Friday, February 3, 2012

Ledil optic improves Sturmey Archer Cree XM-L upgrade

I recently posted about a Cree XM-L upgrade I designed for the vintage Sturmey Archer head lamp (at least the one seen most commonly on North American Raleighs from the 1970s).  The light output is high, but the beam shape has some significant shortcomings, rather embarrassing ones considering the centre of the beam from the original bulb was still brighter than the LED upgrade.  Yikes!

The wise and wonderful people at CPF suggested I try a small modern optic instead of relying solely on the vintage reflector.  The choice of sub 20mm optics for the XM-L is quite limited.  The best I could find was this 16.1mm  37º optic from Ledil (This 23º optic would probably be better but wasn't stocked by either Mouser or Digikey).  It arrived from Mouser today.  I had to snip off the mounting posts, but it otherwise fit perfectly over the XM-L's PCB, attached firmly with its own adhesive tape:

Ledil 16.1mm 37º optic mounted on XM-L
As before, I measured the beam profile at 2.5m from my lux meter's sensor.  Here's the data I would show if I was a savvy marketer:

Beam profile of Cree XM-L with Ledil optic compared to original incandescent bulb
Well, that looks great.  Big fat wide beam compared to the puny and narrow beam of the original bulb. Of course, that doesn't tell the whole story.  Here's the same beam profile compared to not only the original but also to a 0.35A incandescent.  The profile of the untreated XM-L's beam is also plotted:

Beam profiles of Sturmey Archer lamp with different light sources
While the Ledil optic does a nice job of evening out the profile of the XM-L's beam and doubles its brightness in the center, the peak output is still a lot lower than a simple incandescent upgrade.  Granted, an original Dynohub barely puts out 0.35A at high speeds and the front bulb is usually run in parallel with a 0.1A taillight bulb.  What you get from the Cree XM-L is a bright wide beam with lots of spill.  This results in a nice amount of off axis visibility, which is important to me in the city traffic where I do most of my riding.

With the Ledil optic I'm willing to declare my Sturmey Archer LED upgrade a success.  There's even the possibility of upgrading to the 23º optic if it ever makes its way over to North American shores.


Bob Derringer said...

From someone oddly obsessed with high-power LEDs and secondary optics, you are featuring LEDiL's FA11902_TINA3-W and I will make it my personal mission to get the 23º FA11905_TINA3-S stocked in North America along with the wider angle beams of 56º and 71º.

minisystem said...

Thanks Bob. You are with Ledil, right? It'd be nice if Digikey had a wider selection of Ledil products. Their shipping to Canada is only $8 compared to the $20 that Mouser charges for the same service!

Bob Derringer said...

Yes, I am with LEDiL and I work closely with our distributors. We value the input we receive on this blog and others and will work to get the spot version in stock in the channel.