Tapetum lucidum

[8] The color in reindeer changes seasonally, allowing the animals to better avoid predators in low-light winter at the price of blurrier vision.

[3] Strepsirrhine primates are mostly nocturnal and, with the exception of several diurnal Eulemur species, have a tapetum lucidum of riboflavin crystals.

Its color is heterogeneous, varying with age and species due to factors like rodlet spacing, refractive index, and light interactions.

Young cats exhibit a blue appearance, which shifts to yellow with age, with adult coloration ranging from light orange to green.

[14] Kiwis, stone-curlews, the boat-billed heron, the flightless kākāpō, and many nightjars, owls, and other night birds such as the swallow-tailed gull possess a tapetum lucidum.

Four general patterns can be distinguished in spiders:[17] Animals without tapetum lucidum include haplorhine primates, squirrels, some birds, red kangaroo, and pigs.

In low light, a hand-held flashlight is sufficient to produce eyeshine that is visible to humans (despite their inferior night vision).

Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink, and red.

However, since eyeshine is a type of iridescence, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum lucidum crystals.

Although human eyes lack a tapetum lucidum, they still exhibit a weak reflection from the choroid, as can be seen in photography with the red-eye effect and with near-infrared eyeshine.

[18][19] Another effect in humans and other animals that may resemble eyeshine is leukocoria, which is a white shine indicative of abnormalities such as cataracts and cancers.

[21] Traditionally, it has been difficult to take retinal images of animals with a tapetum lucidum because ophthalmoscopy devices designed for humans rely on a high level of on-axis illumination.