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Fenix LD41 prototype Review

6 Januar 2012
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Fenix are updating the LD40 with an XM-L emitter. On test here is a prototype (not even pre-production) of the LD41.

All comments in this review are limited to the prototype sample I have. The final LD41 may differ slightly from this early prototype.

Initial Impressions:

The LD41 prototype is all metal, and has a similar rectangular battery tube to the LD40 to hold the 4xAA that power this light. In this sample the edges of the rectangular profile would benefit from some rounding off to give a more comfortable shape to grip.

The LD41 is a very good size and weight, and odd though it may seem, the light fits well in your hand and makes it extremely resistant to rolling.

What is in the box:

Being a prototype, there was no packaging or spares provided, so this section is not applicable here.

LD41 - as supplied


Looking inside:

Now looking from the top to show the rectangular cross section of the LD41 (compare to the side view shown above)


Looking directly at the tail switch shows the larger forward clicky switch and smaller mode changing switch. The plate holding the switch rubber covers is fixed with four screws and the plate itself in this prototype is steel. There is evidence of minor corrosion from finger prints showing on this plate.


Looking directly at the LED


The contacts in the head are typical Fenix style, but as this is a prototype, the finish is not that neat.


The threads are trapezoid and a single o-ring is used


With the battery holder still in the battery tube, you can see the circular negative contact surrounding the central positive contact. Again as a prototype, the parts used are not of a final production quality


The battery holder has a 4S circuit with 2x2 layout (measure voltage is around 4.7V with Ni-mh depending on charge level). This holder looks like it is the same as the LD40’s.


And switches which are integral to the battery holder (the battery tube only holds the switch rubbers, not the switches themselves)


Modes and User Interface:

The tail-switch is a forward clicky giving instant access to the previously set constant output mode. Next to the power switch is the mode changing switch. The previously set constant output mode is remembered so the light will come on (or can be flashed using the momentary feature of the forward clicky switch) in this memorised mode.

To change modes the mode selection switch is used in two different ways:

To cycle through the four constant modes, a brief press of the mode change switch changes from low->med->high->turbo

If you press and hold the mode switch while the light is on, after ~2s the light enters the two speed strobe, and if you keep holding it for a further 2s an SOS mode is activated.

Switching the light off and on, or a brief press of the mode switch changes back to the constant mode.

Batteries and output:

Supporting only standard AA battery types the LD41 is likely going to be optimised for ni-mh as most Fenix lights are. As there are no specifications or instructions it is not clear if Lithium AAs (1.7V) will be supported, but it is likely they will be. Testing has been carried out with Eneloops.

As this prototype is not the finalised I do not have the output specifications, however I have just built an integrating sphere and have used this to get an approximate measurement of the constant output modes.


Based on my integrating sphere measurements and comparisons to other well known light sources to calibrate the sphere, I have found the following results:

Low – 5lm
Med – 76lm
High – 190lm
Turbo – 510lm

An early promotional material for the LD41 says it will have 458 max output, so this prototype may be performing above the final production models.

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The Use of photos/pictures and material you don't own the copyright for is not allowed in the TLF because of the German law.
You can, however, post a link to the pictures. :)

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The modes are regulated and do not use PWM. When the batteries can no longer provide maximum output, the high mode simply dims gradually. As there is no sudden cut off so you will not be left in the dark and have plenty of warning.

In The Lab

In an attempt to quantify the actual beam profile I developed the following test. 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 light 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.

For the best throw you want to see a sharp peak with less of the distracting spill. For the best flood light the trace should be pretty flat.

The LD41 is shown here with the RC10 pre-production model I have on test (very similar beam to the TK15) and a ‘standard’ Cree R2 P60 module. The LD41 has a quite diffuse hotspot and nice floody beam.


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 LD41 has a lot of light in its spill and a very impressive output.


The beam of the LD41 protoype

As a general purpose light, the LD41 has a well rounded beam. Great for shorter distances with a bright floody spill and still enough power for mid range use.

With the exposure set for a similar look to the naked eye (indoors on maximum is bright!)


And a reduced exposure to highlight the shape of the beam


Taking the LD41 outside I ran it next to the TK41 on a driving range

First the awesome TK41


Then the LD41


Moving to a nearby track, again first the TK41


Then the LD41


Using the LD41

The LD41 has proven itself to be an excellent midrange light. A bit too large to EDC, but not as large and heavy as the TK41 and TK45.

There is plently of power on offer here and a really useful beam for everything but searching long distances.

A rectangular cross section is not that common (though there are some other examples on the market) but it actually give a very positive grip and feels good in the hand. I have asked people with both large and small hands to try it, and they all liked it saying they felt they could hold it firmly and even the person with the smallest hands was happy using a cigar grip to turn it on and change modes.

Hopefully the final version will have the same maximum output as this prototype (which measure over 500lm) and the same beam profile.

With its unusual shape, the LD41 has proven a surprisingly good performer in every regard and gives a real AA powered alternative to the common P60 host 18650 form factor lights.

Test sample provided for review by The Photon Shop.

I’ll update post 2 of this thread once I have some more comments to add....
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