Dark Adaptation

Modified: 2024-06-17 9:33 PM CDST

• An interesting visual phenomenon is the process of dark adaptation and night vision.
  • When I was six, I hated to have to turn the light on in my room at night because, when I turned it off, it was so dark.
  • But, after a while, it would get less dark.
  • Of course, it was just as dark all the time.
  • What was happening was that I was dark adapting.

• After being in the light, it takes about 30 minutes to dark adapt completely.
  • The reason involves the restoration of two chemicals, iodopsin in the cones, and rhodopsin in the rods.
  • Light bleaches these chemicals.
  • When you go into the dark, the bleaching stops and the chemicals reach their maximum levels in about 30 minutes. See this diagram, as the curve drops, night vision improves.
    • Note several points.
      • First, the curve for dark adaptation from white light is actually composed of two parts, one from the cones, and one from the rods.
      • Second, if you had no rod cells, your night vision would not be very good (night blindness).
      • Finally, the distance between the bottom of the curve (best night vision) and a line drawn horizontally from the cone adaptation portion describes the difference in sensitivity between the rods and cones.
      • That distance gives us a good estimate of the night-vision differences between rats and pigeons, for instance. Because rat's eyes are nearly all rods and pigeon eyes are nearly all cones
  • Because red light is of a lower energy, it bleaches these chemicals less.
    • So, now you know why submariners and bomber pilots are briefed in red light.
    • They will dark adapt faster .
• You too can take advantage of these physiological facts.
  • When you are driving at night, turn down the dash lights.
  • But, do not turn them completely off like one of our students did; she got a ticket for speeding.
  • Also, wear your sunglasses when you stop for gas at night.
    • That will minimize the bleaching while you are in the light.
    • Be sure to take them off before you get back on the road, however.
    • Corey Hart (1983), I Wear My Sun Glasses at Night (Audio)