It is difficult to have a discussion with you because you don't seem to understand how light works. The reflector just gathers the light from the source and directs it outward. At the lens of a car light, you will have all the light that can be gathered by the reflector. You can't get any more. If you were to place a target at the lens of a car light (or any focused light for that matter), you can measure the size of the light beam and you can be reasonably certain of what the lux measurement of the beam exiting the light is. Many car lights have an aperture of about 2"x4". If they were 3"x5" or 6"x6", it wouldn't matter that much because the area in square meters is going to be about the same magnitude. For the sake of the discussion, assuming a 2"x4" aperture is reasonable.

Why would I need to do the calculations for daytime. They would be the same. The car light would still put out the same amount of light. The reflector would reflect back the same amount of light and the bike would put out the same amount of light. The only difference is that the whole calculation would be moot since the sun light would swamp out any contributions from any light source.

Sunlight falling on the reflector is only going to be reflected to an observer if the observer/sunlight/reflector is in the proper orientation. That proper orientation will only occur in the morning and evening and will be dependent on the direction of travel of the reflector. It will also be dependent on the location of the reflector on earth and the seasons. Early in the morning around this time of year, that lines up if front reflector is traveling east at around 0700. After about 0900, the sun is too high to reflect off the front reflector towards an observer sitting in an automobile. If they were laying on the ground, they might get some reflection depending on how far away from the front wheel they are but you would probably run over them before they would see the reflection and it would be swamped out by the ambient light anyway.

If you set a reflector side-by-side with a 600 lumen bicycle light, then shined a car light at it from 25 feet directly in front of the reflector, you would not see any light coming off the reflector back at the car. The amount of light from the bicycle light is going to completely overwhelm the small amount of light reflected back by a tiny little reflector. And, I'll say it again...the ambient light during the day will swamp out any light coming from the reflector at just about any angle you care to measure it at.


No, the values that I provided are for current technology LED lights. The output of a Cree XML T6 is around 600 lumens for a single emitter. A Cree XML U2 has an output of around 700 lumens. I'm not sure what LED current automotive lighting is using but the most common light you'll run across for automotive lighting is still going to be tungsten or halogen with a lumen output of 700 lumen. High beams are 1200 lumens in a flatter trajectory and I used the high beam value for my calculation. 1200 lumens is not "3 to 4 times as much..."

Nor are cyclists in an urban environment going to encounter a whole lot of high beam lights. There are a few people who commute and night time ride on dark country roads but, by far, most night time cyclists are going to be commuters in an urban environment where high beams are discouraged.