The question of whether humans can possess red eyes is complex. Truly red eyes are not a naturally occurring, pigmented color like brown or blue. However, a reddish or violet appearance can manifest under specific conditions, primarily due to a lack of pigment in the iris. This lack of pigment allows underlying blood vessels to become visible, or the appearance may result from a temporary medical issue. Understanding the biology of the eye helps separate rare biological reality from common photographic and medical illusions.
The Photographic Illusion
The most frequent instance of “red eyes” is not a biological change but a common occurrence in flash photography, known as the red-eye effect. This happens when the camera’s flash is positioned too close to the lens in a low-light environment. In the dark, the eye’s pupils dilate to maximize light entry.
The sudden, intense light from the flash travels through the dilated pupil and reaches the retina at the back of the eye. Before the pupil can constrict, the light reflects off the retina and the vascular layer beneath it, called the choroid. This underlying tissue is rich with blood vessels. The returning light picks up the red color of the blood, reflecting it directly back into the camera lens to create the illusion of glowing red pupils. Individuals with less pigment, such as those with albinism, often exhibit this effect more prominently because their eyes absorb less light, reflecting more red hue back to the camera.
True Red Eyes Genetic Causes
The closest humans come to having red or violet eyes occurs in individuals with severe forms of Oculocutaneous Albinism (OCA). This genetic condition is characterized by a reduced or complete lack of the pigment melanin in the hair, skin, and eyes. Melanin typically gives the iris its color and provides opacity. In its absence, the iris becomes largely translucent.
Light entering the eye is not absorbed by the iris tissue. Instead, red light reflecting off the hemoglobin in the blood vessels of the retina and choroid shines through the transparent iris, giving the eyes a pinkish or sometimes violet appearance. This visual effect is structural; the eyes are not pigmented red but reveal the color of the internal blood supply. The condition is also associated with visual impairments, including photophobia (sensitivity to light) and nystagmus (involuntary eye movements).
Temporary and Medical Causes of Red Appearance
Beyond genetic causes affecting iris color, the appearance of redness is a common sign of a temporary medical condition or superficial irritation. This redness typically affects the sclera, the white outer layer of the eyeball, rather than the iris. The red, or “bloodshot,” appearance results from the dilation or swelling of tiny blood vessels on the surface of the eye, specifically in the conjunctiva.
Conditions such as conjunctivitis (pink eye) cause inflammation and swelling of these surface vessels due to bacterial, viral, or allergic reactions. A subconjunctival hemorrhage, often caused by straining or minor injury, involves a small broken blood vessel beneath the conjunctiva, leading to a bright red patch on the white of the eye. These forms of redness are temporary and external, representing a reaction to injury or irritation.
How Eye Color Works Normally
The vast spectrum of human eye colors, from brown to blue, is determined by the concentration and distribution of melanin within the iris stroma. High concentrations of the pigment eumelanin lead to darker colors, resulting in brown eyes, the most common eye color globally.
Conversely, lighter eye colors like blue and green are not caused by blue or green pigments, but by very low levels of melanin. In these lighter eyes, the lack of pigment allows light to scatter as it passes through the stroma, a phenomenon similar to Rayleigh scattering, which makes the sky appear blue. This scattering effect reflects shorter-wavelength blue light back to the observer.
Green and hazel eyes result from a combination of this light scattering with small amounts of yellowish pheomelanin pigment present in the stroma. This interplay of pigment and physics is why the structural absence of color in albinism is striking, as it bypasses the normal mechanisms that determine eye hue.