|MadSci Network: Anatomy|
Steve Horner: I apologize for the delay in answering your question. I'll try to answer your question as thoroughly as I can without being too scientific. I am assuming that the officer has not only 20/20 vision, but normal retinal functioning in which I mean not only good central vision (20/20) but normal night vision. Night vision depends primarily on "rod" photoreceptors in the retinal layer of the eye. If there is a deficiency in rod functioning, night vision also decreases. During the day, we depend more on "cones" for vision which accounts for all color vision and good central vision. Unless you desire a mathematically based answer as to how much illuminance of the object is needed, I will just give you some basic ideas and theories. The kind of object the officer is trying to see is important because a "complex" object with various patterns and structure, like a pistol, will perhaps be more difficult to recognize than a "simple" object like a club. It takes at least 20 minutes(in the dark) for an average human to become completely dark-adapted, in which case one should be able to see a club just using luminance from a full moon. If you are not dark- adapted, however, you need more light to accurately identify the object against the dark background. Furthermore, a stationary object will be easier to recognize than a moving one. Incidently, there is an inversely proportional relationship between "sharpness" or acuity of vision and dark adaptation. Increased dark adaptation means decreased acuity or sharpness of vision. Also, some people have, if you will, an inate visual sense in which they are better able to discriminate small differences in shapes of objects, thus more correctly identifying an object. This probably has to do with visual processing in the cortex, which differs from human to human. I hope this gives you some kind of idea to answer your question. By the way, luminance is usually measured in foot-Lamberts. Best visual acuity in humans is attained at about 100 foot-Lamberts. Let me know if you need clarification. Kenton McWilliams
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