Tapetum lucidum

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Tapetum lucidum in a calf eye, with the retina hanging down.

The tapetum lucidum (Latin: "bright carpet") is a reflecting layer immediately behind, and sometimes within, the retina of the eye of many vertebrates (though not humans); it serves to reflect light back to the retina, increasing the quantity of light caught by the retina. This improves vision in low light conditions, but can cause the perceived image to be blurry from the interference of the reflected light. It is therefore primarily found in nocturnal animals with good night vision, such as cats.

In various species, the tapetum lucidum is responsible for the equivalent of the red-eye effect in humans. In flash photographs, the eyes frequently appear to be glowing in one of a wide variety of colors including blue, green, yellow and pink. Bottlenose dolphins and dogs also have a tapetum lucidum.

The tapetum works roughly on the interference principles of thin-film optics, as seen in other reflective tissues such as butterfly wings (see Blue Morpho). However, different species have different types of structured tissue that lead to different mechanisms of reflective interference (Ollivier 2004). Known tapetum structures include:

• Retinal tapetum in teleosts, crocodiles, marsupials and fruit bats.
• Choroidal guanine tapetum in elasmobranchii.
• Choroidal tapetum cellulosum in carnivores, rodents and cetacea.
• Choroidal tapetum fibrosum in cows, sheep, goats and horses.

References

• Ollivier, F.J. et al. ["Comparative morphology of the tapetum lucidum (among selected species)"], Veterinary Ophthalmology 2004 Jan–Feb;7(1):11–22. PMID: 14738502.

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