DARK ADAPTATION
Allowing adequate time for your vision to adjust to darkness can deliver huge gains
DARK ADAPTATION ALLOWS our eyes to see more clearly in conditions of low light. This key technique relies on two changes in the eye, one fast and one slow.
The rapid change is where the eye’s pupil dilates to its maximum diameter, allowing more light to reach the retina where it triggers nerve impulses. For the naked eye, dilation increases the eye’s light gathering capability by about 16 times. When using a telescope, the pupil’s maximum dilated size determines its lowest useful magnification as well as an object’s maximum surface brightness. So for high and medium magnifications, where the eyepiece exit pupil is much smaller than the dilated pupil, dilation gives little benefit.
The second slower but potentially much larger improvement in eye sensitivity happens thanks to chemical changes that occur in the retina in the absence of light. Such changes can increase its sensitivity by an amazing one million times.
The retina contains two kinds of lightsensitive cells, rods and cones. Cones can detect colour and brightness and are generally used for daytime vision. Rods can potentially be much more sensitive but don’t detect colour, and because their density is lower their resolution is limited. When you go from a bright environment to a dark one the retina’s sensitivity begins to improve immediately. To start with, the colour-sensitive cones adapt most quickly and are the more sensitive, but after about 10 minutes their improvement tails off and the rods take over, reaching maximum sensitivity after about 30 minutes.
There’s a reason astronomers use dim red lights to work at night. To read maps and equipment settings in the dark, you need to use the better resolution of the colour sensitive cones rather than the lower resolution rods. At the red end of the spectrum the rod and cone sensitivities almost match, allowing for high-resolution colour vision from the cone cells with minimal excitation of the rods, so preserving dark adaption.