The tapetum lucidum helps nocturnal animals see in the dark; it exists as a thin layer of tissue behind the retinas of nocturnal animals, reflecting light and producing shine. The eyes of many animals contain photoreceptors, which function as light-receiving organelles. These components work in conjunction with the brain and brighten images, helping animals see during the day.
The tapetum lucidum has the opposite effect of photoreceptors. These equally important organelles reduce brightness, providing night vision in nocturnal animals. Typically light hits photoreceptors from direct sources, but tapeta bounce light back towards the source, ultimately preventing absorption. This structure appears in some primates, raccoons, opossums, dogs, cats, foxes, horses and even some fish. The tapetum lucidum appears in photographs as a bright light within the animals' eyes. Like the animals in which they reside, the tapetum lucidum organelles come in various shapes, sizes and colors. They appear as white structures in some eyes, and as yellow or blue objects in others. The color of tapeta depends largely on the chemical and physical composition of eyes, including the amount of pigment in them and presence of certain substances such as zinc and riboflavin. Scientists classify tapeta lucida into four distinct groups; organelles in these groups vary in size, shape and composition.