Originally Posted by
buzzbee
I recall that the inverse square law applies to a point source of radiation.
I'm pretty sure light falls off less rapidly when well focussed... (extreme case of shining a laser spot on the moon)...
Any light source, at a sufficient distance, does behave as a point source (divergent beam)
The inverse square law applies to all kinds of phenomena. Light happens to be one of them. The light propagates in a spherical wave front which increases in area as you move away from the source. The size of the wave front is related to the size of the aperture of the beam. A laser stays well focused because the radius of the aperture is very small so that the wave front stays small for a longer distance.
Originally Posted by
buzzbee
Overall bike light brightness: I was refering to, then and now, the brightness with "off the shelf" bike lights.
I think they are (on average) brighter now, due to LEDs and batteries.
Niteriders of around a 2000 vintage...as "off the shelf" as you can get...were as bright or even brighter than the single emitter LEDs currently available. They were a MR-11 that was overvolted (about a 4.5 V lamp run at 6 V, if I recall correctly) so they pumped out more light then at nominal voltage. There were other 12 V systems available which put out 400 lumens at nominal voltage.
Although no one used an MR16 in an off the shelf unit, there were plenty of home brew systems around that put out 800 lumens at 12V and a few tinkerers that made lights that put out the 1500 lumen lights. Halogens are power hogs, especially the MR11 but an overvolted MR16 has about the lumen per watt output as is currently available with LED. LED has the potential to go to a much higher lumen/watt output but they aren't there yet. I currently get about the same run time on the same amp-hour battery with LED as I did back in my days of halogen.
Originally Posted by
noglider
cyccommute, I agree that lux ratings don't mean anything without specifying distance. I'm going to bet that bike light makers are assuming some sort of standard distance such as ten meters, which means that the ratings might very well have meaning.
I agree that the distance is probably 10 meters but that's only speculation without further information. To state that the lux is
the measure of light to use without knowing what the distance to the target is and further state that lumens is meaningless is a fundamental misunderstanding of the issue at hand. Yes, lumens are often inflated. But we know that and, from measurements that other people have performed as well as the specifications of the emitter, we can short through the chaff and get to the wheat.
Lux is a measurement that could, potentially, be even more inflated than lumen measurement. As I showed above, I have a million lux light if I measure in the right place. Because the lux measurement is so distance dependent, even a little variation in the distance can throw the measurement off. For example, Telly says he has a 60 lux light. Assuming 10 m distance, that means that the light is pumping out 1300 lumens. I find that value a bit hard to believe. If the lux measurement is made at 5m, the lumen output of his light is 330 lumens...a value that is more believable.
I went over to
Peter White and looked at some of the Lumotec lights. He has a chart on the Luxos lamps. The claimed lux is 70 or 90, depending on model. At 10m, that's a lumen output for a dynamo lamp of 1500 lumens and 1971 lumens, respectively. I kind of doubt that you can get that kind of output from a dynamo. At 5m, the lamps have a lumen output of 380 lumens and 500 lumens, respectively. Both of those lumen values are more in the range of what I've seen for dynamo outputs.
This just goes to show that assumptions can be tricky. They are often wrong.