Fenix TK21 U2 – A review in four parts

A few days ago I received a TK21 for review along with the Fenix AR102 a remote tail switch, AF02 a bike mount, and two types of gun mount (scope and rubber block).
Before I look at how this torch performs while out hunting (part 3) and on bicycle rides (part 4), part 1 will cover initial impressions and part 2 some measured performance tests.

PART 1 – Initial Impressions:

The TK21 U2 is the latest single 18650 or two CR123 powered torch from Fenix with an XML U2 LED (updated from the XML T6). It has a combination of features that I have found make it incredibly versatile as a tactical/hunting and general purpose light.

What is in the box:





The TK21 U2, Holster and spares (lanyard, o-rings, switch rubber)



Two CR123 batteries were already in the torch with the tail-cap slightly undone to lock it out. This is because the Photon Shop provides a set of batteries with each retail sale, not because Fenix provide them.



Overall quality is excellent. This torch feels like it will take hard use in its stride. Fit and finish is excellent. The threads are well formed and smooth in use. The LED is well centred.



The tactical grip ring near the switch can be unscrewed if you don't want it, but this does leave the screw threads exposed.




The positive and negative terminals are both springs. This gives excellent continuity of connection when the torch is subjected to recoil or other shocks. This means you can also use flat top 18650s.




Modes and User Interface:

The TK21 has a forward clicky tail-cap switch giving momentary operation, and the modes are changed by a side switch. The output modes are Low, Medium, High, Turbo and an automatically cycling strobe which alternates between a 15 Hz and 2Hz strobe. The strobe is hidden in that you have to hold the mode changing button down for 2 seconds to enter strobe. Turning it on and off again clears the strobe, so in normal use you never accidentally get to the strobe.

The use of a separate mode-changing switch gives this light a totally reliable memory, always coming on in the last used mode (apart from strobe). You will not accidentally change modes by using the tail switch (which being a forward clicky is vital if you use this with the momentary output). This also makes it ideal for use as a hunting light in combination with the momentary operation remote switch.

Batteries and output:

The recommended power source for the TK21 is a set of two CR123s, but it will work with an 18650. My preference is for a rechargeable battery (and the TK21 is not compatible with RCR123) so I wanted to use an 18650.

Despite not being the recommended power source, output is actually slightly higher with an 18650 when compared to two CR123s possible due to the 18650 being better able to deliver a high current.

On 18650s, the TK21 is quite fussy on two counts. Some protected cells are too long to fit inside. I marked one of mine when tightening up the tail-cap and it developed a circular pressure mark around the positive terminal button. Changing to a different brand which was 1mm shorter and there was no problem. Secondly, some older cells I had would only keep the Turbo output on for around 5-10 seconds before dropping to high. Newer 18650 cells managed 40mins+ on Turbo.

Running a non-protected 18650 to the point the torch turned off resulted in a cell voltage just over 3V once removed from the torch, so the TK21 seems to have a built in LVC protection.

Indoors and just sitting on a table, the whole torch gets pretty warm after 15 minutes on Turbo, but never too hot to touch. Once it has reached this temperature it stays stable at that temperature and doesn't get any hotter.

There have been comments about a green tint, but I saw no evidence of this. If anything the tint of the TK21 seems more neutral than cool. Directly next to a TK45 which is cool white, the TK21 did look a bit warmer.

The remote tail switch is a hunting related item and is covered in Part 3 – Out In The Field
The bike mount is examined in Part 4 – Riding Into The Night

Standard Beam shots have already been done on Fonark and Lygte, so I won't repeat these.

PART 2 – In The Lab

Beam shots are one thing, but I decided to try and quantify the actual beam profile. There are probably many flaws in my method, but it is simple and easy to carry out and seems to provide a good enough comparison.

The method used was to put the torch on the edge of a table 1m from a wall, with a tape measure on the wall. The zero of the scale is placed in the centre of the hotspot and a lux meter is then positioned at points along the scale, with the measurements recorded. Beam shots are often taken with the light shining on a flat white wall, so this method is simply measuring the actual intensity across the beam on a flat surface, not the spherical light emission.

The results are then plotted on a graph with other torch profiles superimposed for comparison.

For the best throw you want to see a sharp peak with less of the distracting spill. My previous hunting light with R2 LED is also profiled here and you can see it has a pretty good throw profile, but the TK21 blows it away!



After creating this graph I realised that it makes the TK21 and TK45 look very similar despite the TK45 having a higher output on Turbo.

Taking this a little further, I calculated an approximate factor to apply to the lux measurements, as each measurement gets further from the centre of the beam, it corresponds to a larger area onto which the light is falling. It seems to me that this should also be taken into consideration, so I applied these area corrections and came up with this odd looking graph.

The key quantity here is the area under the graph line. This should correspond to the total light output. The TK45 with 760lm output and TK21 with 468lm, so the TK45 should have 1.6 times the output of the TK21 and although difficult to tell accurately, it does appear to show is more in keeping with the actual output of both the TK21 and TK45 on Turbo.



I have also done the profile for the TK41 which is insane (the TK41 is a monster thrower). I'll post this in a TK41 thread when I get a chance.

.....to be continued with....

PART 3 – Out In The Field (hunting with the TK21)



PART 4 – Riding Into The Night

Coming soon

(Note: this light was supplied by Fenix for review)

PART 3 – Out In The Field (hunting with the TK21)

The Fenix AR102 remote tail switch is one of the best I have ever come across. Solidly built, it has a high quality pressure switch and a reverse clicky switch that operates independently. This allows you to have the light switched on permanently or momentarily.

This combination proves perfect, as it gives flexibility while hunting of either having a permanently-on light, using the clicky switch, or a momentary-on light, using the pressure switch. With other lights, I have had to change the tail cap to be able to use permanent on or momentary, so the AR102 makes this easy having both built into one tail cap.

The pressure switch itself has four small click switches inside it, along its length. The result of this is that if you apply pressure at any point along the rubber pad, it will reliably turn on the light. Another 'feature' of this, is that if you use either end, a noticeable click can be heard (which is surprisingly loud on a quiet night) but if you use the middle section, the click is almost silent.



Gun mounting the TK21

With the supplied accessories I have a few different options:

Scope mounting using the figure of 8 metal scope mount, or the rubber block mount.
Barrel mounting using the rubber block mount or a standard scope mount on to the Picatinny rail fixed to the barrel near the muzzle of the gun.
Moderator mounting using the rubber block.

My two rabbiting guns are a .22LR Ruger 10/22 and a York Guns Stealth .410 shotgun. On the .22LR as I use a compact scope, there is no room to scope mount the torch, and I dislike the gun being lit by the spill light, so instead I opt for a forward mounting position with an under barrel mounted Picatinny rail and the torch held in a scope ring.

The mounts


The compact scope doesn't have room for the scope bracket.


My preferred mounting method


Close-up of the mounting block


When using the Stealth shotgun, only the rubber block mount will work, holding the torch near the muzzle, again to avoid spill light on the gun.



Close-up of the rubber mount



A quick digression - showing Security the light

While preparing to go onto the golf course, one of the security guards dropped by. He was curious to see what I was up to and had seen some bright light. He had brought his 4D Maglite and Ansmann Future 3D Plus over and was telling me where he had seen some rabbits, so I whipped out the TK41 to 'point' to where he was indicating. 'WOW, that is like a light sabre! What is it?' He turned on his torches and could hardly tell they were even on next to the TK41. The TK21 resulted in another WOW as he couldn't believe something so small could produce that much light. I gave him a little run down on his lighting and battery choices and he said he would be ordering the TK60 the next day with full capacity Ni-Mh D cells. It seems a security guard really does want something that can be used as a club as well as providing light. It is always fun to show someone 'the light'



But, back to the TK21....



Hunting with the .22LR

My previous gun light throws a spot roughly filling the scope's field of view. The TK21 lights up a much larger area making tracking a target that has decided to run much easier.

Set up on a golf driving range to get a beamshot, the yellow 100 yard marker is clearly lit (with a post at 50yards)


The spill light hits the ground close to the shooter and despite being mounted very near the front lights up the bottom of the sound moderator brightly. Being mounted under the gun, this was not a real issue. If mounted on the scope itself I suspect the spill light would be quite distracting


While finding the rabbits I often use a permanently-on red light that they cannot see clearly (but neither can I) and the TK21, activated by the pressure switch, then floods the target with light making the final aim easy to acquire.

Surprisingly the R2 light still performs comparably with the TK21 for extreme ranges as its very narrow beam allows target identification to approximately the same distance. The wider beam of the TK21 has a tendency to obscure longer distances as the spill is brighter and slightly distracting. For nearer targets, they are flooded with light and are very clearly lit.

With my eyes only partially adapted to the dark I was able to clearly make out the 150 yard marker and just about the 200 yard marker.

The 18650 batteries worked less well than in the original testing, only managing turbo for a few minutes or less despite being fully charged. Swapping for two CR123s solved this, so for reliable performance this light will need to use CR123s as recommended by Fenix. However when using the momentary remote switch, and only lighting the field for a few seconds at a time to acquire and shoot the target, the 18650 was working well enough.

Here the camera is positioned closer to the gun to give a better impression of the shooters view when not looking through the scope.


Just for fun I put my TK41 on the table I had used to support the rifle when photographing the beam, and used the same exposure and camera position. I did this just to show how much of a monster the TK41 is!


Hunting with the .410


In contrast to the rifle which requires precise considered aiming, hunting with the shotgun allows for moving targets to be taken with an instinctive 'pointing' of the gun. This was previously very difficult using the torch with an R2 as it had more throw than spill. With the combination of throw and spill provided by the TK21 it makes target acquisition so much easier and the softer hotspot lighting a wider area well enough to easily track the target, attain lead and fire.

A final factor to mention, especially as I mount the light near the muzzle is weight. The TK21 doesn't upset the balance of the gun, however, going mad and mounting three lights, does!


Although a very capable light, the spill of the TK21 is a little too bright to be an ideal rifle hunting light. The TA20 or TA30 may be a better choice. For the shotgun, the TK21 worked excellently.




PART 4 – Riding Into The Night

The AF02 bike mount is satisfyingly robust. The construction is mainly plastic, but with metal screws that go into metal nuts held within the plastic body. There is full rotational adjustment to allow for crossbar, stem or handlebar mounting options with positive click stops for the fine adjustment.

The torch mount has a removable rubber section which is used for smaller torches such as the LD10, but had to be removed for the TK21. Once fitted to the bike this mount is solid. One feature I would like to see is a quick release to allow easy removal of the whole mount from the bike. If I did leave the bike locked up outside I wouldn't leave the empty mount that anyone could easily unscrew, and it is a bit of a pain to have to unscrew each time yourself.

The AF02


The threaded hole is metal (a metal nut held inside the plastic body)


Fitted to the handlebars




As I have an unusual suspension stem (meaning the bars themselves move up and down to cushion the rider's hands) the light will end up bouncing up and down, so instead I fitting it to the stem extension instead. The fully adjustable rotation then allows the beam to be aligned to best effect. (Note the mount would not quite fit the frame tube). To try and avoid any potential rotation I fitted the torch into the mount so that it was held at the point of balance. With no weight acting forward or behind the mount's adjustable pivot, it should be less likely to be knocked out of alignment.




Out on a ride, the mount is solid as a rock and didn't move even when I purposefully hit bumps hard. Any more abuse would have buckled the wheel, and the mount and TK21 held up perfectly. The beam lit up the trail beautifully with enough spill to light us the ground near the wheel and the throw lighting up far enough ahead to be able to stop before encountering any hazard.



As with the hunting trip, 18650s were only giving a few minutes on turbo, however, high was still perfectly good enough to ride by. CR123s were reliable at maintaining turbo, but may get expensive for regular use.



Whenever I did pass by other people various statements of amazement (including many expletives) were uttered along with me apologising for dazzling them. Good fun!





Mount and light worked extremely well for night time cycling, and knowing that as the batteries get low, the warning is graceful as it just drops to high, gives confidence of not being suddenly plunged into darkness.

Very Nice thanks. Cant wait to Pt 3+4. I have never liked the look of the Tactical rings and would not have it on but I couldn't purposely expose the threads to the elements so would have to keep it on. Although this is the smallest tactical rings that I have seen so far. I am glad you have a non green version. I like the look of this light with the thumb button but have gotten green tinted lights of fenix before and hate it (LD10) although all my other fenix torches are a nicer white (apart from the L01).

Cant wait to Pt 3+4 too.

the pt 2 is excellent and detailed.

BTW,i really like the side swith on the tk21.

Thanks for that review. I'm using my TK21 sparingly until I get my charger and can run it on 18650's. I'm glad to hear that it is as bright on 18650's as on cr123's. I know it drops down from turbo more quickly on 18650's but I'm amazed by how bright a light that size is.

I really like those beam profile graphs, I think that is a great way to show a beam profile, and the methodology seems straightforward enough for others to use it also.

Nice review man!

How do you like it though?

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Duplicate post

Parts 3 and 4 have now been added to post 2 of this thread

Thanks for the review - Got mine from Rob too.

Excellent work and pictures, but your butterfly graph needs extra explanation... what are we looking at exactly????

Cataract said:

Excellent work and pictures, but your butterfly graph needs extra explanation... what are we looking at exactly????

Click to expand...

The first graph is very simply the light intensity measure across the beam. However, I found was that lights with quite different output levels could look quite similar on this graph (for example the TK45 - 760lm and TK21 - 468lm look quite similar) and it does not do enough justice to floodier lights where the light is more spread out.

What I then I tried to do was to take into consideration that each spot measurement taken actually corresponded to a 'ring' of light, the further you move away from the centre of the hotspot, the bigger that ring was and therefore if it is a large area, the total light output into that ring would make up a significant part of the total output of the torch. So I did some very rudimentary differentiation type calculation to come up with a multiplier to apply to each spot value to adjust each measurement according to the actual area it is lighting.

The second graph should therefore show an estimate of the total light output, represented by the area under the line. (and how much light is output across the beam profile). I think my multipliers are a little too coarse making the graph look very spiky.

What I have done is very basic but should give a rough indication. Unfortunately most of the mathematics I used to know has faded away through neglect and I know this could be done better and produce a smoother looking graph.

If you were to design a perfect flood light, as you move away from the centre of the beam, more and more light would need to be output to keep the brightness the same in each successively larger ring. This would make the first graph a flat horizontal line, but the second graph would look like a U with actual output increasing as you move out from the centre of the beam.

Does this explanation make sense?

P3&4 WOW.

This is amazing. I love the real world reviews better that the lab tests. Amazing work. Thank you very much Sub Woofer.

Thanks your sharing. When hunting, I used to my torch TK21 sticky on my shortgun.
The CP of light is good. For a new comer, it's easy to use. Your introduction is so detail. That's benefit for me.

One of the better reviews I've read, thank you

Very nice review indeed, just a note though; after you take off the cigar grip you can use the optional fenix ring/sleeve that can be bought elsewhere that will nicely cover-up the threads that would otherwise be exposed.

picrthis said:

Very nice review indeed, just a note though; after you take off the cigar grip you can use the optional fenix ring/sleeve that can be bought elsewhere that will nicely cover-up the threads that would otherwise be exposed.

Click to expand...

It is a pity that is not supplied with the torch then, as it can't cost much to include a small ring that would cover the threads if you choose to take of the tactical grip ring.

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And thanks everyone, for the encouraging words, I may well be tempted to do another review sometime soon.

I agree the tacticool grip things look ugly and ruin the look of the light IMHO. I have not nor will buy a light with them unless a ring is provided. Not all people own weapons and would like the clean lines that the rings could provide.

I do not see the rubber mount you used for the shotgun on Fenix's website? Is it a product of theirs or another brand? I assume the bike mount is too big to use as a shotgun mount? Thanks!

I use mine for hunting on my SIG 556 and it is an excellent weapon light.