Pigment Behind the Retina Either Absorbs or Reflects Light, Depending on the Animal's Habits

In animals that rely heavily on acute, daylight vision, there is a dark pigment in the epithelial layer between the photo­receptors and the choroid. This pigment absorbs light that has passed by the photoreceptors without stimulating them. If such light were reflected back into the retina, the sharpness of the visual image would be blurred. In nocturnal animals, however, these pigmented layers contain a reflecting pigment and are called the tapetum. This allows the retina to make optimal use of what light it receives, but at the expense of visual acuity. Reflection of light off the tapetum causes the familiar “night shine” from nocturnal animals’ eyes.