My Rides

Sunday, October 9, 2011

Bike Light - Experience

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I've really gotten into these bike lights.  I've built 2 now, and have even gotten into some light pattern simulation  - mainly to compare patterns of something I don't have to something I do have, to decide how bad I want to have.  I had been holding off writing about them until I can follow up commentary with objective photos.  My digital camera doesn't have a manual mode, so I can't set the exposure and aperture and take apples to apples shots, so those will have to wait.  I still want to note some of those "first impressions" for those tentative about taking the plunge - with questions like:

  • How much is enough light?
  • Should I mount my light to my bike or helmet?
  • How wide of a beam do I need?
I'll "answer" (offer one opinion) those in reverse order.  I'll frame my answers by first describing where and how I ride.  I live in a suburban area.  I mostly ride on unlit paved and gravel roads, with little traffic, sidewalks, and on double-track, single-track trails and in open fields (at a community college near the house).  I ride 15-25 mph off-trail, and 10-<20 mph on trails.  I consider myself an experienced amateur and rode all these places in the daytime for a year before riding at night.

How wide of a beam do I need?
For these areas, and with 200-1000 lumens, I think anything wider than 20 degrees FWHM (full-width, half-magnitude) is too wide.

source:  carclo-optics.com

The above is the (initial) beam pattern of (not scaled for my application) of my 3 x XP-G light.  On the sidewalk or single-track trail the reflection from nearby foliage is too bright - limiting how far I can see down the trail.  On a double-track or on a road, I'd just prefer to have more "throw" or redistribute some light from the side down the trail.  One time, while waiting for a friend, I sat at a wide intersection ( 2 lanes, separated by a median), and my light was lighting the top of a 50 foot tall tree across the intersection.  When I compare to my van's head-lights this light seemed to produce more light (than 1 dim light), but was spread out more...

With somewhere near 1000 lumens (my 3 x XP-G light is rated for up to 1280 lumens, excluding lens losses), having the light too wide isn't the end of the world, but it's not optimal (I've changed to the narrow clear lens, which is technically the same width, but 40% brighter in the center).  So, my advice is (especially after studying optics a bit more), if you're using 20mm TIR optics, for trail riding stick with "narrow-clear" (even for single LED lights, even if the lens is  - I'll explain in more detail in a later post).  Stay away from medium, and  especially wide.

How much is enough light?
That depends if you follow my advice on beam-width, and where you mount it.  After riding a couple of weeks with the 1000 lumen, 24 degree wide light, it broke (I broke it - don't ask).  So, I quickly put together my helmet light.  It's a "warm white" XP-G LED, driven at 1A, and should be producing around 250 lumens.  I rode with this for about 3 weeks, using a 20mm narrow-frosted lens (14.3 degrees, FWHM).  Mounted to the helmet, I found 200ish lumens "just enough" for the riding I do.

If you insist that you need or want something wider (excluding elliptical pattern lenses), 200 lumens is NOT enough.  As you spread those lumens over more area, the intensity diminishes rapidly (and, at least for TIR lenses, the total amount of light you get reduces as you attempt to spread it out).  So, for anything over 20 degrees wide, I'd say you need at least 500-600 lumens (rated pre-lens, as most lights seem to be).

But, for a friend, about to spend hard earned money, or precious time building, I would recommend 500-600 lumens for a single light.  Less light, and you'll be wanting more before too long.  More light, should be in a second light.

Should I mount my light to my bike or helmet?
Helmet!  I'll qualify that.  If you're planning 1 light, at least at first, and it's 500-600 lumens, or less, then mount it on the helmet.  The ability to steer the light - to look around an upcoming turn in the trail, or up the trail when things are smooth or down in front of the tire when things are rough, to look down at the bike computer or gears... ...really makes the most of a single light.

If you've got more than 500-600 lumens, you've probably got a big battery pack.  With newer LEDs you can get 500-600 lumens for about 5W power input, and a single (small, light) 18650 battery is good for about 10 W*hr (10 Watts for an hour).  So, you could go for a < 2 hour ride with a battery that doesn't feel too heavy mounted to your head...

Drawbacks to mounting to the helmet:

  • Mounted on top of the helmet, where most convenient, I've bashed the light on tree branches.  
  • I have to get my light right at the front of my helmet, or else the helmet cuts off the light spilling down right in front of my bike.  I'm not sure all helmets and or light mounting schemes will allow mounting to avoid this drawback. 
  • If you look up as you take a drink, or down at your computer you take the light off the trail.
So, for DIY light builders, based on my experience so far, I think the best value (light where you need it vs. cost, weight, complexity, etc.) in lighting is:

  • Cree XM-L LED, with Carclo 26.5 mm elliptical TIR lens, mounted to the handle-bars, and driven at 1-1.4A, by single 18650 Li-ion cell.  This will provide good illumination of the trail in front, and to the sides.  The beamwidth of this light alone breaks my 20 degree rule, but not when considered as a set.  
  • Cree XM-L LED, with Carclo 26.5 mm narrow clear or narrow frosted TIR lens, mounted to helmet, and driven at 1-1.4A, by single 18650 Li-ion cell.  This is your high-beam when riding straight ahead, and the light you steer as the trail turns.  The XM-L gives a nice spill, so a little glance down should illuminate bike computer of gears, without bringing the bright part of the beam down enough to blind you.  
These 2 lights together, will consume about 7-10 W, and significantly outperform (total light output, "throw" of the light or peak intensity, steer-ability) my 3 x XP-G light (also a 10W light).  The batteries for each will be easy to mount, and the bike mounted light could even be driven with a generator hub...  The 26.5 mm lenses are more efficient than the 3x10mm lenses, putting a lot more light down the trail.  The XM-L is more efficient than the XP-Gs...  In a future post, I'll take you through the analysis, calculations, and beam-pattern simulations that lead me to this advice / conclusion.