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aggasalk t1_j6gyf6k wrote

The pupil's contribution to light adaptation is relatively minor.

The main action is in the retina. There are two types of photoreceptors in the retina, rods and cones. They work in similar ways: they are constantly producing substances called photopigments, and when the right kind of light hits a photopigment it transforms in such a way that it can activate the photoreceptor so that it sends a "light detected" signal.

Rods are extremely sensitive. A rod can potentially detect a single photon. So, you use your rods to see in very very low light conditions. But because of this great sensitivity, a moderate amount of light will 'bleach' the rods, destroying their photopigments and making them useless.

Cones are very insensitive. It takes hundreds or thousands of photons to trigger a cone. But this is fine, because most of the time you are in environments that are totally flooded with light, so there's almost always enough to trigger your cone detectors.

Since they are so insensitive, it's just about impossible to bleach the cones (a very bright flash, or glancing at direct sunlight can do it). So, they are always producing pigment that is available to detect photons and excite the cones.

Both systems are very homeostatic in their light regimes, producing pigment in decent amounts so that available light can be detected, without ever needing to know the current light levels. Luckily they overlap in their sensitivities, so there's no light level at which you have no functioning receptors.

Basically, long story short, when it's very dark your rods become useful because they're so sensitive to light and your cones become useless because they're so insensitive. When it's not very dark, your rods are bleached and useless, while your cones are now useful because there are enough photons to stimulate them.


Ryan949 OP t1_j6h4e6u wrote

So if I understand you correctly, when suddenly going from a bright to dark environment or vice versa, the adjustment period is caused by our rods and cones adjusting the rate of photopigment production to return the photopigment to ideal operational quantities.

So when we go into a dark room, there aren't enough photopigments to catch what little light enters our eye and no signal gets sent to our brain and we're effectively blind until the photopigments get to high enough levels to detect the low light.

And in the reverse situation, there's too much photopigment so all our light cells are blaring/bleached and we're again blinded until the photopigment levels drop.

Is that right?


aggasalk t1_j6h5wm4 wrote

right except for the reverse situation. though it is a more-or-less passive process, the rod/cone pigments are constantly regenerating at the same rate regardless of light level.

when you go into a dark room, the cones are stocked with pigment but it's useless - but it will take a few minutes for the rods to be fully stocked, since they were bleached by your earlier exposure to light.

when you step from darkness to light, there is a brief flash since suddenly all your rods pigments are isomerizing, but the cones are functioning from the get-go.


aztronut t1_j6kbysc wrote

Full dark adaptation can take up to an hour but it varies with age and individual, typically 30-60 minutes.