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Allowing the eye a period of adjustment in the dark during which it becomes more sensitive to faint sources of light as the pupil gets larger. The time taken varies with individuals, but is likely to be at least ten minutes, and improvements may be noticed for up to an hour. Dark adaption is essential for visual astronomical observations. It is instantly destroyed by any exposure to bright lights. For that reason, dim red lights are normally used by astronomers where some lighting is essential, such as for reading charts.
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