red filter

A red filter is a piece of tinted glass or plastic that blocks certain wavelengths of light while allowing red light to pass through.

In photography, red filters darken blue skies and make clouds stand out dramatically in black-and-white images.

In darkrooms, red filters provide safelight that won't expose light-sensitive paper.

Astronomers use red filters to enhance visibility of certain celestial features, and in color theory, red filters demonstrate how we perceive color by selective absorption.