Tapetum Lucidum

"The tapetum lucidum (/təˈpiːtəm/; from Latin for "bright tapestry; coverlet", plural tapeta lucida) is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep sea animals."- Wikipedia   While most primates lack these, many supernatural creatures such as Vampires and Weres have them. This is used to identify these creatures on sight, although vampires have developed contact lenses that drastically reduce the flashiness of their eyes, called anti-flash contact lenses.