Dark Adaptation

Modified: 2020-09-30

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.

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 . In the diagram referenced above, 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.

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.

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