Demonstrations: Light/Dark Adaptation
1. Rod vision. When you are reasonably dark adapted, dim lights
are detected only by the rods, and bright lights are seen by both rods
and cones. Here are several demonstrations of rod versus cone vision
and dark adaptation.
(a) Close and tightly cover one eye (e.g. with you hand) for several
minutes (at least 5, but preferably 10 minutes), while the other eye
remains open and you are in a reasonably bright room. Next, turn the
lights off (and cover all windows) so that the room is quite dim. Open
your eyes one at a time and notice the huge difference in sensitivity
between the two eyes (thus, dark adaptation is local to a given eye).
Turn the lights back on and open your eyes one at a time. Note that the
dark-adapted eye is dazzled (and takes time to "light adapt"), and the
other eye is still reasonably light-adapted (because dark adaptation
takes several minutes).
(b) Turn the lights off again, leaving very little illumination, and
dark adapt for several minutes. Notice that you are color blind in
regions that are the most dim, but that if objects are sufficiently
bright, they appear as colored (this is the difference between rod and
cone, or scotopic and photopic threshold).
(c) In these dark-adapted conditions, find a very dim (dark)
object on a darker background. Note that this distinction may not be
visible if you look at it directly, but should be more visible if
viewed slightly peripherally. This failure of foveal vision in the dark
is a result of the rod-free area in the fovea (it is called "Arago's
blind spot").
(d) In these dark-adapted conditions, turn on a small light
source (e.g. a penlight) and move it around. Note that you see trails
of illumination which remain where the penlight has been. These trails are a result
of the slowness of the rod system. The rods continue to respond with
hyperpolarization even after the light landing on them has been
extinguished. Of course, if the light lasts a reasonable amount of time
concentrated on one area of the retina, then light adaptation will
occur, and the spot which is still visible after the light is
extinguished is an after-image. We can distinguish between trails and
after-images by noticing that the trails occur even with very brief
exposures to quite dim lights which are insufficient to cause
appreciable light adaptation.