Body:As a while, the TK40 has quite a massive sight considering that it runs on AA batteries. The reason behind this is because it uses 8x AA batteries, although it can also run in 4x AA configuration (more on that later) the size still remains the same. The main battery tube has a simple cylindrical design with more than ample knurling covering its entire exterior surface. Apart from this, there are two narrow flat surfaces machined on the battery tube where branding text is printed on. Threads on both ends of the battery tube are well cut and of average thickness, similar to those on most other Fenix lights. The threads on the front end has a thick rubber o-ring sitting just below it which provides water resistance. Towards the rear end of the battery tube is a slightly raised hexagon shaped ring. It doesn't seem to serve any functional purpose since the ample knurling already provide sufficient grip and this ring isn't raised enough to make contact when the TK40 is placed on flat surfaces. Inside the battery tube is a battery magazine which is useful considering the number of cells needed to power this light. The "+" and "-" contacts are gold plated for improved electrical conductivity. The ends of the battery magazine are made of plastic, reinforced along its length by aluminum bars. The tail cap is shorter than expected but still is wide enough to allow easy removal for battery changes. It has a thick rubber o-ring just behind its threads for water resistance. Threads on the tail cap aren't anodized and as such, loosening it will not lock out the switch to prevent accidental activation. Its exterior has a somewhat busy design. There are two wide scallops cut in the end for easy activation of the switch while still allowing the TK40 to tail stand. There are two large holes towards the end for lanyard attachment. This is where one of the main design problems of the TK40 surfaces. While the included shoulder strap is useful for carrying a light this large, the way it's attached to the light itself wasn't well thought out. One would expect the shoulder strap to attach towards the tail on one end, and the head on the other. This method has been employed by many large lights, especially HID's and provides a good balance when carrying. TK40 requires both ends of the shoulder strap to clip on the tail cap, although this does allow the light to be carried head-down. Another issue it that the holes in the tail cap itself aren't wide enough to fit the strap clips, so a braided nylon strap, which seems like an afterthought, is required as an adaptor of sorts to allow attachment of the shoulder strap to the tail cap. Inside the tail cap is a toggle switch push button which has minimal travel when engaged, providing good tactile feedback and a muted clicking sound.

Bezel/Head: The TK40 is Fenix's first product to utilize the quad-die Cree MC-E emitter, and also the first among any manufacturer to do so running on common AA batteries. Up the front the head has a bezel with subtle crenellation, quite different from other strike/self defense bezels. It just has four wide and shallow slots cut along its bezel which is sufficient to allow light to shine through when placed head-down on flat surfaces. Behind this bezel sits the usual toughened AR-coated glass lens used my most Fenix flashlights. The AR coating is good, close to that of Olight products. The TK40 uses an aluminum reflector with consistent and well polished textured surface. The LED sitting at the bottom of the reflector cup is perfectly aligned upon visual inspection. On the exterior, the front end of the head has some shallow scallops which serve as an anti-roll feature. However, due to the hefty weight of the TK40 with batteries loaded, this feature becomes less effective. The middle area of the head is a simple, smooth cone shape. Towards the end are a series of cooling fins machined to help dissipate heat. Looking in from the rear end of the head we can see the PCB contacts which make contact with the battery magazine. Threads on the head mate well with those on the front end of the battery tube well with no grinding or cross-threading encountered.

Output: The TK40 has 8 type of output include 5 modes in total, Each of these levels has its own corresponding strobe mode in the following order according to the position of the bezel. Cycling through modes in either set is done by half-pressing the push button, sort of like how the auto focus works on a digital camera.

The user interface works differently from other Fenix lights. All output level and mode changes are done solely by its switch in the tail cap. From the Off position, the TK40 always turns on one of the constant output levels which it memorizes based on the last level used. To change brightness level, simply press and hold the switch for about half a second. To access any of the strobe modes, double press the switch when the light is on its corresponding brightness level.

As mentioned earlier, the TK40 runs on 8x AA cells and advertised to output up to 630 lumens. It can also run on 4x AA cells when needed to, although this is not officially supported and would require the use of good quality NiMH cells capable of performing on high current drains. The TK40 produces one of the most useful and well-balanced beam profiles among the quad-emitter LED flashlights reviewed. The hotspot is of a medium size which illuminates a large enough area to practically be used in close indoor environments. Around the hotspot is a subtle corona, softly tapering off towards the spill beam. Speaking of which, the spill beam is very wide, adding to the usefulness of the TK40's beam profile. It provides more than enough peripheral illumination, easily covering an area from floor to ceiling as close as 3m away. Beam color temperature it slightly cool when compared to sunlight white. While the beam is relatively clean with no artifacts, the hotspot does have a slight dark area in its center which is noticeable up to a distance of about 5m. However this is only if you're intentionally looking for it, in practical use though, it poses no issues.

Regulations is maintained very well thanks to the high voltage provided by the 8x AA cells. Runtime is also generally very, but the high drain and heat produced when continuously running the TK40 does seem to affect its performance on Turbo brightness level. On the other output levels, performance is improved considerably. As shown below, while the High level produces about half the output as compared to the Turbo level, overall runtime is improved almost three times as long.

Runtime Plot:Regulations is maintained very well thanks to the high voltage provided by the 8x AA cells. Runtime is also generally very, but the high drain and heat produced when continuously running the TK40 does seem to affect its performance on Turbo brightness level. On the other output levels, performance is improved considerably. As shown below, while the High level produces about half the output as compared to the Turbo level, overall runtime is improved almost three times as long.

Switch: The switch is a rubber covered clickie and is fully recessed in the tailcap which allows the light to stand on end and be used as an electric candle. Despite the fact that the switch is recessed, it is very easy to use. Click on, click off. You can lock-out the switch by unscrewing the tailcap 1/2 turn. This will prevent accidental activation when packing the light in your backpack or luggage.

Seals / Water Resistance: The lights are protected from the environment by an o-ring at both ends of the body tube, a rubber switch cover and a sealed bezel. Fenix advertises the light as "waterproof" with no specific depth rating. I would call it "dunkable" but not "diveable".

If either gets wet inside, just disassemble as much a possible without tools and let dry before using again.

Batteries: For batteries, this light is approved by the manufacturer for use with AA alkaline, AA rechargeable NiMH cells, and AA L91 Lithium cells. All others are at your own risk. Tk40 is also suitful for 4x AA batteries. however, after the test performence, the brightless and runtime are not as good as powered by 8x AA batteries.

To change out the batteries: unscrew the tailcap, drop out the old cells, place in new cells observing proper polarity. Reattach the tailcap and you're ready to go.

Accessories: Several accessories are included with the light. Inside the package you will find a large adjustable nylon shoulder strap, Spare rubber switch cap, Spare rubber o-rings, Key ring and a Braided nylon lanyard. The Fenix brand is one of the few I have received that included this many accessories, and I have never seen a light include a spare rubber switch cover.

What I Liked: Solid build quality and cleanly machined. Well-maintained regulation and good runtime on most output levels. Easy access to output modes solely by the switch, without requiring head twists. Well designed and useful beam profile

What I Didn't Like:Hefty size and weight may not suit everyone. Shoulder carry design not properly thought out.

Picky Little Things:

Conclusions: The Good - Very bright output on Turbo mode, great build quality as expected by Fenix. Fit and finish are well done. Runtime and regulation are above average. Lots of output performence is able to select and mulitiple functional flashlight
The Bad - Nothing significant.
Beam has a slightly cool color temperature. Is able to tail stand. Toggle switch doesn't allow for momentary activation. Comes in a convenient plastic carrying case.

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