Another thought just came to mind:
(hopefully this hasn't been mentioned already)...
For the spot beam, use a lens/optical device that produces ONLY a spot beam and nothing more (minimal or no spill). These are ideal because they can generate massive throw from relatively small sizes. the idea being here that one can actually choose not to have any flood at all if that is what one wants, there are situations where trying to spot something off in the distance can be made much easier if you have NO flood around you, the excess light in the flood from common reflector beams can cause your eyes to adjust "down" so that things off in the distance are harder to see. Being able to really "hone in" on the perfect spot beam and flood beam combined would really be NICE!
Ordinarily I HATE these types of focusing optics on flashlights because I prefer the spill-beam to be there for 90% of tasks, but in a headlamp where you could control both the spot and spill being able to isolate the spot beam would be amazing!
This is an EXCELLENT point, one that I hope you take to heart. If you have a spill-less and flood-less optic, not only does that mean that your pupils won't shrink in response to light close up, you can also eliminate the flood/spill to help with light reflection in poor visibility conditions and get the same throw for less current to the emitter, and thus longer runtime. This setup makes perfect sense - because you always have the option of turning on the flood beam if you want flood! Plus, there's always the efficiency bonus of under-driving the emitters...
If I could design a headlamp myself, it would be a combination of my PT Apex and my Zebralight H50: Tough. Waterproof. Approximately the physical user-interface of the Apex, with off-low-med-high-off settings for both throw and flood (more on UI below). Two Cree emitters, one with a spill-less optic as described by mdocod just above, one with a total-flood emitter like the Zebralight with one difference, the current Zebralight is a 120 degree circle. I would make the output elliptical: keep the 120 degree horizontal angle and decrease the vertical angle to 75-90 degrees or so. This should keep your peripheral vision good but prevent quite so much light from going into the sky, and result in slightly higher lux readings for any given drive level (more brightness/runtime).
Batteries: I have chosen 2 battery options to base my light collection on: AA and 18650. AAs because they're common, small, have decent energy density, and are available in various chemistry options - I use Eneloops in literally every AA light I have except my ROP, but would use Energizer lithiums for backpacking. I chose 18650s because of their LSD, energy density, and size... unless you're putting out a lot of power, you only need one to run the light. I'm hoping to see a light like the one you're describing available in 2AA and 1x18650, both with the battery on the forehead, with the possible option of a 6xAA or 8xAA and 3x18650(3p) or 4x18650(2s2p) belt pack.
Brightness: You've probably figured out by now that the Apex and the H50 are the lights to beat. The Apex is versatile but crude in comparison to the simply elegant execution of the H50, particularly in one main way: the light levels. The light levels in the H50 seem to take into account the logarithmic way the eye perceives light, and so the three levels (three are DEFINITELY needed when the highest gets only 2 hours of runtime) are arranged such that there is a marked difference between each one. Low is quite low, for those times when you don't need much light - medium is brighter, for those times when you need more than just a few lumens but still don't need much, and high is pretty darn bright! The nature of the beam prevents it from reaching out too far into the night, but at the distances its intended to be used everything is evenly lit-up like the light of day. I hope your engineers purchase/use an H50 when considering drive levels and beam qualities. The brightness of the throw emitter will be slightly harder to pin down, but I feel that it too should have three settings - that way the flood and throw can be used together based on the light and runtime needs of the task at hand. Sometimes flood by itself isn't enough, you need just a little throw - but not much, so you put flood on medium and throw on medium. Maybe you want to use throw primarily, and just have the flood on low to keep you from tripping on your own feet or walking into a spiderweb, rock, or hole. Maybe you want as much as you can get of both and put both on high - or maybe you want the runtime offered by low with the versatility offered by both flood and throw on low. In order to make the throw emitter worth having it needs to be good enough to negate the need for a second light for throw.
Buttons: Three buttons, one for flood, one for throw, and one for "both." Position them so they can't be mistaken for each other - flood right-bottom, throw left-bottom, and "both" in the middle-top. All operate low-med-high-low-med-high-low-etc. Hold any button to turn off the associated beam regardless of current power level. Like the Apex, put some kind of easily identifiable tactile form on each button to make it identifiable by touch. This would mean that if you wanted both beams on medium, from off you press the "both" button twice. If you wanted flood on low and throw on high, you press either flood once and throw three times, or "both" once and throw twice. If you wanted flood on high and throw on low, you press either throw once and flood three times, or "both" once and flood twice.
Beam-alignment: It occurs to me that one of the essential elements that makes the H50 great is that it can be rotated in its mounting to direct that flood beam so it's placed just as you like it. The Apex also has a hinge which allows the same function. This new light would be much less user-friendly if it didn't have something to allow for the same kind of adjustment. The question is, are the two beams going to be independently adjustable? If they are sensibly aligned relative to each-other during the design/testing process, it may be that independently-swiveling beams isn't necessary - but it would be VERY nice. Some people may want to adjust the flood beam to illuminate their feet as they walk with the throw beam higher for longer-range objects, but some others may want the throw-beam centered in the flood beam (cavers?). As small a detail as it might seem, I think it might be the difference between a good headlamp and a great one.
Hope my input was helpful... :thumbsup:
EDIT: Was just reading through to make sure I didn't miss anything and I did. Please please please take mdocod's advice and dispense with "turbo" modes, strobes, or any other such foolishness. Those are for torches, not headlamps. Even if that weren't the case, I imagine it would be worth eliminating the turbo mode just to avoid customer complaints about overheating or damaged lamps...
Now... others have mentioned that if you can't make "high" a selectable continuous output, don't bother, don't mess around with "turbo" modes that are temporary. "Turbo" modes are cheesy and not practical in the field. Figure out what the highest practical mode is that you can dissipate heat for on your design and make that the high, even if it's only 350mA per LED. As other have said, no strobes or SOSs, strobes and SOS modes will simply get in the way and be a nuisance.
) Just my two cents:thumbsup:
So I like where things are going with this light. :thumbsup:
