…continuedA Pupil Primer
Measuring Your Pupils
It's easy to find your pupil diameter and observe how it changes in varying light. For a quick test, hold a pencil vertically just in front of your eye, resting it against your cheek and eyebrow. Close the other eye. A standard pencil is about 7 mm in diameter. Against bright light you'll see a fuzzy fringe surrounding an opaque core. Block most of the light from view by cupping your hands, and watch the core narrow. If the core thins away completely in dim light, so that you can see a little light right through its center, your pupil has enlarged beyond 7 mm.
A better method is to use a pair of small slits in an opaque sheet with their inner edges separated by a measured distance. Look through the pair of slits while holding the paper against your eyebrow and cheek. (The holes should be about 14 mm in front of your eye, but this is not critical unless you are strongly nearsighted or farsighted and aren't wearing contact lenses.) You'll see two dim disks of light. If their edges barely touch, your pupil diameter equals the separation of the holes.
The pupil does most of its dilating in the first second or two after you enter the dark, but it takes a few minutes to reach its absolute maximum size. Pupil dilation should not be confused with true dark adaptation, a chemical process that happens more slowly in the retina.