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Almost all animals have eyes to sense light in their environment \u2013 even in dark habitats such as the deep ocean, where the only source might come from the odd burst of bioluminescence.<\/p>\n
But although species across the animal kingdom have evolved various structures for sight, they all need special cells called photoreceptors.<\/p>\n
Photoreceptors are cells that transform light energy into electrical signals that can then be interpreted by the brain. There are two types based on shape: rods and cones.<\/p>\n
Rod cells are built for contrast (black versus white) and contain the light-sensitive pigment rhodopsin.<\/p>\n
A cone cell will carry one of many pigments, each tuned to a specific range of wavelengths \u2013 matching a colour \u2013 on the visible spectrum.<\/p>\n
The number and type of photoreceptors an animal has dictates which colours it can perceive. A human retina has roughly 120 million rods and six million cones. The cones are split into three kinds or \u2018spectral classes\u2019 for trichromatic vision (around red, green and blue wavelengths).<\/p>\n
In comparison, butterflies and mantis shrimp have a dozen classes to cover a broader range of the spectrum (from deep ultraviolet to far-red light), which gives them hyperspectral vision.<\/p>\n