Light Adaptation

In another part of this book we discussed how the visual system adapts when going from a bright environment to a dark one.  We called it dark adaptation.  Let us now look at what happens when we go from a dark environment to a bright one.  This is called, yes you guessed it, light adaptation.

Surely you have all had this experience.  You spent the afternoon in the movies and when you left and the sun was still shining recall how your eyes almost hurt when you got outdoors.  Or perhaps, on a cold winter day you go from indoors where the lighting is moderate and you could see very well to the outdoors.  Especially, at high noon when there is lots of fresh snow on the ground you find that it is difficult to see for some seconds, perhaps as long as a minute.

One of the major differences between dark adaptation and light adaptation is their time course.  Whereas dark adaptation takes about 30 minutes to be complete, light adaptation happens very quickly, usually in less than a minute.

Another difference between these to type of adaptation is that when you are light adapted and then go into a very dark room for a while you may not see anything at all.  As you dark adapt more and more things become visible. When you go from a darker area to a very bright one you usually are not temporarily blind.  It is just that your vision temporarily is not very good.  In technical jargon, your contrast sensitivity is poor until you become light adapted.  By that we mean that you will have difficulty in perceiving areas of low contrast.  It is like every thing is all washed out.  But as you quickly light adapt the darker areas become darker and the lighter areas become easier to see.  Click on this demo to obtain a graphical idea of how contrast changes before light adaptation and after.

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