Stray light is a stray subject, little mentioned in books and courses, usually learned about the hard way— by bumbling into it. Stray light is everywhere, degrading the performance of optical systems. Like a chronic disease, its effects are persistent and gradually increasing, so that victims may not recognize their affliction. We typically notice stray light—as everything else—when there is a sudden change for the worst. The scratches and dust on our windshield are given little thought until the car is turned toward a low sun and we are overwhelmed with scattered light. One reason for overlooking stray light is the tendency to think about the light we want, not the light we don't want. Also, the path of the wanted light is more predictable.
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