The effect of colorful spots and glow in eyes, which can be seen in the pictures of humans and animals, is sometimes referred to as the “red-eye effect,” even though it is not necessarily red (Battiato 1302). Apart from that, animal eyes can glow while illuminated regardless of whether they are photographed or not (Wolpert 269). In both cases, this effect is the result of a combination of biological factors and light reflection, which, when studied, can help photographers to avoid the appearance of the spots and glow in their pictures (Battiato 1302; Battiato et al. 217).
The red eye effect is produced by the reflection of light, which is why it is typically the result of flashlight (for photography) or other types of illumination (Battiato et al. 218; Wolpert 269). In animals, the effect is explained by a particular layer in their eyes, which is called “tapetum lucidum.” Tapetum lucidum is a “biological mirror,” which exists to improve a creature’s vision in scant light by ensuring a “double pass” of photons through its retina, which enhances photon absorption (Wolpert 269-270). This eye layer is typical for nocturnal animals, including dogs and cats. The “double pass” is ensured due to the reflection of the light by the tapetum (the layer is usually found behind the retina, although it can sometimes appear within it as well), which means that the red-eye effect in animal pictures can be regarded as a side effect of tapetum’s natural function. It is noteworthy that the animal red-eye effect can result in a glow of different colors from blue to pink (Wolpert 270).
No tapetum can be located in the human eye, which explains the fact that the human ability to see in the dark is rather deficient. However, the human eye still has a surface that can reflect light, the choroid, which contains blood vessels and is located in the back of the eye (Battiato et al. 218; Wolpert 270). The fact that the light reflects from the blood-rich layer of the human eye explains the red color of the effect. Here, it should be pointed out that the primary function of the choroid is to provide nourishment for the eye, which explains the presence of blood vessels in it (Millodot 61-62). Thus, the red-eye effect can once again be described as a side effect of a function of an eye layer. However, it is noteworthy that this form of reflection does not always result in a red color. Indeed, if the part of the retina, which reflects the light during a particular photography session, does not have many blood vessels, the picture may have yellow or white spots (Battiato 1302). Thus, the red-eye effect for humans is not limited to shades of red.
To sum up, the red-eyed effect is a side effect of the functions of certain parts of human and animal eyes and a direct effect of light reflection. It is also noteworthy that the differences in the intensity of the effect depend on the angle of the reflected light, which, in turn, is determined by the opening of the pupil (Battiato 1302). Battiato et al. point out that this factor is exploited, for example, by flash brackets, which reduce the possibility of the red-eye effect by modifying the angle of the light (218). Thus, the study of the factors that cause the red-eye effect has a direct practical outcome: it allows finding the means of removing it from pictures (Battiato 1302; Battiato et al. 217).
Battiato, Sebastiano. “Single-Sensor Imaging Devices: An Overview.” Handbook of Digital Imaging, edited by Michael Kriss, John Wiley & Sons, 2015, pp. 1281-1310.
Battiato, Sebastiano, et al. “A Cluster-Based Boosting Strategy for Red Eye Removal.” Computational Intelligence in Image Processing, edited by Amitava Chatterjee and Patrick Siarry, Springer, 2013, pp. 217-251.
Millodot, Michel. Dictionary of Optometry and Visual Science. Elsevier, 2014.
Wolpert, Donald. “Biological Optics.” Biomimetics: Nature-Based Innovation, edited by Yoseph Bar-Cohen, CRC Press, 2012, pp. 267-307.