With typical outputs of 10 to 20 lumen, flashlights pose little or no threat to the eyes. One reason for this is the built-in defense ability that the eyes possess. Through the coordinated efforts of the pupil, iris, retina and the optic nerve, the eye can adapt to a variety of light conditions, including that pesky flashlight.
The pupil is the portal through which light and images enter the eye. It varies from 3mm to 9mm, depending on conditions. This opening is covered by the cornea. It only looks black because the internal tissues of the eye, which would otherwise be visible, absorb almost all light.
The iris, the colored part of the eye, is like the retractable sun screen of the eye. In dim light, dilator muscles in the iris contract and open the pupil to allow more light in. In bright conditions, such as that flashlight shining in your face, sphincter muscles in the iris contract to constrict the pupil and block any potentially damaging or uncomfortable light.
The Retina and Optic Nerve
The retina is the nerve center of the eye and the optic nerve is the information conduit to the brain. Together, they control this protective mechanism, which is called the pupillary light reflex. When the rod and cone receptor cells of the retina become overstimulated with light, the optic nerve triggers the sphincter muscle and the pupil constricts.
The Fall Back Plan
If the offending light is too strong, or the eye needs a few extra seconds to make the needed pupillary adjustment, our brain calls in the big guns--the palpebrae--also known as the eyelids. An impulse shoots to the obicularis oculi muscles and the lids slam shut involuntarily.
Dilated Eye Exam
If you're curious about what life would be like without the pupillary light reflex, call your eye doctor and schedule a dilated eye exam. During a dilated exam, the doctor uses medicated drops to immobilize this light reflex. This allows him to keep your pupil wide open while he uses his lighted instruments to take a good look at the back chamber of your eyes. When the exam is done, it often takes an hour or two for the drops to wear off, and if you walk from the dim exam room into bright sunlight you'll probably get a quick lesson in light sensitivity. It should make you appreciate just how good your eyes normally are at controlling light.