Above is the rear of the battery compartment, where it rests against the back of the head. The transverse headstrap is not in place in this picture, but I've indicated the clasp over which it is to be mounted. The battery compartment has been locked in place with the thumbscrew, and the lamp is ready for operation. Of note is the curved elastomer cushion that's designed to cup the back of the head. It's extremely soft and comfortable, and quite stable and difficult to dislodge, even with rapid motion.
As noted in the Product Information section of this report, the Apex can use a variety of battery types. Below (courtesy of Princeton Tec), I've reproduced a table from the instructions showing the claimed longevity for alkaline and lithium batteries under different modes of headlamp operation.
I find the stated differences in voltage regulated burn times between alkaline and lithium batteries (especially those for the 3 Watt Maxbright) in this table quite noteworthy, though by no means unexpected. I will test some of these combinations of battery and brightness setting in due course, as I'm interested to see how performance declines once the power drops below the point where the voltage regulation circuitry can control it. Given the substantial weight difference between four alkaline and four lithium AA batteries (lithums are about 1.5 oz, 43 g lighter), the known superior performance of lithium batteries at low temperatures, and the significantly superior regulated burn times indicated in the Apex literature, I will likely be using the headlamp primarily with lithium cells. Still, I intend, when testing, to give some account of my experience with both battery types, as they have very different characteristics. I will be also be interested to see how well the battery power indicator predicts the remaining life of lithium cells. These, unlike alkaline batteries, decline from full voltage output to almost nothing, without much notice in the way of power reduction. Lithium cells are also substantially more costly per unit than alkaline, and are not as readily available.
While rechargeable batteries may be used in this headlamp, Princeton Tec's instructions read "Rechargeable Nicad or NiMH battries may result in reduced brightness in some modes due to lower voltage." The instructions also indicate that some battery types will emit hydrogen gas, capable of explosive potential within a sealed container (not a pleasant thought, given the proximity of the battery case to the head)! However, the battery compartment contains a platinum catalyst designed to remove the hydrogen. The instructions note that a severe impact could fracture the catalyst, causing it to spill into the battery tray. It is advised that the headlamp should not be used if this occurs. On a further battery-related note, the waterproof seal should be inspected for damage each time the batteries are changed, and should be kept clean from dirt or chemicals. A cotton swab and mild soap are recommended for cleaning the gasket area, should this prove necessary.
Lamp design and operation
The key to the operation of the headlamp is the heatsink for the LEDs (Light Emitting Diodes), shown above. LEDs are pretty efficient at converting electrical energy to light, but still, a good deal of the supplied energy (as with an incandescent bulb) is dissipated as heat. This is particularly true for the 3 Watt Maxbright LED incorporated in this unit. The first headlamps to include these powerful LEDs to come on the market could only be operated in a "burst mode." This provided some seconds of intense light before protective circuitry dimmed them. The Apex's heatsink should enable the main LED to be continuously operated.
So far as I can determine, the heatsink has about 20 fins extending from a metal plate, within a housing that has numerous slots open to the air. The heat emitted in operation is noticeable, but for the brief time I've worn the headlamp so far, the heat has been very effectively dissipated. This will be something I will be closely monitoring during the test. So far, I have experienced only a slight sensation of warmth where the support plate for the lamp unit rests against my brow. The heatsink and plastic cover should be kept free from mud and dirt and similar obstructions, and the heatsink cover must be kept in place. If it is damaged or broken, the headlamp should not be used (Princeton Tec instructions).
The lamp is hinged at the bottom of the support plate, and can rotate through slightly more than ninety degrees. The hinge is fairly stiff, and it can be something of a fiddle to adjust the lamp angle while the headlamp is being worn. The lamp may be rotated to any angle below the horizontal plane, but it can't be directed upwards without tilting my head back.
As previously noted, the Maxbright LED is housed in a collimator assembly, in order to provide a focused beam. Below the collimator is the battery power meter. This blinks periodically (every two or three seconds) for 24 hours after the lamp has last been used, after which it turns off in order to conserve power. It is itself a small multicolored LED. If the LED shows green, this indicates that greater than 40% of the total battery capacity tremains. Yellow indicates that 40% or less capacity remains, and red that approximately twenty minutes of run time remain. The instruction sheet explains that "Capacity percentage is the minimum expected remaining power needed to operate the Maxbright LED at 0 degrees C [32 degrees F]." Princeton Tec further explains (partial quote) "The purpose of the battery power meter is to give you a general idea of the remaining burn time. Switching modes can create an abrupt change in battery voltage which may cause the indicator to change color. Once the color turns yellow, the burn time remaining depends on the typical discharge curve for the type of batteries you are using." [italics mine]. This proviso is important, because lithium batteries in particular will drop from full voltage to almost nothing very swiftly. Clearly, some degree of interpretation is needed with the meter. I will be closely monitoring its operation during the test period.
A pair of the Ultrabright LEDs are housed to either side of the Maxbright LED assembly. These are mounted within a protective plate, and are protected by a plastic cover. The power cord comes in from the right of the lamp unit. The Maxbright and Ultrabright LEDs are on separate switches; these two are housed on the underside of the headlamp assembly. The Maxbright switch is at left, and the Ultrabright switch is at right. They can be distinguished (at least, with bare hands) by a pattern of four raised dots on the Ultrabright switch. There is a circular indentation on the Maxbright switch, but this is almost impossible to feel.
There is no mode memory (i.e. after turning the lamp off, the previous setting is not "remembered"). The Maxbright LED (or Ultrabright LEDs), when activated, turn on at full power. Each press of the Maxibright button toggles the LEDs between high- and low-beam mode. The arrangement is similar with the Ultrabright switch, with the addition of an added flashing mode for signaling. To turn the lamp off, the switch is depressed and held for a second or so. I have made preliminary measurements with fresh lithium batteries of the light intensity of the Maxbright LED. At beam center, at two feet, I registered about 3500 lux (using a digital luxmeter), with the Maxbright set to high. This is well in line with the claimed 3 Watt output; a 1 Watt LED I have previously tested read 1050 lux under the same set of conditions. Low beam registered about 1500 lux, by extrapolation approximately equivalent to a 1.5 Watt LED. This is still a very respectable level of illumination, and may well prove adequate for all but the most demanding route-finding. All-in-all, this is an absolute powerhouse of a lamp. I strongly recommend avoiding looking directly at the Maxbright LED from close-up.
Straps and cables
The circumferential headband of the headlamp feeds through two slots on the battery case, and similarly through two slots in the headlamp assembly. Length is adjusted with a buckle assembly on the left-hand portion of the strap. The power cable runs from the battery compartment to the headlamp through a pair of nylon clips on the right-hand portion of the strap. Adjustment of the strap length is straightforward. The strap itself is elasticized. It bears the Princeton Tec logo prominently in white, and is patterned in gray and black
In addition, there is a transverse strap provided to run over the top of the head. This fits into slots on the top of the battery compartment and the lamp, but is supplied unattached. While I will test the lamp with this in place (some of the time, at least), at first glance it appears largely superfluous, as the headlamp is very stable without it. Again, this is something that I'll be examining; my preference is to avoid the additional strap.
Future Testing Strategy
Testing will take place over the next four months. Over that period I will be hiking at least once or twice a week, and backpacking periodically. I frequently hike after dark, sometimes over difficult off-trail terrain, and I'm very much looking forward to testing this headlamp in such settings.
I will examine the following aspects of the headlamp performance, in addition to those mentioned in the text above, as well as any other "matters arising."
1. How stable over time is the light output? Does the voltage regulation circuitry indeed perform its job appropriately? This will be tested both in the field and (since subjective assessment of light output is tricky) with a digital luxmeter.
2. In each of the standard illumination modes, does the illumination pattern match the website claims? How easy to operate is mode-switching with the dual switch system that Princeton Tec has incorporated? Is the heatsink cooling of the LED adequate, or does the casing become warm (and potentially uncomfortable)? Is it adequate to protect both me and the LED?
3. What are the limitations of each of the five modes in practical terms? Which mode is optimal for trail use, which for bushwhacking, which for in-camp use? Is dazzle a serious problem in the Maxbright mode, either for others or myself? Is the flashing mode of any use?
4. How effective is the LED Optic Collimator at focusing light into a beam? Is the effective range of the Maxbright on the high setting indeed "only" 56 m (184 m)? Is the intensity of illumination within that range equivalent to daylight, as Princeton Tec claims? Is illumination reasonably even across the width of the illuminated field? How about the low setting of the Maxbright? How does that compare? What's the quality of illumination with the four Ultrabright LEDs in each setting? Is there any color cast to the light on any of the setting? If so, how does it affect perception? How focused is the Ultrabright LED beam?
5. What is the swivel range of the lamp? How comfortable is the headlamp? Is the strap system stable? Can it be dislodged by head movement? How well does it fit over a hat or helmet? How adjustable is the strap to different head sizes?
6. Is the lamp indeed fully waterproof? Is the battery-compartment gasket adequately mounted? Can it be accidentally dislodged?
7. How easily is the battery compartment accessed? Can batteries be changed with gloved hands? How easily is the switch operated? How easy is it in gloves?
8. Is the lamp resistant to moderate impact? How durable is it overall, during the test period?
9. How useful is the battery power meter? Will it enable me to avoid needlessly carrying spare batteries, a weight saving?
I thank Princeton Tec and BackpackGearTest for the opportunity to participate in this wonderful test!
