How long does it take our eyes to fully adapt to darkness?
Category: Biology Published: August 9, 2013
First of all, it is impossible to see anything at all in total darkness. Total darkness means the absence of light, and our eyes depend on light to see. With that said, it is quite rare to be in a situation with total darkness, even at night. City lights reflecting off of clouds, car headlamps, the moon, the stars, and even the airglow of the night sky itself all fill the night with faint light. Most of our experiences with darkness are actually cases of partial darkness; where there is still a small amount of light present. With enough time, our eyes can adapt and see the low levels of light present in partial darkness.
Human eyes take several hours to fully adapt to darkness and reach their optimal sensitivity to low light conditions. The quickest gains in vision sensitivity are made in the first few minutes after exposure to darkness. For this reason, many people think that after only a few minutes, their eyes have reached their peak sensitivity. But several hours into darkness exposure, the human eyes continue to adapt and make small gains in sensitivity.
There are several factors that contribute to our eyes adapting to darkness. As described in the textbook Optometry: Science Techniques and Clinical Management, edited by Mark Rosenfield and Nicola Logan, the three main players in dark adaptation are the pupil, the cone cells, and the rod cells.
The pupil is the dark hole near the front of your eye that lets the light into your eye so that the light can form an image on the back (the retina). The iris that surrounds the pupil contains muscles that control the size of the pupil. When confronted with low light conditions, the iris expands the pupil as wide as possible. This dilation lets as much light as possible into the eye so that sensitivity is enhanced. The pupil's contribution to dark adaptation takes only a few seconds to a minute to be completed.
The cones cells along the retina are responsible for color vision. Similar to a grid of pixels in a digital camera, a vast spatial array of cone cells along the retina detect the different bits of colored light that make up the image we are seeing. Human eyes have red-, green-, and blue-detecting cone cells. All other colors that exist are experienced by humans as a mixture of red, green and blue. The cone cells themselves can adapt to partial darkness. Cone cells contain rhodopsin, which is one of many light-sensitive chemicals. Rhodopsin is very sensitive to light and is the primary chemical used by the cones when seeing in low light conditions. The problem is that rhodopsin is so sensitive to light that under normal light levels, the light deforms and deactivates (photobleaches) this chemical. Most of the day, when we are walking around in normal light, the rhodopsin in our eyes is deactivated. Upon exposure to darkness, the rhodopsin is able to regenerate and reactivate, becoming sensitive again to light and improving our night vision. But this regeneration process takes time. Cone cells take about 10 minutes to adapt to the dark.
Lastly, the rod cells in our eyes are responsible for black and white vision. They are the heavy-hitters when it comes to vision in low light conditions. The rods in our eyes achieve this great night vision through several mechanisms:
- Like cones, rod cells contain rhodopsin, the chemical that is highly sensitive to light. In fact, rod cells rely more heavily on rhodopsin than cone cells, leading each single rod cell to be about 100 to 1000 times as sensitive as a single cone cell once fully adapted.
- There are far more rods on the retina (100 million) than there are cones (5 million).
- Several rods all connect to the same output signal (the same interneuron). This fact allows lower levels of light to be detected at the cost of image resolution.
- Rods respond slowly to light (they collect light over long time periods). This slow response means that lower levels of light can be detected at the cost of sensing rapid changes in time.
Rod cells take several hours to become completely dark adapted. Expert naked-eye astronomers know this fact well. They will give their eyes several hours to adapt instead of a few minutes in order to maximize their vision of the dim stars. In summary, upon exposure to darkness, our pupils dilate in a matter of seconds, our cones adapt in 10 minutes, and our rods adapt completely after several hours.