Issue 104 / March - April 2015
How Is Vision Possible in Total Darkness?
Animals can move and even hunt in the pitch-dark of the night. How do these animals see comfortably in the dark? How are their eyes different from ours?
Anatomically, the eye consists of three layers called the Tunica fibrosa (outer layer), the Tunica vasculosa (mid layer), and the Tunica nervosa (retina). The outer layer of the eye is composed of the sclera and cornea; the middle section of choroidea, corpus ciliare, and the iris; and the inner section of the retina.
Light rays reflected off objects first pass through the eyeâ€™s translucent layer (the cornea), then into the black round part located in the front of the eye (pupillae, or pupil), and later go through the lenses. The colored section around the pupil, covered by smooth muscles, is in charge of regulating the amount of light entering the eye. The real and inverted image of an object is projected on the retina by the refraction of light through the lens; the image is transmitted to the optical center of the brain as a result of nerve cell stimulation via the optical nerves. Sight happens as a result of these causal chains.
Now, there is a twist to all this: the optical center in the brain accepts spots that do not emit light as black. For humans, this means a loss of sight. Black objects are not visible because they absorb and keep all the light reaching them.
How could these animals, without night vision goggles, see in pitch blackness despite having the same optical mechanisms as humans?
The answer to this question was hidden in an anatomic structure (tapetum lucidum) located in the eyes of some vertebrates. This special structure, lacking in humans, monkeys, squirrels, birds, red kangaroos, and some other mammals, is found in equines, ruminants, and many carnivores. This structure is located in the cytoplasm of the choroidea layers in between the innermost light sensitive ocular layer and sclera, and it acts like a biological reflector. With its crystalline composition of varying colors from golden yellow to white, tapetum lucidum is a wonder of creation. The eyes of these animals shine when a light source is projected due to the aforementioned structure.
The primary task of this formation is to reflect the light which is projected to the rear section of the eye again without absorbing it. This happens due to the crystalline makeup. Thus, lower light levels are enhanced by the repeated reflection in the eye, and sight is enabled for animals. The second job of Tapetum lucidum is to elevate the sensitivity of the retina to light in order to transmit signals with no stimulation strength to the optical center.
This reminds us of night vision goggles. Night vision cameras are electro-optical devices that strengthen the light that is present. Light enters this device through the lens and hits the charged cathode, which has a lot of high energy. The energy load hits the phosphorous screen where the image is focused after passing through the vacuum inside the charger. The image is an enhanced picture on the phosphorous screen and it is not visible directly through the object. Tapetum lucidum however, is such an artistic work that it cannot even be compared with the night vision systems of technology. While the lifespan of a night vision system is only 2,500â€“4,000 hours (104â€“167 days), animals with Tapetum lucidum can benefit from this for a lifetime. The newest night vision systems, with a maximum optic range of 30-120 meters, cannot provide sight under light conditions one fourth of the moonâ€™s illumination strength, whereas animals with Tapetum lucidum can see objects hundreds of meters away in much dimmer light.
This isnâ€™t the only example of how masterfully and diverse eyes can be. Damseflies have over 30,000 simple eyes, called ommatids; eagles can see their prey from thousands of feet above them.
By the principle that "certain things are appreciated best in their absence," if we imagine a dark night in which we are unable to see anything and cannot even step a foot safely, we can perhaps understand that Tapetum lucidum is a great blessing for these animals.
Veterinary Ophthalmology (2004) 7, 1,11â€“22. Comparative morphology of the tapetum lucidum (among selected species), F. J. Ollivier,* D. A. Samuelson, D. E. Brooks, P. A.Lewis, M. E. Kallberg and A. M. KomÃ¡romy.