Department of Psychology
Psychology Instruction Project
369 Sensation & Perception
  1. PSYC 369

Brightness, Lightness, Darkness & Contrast: Light Adaptation

After you've left a matinee movie show and walked out into bright daylight conditions, you may have noticed how everything seems glaringly bright and “washed out”. In a short period of time the visual discomfort passes and details come into better view. This relatively rapid process by which the eye adjusts to much brighter conditions is referred to as light adaptation. What's the basis for this change?

To understand light adaptation, it would help if you've already reviewed the prior page on dark adaptation (accessible via the drop down menu in the top right). Adaptation to light is a rapid reversal of the dark adaptation process. Incoming light "bleaches" the rod and cone photo pigments leading to massive firing in the retinal ganglion cells. This accounts for the white "wash" over everything at first.

The light adaptation effect is simulated in the picture below. When the blinds are first opened to the daylight, everything appears to be obscured by a white overwhelming glare. In a few minutes, the rods and cones adjust to the higher lighting levels, and details in the scene outside begin to show properly.

 

Light Contrast

Notice that as the blind opens to reveal the brightness of the day, everything appears white (because the photo pigments have been bleached), but over a short period of time the image out the window begins to show properly.

 

 

 

 

 

 

 

 

 

 

 

One of the main differences between light and dark adaptation is that light adaptation occurs in a couple of minutes compared to the 20-30 minutes it takes to dark adapt. To test this, try the exercise below when you have a half hour or so to spare.

Exercise:

   1. Dark adapt your eyes by sitting in a dark room or closet for 20-30 minutes. (Perhaps you’ll want to try this when you have difficulty sleeping.)
   2. When both eyes are adapted, cover one eye completely so that no light can get to it
   3. Turn the lights on in the room for a few seconds, then turn them off again
   4. Now switch between covering your left and right eye and notice the difference between the dark-adapted eye and the one exposed briefly to light.

You'll notice how fast and easy it is to eliminate a half hour of dark adaptation. There are many occupations and hobbies such as nighttime flying or security duty that rely on dark adaptation, so the unwanted inclusion of a light source can be quite a costly one in terms of time. Settings such as dark rooms where its important to maintain dark adaptation often use red (i.e., long wavelength) lights. Because rods are relatively insensitive to long wavelengths (650 nm or higher), they are not bleached out by them.