The glowing reflection seen in a dog’s eyes at night or in flash photographs is a common phenomenon called eyeshine. This luminescence does not come from the eyes creating their own light; it is simply a reflection of an outside light source. The color of this eyeshine can vary dramatically between individuals, appearing anywhere from a vibrant green or yellow to a red. Understanding the difference between these colors reveals distinct underlying anatomical structures within the canine eye.
The Biological Mirror: How Eyes Produce Eyeshine
The ability of a dog’s eyes to glow is due to a specialized structure located behind the retina called the tapetum lucidum, which is Latin for “shining layer.” This layer acts as a biological mirror, a retroreflector designed to enhance low-light vision. When light enters the eye and passes through the retina without being absorbed, the tapetum reflects it back across the retina. This process gives the photoreceptor cells a second opportunity to capture the light, effectively boosting the dog’s ability to see in dim conditions. The characteristic glow we observe is simply the reflected light exiting the eye.
Understanding the Green and Yellow Glow
The green, yellow, or sometimes blue reflection is the true eyeshine produced by the tapetum lucidum itself. This reflective layer is a cellular type, typically composed of about 9 to 20 layers of specialized cells. These cells contain zinc-rich crystals, specifically zinc cysteine, which act as the primary reflective agents. The exact color reflected is determined by the density and composition of these crystals and the angle of the light source. Most adult dogs exhibit a yellow-green hue, though it can range from a deeper green to an orange-yellow depending on the breed.
The Reason for Red Eyeshine
When a dog’s eyes reflect a red glow, it is usually not the tapetum lucidum being seen. A red reflection occurs when light passes through the retina and encounters no tapetum to reflect it back as green or yellow. This lack of tapetal reflection allows the light to hit the vascular tissue at the back of the eye. The red color is caused by light reflecting off the abundant blood vessels in the choroid layer, which lies beneath the retina. This phenomenon is identical to the “red-eye” effect seen in human flash photography and is most commonly observed in dogs with certain genetic traits, such as those with blue eyes or very little pigment.