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Have you ever wondered how you are still able to see your bed when you turn off the light at night? Or how people don’t trip over themselves during nighttime walks? The answer lies in night vision. Night vision, also called scotopic vision, is the ability of the eyes to naturally see in the dark. It allows you to see in low light environments and is facilitated by two main actors, the pupil and the retina.
The pupil is the part of the eye that acts similar to a camera’s aperture. It expands and contracts in order to control the amount of light that enters the eye. In settings with high light intensity, the pupils will contract in order to limit the amount of light exposure, as too much light can be damaging for the eyes. Likewise, in low light settings, the eye will want to absorb as much light as possible. Thus, the pupil will dilate to help with one’s night vision.
The retina is a thin tissue that lines the inside of the back of the eye. It is made up of mostly millions of photoreceptor cells, tiny cells that are sensitive to light. Of these photoreceptor cells, there are rod-shaped and cone-shaped cells, simply called rods and cones respectively. Cone cells are used to see very fine details, and are the reason why we can perceive colour. However, they require much brighter lights to do so. Rods, on the other hand, can only see in black and white and have a much poorer resolution, but they are much more sensitive to low intensity light. Rod cells are the photoreceptors that regulate scotopic vision.
Night vision also differs from species to species. Animals such as owls and cats have stronger scotopic vision than humans because they have a much greater rod density, meaning that owls and cats have many more rods than cones. In owls, rods outnumber cones in a 30:1 ratio, far exceeding the human ratio of 20:1. Additionally, owls and cats have a reflective layer of tissue behind the retina, called the tapetum lucidum. It reflects light back into the retina, and greatly increases the amount of light available for the photoreceptor cells, making these animals excellent at seeing during the night.
Overall, night vision is a remarkable thing that enables all animal species to operate in dark environments. It allows nocturnal animals to hunt and thrive in the night, and allows us humans to take nice, long, nighttime walks. The next time you lie in bed while getting ready to fall asleep, or take a step outside during these long winter nights, think about all the tiny rod cells inside your retina, working hard to let you see the wonderful things around you.
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