The eye’s ability to adjust to dark surroundings is called dark adaptation. Dark adaptation is an important physiological factor to understand because it can affect the overall comfort and enjoyment of your night dives.
Once your eyes have adjusted to the dark surroundings, it takes only a brief flash of bright light to desensitize them to the darkness. When the eyes are exposed to bright light after they have undergone dark adaptation, they are not able to immediately recover because the bright light triggers a physiological response in the rods and cones. The pigment found in the retina’s cells undergoes a discoloration, known as photobleaching, in response to bright lights. The rod cells, that are responsible for night vision, are more sensitive to light and take longer to adapt after they have undergone photobleaching. It takes rods approximately five minutes for a 50 percent recovery and 30-40 minutes for complete dark adaptation. This means that your dive may be ending before your eyes completely readjust to darkness. Other notable factors that can affect your eyes ability to adapt in the dark are breathing poor oxygen mixtures, breathing increased levels of carbon monoxide or inhaling cigarette smoke. Each of these may cause an increase in the amount of time required for darkness adaptation.
To help prevent problems commonly encountered with the amount of time required for darkness adaptation, some divers choose to wear masks with red-tinted lenses that filter white light. Lowering the amount of white light that reaches the eyes reduces the amount of time required for darkness adaptation, to as little as ten minutes in some cases. The photoreceptive pigment in rods is less sensitive to the color red’s wavelengths; therefore, red light does not typically cause the same photobleaching of the rod’s pigment that white light does. This is why red light is typically used for night vision. It is also the reason that aircraft pilots often use red-tinted flashlights at night to illuminate their charts and checklists and their instruments glow red. Many diving equipment manufactures offer computers, instruments and gauges that have red tinted displays specifically for night diving.
Remember, when twilight or night diving, it will take some time for your eyes to adjust to the darker conditions. Avoid looking directly at bright light while diving and do not point your dive light directly into another diver’s eyes, especially your buddy’s.