Background Affects Perception
Marc GreenWhat we see depends not only on objects but also on their backgrounds. A bright object will be had to see against a bright background and but easy to see against a dark one. Conversely, a dark object might still be highly visible if the background is bright. The following pictures illustrate that we perceive "contrast," the difference between objects and their backgrounds, rather than the simply the amount of light coming from the object.
The top figure shows a brightness contrast while the bottom illustrates color contrast. Note also that the background changes the appearance of the foreground object. The 3rd line is an intermediate brightness in the top figure and yellow in the bottom. The letters "Sim" at the beginning appear a darker gray than the last "ast" of contrast, although they are exactly the same luminance. Similarly in the color example, the "Sim" appears a redder yellow than the final "ast", which looks a greener yellow. Again, the shades of yellow is really the same. In the real world, the effects are even bigger because computer screens don't produce good colors.
Here's another good example of brightness contrast. The disks in each square are actually identical.
The moral is that what witnesses perceive will be greatly influence by the background. Any estimate of what a witness could have seen or reported to have seen must take background into account. This means that it is important to place yourself (or your light measuring equipment) at the witness eye position so that you are looking at the same background.
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