Photon Rex. Photo courtesy LRI.
Flashlight progress comes in unpredictable spurts, interspersed with long plateau periods where little notable progress occurs. The first generation of LED lights used several batteries and a current-limiting resistor. The second generation used several batteries and a fairly crude electronic chip to provide current-limiting. The Freedom chip used in the ReX is a third generation device that is somewhat more sophisticated – and it shows.
Slightly larger than their existing Photon line of button-cell flashlights, the ReX differs in three significant respects: its battery is a single rechargeable lithium-ion cell, it has four (instead of the usual one) good white LEDs, and the ReX can be recharged on the go using common inexpensive batteries.
Photon Rex. Photo courtesy LRI.
The ReX is fitted with the LRI Freedom controller chip. This chip is reportedly also used in the LRI Photon Freedom Micro single-LED light, but that unit does not have the recharging facility. The battery in the Photon Freedom Micro unit is replaceable, while the battery in the Photon ReX unit is not. The Freedom chip delivers "stepless" variable output and a number of flash modes as well, all operated via the single button. The button is phosphorescent, by the way, making the ReX a little easier to locate in the dark. A spring clip connected to the ReX with a split-ring gives one a way to anchor the little flashlight to hopefully keep from losing it.
Photon Rex. Photo courtesy LRI.
The ReX is charged by connecting to a battery (1.2 volt to 3 volt, see below) using a pair of magnet-tipped wires. The wires are anchored to a yoke that connects magnetically to the ReX itself, and the other ends attach to the donor battery terminals. To keep the positive and negative separate, one yoke terminal has a small plastic collar to prevent it from being reversed inadvertently, creating a polarized connector. This is where several of us had some problems: a mismatch in the dimensions of the polarized connector can actually prevent a connection from being made. At the very least, some fiddling is required to seat the yoke correctly and complete the circuit. Proper connection is clearly signaled by a slow flashing from the ReX, which ceases when charging is complete (in approximately two hours with a fresh battery, according to LRI). This is further discussed under the ‘Problems’ section at the end of this review.
Photon Rex. Photo courtesy LRI.
The Freedom chip does far more than just control the flashing, however. It is a fairly sophisticated switch mode controller chip, and actually performs four distinct functions:
- Sensing the button to turn the power on and vary the power level
- Converting the four volts from the little cell to a variable mark/space ratio supply to drive the four white LEDs at varying power levels
- Converting the incoming battery voltage to the required voltage to recharge the internal cell
- Monitoring the charging process and turning it off when the cell voltage is high enough
It is, therefore, far more sophisticated than the more common chips found in other LED headlights, which just turn the LEDs on and off at three different brightness levels. This is clearly a third generation device.
LRI claims two full charges from an alkaline AA battery and twenty or so full charges from an alkaline D-cell battery. They champion the use of NiMH rechargeable cells for the least expensive and most environmentally friendly approach (with an average three charges from an AA cell). The ReX onboard Li-ion battery is claimed to have a useful life of three hundred to five hundred charge cycles.
The little charge wire assembly comes with a snap connector that can (and should!) be anchored in some safe place to prevent its loss – it’s hard to envision fashioning temporary connectors from stray or salvaged bits you might have in your pack! The ReX is too small to store the wires on board, unlike the rechargeable Aquastar mUV, for instance.
Interestingly, the ReX isn’t water-resistant, despite not having a battery door. It may be they weren’t able to sufficiently seal the two halves of the clamshell around the four-LED bezel to supply this desirable trait, which is a shame, since the Freedom is splash-proof. However, it seems likely that a few splashes would not do much harm to the unit. The ReX ships with a clip similar to that of the Freedom’s for attaching to a hat, etc. (ReX accessories, including a clip, headband, and solar charger, are promised in the future.)
At the time of writing this review, the LRI website was claiming that "The ReX will charge from any standard battery of six volts or less." This is not strictly correct and should not be followed. Previously, the website was claiming a maximum of three volts, which is correct. The six volt claim relates to charging from a USB connector, but the supply on a USB connector has a protective ten ohm resistor in series, and this reduces the output to under three volts when used to charge the ReX. Our understanding is that a USB charger is planned for the ReX in the future, along with a solar panel.
However, in bench testing, we were sometimes unable to get the charging circuit to start working when the incoming voltage was much above 2.5 volts. The microprocessor is a shade cautious about overload. On the other hand, we found it would work at very low incoming voltages, down to almost 0.6 volts, which approximates a very flat single alkaline cell, which is actually more important.
A few notes are warranted regarding the ReX modes of operation, especially about how to turn it on. Starting from the off state, a single brief click of the rubber button turns the ReX on to the high setting. Stepless dimming is performed simply enough by pressing and holding the button again. It dims until it reaches the lowest level, which is signaled by a blink. Other flashing modes can then be accessed. Actually, the dimming isn’t really stepless: there are just lots of steps which the eye can’t resolve, except at the lowest power, which is fine.
However, many of us don’t want to start at the brightest setting and kill all our night vision, and this is in fact a common complaint about many LED lights. The ReX has solved this problem: Instead of giving the button a brief press, you can press and hold the button for a moment. The electronics inside senses the longer button press and obliges by turning on at the lowest setting. Holding the button down will then, after a moment, start ramping the brightness upwards. This has got to be one of the biggest improvements in headlamp circuitry for a long while!
If you keep the button pressed beyond the ramp, the ReX eventually enters the signal mode, cycling though slow, medium, and rapid blink modes and an SOS mode. There’s yet another Morse code mode, reached via a series of fast clicks from the off state. In this mode the ReX blinks briefly, once, with each subsequent click. If you don’t know you’re in this mode, the light will drive you nuts until you recall/accidentally discover how to clear it (press and hold).
Most folks most of the time will only have use for the full and stepless dimmer modes, but it’s important to at least learn the others well enough to be able to get rid of them and regain control of the light! One sometimes wishes the marketing guys could be persuaded to skip the frills and concentrate on the basics, despite how nice some of the frills can be at times.
There is an undocumented auto-off point-of-purchase protection mode, but we haven’t found out how to enter this mode in our testing. If you discover this behavior while using your own ReX, then you’ve solved the mystery. You get out of it by pressing the button down for fifteen to twenty seconds. Instructions for this are included with the ReX.
How do they get all these different modes of operation into such a small device? Quite simple really: the central control chip is (we believe) a PIC microprocessor. There’s no part number on it, just the word "REX". All the smarts are in the software. Isn’t technology wonderful?
The ReX’s bright wide beam is a revelation – how can a flashlight so small do that?!? Fully charged, it’s more like a three- or four-LED headlamp like a Petzl Tikka or Princeton Tec Quad than a button cell light. Without question, this is a light one could hike by – not for hours, mind you, but certainly for a quarter-hour stretch with judicious use. Yes, the little battery inside does have a very finite life when used at high power. More details of this are given below in the technical section.
The beam is the familiar purple-white center with white halo that’s the hallmark of five millimeter white LEDs. The Nichia-brand LEDs have a small shield above and below them, but are more open to the sides, allowing sidespill while controlling top and bottom glare. This configuration makes the beam spill fairly rectangular.
In the Field
The ReX’s four LEDs place demands on the little battery, draining it fairly rapidly on high. Dimmed, it lasts a good while longer, certainly long enough for camp chores and some tent time. If you are making a lot of use of it, you may find it convenient to recharge the ReX overnight, and a single 2,000 mAh NiMH cell should yield two or three full charges. However, if you don’t drain the battery completely each night, many more chargings are possible, and it is hard to give a definite number of possible recharge cycles like this per recharge battery.
The blinking charge indicator makes the ReX easy to find in the dark and makes it very easy to monitor the recharging. Charging on the go is also possible, but the battery, flashlight, and contact wires need to be carefully packed so they don’t disconnect. One won’t know if this happened until unpacking that evening, and our experience is that the connection at the ReX end can be somewhat tenuous.
I’m used to carrying a technical headlamp and don’t have any misconceptions about the ReX replacing it. For trips where I don’t have to navigate after dark for more than a few minutes, it probably is up to the task. And, because it can always be charged up, it always starts at peak performance, unlike button cell lights with disposable batteries, which generally are partly spent. Also, unlike button cell lights, cost of operation for the ReX is miniscule, especially if you rely on NiMH batteries to charge it. For long trips, lower-capacity eneloop-type cells that hold their charge are preferable to high-capacity but common formula NiMH batteries, which self-discharge rather quickly.
We recently spent a week in the Australian Alps. This should have been a late summer trip, but the weather decided otherwise, with nights down to -7 C and some rather violent winds. While I normally take some white LED headlamps, I made myself with Energizer lithium e2 batteries, I also decided to take the little ReX on this trip.
Well, what happened was that I mostly used the ReX each evening instead of one of my headlamps. It was just so easy to use in comparison with getting a headlamp out and putting it on my head. I usually needed some light by the end of cooking dinner, so I general hung the ReX from the roof of the tent and turned it on to the lowest power setting while packing up and getting into my sleeping bag.
Dinner by ReX-Light. The glow at the top edge of the photo is the ReX.
On another weekend trip down a very narrow and rough rainforest-filled valley in winter time, it was dark by the time I had cooked dinner. Again, I hung the ReX from the roof of the tent, this time so my wife could see to eat her dinner. On one notch up from Low, she found it quite adequate, as the lowest setting was just a bit too dim for ease of use. The photo here was taken by the light of the ReX, although I did turn up the brightness a bit to reduce the exposure time.
I should point out that the lowest power setting really is moderately dim. It was bright enough for getting into my sleeping bag, but if I lost something in the tent I usually had to turn the brightness up at least one notch. (Stepless? Not quite!) It was then bright enough for all my needs, and very seldom have I needed to use it at full power.
For these two trips, I had charged the little rechargeable lithium battery inside the unit fully before leaving. In a calculated gamble, I left the recharging accessory at home each time, figuring that the expected light duty it would get would be within the capacity of the little battery. I was right: the single charge lasted the whole of each trip at generally low power.
The insides of a ReX.
For those who might be interested, the ReX may be disassembled fairly easily, and the insides look like this. There is a shell in black, a rubber seal and button cover in white, some electronics on a green PCB with the four LEDS soldered to the edge, and the rechargeable lithium coin cell under the PCB, with a yellow plastic cover.
The Printed Circuit Board (PCB) holds the four white LEDs and the Freedom chip, which is the small integrated circuit (IC) to the right with a blue dot on it. There are also a couple of discrete devices including the essential inductor for the switching operation: the copper coil in the middle. Behind that is the little brass-colored flat push-button assembly in the middle. The red wire at the front goes to one of the metal contacts for recharging. It really looks rather neat and simple.
The steady illumination ranges from high power to low power. It does this by very rapidly switching the bank of four white LEDs on and off. The ratio of on-time to off-time (mark-space ratio) sets the perceived brightness. The ratio is set by a digital counter in the Freedom chip, but the steps are quite small. It is easy to see this as being stepless at anything much above the lowest power. This digital mark/space ratio technique is more efficient than placing a current-limiting resistor in series between the battery and the LEDs.
It should be added here that high power really is very bright and low power really is quite dim: there is a wide dynamic range on this light, much more than on most LED lights.
The result of waving the ReX around.
You can actually see the pulsed illumination if you want to. Turn the ReX on medium to low power on a dark night and wave it rapidly across in front of your face. If you do this correctly, you will see a series of bright white dots in space. Each dot is one of the on-periods. We managed to photograph this, as seen here. Each pulse makes a column of four white dots (the four white LEDs), and there are several passes in front of the camera.
Two methods were used to measure the performance of the ReX while switched on and discharging. For one method, Roger’s, the ReX was mounted in front of a photo-sensor inside a dark box, and the output signal from the light sensor was monitored and recorded with a PC-based data logger. For the other method, Rick’s, the ReX was mounted in front of a calibrated Luxmeter (the Lux is a standard unit of brightness) and readings were taken every five minutes. For charging, several parameters were monitored by the PC data logger.
Test on High.
For the tests shown here, a fully-charged ReX was turned on and the brightness level was measured over time as the battery decayed. As may be seen in Run 1 of Rick’s results (left hand graph), the light output decayed steadily for a while and then seemed to drop a bit more, around fifteen minutes. The light then decayed away to a low level with a long tail. The second test run (Run 2), gave very similar results. Roger’s results (right hand graph) show a similar behavior, with the decay of the battery voltage also shown.
Clearly the behavior of the ReX changes when the battery voltage gets low: it seems to go into a low-power survival mode. This is very useful. The light level during the survival mode tail might be adequate for settling down in your sleeping bag for the evening.
After one such test, the ReX was given a couple of minutes of off-time while the battery rested and was then turned back on to see what sort of recovery the battery can make. It was found that while there was a little recovery, the effect was hardly worth the effort. Don’t rely on it.
One could say that the battery life of the ReX at high power is not great: only about twenty-five to thirty minutes. You certainly would not want to rely on it for an alpine start up the mountains, for instance. But let’s be realistic: you would hardly expect that sort of performance from something this small anyhow. The unit is so bright that it is tempting to compare it with a headlamp using three AA lithium batteries and a 1 W Cree LED, and that comparison is just not fair! On the other hand, it is surprising just how much power is contained in those little rechargeable lithium button cells.
The discharge test was repeated with the ReX set to its lowest power, and the results were similar – but stretched over a much longer time. For a fully charged ReX, the useful time could be several hours. Certainly there would be plenty of time for cooking dinner, washing up, and getting to bed, and probably for more than one night if you are careful. This is something which would depend very much on how you use the light and how well-charged it was at the start, but it should be possible to rely on the internal battery alone for a three-day trip. That’s an awfully light way to illuminate things.
Recharging – Basics
This is typically done with the little recharging accessory seen in previous pictures. This accessory attaches to both the ReX and an external battery by means of small gold-plated high-power magnets (but see below under the ‘Problems’ section). While recharging, the ReX periodically flashes briefly: if you don’t see the flashes, then the unit is either not connected properly or it has fully charged. The instructions that came with the ReX say the flashing will stop when the internal lithium battery is fully recharged, and yes, this does happen. It may take a while, up to a couple of hours if the internal battery started out flat, but it does work.
The bit which attaches to the ReX unit is physically polarized: one magnet has a little plastic sleeve around it which can only mate with the right contact on the ReX. This is meant to prevent you from attaching the battery back to front. Note that the gold contacts on the ReX are just steel and not magnetic: they won’t prevent you from trying to connect the recharging accessory back to front, but the plastic sleeve should prevent you from making it work. However, be warned: The physical polarization does not stop you from attaching the magnets out at the ends of the wires to the battery the wrong way around (reversed polarity), and while reversed polarity is not fatal for up to fifteen seconds (according to the company), this doesn’t do the ReX any good. A longer period may well do some harm, as the components overheat.
While the company claims you can use a three volt battery, we found that the recharging circuit would often only start to recharge the internal lithium button cell when the applied voltage was under 2.5 volts; three volts was sometimes too high, and the charging unit would not activate. This doesn’t matter if you use a common 1.5 volt battery, of course.
Interestingly, we found that it is also possible to be charging from an external battery while the unit was switched on and giving out light. The company confirmed that this is possible, adding that in this mode the recharging is done with very conservative rules, as the microprocessor is fairly busy looking after the LEDs. Finally, there is another undocumented feature which can sometimes be invoked when the ReX reaches full charge. It gives a very brief flash which signals that the unit is charging, but when really fully charged it can give an intense flash of about one second once every few seconds. This is called the "lighthouse signal," and it is meant to alert you to take the unit off charge. In the meantime, it dumps a bit of excess power every so often. Wonderful what you can do with software!
Recharging – Experiments
For the recharging tests, a bench power supply was set to about two volts and was applied through a current-sensing resistor. The ReX would not start charging with a higher voltage. A number of recharging runs were recorded; some typical results are quoted here. Please note that the length of time taken to recharge the ReX on any occasion will depend on the initial state of discharge and on the power supply used (normally another battery). This means that measured charging times may not be very meaningful. The following will, however, serve to illustrate what happens during a charging cycle.
Here we see a typical charge cycle, starting from a fairly low battery condition. The battery voltage (black) starts to rise as charge is poured in. The charge current (blue) settles quickly to a steady level. Eventually the recharger in the ReX decides the battery is fully charged (about 4.22 volts, in this case) and stops pouring current in. The battery voltage relaxes slightly while the charge current drops to zero. The input voltage (green) rises to the power supply level (in this test set-up) as the voltage drop across the current-sensing resistor drops to zero. The ReX has absorbed about 0.84 watts for about 3,400 seconds, in this case, which seems typical for this unit. However, input power can vary significantly depending on the external voltage source: values from 0.3 watts to 0.9 watts have been encountered, and a wider range may be possible.
Choice of External Battery
The ReX is designed to recharge from an external battery, not from a power supply as used for these measurements. Testing showed that the recharger inside the ReX is able to draw power from an external source down nearly to 0.6 volts. This is below the fully-discharged rating of 0.8 volts quoted for most single cell batteries in the AAA to D range, although the final decline from 0.8 volts to 0.6 volts is usually very swift. This is quite impressive.
For those with a technical mind, it should be noted that the internal impedance of a battery, be it a AA or a D cell, will be far less than the one ohm external resistor used in the tests above. The external resistor was used to allow current monitoring. It also means that you, the user, need to be just a little careful, as a short-circuit across the external battery can be somewhat destructive (to the external battery).
It is relevant to ask what capacity a single-cell battery might have, but this is not an easy question to answer. For instance, the data sheet for the Energizer alkaline e2 AA battery states 2850 mAh to 0.8 volts, but the voltage is falling over the discharge cycle. Assuming a 600 mA load, the AA battery is quoted as providing just over two hours of life. Just how many recharges you might get from one of these batteries is totally dependent on the initial state of discharge of the internal battery. If the internal battery was really flat to start, one might get only a few recharge cycles from such an AA battery.
The data sheet for the standard Energizer alkaline AA battery (not e2) gives significantly lower performance figures at high discharge rates. This battery would seem to provide about half the performance of the Energizer e2 battery. It is not as good a choice for high loads.
The data sheet for the Duracell Ultracell AA alkaline battery uses a different method of presenting performance, so the two brands cannot be directly compared. However, looking at the available data suggests that the performance would be similar to that of the Energizer e2 alkaline AA battery.
The data sheet for the Energizer e2 lithium AA battery is again slightly different. This lithium battery excels at high loads and cold temperatures, and is the one many of us use, despite the higher upfront cost. At a load of 0.9 watts, the battery should be able to provide four hours of life: twice that of the Energizer alkaline e2 cell and the Duracell Ultracell. However, if the power drain is reduced, the life goes even higher. What’s more, this lithium AA battery is half the weight of an alkaline AA.
The ReX literature suggests using a D cell to get twenty-plus full recharges. These D cells are heavier than the AA cells, and we question whether the extra weight is worthwhile for normal walking trips. An Energizer D battery may give up to four or five times as many recharge cycles, but it weighs as many times more than an e2 AA alkaline battery and ten times as much as a lithium e2 AA battery. We would prefer to carry the e2 lithium AA – and they can often be used in cameras as well.
Problems when Recharging and Recovery
Applying the wrong polarity does not immediately damage the ReX, but it can overheat the internal electronics in about ten to fifteen seconds. Failure is certainly possible. Doing so is not a good idea, and it may also flatten the battery. It may also be possible to flatten the internal battery completely just by leaving it on for too long. Once the battery is seriously discharged, the recharging circuit inside may not turn on. This can make it difficult to rescue the ReX from a seriously flat state.
If the battery is only seriously discharged (not damaged), it can be recovered by one of two methods. The first is to allow the lithium battery about an hour or so to recover all by itself. It can sometimes do this, to the point where the recharging circuit will start up again. The second method, which should be used with some caution, requires that you open the plastic case up and apply a carefully controlled and current-limited three volts to the rechargeable cell itself. There are some solder connections on the PCB where this can be done, and doubtless this will void the warranty. Both of these methods have been tested – and worked. (Yeah – some mistakes were made during testing!)
The recharging adapter and the polarized connector.
It is also possible (and unfortunately common with the early model units) that the ReX will appear to not accept a charge when you think you have attached it correctly to the recharging accessory and an external battery. The symptom here is quite obvious: it does not flash. Two out of three reviewers, Roger Caffin and Will Rietveld, had this problem. The cause might be a seriously flat battery as above, but more commonly it is due the the polarized connector not making proper contact. In the picture here the gold connector with the red dot is the one described as being polarized. It has a thin plastic shroud around the central gold-plated contact, which otherwise looks like the connector with the green dot.
The problem appears to be that in the early models the plastic sleeve or shroud around the gold connector was effectively a shade too large for the matching hole or depression in the plastic case of the ReX – by a whisker. As a result the gold-plated connectors do not make contact, and there is no circuit. The instructions do say "Do not press or force them into place" and that they will "self-align," but for the early-model units we had, it just didn’t happen that way. Will Rietveld reported having this problem, and the company quickly replaced his unit, in case the problem lay elsewhere. The replacement unit had the same problem, and this persisted until Will tried the "gentle squeeze" described next.
We carefully gave the little accessory part a gentle squeeze onto the ReX body at the appropriate connector. This seemed to push the plastic sleeve on the accessory into its socket on the ReX. When done properly, the circuit was made, and the flashing was visible. We successfully tested this quite a few times. However, Roger found that the repeated squeezing together of the polarized connector had fractured the shroud on one unit. The two pink arrow heads in the photo above point to a pale grey line on the shroud. Viewed under high magnification, this grey line was seen to be a fracture: the plastic had cracked. Mind you, being cracked didn’t mean the unit was unusable.
We discussed this problem with the company. The company attributed the problem to the manual assembly method used for early units: it meant the two halves of the plastic shell were not always properly aligned when the ultrasonic welding was done. They have since moved to an automated system of assembly, which they believe will provide more consistent joining of the parts.
A third problem encountered (by Roger) was that some of the rechargeable batteries seemed to have extremely low capacity, lasting for only a few minutes (about four) at full power rather than the thirty minutes seen in good units. The company reported that some batteries appear to have been shorted out during assembly, and this had severely damaged them (it would!). Again, a replacement unit was quickly provided. In the meantime, Roger spent some time cycling the battery through discharge/recharge cycles, with long float times at the ends. This seems to have restored the faulty battery to better capacity, but with an uncertain life.
Enhancements for the Dedicated Ultralighter
Small enhancements for the UL hacker.
Of course, no dedicated ultralight enthusiast ever misses out on an opportunity to further reduce weight and fine tune his gear. The photo above shows two enhancements which Roger uses.
The red thing on the left is a 200-weight fleece cozy or sleeve, designed to protect the ReX from abrasion inside your pack. It will protect the rubber button and the gold contacts. It’s made from some scraps of generic fleece.
On the right we have the metal clip which came with the ReX. Now having a clip is an excellent idea, of course: it lets you hang the ReX from the roof of your shelter at night. Granted, the clip weighs only about three grams, but it looks so solid (heavy). A suitable UL replacement is shown on the ReX in the middle: a small shaped hook made from 0.9 millimeter stainless steel MIG welding wire. Weight – under one gram. You could probably use part of a paper clip instead – preferably plastic-coated. The hook is folded sideways so that it lies flat against the body of the ReX inside the red sleeve.
|2007/8, Photon ReX|
|internal rechargeable lithium, not replaceable, 300 to 500 recharge cycles|
|four 5 mm Nichia|
|From twenty-five minutes upwards at high power to several hours at low power|
|16 g with metal catch (very useful); recharger with clip 3 g|
- Reasonable price
- Good company support
What’s Not So Good
- Tricky attachment of the charging accessory (reported to be addressed in the next production run)
- No way to clip to cap (reported to be coming)
- No solar charger (not yet, but a prototype has been seen)
- Not waterproof
- Improve charge wire anchoring or attachment to the ReX itself (priority)
- Integrate a cap clip or provide a minimalist head strap
- Make water-resistant (tricky)
- Provide a truly UL hook