A New Headlamp for a New Season - The “QuadCube”
The first Light Emitting Diode (LED) headlamp that I designed and built was a Technology Demonstrator: “TechDem1” – see the original Night-O Headlamps Article for technical details of that lamp. TechDem1 was the test bed for various ideas that I wanted to apply to a hew headlamp. This lamp turned out to be very successful as a prototype and the (generally) robust build of it and performance naturally led to it becoming my main headlamp for the balance of the 2007/8 night-O season (in preference to the tried, tested – but very tired Silva 10/20W halogen headlamp).
This article describes the next step in the LED Headlight development, culminating in the powerful “QuadCube” design.
Improvements to Technology Demonstrator
There were some early problems that needed correcting, most notably the battery lead, which sheared during the British Night Champs in the Forest of Dean. This left me with nothing better than a very low powered back up torch with which to return to base to retire the course. I suppose if it was going to fail, then it might as well be at one of the two prime night-O events of the season! The reason was down to poor choice of connector leads (essentially, I had opted for something that wasn’t really designed for the flexing that a headlamp cable needs because it was cheap!). Lesson learnt and some new wire and connectors chosen and fitted: the headlamp performed flawlessly for the rest of the season – including during the other main night event – the Harvester Relays.
Review of Performance Requirements
TechDem1 has been superb for what it was designed for and outperformed it’s expected usefulness, but there was a compromise in using two LEDs. I met the prime requirement of having a nice safe pool of light to run through, but TechDem1 lacked a really powerful throwing beam with which to see far into the distance – something that the traditional lamps have been better at. 30 odd years of evolution of specialist running lamps had led to a nicely balanced beam pattern with a single/twin bulb and reflector. Power LED technology is not mature enough for a single light source to be used to achieve a perfect beam and light distribution for a relative minority interest like night orienteering. The current technique used by both commercial and home producers of lamps is to shape the beam by using discrete secondary optics. This is partially due to the limit on the output of traditional power LEDs (though that may soon be a historic factor, as single devices are now becoming relatively widely available capable of best part of 1000 lumen of light output (comparable to that of a 40-50W of incandescent halogen bulb).
New (Improved) Headlamp
The approach of the new Night orienteering season prompted me to continue development to produce a headlamp with less compromise on performance. The outcome of this was the powerful “QuadCube” headlamp, with four LEDs and shaped like a cube.
Light Source and Electronics
QuadCube is essentially two light engines in one housing. The first of the light sources (“Dipped Beam”) was based very closely on the original TechDem1 design. One of the differences was that I used slightly more powerful LEDs. These were Cree XR-E Q5 bin devices – not actually the brightest single die LEDs available (those being XR-E R2 bin – up to 15% brighter), but not far off, and I had already bought them early in the project. I again used square Ledil optics, but found that the beam angles available for the Cree LEDs did not quite correspond to the SSC ones and settled on one medium optic (+/- 14 degrees) and one narrow “diffuser” optic (+/- 7 degrees). The slightly wider overall beam was compensated for by the slightly higher light output of the Cree LEDs and some extra fill-in from the main beam. The control circuit was identical to the one used by TechDem1 and provided a full brightness setting, half brightness and a few (less useful) strobing modes. The dipped beam had it’s own switch. The second (high beam) light engine was itself two parallel circuits: each of which consisted of a Cree XR-E Q5 LED (fitted with a narrow “Real Spot” optic (+/-6 degrees) and a single mode control circuit providing 1A to the LED. Both high beam LEDs were activated simultaneously by the same switch.
The housing was a bespoke assembly made entirely from stock aluminium materials – much sawing, drilling, filing and thread tapping was involved! The LED optics were protected by a square polycarbonate flat lens cut from some scrap material (actually originally used to protect the old club noticeboard from the elements!). The switches and cable gland all incorporated watertight seals, so it could easily be practical to make the lamp water resistant (not dunk proof!).
The Cable from the headlamp to the batteries (intended to be carried in a bumbag) was made from some heavy gauge, but highly flexible silicone insulated test lead wires. I used the same heavy duty connectors from the revised TechDem1 design. QuadCube can be powered from either one or two 7.4 Vbatteries (of the same design as before) in parallel: this is because the headlamp is capable of drawing over twice as much current, and is needed to maintain a comparable runtime (designed to be in excess of 2 hours).
QuadCube was a robust beefy affair and significantly heavier than TechDem1 (though not as heavy as the overall mass of a halogen headlamp and battery). The head mounted part turned out heavier than was originally hoped for, but not excessively heavy to carry on the head (NB around 60g of the overall mass was in the heavy duty cable and connector linking the head mounted part with the waist carried battery). QuadCube is currently mounted onto a set of straps from a cheap commercial headlamp (similar to TechDem1) – these may prove to be the weakpoint in the overall design and work is continuing on trying to improve this.
There are still several proving tests to be done to confirm performance, but experience gained from TechDem1 gives me confidence to use the lamp in less critical situations and competition in the mean time.
Mica SL 10/20W
Light Source Halogen Incandescent Bulb
Battery 4Ah 6V NiCd
Run time 1.25hr on 20W, 2.5hr on 10W
Weight Overall 990g
Lamp only 226g
Battery Only 764g
Est Light Output 400lumen (max)
Power Consumption 20W
Light Source 2xSSC P4 U Bin LED
Battery 2.5Ah 7.4V Li-Ion
Run time Est. 3hrs + on High; 6hr + on Low
Weight Overall 294g (With Revised connectors)
Lamp only 172g (With Revised connectors)
Battery Only 122g (With Revised connectors)
Est Light Output 290-320 Lumen
Power Consumption 4-5W est
Output of TechDem1 on high approximately equivalent to 15W halogen (i.e. midway between 10 and 20W settings on old lamp). Maximum lamp body temperature (still air in ambient room conditions) 55 deg C.
Light Source 4 x Cree XR-E Q5 Bin LED
Battery 2 x 2.5Ah 7.4V Li-Ion
Run time Est 2.5hrs+ High; 12hr+ Lo
Weight Overall 576g
Lamp only 334g
Battery Only 242g
Est Light Output 685-730 Lumen
Power Consumption 10W Est
Output of QuadCube on high approximately equivalent to 40W standard halogen. Maximum lamp body temperature (still air in ambient room conditions) TBC.
Headlamp evolution currently seems to be tracking the developments in Light Emitting Diode technology. There are more new offerings from commercial manufacturers (e.g. Silva, Petzl, Mila) traditionally associated with halogen head lamps. Future evolutionary applications using halogen incandescent bulbs seem unlikely unless power source technology becomes such that a much higher energy density can be obtained in a portable source (but then this could equally be used for the more power efficient light technologies). HID may increase in efficiency slightly (though the technology is inherently more complex), but LEDs seem to currently have the most scope for improvement.
Power output improvements from single die LED devices (like the SSC P4 and Cree XR-E) have slowed down (but are still gradually creeping up). The main applicable advancements in LEDs relevant to headlamps are in packaging of the devices. There have recently been a number of multi-die devices released by various manufacturers. Multiple dies in the same device are not new. The original innovators in high power LEDs, Lumiled, produced a 5W “Luxeon V” device with 4 dies. These proved to be less reliable than hoped for. Osram released a 6 die “Ostar” LED device about a year ago and SSC and Cree have more recently produced 4 die LEDs. The Ostar is relatively difficult to drive from batteries (needs about 20V), expensive, and has a scarcity of secondary optics. The SSC (P7) and Cree (MC-E) devices are more affordable (£15-£20 each), easier to power from existing systems and we are starting to see readily available reflectors and optics. There are already a number of SSC P7 hand torches being sold. The four LEDs of the QuadCube are virtually equivalent to a single SSC P7 or Cree MC-E. Something that needs to be bourne in mind is that the individual dies of these new devices are not any more efficient than the high output single die devices (Cree XR-E Q5, R2 or SSC P4 U-bin) and they will still need to be mounted in such a manner to take waste heat away – thus the enclosure size of a MC-E based headlamp may need to be similar to that of a headlamp using four individual XR-E devices because the enclosure acts as the heat sink to the atmosphere. There is some scope for simplifying the design (and lowering cost) with the multi-die LEDs, and this may be the next step in the search for the ideal headlamp.
Posted by Peter Chapman on 02nd Oct 08