Why do people have red eyes in flash photos

Many animals in the world are known to develop a glassy “glare” of various colors at night should a light be shined into their eyes, with some of the most common examples of these being cats, dogs, deer and other animals living a highly nocturnal life. This glare in generated from a layer located within the eye itself known as the tapetum ludicum that acts as an organic mirror of sorts to reflect light waves back out of the eyes – thus generating the glare seen when a light is shown into the eyes of these animals when it is dark and not at other times.

The purpose of the tapetum ludicum is to reflect light waves that filter entirely through the eye back out of the eye in order to allow for a second chance at detection by visual receptors. This process increases the overall effectiveness of eyes in low-light settings and thus enables animals to remain more functional in nocturnal environments.

Unlike these animals, however, human beings do not possess a tapetum ludicum within their eyes, thus no light is generated when a standard light bounces into a human’s eyes in a low-light environment. A high-intensity camera flash, on the other hand, is a different matter. Generating a powerful-enough burst of energy and capturing the resulting light waves in an aperture immediately after the light is generated creates a reflection to be generated off of the standard pupil of the eye. The resulting reflection is what is captured in a photograph, with the red color coming from the reflection of blood vessels within the eye.

Because the “red eye” of humans is generated by a reflection of blood vessels within  the pupil necessary for eye function the red-eye reduction capabilities of modern portable cameras works to reduce this by taking pictures with a double-burst of light. The first flash generates a strong enough light source to cause a contraction of the pupil and thus significantly reduce the overall possibility of reflection from occurring, while the second burst is used to provide luminescence for taking the photo itself. Alternatively flashes can be bounced off of nearby walls or light generated from an alternative angle as well in order to cause the reflection to move away from the camera rather than be returned directly into the camera’s lens, thus removing the glare from human eyes (though unfortunately not that of many animals possessing a highly reflective tapetum ludicum as this will return light from virtually any angle in a dark setting).