flashlight and lit with the SP31UV. As there’s no white material, there’s barely any violet light, like you get from cheap UV lights.

UV is great for spotting scorpions. There aren’t any wild ones in the UK but we have stuffed toy lobsters instead, which is pretty close.



  • Very bright UV
  • Fun to play with (be careful)
  • SP31 host is good quality


  • Didn’t come with manual or safety information (but that could be just my sample)

Potential changes and improvements

There isn’t really much to change or improve here. The only thing I can think of is a third mode that’s very low for close up work.

As an option, Sofirn could also supply an ZWB2 filter for $1 or $2 extra, which would filter out any visible light.


The SP31UV is by far the best UV light I have. If you need a dedicated 365nm UV flashlight for your job then the SP31UV would be a great option. If you don’t need a UV light, then at around $25 the SP31UV is worth getting just to play with (carefully).

This flashlight was sent to me for review by Sofirn, without payment. This is an honest review and I have not hidden any issues with it.

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