1. colour index; color indexAstronomy, etc. term, Geology, etc. term
Wikipedia definition
2. Color indexIn astronomy, the colour index is a simple numerical expression that determines the colour of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light. The set of passbands or filters is called a photometric system.
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Read “色指数” on Japanese Wikipedia
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