But both, with the proper information, can give you an idea of how they will work for you.

What you say is partly true. Lumens are for the luminous flux in all directions...if the light isn't constrained. For bicycle lights all of them have some kind of reflector to constrain the light to a certain path.

Light doesn't just "fall off with distance". Light intensity decreases in proportion to the square of the distance. So if the light has an intensity at point A of 1, if you measure the light at point B which is 2 times the distance of point A, the light isn't half as bright but the intensity decreases by 4 times. That's the problem with stating that only the lux of a light should be used for comparison. If you don't know how far away from the source the measurement is made, you can't really determine how much area is being illuminated. With lumens, you can calculate a lux...which is a useful measurement...at any distance. That will tell you a lot about how bright the beam will be. Without the distance measurement, you can't back calculate the lux for varying distances.

On any beam, you can assume that the beam shape is going to be roughly round. Even with cutoffs, the beam will eventually assume a round shape since it is going to be spreading out in a cone from the source. It may take a little while to assume a conical shape but it will get there.

Hence my point about not riding on bike paths at night. If you are really concerned about "blinding" a fellow path user, don't do it. Even then "blinding", i.e. the complete loss of visual acuity, is too strong a word. You might cause someone looking at your lights discomfort but they aren't going to lose their eyesight completely.


I disagree. Go stand a few dozen feet from in front of a car with an abrupt cutoff. The light is coming at you in a cone. You get a lot of upward spillage as an observer and the light will appear conical. If you are lower than the light source, the intensity of the light will increase. The world isn't flat and there are plenty of times when you might find yourself above an observer. A helmet light...something that I use...is even worse because the source is almost always higher than someone coming at you.

As for light brightness, I have to disagree here too. LED lights are currently about where high end halogens were around 2000. You can force about 700 lumens from an MR11 halogen by overvolting it. LED emitters are currently putting out about that amount of light. If you go to an MR16 halogen, you can overvolt it and get out 1500 lumens from a single source. You can gang up several LEDs and get that kind of output but the throw of a single source 1500 lumen lamp is much, much further than that LED. Back when I used halogens, I could throw a coherent beam across Crown Hill Park lake and illuminate the trees on the other side of the lake. That's a distance of almost 0.5 miles. Haven't been able to do that with any LED I've used so far.

Just to show that I'm not always disagreeable, I wholeheartedly agree with you here.