This discussion is becoming quite tedious even for me...and that's saying something. Lux is pretty easy to calculate. It's lumens per unit area. I am assuming a square lamp face and that the light is spreading out in an even manner across the beam profile. That's not a bad assumption considering that a lot of optical engineering has gone into providing drivers with light that is evenly distributed. In other words, car lights don't have a lot of hot spots. The beam isn't going to remain square, of course but, for the purposes of the example that won't make much difference. Lux is actually a much more useful measure of light intensity than lumens...as long as you specify how far from the light source the measurement is made. Lumens measure just the raw output. For the discussion of how the light interacts with the reflector, using lumens isn't all that useful because not all of the light put out by the car's lights is going to hit a 2"x4" target. You can calculate the lux at 25 feet, then calculate the lumens hitting the target, reflect the light back to the source and calculate the lux of that light going back. Saying that 800 or 1200 lumens hits the reflect from 25 feet is just wrong and ignores a whole lot of physics.


The car's lights still put out the same amount of light and the reflector still reflects only a small fraction of that light back. Any reflection you get from the sun is going to be highly dependent on the location of the sun, the location of the reflector and the location of the observer. And the amount of light reflected back is going to change minute to minute as the angle of the sun changes.


The retroreflectors can't violate the laws of optics. They bend them a little but they don't break them. Light hitting the retroreflector will bounce back towards an observer only if the angle is relatively small. Get much the sun much past 25 degrees above the horizon and the light doesn't reflect back towards a driver sitting in a car but reflects down onto the ground. And the intensity is still small due to the small size of the reflector. I watched cars this morning on the way to work. The taillight reflectors of the cars traveling exactly west, did light up from the sun at 0700. By 0730, they didn't. I turned south and the reflectors didn't reflect light at all from the sun. The window for a reflector to have any effect is very narrow. And if I turned back to the east into the sun, reflectors had no reflection whatsoever, because, well, they can't reflect without light shining on them.

I suggest you go try observing this yourself.

If you think that you'll encounter high beams in urban situations, try driving around with high beams on in a car some time. If you don't get flashed by every other car on the road, you'll get a ticket from a cop for operating with your high beams on. That high beams are discouraged in towns is most certainly a true statement.