But wait! As you get closer you see what looks like a "Do Not" circle on the small sign. You shine the car's headlights toward the small sign. (See picture below.) Oh! camping is NOT permitted here.
The red "Do Not" circle is easy to see during the day because red is highly visible under bright light. However, after the sun has set, your eyes begin to adapt to the darkness. As this happens the subjective brightness of red decreases in comparison to other colors. This phenomenon, called the Purkinje shift, makes it more difficult to see the red "Do Not" circle, especially on a brown background, because there is little contrast. The fact that this small sign is difficult to read in low light conditions is ironic since those are likely conditions under which people are looking for a place to camp.
Foley, H. J. and Matlin, M. W. (2009). Sensation and Perception. Needham Heights, MA: Allyn and Bacon.
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