Fireflies ROT66 review (3x18650, 9x219B)

Disclaimer: The Fireflies ROT66 was provided for testing by manufacturer free of charge

Product page:

A new brand called Fireflies recently surprised the community with a release of a flashlight combining many features we love.

The ROT66 drew attention with its
-Nichia 219B sw45k R9080 high CRI emitters (XP-L HI optional)
–5000 lumen output from those (10000 lm for the XP-L HI version)
-Fantastic NarsilM ramping UI by Tom E
-Compact soda can format with 3 18650s

For many, this was a dream come true and unsurprisingly, it has apparently sold well after the release.

If you’re here only for the numbers, here they are:

Manufacturer’s specifications
Battery: 3x18650 (accepts unprotected button and flat tops, protected too long to fit)
LED: 9x Nichia 219B sw45k D220 R9080 4500 K, or optionally XP-L HI V3 in 6500 K, 5000 K or 4000 K
Optics: 25° TIR lens
Lens: AR coating and 98% light transmission
Waterproof: IPX8 2 meters
Driver: 3 channel (1x7135 + 6x7135 + FET) with NarsilM 1.2 firmware
Switch type: electronic side switch
Mode memory: configurable
Low voltage protection: yes
Thermal regulation: yes
Lockout: electronic and physical
Tripod socket: no
Body material: 7075 Aluminium
Anodizing: HAIII
Color: black or champagne

Manufacturer’s output specs
Maximum output: 5000 lumens
Other output levels: infinite ramping or several programmable mode sets (NarsilM)
Light intensity: -
Beam distance: -
Special modes: several strobes and beacons

Measured dimensions and weight
Length: 109.6 mm
Head width: 53.5 mm
Handle width: 44.6 mm
Weight: 237 grams plus 140 grams for three 18650 batteries

Box and contents
The light comes packaged in a better than typical hard cardboard box with magnetically closable flap.

Bundled in the box with the light:
Two spare o-rings

My review sample didn’t have a printed manual even though it is supposed to come with one.

Physical appearance

The nine Nichia 219Bs are behind a clear TIR optics. Fireflies says the beam angle of the TIR is 25 degrees. My measurement shows 22° to 50% lux. The glass lens has an anti reflective coating and specified light transmission of 98% which I verified at 98.4%.

Battery carrier holds three 18650s. The carrier is inserted to the body with the end with contacts facing the head. The battery configuration is parallel so the driver receives 4.2 volts.

There is double springs on both the driver and on the battery carrier. They seem very nice and I’m told are all gold plated beryllium-copper. They are quite short which makes for a solid but quite a tight fit even with unprotected button tops. Protected batteries will not fit in the carrier. Flat tops are fine, since the positive end has a protruding button on the carrier.

I would periodically check the screws that hold the posts together to the end caps . Continuous inserting and removing of batteries may loosen them as you twist and turn the carrier in your hands.

The threads and o-rings on my sample were dry. The v-threads are quite thin. Tailcap takes about 3.5 turns to close.

The side switch isn’t recessed and might get accidentally pushed in in a pocket or bag. Just open the tail a quarter turn to physically lock out the light. 4 shorts clicks from off also activates (or deactivates) electronic lockout. When locked, only two of the four switch backlights are lit. In addition to the four clicks, you can unlock the light with a power cycle (open and close the tail).

The screw thread on the opposite side to the switch is not for a tripod but a mount for the lanyard.

The bezel is not glued and comes off easily. However, the driver retaining ring is glued. I didn’t try to open it.

The MCPCB is screwed on the integrated 7.5 mm shelf. The emitters are not rotated, but the sheer number and positioning of them avoids the square beam syndrome.

User interface

The ROT66 is operated via an electronic side switch. It uses Tom E’s NarsilM 1.2.

The UI is way too advanced to be described in its entirety here, but here are some major features:
-Ramping output selection, hold switch to adjust brightness
-Instant one click on/off
-Shortcuts to moon (long click from off) and max output (double click from off or on)
-Several configurable mode sets with discrete output levels
-In mode config it’s possible to advance (single click) and reverse modes (long click)
-Adjustable temperature limits or timed stepdowns

Here’s the user manual for the ROT66 (click image for full size)

Alternatively, you can use the NarsilM reference manual, the manual for the BLF Q8 or the cheat sheet. If you ever need a manual for any of the popular UI’s, check CRX’s firmware manual thread.

The UI is intuitive even if you’re not familiar with all the aspects of NarsilM. Give the light to someone and they’ll figure it out immediately. You don’t necessarily need to delve into the configuration menu (hold switch for 8 sec) at all. Only thing I would do is to adjust the temperature limit for your particular sample, as I suspect that it doesn’t come calibrated from the factory.

Size comparison

Size comparison to other multi emitter TIR optic lights. In size the Fireflies ROT66 is very similar to the previously tested YLP Gryphon G180. I find the size and weight distribution excellent. It’s much more pocket friendly than the bigger 4x18650 lights.

Astrolux MF01, Noctigon Meteor M43, YLP Gryphon G180, Fireflies ROT66

Beamshot comparison

The TIR shootout comprises of
Fireflies ROT66 (9x219B CRI90)
Noctigon Meteor M43 (12x219C CRI80)
Emisar D4 (4x219C CRI80)
Astrolux MF01 modded with TA driver (18x219C CRI90)
Acebeam EC65 (4xXHP35 HI CRI70)
YLP Gryphon G180 (9xXP-G2 CRI70)

Individual beamshots:
Fireflies ROT66 (9x219B CRI90)
Noctigon Meteor M43 (12x219C CRI80)
Emisar D4 (4x219C CRI80)
Astrolux MF01 modded with TA driver (18x219C CRI90)
Acebeam EC65 (4xXHP35 HI CRI70)
YLP Gryphon G180 (9xXP-G2 CRI70)

Fireflies ROT66 (9x219B CRI90)
Noctigon Meteor M43 (12x219C CRI80)
Emisar D4 (4x219C CRI80)
Astrolux MF01 modded with TA driver (18x219C CRI90)
Acebeam EC65 (4xXHP35 HI CRI70)
YLP Gryphon G180 (9xXP-G2 CRI70)

Fireflies ROT66 (9x219B CRI90)
Noctigon Meteor M43 (12x219C CRI80)
Emisar D4 (4x219C CRI80)
Astrolux MF01 modded with TA driver (18x219C CRI90)
Acebeam EC65 (4xXHP35 HI CRI70)
YLP Gryphon G180 (9xXP-G2 CRI70)

Beam and tint

In addition to the firmware, one of the most attractive features of the ROT66 is its choice of emitters. Fireflies somehow managed to score loads of the now discontinued Nichia 219B sw45k R9080 LEDs. They are known for their superb color rendering and famous rosy tint.

The combination of the optics and lens in the ROT66 make the emitter look a bit more neutral than you might be familiar with. This is a good thing, as the light has just the perfect amount of below black body rosiness on the lower levels up to the maximum regulated mode (7x7135, 700 lumens). On maximum output the beam is visibly more pink which is normal for the 219B.

The beam is also very smooth and exceptionally consistent in tint despite the clear optics, even though lights with those often exhibit visible tint shifts. In my opinion the 219B ROT66 easily takes a top 3 spot in my collection of lights with regard to light quality.

Spectral data and color rendering

For spectral information and CRI calculations I use an X-rite i1Pro spectrophotometer with HCFR, Babelcolor CT&A and ArgyllCMS spotread for the graphs and data. For runtime tests I use spotread with a custom script and an i1Display Pro because it doesn’t require calibration every 30 minutes like the i1Pro.

Explanation of abbreviations

If you have an hour to spare, I recommend watching this presentation on IES TM-30-15 which also shines light into color rendering in general.

CRI data on turbo measured from the hotspot

CRI data on 7x7135 (700 lumens)

CRI data on 1x7135 (100 lumens)

CRI data on low (4 lumens)

Here’s the effect the lens has on the output, CRI and tint.

Transmission: 98%

Without it the beam is a bit rosier (duv –0.0030 vs. duv –0.0018).



Output and runtimes

Comparison on ~100 lumen level, 1x7135 mode on the ROT66 and D4. Test done with just a single 18650.

In stock form my ROT66 stepped down before or right after 30 seconds, depending on ambient temperature and whether I had used the light or made a complete cold start. After adjusting the temperature limit it steps down after 60 seconds with the head reaching 65°C.

With half drained batteries the maximum output drops to 3259 lumens (at 30 sec), since on the FET the output is tied to battery voltage.

Standby drain

There’s some parasitic drain on the batteries when the light is switched off. This is understandable due to the electronic soft switch and especially its backlight. The drain is not very high though.

Standby current: 164 µA

It would take about six years to drain three 3000 mAh batteries.

Low voltage protection

Low voltage protection at 2.89 V

The light will drop output as the battery voltage drops until the voltage drops below 2.9 where the light shuts off.


There is no visible flicker, but PWM is used on modes other than 1x7135, 7x7135 and full FET.

I measure the flicker optically from the LED with a Thorlabs DET36A/M photodetector and an oscilloscope.

Flicker frequency: 15.6 kHz below 7x7135, 31.2 kHz above 7x7135
Snob index: 0% (not visible)

Flicker measurement on low (4 lumens)

Medium (between 1x7135 and 7x7135)

High (between 7x135 and full FET)

Almost max

Brightness is varied using PWM on every brightness setting except for three exact modes where either one or seven 7135 current regulators or FET are are fully active. The 7135 modes cannot be directly activated in ramping mode, but they are part of the mode cycle on the default mode setting. With moon activated (default), these PWM free modes are the 3rd and the 4th mode with the 5th being full FET turbo.

PWM frequency is either 15.6 kHz below 7x7135 level (700 lumens) and 31.2 kHz when the FET activates above that. Flicker is not visible to the naked eye.

Here is how the output looks like on a scope when the brightness is increased.

First the output is PWMd between zero output and a single 7135. Above 1x7135 the six other regulators turn on and PWM switches output between 1x and 7x7135. Finally the FET activates direct drive, first by PWM between it and the regulators and finally transitions fully to the FET.


My testing was done indoors in 28°C ambient temperature.

The ROT66 gets very hot very fast. It’s not quite as aggressive of a hand heater as the Emisar D4, and is able to retain turbo longer.

The temperature on turbo depends on the sample’s calibration and the temperature limit set by the user. I configured mine to step down when the surface has reached 65°C, which allows the turbo to run about a minute before stepping down. In the factory default configuration it stepped down before 30 seconds and barely reached 50°C on the surface. After stepping down to ~700 lumens, the light will quickly cool down to about 55°C

If you start the light at the maximum regulated output of about 700 lumens, it also gets a bit too hot to touch after some minutes, but is safe for the electronics and batteries.

Compared to a typically sized triple or quad hotrod, the ROT66 does shed the heat more efficiently. For example the Acebeam EC65 heats to over 60°C on its medium level of ~550 lumens.

On the 7x7135 level the light will probably be hand holdable indefinitely. When freely tail standing without a hand helping in the heat sinking, the light heats up to 53°C in 40 minutes.


Fireflies ROT66 is the ultimate tint snob light. It’s the only factory light with the Nichia 219B sw45k super high CRI emitters (R9080) and offers thousands of lumens of excellent light quality. It does 700 lumens of regulated output for 3-4 hours depending on battery capacity.

Combine all this with the flexible (ramp/modes) NarsilM UI, jacket pocketable size and no obvious letdowns it’s easily one of the must haves for 2018. Act fast since the now discontinued sw45k stock will not last forever. The driver hardware and temperature regulation is not as advanced as on some higher end lights, but the efficiency isn’t bad either.

Only thing I experienced with my light and was confirmed a possible issue is the memory bug, which makes the light forget the ramp/modes setting when changing batteries. This does appear to resolve on its own after some use though.

+Beautiful tint and superb color rendering right ouf of the box
+Handy size
+Regulated output up to 700 lumens
+Flexible NarsilM UI
+Very low moonlight mode
-Possible random problems with memorizing settings

Awesome review as always, with useful quantified measurements well beyond what most people provide. I wonder how they got 219b emitters.

I’m not happy to hear that the driver is glued, but hopefully there will still be a way to access it.

I couldn’t tell from the pictures, but are the batteries parallel or in series? Does the lighted switch turn off completely sometimes, or only halfway off?

Thanks for the comment and thanks for helping Tom E and Fireflies with the memory bug. Let’s hope it works and they have the fix applied soon.

The batteries are in parallel. Haven’t seen the switch backlight turn off, other than half of them are off when the light is on or it is in lockout mode.

Half are probably wired to Lexels L pad and the other half to the LS pad. L is the normal mcu controlled pad and LS is the permanent on. This is a new thing on Lexels drivers.

Damn that light is nice.

It’s using BeCu C17500 springs, has a NarsilM FET 3 channel driver, using rosy 219Bs R9080, a bit too rosy though, and a very nice form factor.

But that’s just it, they aren’t too rosy. Just makes everything else look green, but objectively at –0.0020 to –0.0030 duv on the mid output levels, the end result seems pretty much ideal to me.

Nice pics and collection. I would like to have one too. And thanks for the measurements.

BTW recently watched maglite mods :slight_smile:

Here’s the effect the lens has.

Transmission: 98.4%

Without it the beam is a bit rosier (duv –0.0030 vs. duv –0.0018).



A relatively small change, but I’d probably like it better without the AR lens. A bit closer to my ideal CCT, with a bit of extra pink, is a double win. :slight_smile:

The difference with vs. without lens is practically imperceivable with a CIE 2000 deltaE of 2.2. Everything under delta E 3 is difficult to notice with the naked eye.

Really good review, thanks maukka :THUMBS-UP:

Awesome review!

Glad I ordered the Nichia version and not the XPL HI version.

Excellent review once again! Think I do have a potential fix, can't be sure but potentially. I'll be talking to them today.

Still wondering about the lumens. In theory this light should be equal to 3 triples, 1 batt per 3 LED's. I don't know how the path from the LED's through the battery is, ex: thickness of LED wires, etc., quality of springs, but if an equally tested 219B triple could be tested for comparison, it would be nice to know.

3860 lumens means 429 lumens per LED, or 1287 lumens for a FET based triple.

I only happen to have an Astrolux S41S with 219B sw40k R9080 and MTN17ddm FET driver with bypass on the driver and double springs in the tailcap. It only does 900 lumens max (at 0 sec) with a fully charged 30Q.

edit: another point of comparison is a triple 219C R9050 BLF X6 with a FET (h17f driver), it does 1620 lumens at 0 sec, 1560 lm at 30s.

That's interesting, not the same LED's but big difference per LED.

Thanks Maukka, great review as always.

At 65 degrees, is it still possible to hold the flashlight?

Found another 219B triple, edited my previous comment.

edit: and realized it is actually a 219C CRI90 triple…

Nice review, thanks! :smiley:

65°C was at the head, it takes time to travel to the rest of the body. 65°C is too hot to hold, about 50°C is what I can handle for a short while.

Thanks Maukka