Night vision

The vision of human beings is well-adapted to daylight; the human eye has evolved to see in the range of wavelengths that are brightest in the spectrum of light that the Sun emits.

The intensity of the light the Sun emits by wavelength, with the visible region highlighted.

But humans don’t see par­tic­u­larly well in the dark. The cones that are respons­ible for colour vision don’t function well at low light intens­ities, which is why night vision is almost entirely mono­chro­matic — in the dark humans see in black and white.

When moving from bright light into darkness the first thing that happens to the eye is that the pupil dilates to allow in more light. The iris dilator muscle causes the pupil to increase in diameter by a factor of five (from 2 mm to 10 mm), increasing the amount of light entering the eye by about twenty-five (5²) times, but this isn’t enough for true night vision.

The chemical rhodopsin that is present in the rod (brightness-sensing) cells is respons­ible for night vision. When exposed to light, rhodopsin imme­di­ately (within 200 femto­seconds*) splits to form a chemical called pho­torhodopsin, and then soon after­wards (within a few pico­seconds) another chemical called bathorhodopsin.

The split­ting of rhodopsin is accom­panied by the form­a­tion of other chem­icals called retinals, and during this split­ting process a signal is sent down the optic nerve to the brain, regis­tering the detec­tion of light. (Retinal is created from vitamin A, and so people with a diet lacking in vitamin A fre­quently suffer from night blind­ness.)

A molecule of rhodopsin (rainbow-coloured) embedded in a lipid bilayer.
A (black) retinal molecule is bound within the rhodopsin.

Over time, and at a con­sistent rate, the opsins and retinals recom­bine to form rhodopsin. If the eye is exposed to bright light all the rhodopsin splits at once (a process called pho­tobleaching). When sub­sequently exposed to darkness there is there­fore no rhodopsin to split and the eye cannot detect light properly. The person in question must wait for the rhodopsin to nat­ur­ally recom­bine over time before proper vision can return, a process that takes between ten and thirty minutes to occur. When fully accus­tomed to the dark, the eye is between ten thousand and a million times more sens­itive to light than previously.

The rhodopsin in human eyes is less-sensitive to red light than to other colours and there­fore night vision is not par­tic­u­larly effected by red light. This is why red light is used in dark­rooms and in aircraft before night-time para­chute jumps.

Human eyes, unlike the eyes of many animals, do not have the tapetum lucidum which gives those animals superior night vision. The tapetum lucidum sits behind the retina and acts like a mirror, reflecting back photons of light that were not ini­tially absorbed by the retina, giving the retina a “second chance” to detect the light. This improves their night vision and is what gives rise to the phe­nomenon of “eyeshine” often seen when taking pho­to­graphs of animals.

The tapetum lucidum seen in a dis­sected calf’s eye.

“Eyeshine” is very obvious in this pho­to­graph of a raccoon.

* Inter­est­ingly, the split­ting of rhodopsin into pho­torhodopsin and retinal seems to be the fastest chemical reaction that has been directly studied.