The canine visual system is capable of functioning under a wide range of lighting conditions, but the adaptation to low light conditions enhance their ability to function as predators. The minimum threshold of light for vision in cats is six times lower than that for humans, and the although the minimum threshold for dogs is thought to somewhat greater than that for cats, it is still much lower than that for humans. Several methods are used by dogs to improve vision in low light conditions. The principal method is based on the types of photoreceptors present in the canine retina. The retina of the dog's eye is composed primarily of rod photoreceptors, while the human retina is composed primarily of cone photoreceptors.
Rods function better in dim light, while cones are used for color vision and require bright light.
Humans and canines both have rods and cones present in their retina's, but the relative amount of each is very different. Another method used by dogs to improve low light vision is the use of a tapetum lucidum. The tapetum lucidum is a highly reflective layer of cells located behind the photoreceptors in the canine retina. This reflective layer functions to improve low light vision by reflecting light back through the retina, essentially allowing the photoreceptors two chances to react to each quantum of light. The tapetum lucidum may also function in another manner.