More About Lights
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March 6th 17, 01:26 AM posted to rec.bicycles.tech
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More About Lights
On Sun, 5 Mar 2017 16:03:30 -0800 (PST),
Thanks Jeff. These however all appear to be battery powered
lights. We were sort of looking for lights that operated on
the hub dynamo of 6V 3W or the Globe dynamo of 12V 6W or
four times the power.
Sorry, I thought you were still open to looking at battery powered
lights. Here's what I fished out of my bookmark dumpster. No reviews
and few tests, but some interesting dynamo graphs and numbers:
This one is well worth reading (or skimming) and has quite a few test
This is why the hub generator has so little drag.
Well, let's do some arithmetic. If your dynamo is rated at 3 watts,
and your lighting is rated at 70 lumens/watt, then the most you can
perhaps deliver is 210 lumens. 6 watts will get perhaps 420 lumens.
Usually, it's less as the losses accumulate. Rectification losses,
optical losses, heat degradation, and connector losses all conspire to
produce lower output.
You might find it useful to know how bright you want your light. For
that, you'll need a Lux meter. I have a Lutron LX-102 which works
nicely, and two junk meters I bought on eBay for sanity checks:
Get one that has a wide range. Lowest on mine is 1000 lux, and
highest range is 50,000 lux.
Find a dark night and an accomplice to operate the meter and send them
down the road to the farthest distance that you might want to
illuminate with your headlight. Use a headlight or flashlight to
light up that area. Have your accompli's take a reading. It will
probably be zero. Now, cut the distance in half and take a
measurement. It will be 2x as bright (lux) at half the distance or 4x
as bright at 1/4th the distance. Adjust the brightness for what it
would have been if the light meter was sufficiently sensitive.
Converting the brightness (lux) to luminous flux (lumens) requires
that you know the distance to the accomplice, and the illumination
angle. (1 lux = 1 lumen/square-meter)
The form below makes a mess of bad assumptions but is good enough for
a rough approximation:
Once you know how many lumens you think you need, and have adjusted
for overly ambitious expectations, you can determine which lighting
technology is suitable.
Lets say you want to see 8 meters ahead and 20 degrees to each side
(or 40 degree conical beamwidth) at 20 lux, which is rather dim.
Plugging into the web page above, I get 485 lumens needed. You won't
be able to do that with a 3w dynamo, but might squeeze by with a 6w
and an oval shaped beam.
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