The tapetum lucidum (Latin: "bright tapestry", plural tapeta lucida)[1] is a layer of tissue in the eye of many vertebrate animals.[2]
It lies immediately behind or sometimes within the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors. This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals.
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Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the pupil appears to glow. Eyeshine can be seen in many animals, in nature and in flash photographs. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior night vision); this technique, spotlighting, is used by naturalists and hunters to search for animals at night. Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink and red. However, because eyeshine is a form of iridescence, the color varies slightly with the angle at which it is seen and the color of the source light.
White eyeshine occurs in many fish, especially walleye; blue eyeshine occurs in many mammals such as horses; yellow eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in rodents, opossums and birds.
Despite it being present in some primates, the human eye has no tapetum lucidum, hence no eyeshine. However, in humans and animals two effects can occur that may resemble eyeshine: leukocoria (white shine, indicative of abnormalities including cataracts, cancers, and other problems) and red-eye effect.
Cats and dogs with blue eyes (see eye color) may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In flash color photographs, however, individuals with blue eyes may also display a distinctive red eyeshine. Individuals with heterochromia may display red eyeshine in the blue eye and "normal" yellow / green / blue / white eyeshine in the other eye. These include odd-eyed cats and bi-eyed dogs. The red-eye effect is independent of the eyeshine: in some photographs of individuals with a tapetum lucidum and heterochromia, the eyeshine is dim yet the pupil of the blue eye still appears red. This is most apparent when the individual is not looking into the camera, because the tapetum lucidum is far less extensive than the retina.
A classification of anatomical variants of tapeta lucida[3] defines 4 types:
The functional differences between these 4 different types of tapeta lucida are not known.[3]
This classification does not include tapeta lucida in birds. Kiwis, Stone-curlews, the Boat-billed Heron, the flightless Kakapo and many Nightjars, Owls, and other night birds such as the Swallow-tailed Gull also possess a tapetum lucidum[6] This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.[7]
Like humans, some animals lack a tapetum lucidum and they usually are diurnal.[3] These include most primates, squirrels, some birds, red kangaroo, and pig.[8] Primates that have a tapetum lucidum include the aye aye and sportive lemur.
When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head,[9] such that in all cases the tapetum lucidum enhances night vision in the center of the animal's field of view.
Apart from its eyeshine, the tapetum luminum itself has a color. It is often described as iridescent. In tigers it is greenish.[10] In ruminants it may be golden green with a blue periphery,[8] or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.[8]
The tapetum lucidum, which is iridescent, reflects light roughly on the interference principles of thin-film optics, as seen in other iridescent tissues. However, the tapetum lucidum cells are leucophores, not iridophores.
The tapetum lucidum is a retroreflector of the transparent sphere type. Because it is a retroreflector, it reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The tapetum lucidum reflects with constructive interference,[5] thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum lowers the minimum threshold of vision 6-fold, allowing the cat to see light that is invisible to human eyes.[11]
The tapetum lucidum enables an animal with it to see in dimmer light than the animal would otherwise be able to. This is of use to the animal, but it is of use also to humans. Human uses include scanning for reflected eyeshine to detect and identify the species of animals in the dark, and deploying trained search dogs and search horses at night. Historically, its function was regarded as simply to increase the light intensity of an image on the retina.[12]
Using eyeshine to identify animals in the dark employs not only its color but also several other features. The color corresponds approximately to the type of tapetum lucidum, with some variation between species. Other features include the distance between pupils relative to their size; the height above ground; the manner of blinking (if any); and the movement of the eyeshine (bobbing, weaving, hopping, leaping, climbing, flying).
Manufactured retroreflectors modeled after a tapetum lucidum are described in numerous patents and today have many uses. The earliest patent, first used in "Catseye" brand raised pavement markers, was inspired by the tapetum lucidum of a cat's eye. A more recent use of retroreflectors, helping to provide secure communications between two stations in line of sight, is modeled after the combination of tapetum lucidum and bioluminescent "flashlight" in flashlight fish of the families Anomalopidae and Stomiidae (see Retroreflector). Some of these fish have been shown[13] to use eyeshine both to detect and to communicate with other flashlight fish.
In dogs, certain drugs are known to disturb the precise organization of the crystals of the tapetum lucidum, thus compromising the dog's ability to see in low light. These drugs include ethambutol, macrolide antibiotics, dithizone, antimalarial compounds, some receptor H(sub)2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the tapetal cells."[14]
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