Originally Posted by
wphamilton
Dark adaptation is pretty fascinating when you look into it, not nearly so simple as it seems at first grasp. He's right that rods take 30 minutes to fully adapt, up to 45 minutes according to some. Apparently the timing (of a rod after-image fading) is an exponential function with a half-life of around 4-5 minutes, corresponding with the timing of rhodopsin replenishment - a pigment in charge of night vision. Interesting also that how fast night vision regenerates depends on how bright the light was: brighter "blinding" light makes it take longer.
There is also a neural component in dark adaptation, where the output from the rods are organized and bundled for enhanced vision (up to 10 times sensitivity), and this occurs immediately.
But it's also sort of a red herring. I think that the conditions we're talking about are more generally in the mesopic range where both rods and cones are active, rather than purely scotopic which employs only rods and this is why cyccommute was able to see the white lines in a couple of minutes. Cones adapt much more quickly, and it has to be pretty dark for their cut-off. You can go deeper into this stuff than I have, but I believe that the "dazzling" that we report is primarily the bleaching out of the cones, and the after-images from both cones and rods fading with their corresponding rates.
Your explanation is good but I take some issue with the "red herring". A cyclist using lights has already washed out his rod cells. The cyclist is, essentially, night blind. Turn off the lights and the cyclists will have trouble navigating at most any rate of speed. My point wasn't that the
cyclist has the problems but the unlighted pedestrian on an MUP will have problems. Bike paths in my neck of the woods as well as many others I've used in lots of areas aren't usually lighted at all. Here they tend to follow creek drainages and are wilder spaces. There is some urban lighting but a pedestrian can be walking on a very dark trail for long distances. Their eyes are fully adapted and exposure to light from a bicycle...even light that is shaped...ruins that night vision.