Wednesday, February 1, 2012

Incandescent optics versus LED optics: implications for upgrading vintage lamps

Sometimes facts get in the way of a great idea. I've devoted a lot of time to the problem of upgrading vintage lamps to power LEDs. Power LEDs produce a ton of light, but they get hot and require adequate heat sinking to maintain their output and limit the chance of failure. This led me to develop a series of rather elaborate copper heat sinks. I've made a screw base LED bulb, a wedge base LED bulb, and custom LED heat sinks to fit the classic Sturmey Archer headlight as well as the Luxor 65 series of vintage French lamps, all using Cree XP-G or XM-L power LEDs.  I fired them up from both a DC bench top supply and a hub dynamo and I was impressed with the results.  For the Sturmey Archer and Luxor headlamps, the outputs were bright, producing huge floody beams.  Before I could make claims about the superiority of these upgrades, though, I thought I should take some lux readings and compare the output of the LED upgrades to that of the original incandescent bulbs.

Cree XM-L mounted in Sturmey Archer headlight

Here is some lux data comparing the output of an original 6V bulb at 350 mA (in actual fact, the original bulb was 6V 0.2A with a parallel 0.1A taillight bulb), a 3W halogen bulb (Reflectalite GH106) at 500 mA and the Cree XM-L upgrade at 500 mA (low end modern hub dynamo output) and 700 mA (high end hub dynamo output as well as Dynohub magnet upgrade).  Lastly, I measured the output of a Cree XP-G mounted in a 41.5mm reflector designed specifically for Cree LEDs. The lux meter's sensor was placed 1M away from the light source and the centre hot spot of the beam was measured.   The lux reading recorded represents the brightest spot of the beam.

Lux output of Sturmey Archer headlight with different light sources compared to Cree XP-G with specialized reflector

Well, what the heck?  Is ancient technology really winning?  Well........... yes, in a nutshell, it is. :(

Although I haven't photographed the beams, allow me to emphasize how different the incandescent/halogen beam patterns were compared to the LED patterns.  The hot spot of the incandescent/halogen beams that yielded the highest lux reading was very concentrated. The slightest change in position of the beam could see the lux reading tumble by half or more; the focused hot spot was very small and the overall beam was quite narrow, certainly less than 45º.  In contrast, the LED upgrades mounted in the same reflectors produced a very diffuse hot spot and a very evenly illuminated flood of probably greater than 120º.  The LED upgrade is clearly putting out more light, but it isn't focused like the incandescent bulbs.

To document this quantitively, I placed the sensor of my lux meter on a tripod and focused the center of the beam on it (highest lux reading).  I then took readings at finite distances from the centre of the beam while maintaining the same distance from the light source, essentially creating a brightness profile across the beam's diameter. I rather arbitrarily set the distance at 2.5m.  Some bike light makers measure performance by lux at 5m.  Lux at 1m also seems to be a common standard, however I was limited by the distance between my work bench and my wall, so 2.5m it is, which is fine for a relative measurement.  Here I compare the XM-L's beam to the beams of a 6V 0.35A incandescent as well as the 6V 0.2A incandescent that originally equipped the Sturmey Archer lamp.
Brightness profile of Sturmey Archer headlight beam with three different light sources

As you can see, both incandescent bulbs produce a narrow symmetrical beam that drops off to zero between 40-50 cm from the center.  The 0.35A bulb has a surprisingly bright peak, whereas the original 0.2A bulb is considerably dimmer. The XM-L beam doesn't have much in the way of a center peak, but provides a wide profile of even illumination.  Unusually, it seems to have some asymmetrical artifacts, which I confirmed by measuring the beam in a different axis (not shown).  My room wasn't big enough to measure the full width of the XM-L's beam.  If was able to, I expect the area under the XM-L's profile curve would be a lot larger than the area under both incandescents' curve (ie. greater total light output).

The 0.35A bulb isn't an especially fair comparison as the original configuration was a 0.2A bulb, however it does indicate that a simple incandescent upgrade can dramatically improve the light output. Although I didn't measure the beam profile, you can bet that the 0.5A halogen bulb would be even brighter!

The obvious problem with the XM-L's performance is the optics. The vintage optics are optimized for incandescent bulbs.  The limited viewing angle of the LED means that, although a lot more light is coming out of the lamp, the optics aren't focusing it into a tight beam.  I had originally fretted that the LEDs weren't in the right position relative to the vertex of the reflector's parabola, but changing that position doesn't have much of an effect on beam pattern.  The glass globe of an incandescent lamp allows the filament's light to enjoy a greater than 180º viewing angle. It is my suspicion that it is this backward directed light that is collected and focused by the reflector.

This LED/incandescent performance difference is less pronounced with the Luxor 65 reflector.  Interestingly, the results indicate that the Sturmey Archer lamp does a considerably better job of focusing incandescent light into a tight beam.

Cree XM-L mounted in Luxor 65 head lamp

Lux output of Luxor 65 head lamp with different light sources
So, I think the lesson to be learned here is that there are rather strict limitations to what you can accomplish with a LED retrofit of a vintage lamp. You really are at  the mercy of the reflector, which does a surprisingly good job of focusing a small amount of incandescent light into a tight beam, but isn't very compatible with the narrower viewing angle of power LEDs.

It also really emphasizes how deceiving lumen ratings of LEDs can be.   I suppose this is why Maglite uses 'Beam Distance' to measure the performance of their flashlights rather than total light output or lux at a finite distance (1m, 5m, etc).  This is the maximum distance at which a light source will produce 0.25 lux, thus taking into account both the total light output and the throw provided by the optics.

Modifying or replacing the reflectors of vintage lamps isn't especially practical, so I think I've hit a performance wall here.  While the LED upgrades don't produce a tight focused beam, their output still produces a bright flood that is certainly adequate for riding around town at night and maybe along a dark bike path at normal speeds. I haven't done a side-by-side comparison yet, but I'm pretty sure the greater total light output from the LED upgrade will result in greater noticeability in traffic.  Careening down a long hill on a moonless night on your vintage French randonneuring bike is probably not advisable though...

The bright side (forgive me) to all of this is that the rear red LEDs are much much brighter than their original incandescent counterparts.   I fret constantly about being visible from behind when I'm riding, so I'm more concerned about having highly optimized lighting for the taillight.  The LED upgrade offers a bright standlight and a super visible rear light (with the potential to flash!), which, for my purposes,  makes up for the fact that the head lamp beam pattern is less than optimal.

Still, hope for a better beam shape is not lost.  The opening into the reflectors of both the Sturmey Archer and Luxor lamps is less than 20 mm, but there are modern plastic optics with a diameter less than that.  It's possible some modern optic made for the XM-L might improve the beam shape.  This one from Ledil looks promising:

Ledil Cree XM-L 16.1mm optic
Need to order a couple of these and see if they can improve the beam shape.

Update:  optics arrived and the results are pretty good.


Taras said...

Did you try MC-E? It's a 4die led. You can wire an MC-E in series(which is equivalent to running 2A through it) which should result in more light than 0.5A XML. Additionally mce has a much bigger die area, and requires a wider reflector, so it might actually work with the oldschool reflectors.

minisystem said...

I did not try the MC-E. For them all to get the same current (300-700 mA depending on the hub used), they'd have to be wired in series, no? This would put the forward voltage up to 4 x 3.2V = 12.8V . I think this would affect the low speed performance more than I would like. At higher speeds it would be great though. Definitely something to consider,although I'm optimistic the mini optic described above will do the trick.

Taras said...

Yes, it would have to be in series. But I think the way out of this is to have a micro-controller(or clever analog electronics) add/remove leds in series in response to voltage changes.