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Here's a little demonstration that illustrates the difference between the response of a photometer and the response of the eye. This is adapted from a demonstration found in many introductory texts on visual science or psychology.
The two small squares are of equal absolute intensity, yet the small square surrounded by the larger gray square appears lighter than the small square against the lighter background. A photometer would read the small squares as of equal intensity, but our eyes do not see them as of equal brightness. If the difference in appearance is not obvious, try standing back about 5 -6 ft. from your monitor, or try squinting slightly.
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