Blue Signs

The original posting seems to have been swallowed.

Kinkos is one of a number of firms that use blue lights in their signs. I have trouble focusing on these, and an unrandom sample suggests other people do too.

I think the reason the signs are hard to read comes from the way our night vision works. The night vision is based more on the rods, and cones become hypersensitive when they've gotten little stimulation. Says here that about 2% of our cones are blue sensitive, so we have poorer resolution for purely blue light to start with.

Nevertheless the blue sensitivity is similar to the others, so the blue cones must be more sensitive (which shouldn't be hard to arrange, since the blue light is more energetic). But a single cone, once triggered, has to be less sensitive for a time to subsequent photons, and especially at night, when it starts out hypersensitive. So, for a short time after it registers the blue light, the blue cone will be somewhat deadened. I don't know how long the recovery time is, but I suspect that it has to be somewhat longer for blue than green or red cones. In any event, the recovery of night vision sensitivity is on the order of minutes.

The rods don't see red light, but blue light will fire them and destroy their dark-adaptation, so we lose rod sensitivity in the region around the focus of the blue image.

The eye constantly jitters, which means that the image of a bright blue point source at night will play over a small region around the nominal focus point--deadening the area slightly, and making the image less sharp than you would otherwise expect.

So the combination of sqrt(32)X worse spatial resolution and a deadened area around the central focus spot make blue images harder to resolve at night.

So why in the world do firms spell out words with blue fluorescents when people have trouble reading them?