Originally Posted by
Burton
In theory that all sounds good - in practice the details and specifics mean it doesn't work. What happened to your trig?
A 10 degree beam at 100 feet is about 17 feet wide and only 3.4 feet wide at 20 feet. At a distance of about 40 feet it would encroach on oncoming traffic, and after that it would become increasingly LESS of a problem.
A 35 degree spot is 60 feet wide at 100 feet. That's going to encroach on oncoming traffic regardless of the distance because its still 12 feet high/wide at 20 feet and 6 feet high/wide at 10 feet. And yeah - there's less lux in a flood at any given distance because the beam is spread over a wider area, but light intensity is still proportional to the square of the distance. So the closer you get to a flood the more of a problem it is.
I see your trig and raise you an area. A 10 degree light at 100 ft would cover 227 sq ft of area (assuming a perfectly round beam). At 20 ft, it will cover a 9 sq ft of area. A 35 degree beam would cover an area of 2827 sq ft at 100 feet and would cover an area of 113 sq ft at 20 feet distance. Let's assume that the lumen output of both lights is the same and, for convenience, it's 1000 lumens. Now assuming a completely even light coverage over the entire beam, i.e. a beam without hot spots (not realistic but easier to calculate) we get the following: For the 10 degree beam, the lux (lumens/sq m) is 47 lux at 100 feet and 1200 lux at 20 feet. For the 35 degree beam, the lux would be 3.81 lux at 100 feet and 90 lux at 20 feet. For comparison (and
from Wikipedia), the a full moon has a lux of .2 to 1 lux on a clear night. A light in a living room has a lux of about 50. An overcast day has about 1000 lux.
So at 100 feet, a 35 degree beam is about as bright as the full moon...hardly blinding. At 20 feet it's a bit brighter than a living room. The 10 degree beam isn't as bright as a living room...let's say a romantically lit bedroom...at 100 feet. It is damnably bright at 20 feet...if you consider an overcast day to be blindingly bright.
Don't forget that the measurement for the beam width has to be split down the middle so the wide of the 35 degree beam is only 6 feet into an 11 foot wide lane (standard lane width is around 11 feet without a shoulder). Assuming that most cyclists ride near the edge of the road, that puts the beam at around 5 feet from the center line and cars don't crowd the center line all that much...usually. For the 10 degree beam with 1200 lux at 20 feet, the on-coming cars are going to be about 10 feet away from the edge of the beam.
Originally Posted by
Burton
The problem is under-rated. You evaluate the brightness of your light by how much light is reflected BACK to you - in whats pretty much the opposite direction the light is pointed. Probably only a small fraction of the output. Anyone on the recieving end, on the other hand, has to deal with a much larger percentage of the output.
Nope. We aren't talking about a surface that is even remotely reflective. The light coming off of the bike and striking the ground is being absorbed or scattered by the black pavement. Very, very little of the light is reflecting of the pavement in a forward direction most of the light is scattered backwards toward the rider (or driver). If you ride in the rain, there is more light being reflected...it's why we can't really see our light on the road during rain...but the angle of incidence is all wrong to get light into a driver's eyes...especially if the driver is too far outside the beam for it to matter.
Originally Posted by
Burton
I have 35 and 15 and 10 degree spots and did some pretty thourough real world testing complete with photographs to confirm that. LOL Glare? From a distance of 30 ft onwards on a bicycle path, while walking towards my bicycle (that had my helmet resting on the bars with a SSC P7 Bin D emitter and a 35 degree collumnator, aimed downwards to light up the path) - not only could I not see the bike - I couldn't see the bike path or the grass on either side of the bike. And the closer I got - the less I could see around me - even averting my eyes. At a distance of 10 feet I didn't have a clue what was in front of me.
That basicly forces oncoming cyclists to blindly aim for your left side and pray. Motorists may have a larger lane to deal with but that only reduces the issue - it doesn't eliminate it. The beam coverage on a 35 degree flood is still high enough and wide enough that, unlike automotive headlights that are mounted lower and have a cut-off - it'll hit their windshield.
There is a way to avoid the bike path problem...avoid the bike paths! I don't ride bike paths at night, full stop. I don't do it for 2 reasons. First, all of the bike paths in my area close at dusk. I'd rather not explain to the park police why I'm violating the law and, more importantly, spend my time in court. I particularly don't want to spend my money in court. The second reason is that I don't want to be shining my light in the eyes of other trail users...who shouldn't be using the trails either.
Problem solved.