A vision condition where someone has good eyesight in bright light but has difficulty seeing in dim or dark conditions; opposite of night blindness.
Greek 'hemera' (day) + 'alopos' (blind) + '-ia' (condition). Named paradoxically—literally 'day blindness' even though the person functions well during the day and struggles at night.
Hemeralopia reveals how ancient physicians named conditions based on their misconceptions—they thought the problem was with day vision, not realizing it was actually rod cells not working well in darkness.
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