Johannes Purkinje (1789-1869) published a report in 1823 describing the change in relative brightnesses of the long wavelengths (reds) and short wavelengths (blues) as the ambient illumination decreased. He found that at high illumination reds appeared brighter than blues but at low illumination these same blues appeared brighter than reds.
I tried to set up a demonstration so that you can experience this Purkinje Phenomenon. Because of differences among video monitors this demonstration may not work for you. Start by reducing your ambient illumination by turning off room lights or blocking light coming in the windows. When you click on Demo you will see a red square and a blue square embedded in a black background. Adjust the brightness control of your monitor so it is near maximum brightness you should (at least I do) see the red square as slightly brighter than the blue one. Now, slowly reduce the brightness of your video monitor. At some point you may see that the blue square is brighter than the red square. This reversal in brightnesses will probably occur shortly before you can no longer see the red square, but you may still see a dim light in the position of the blue square, although it may not appear blue any longer..
Try it. I hope you like it. Even if it works for you, it may be more a function of how your monitor operates than your visual system. That is to say, perhaps there are non linearities in your red, green and blue monitor guns. Nevertheless, if it works, for whatever reason, you will experience something analogous to what Purkinje experienced. Now let's try to understand why this phenomenon occurs.Return to Demo
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