ABCs of Optics: N is for Night Vision (part 1 of 3)
Friday, November 8 2024 | 08 h 52 min | Vision Science
This course is accredited for 1 EC for NACOR and 1 CL/EG/RF for COO.
NACOR #110.643 and COO #4250
Completion of online post-course test required at www.OptikConEd.com.
By Thomas Weissberger, RO
For those of you who follow my ABCs of Optics and wonder where M went, I didn’t forget about it, but instead I consider it covered by the Childhood Myopia article and CE course. For those that have no idea about what I am talking about, just enjoy N!
When you hear the words night vision, do you immediately have thoughts of spies and gadgets, or are you more inclined to think about how we see in the dark? Well, here we will cover both aspects.
Night Vision and Adaptation in Animals
All vertebrates share the same basic eye structure: a pupil that dilates or constricts to control how much light enters the eye, a lens to focus the image onto the retina, and nerves that relay the information to the brain. Nocturnal animals (those that are active at night) have evolved physical traits that allow them to see in the dark more effectively than diurnal ones. Their pupils are larger, with wider-set eyes and reflective layers behind their retinas.
Nocturnal animals have more rod cells (discussed below) in their retinas, which enhances their night vision to eight times that of humans. Animals with the best night vision include:
- owls
- cats
- reptiles
- frogs and toads
- bats
- some mammals like opossums and skunks
Owls and cats have pupils that, when open wide, cover the entire front of the eye, as do tree frogs. In owls, eye size approaches the extreme: their eyes occupy over half the volume of their skulls. Many nocturnal animals have a mirror-like layer, called the tapetum, behind the retina, which helps them make the most of small amounts of light. Light that passes through the retina is reflected off the tapetum, giving the retinal cells a second chance to sense it. This causes some animals’ eyes to shine in the glare of car headlights. The reflected colour seen is the pigment on the inner layer of the retina.
Night Vision Science: Rods and Cones Reviewed
As we know, all vision is triggered at the retina, containing two types of light-sensing cells: rods and cones. Cones account for colour vision but require bright, focused light, whereas rods can sense very dim, scattered light, but produce mainly a black and white image. While each cone has its own brain connection, multiple rods are wired to a single brain connector. This pools the information collected from the rods and creates a stronger signal, but the image is less defined. The retinas of nocturnal animals are packed with rods and have few cones, however, because their large eyes create large images that are focused on a big retina, they capture some detail despite the shortage of cones.
Human eyes take several hours to fully adapt to darkness and reach their optimal sensitivity to low light conditions. The quickest changes in light sensitivity are made in the first few minutes after exposure to darkness; many people think their eyes have reached their peak sensitivity in those first few minutes, but dark adaptation has just started.
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The Practical Professor: Optical Solutions for the Real World
I am a second generation optician and have made all aspects of opticianry and optics my life. I have over 10 years of teaching experience as a former professor in the Dispensing Opticians’ program at Seneca College and at Georgian College. I was also responsible for
modernization of the optical curriculum.
I am a trainer and trainee (One never stops learning!), and I am available to share my knowledge and experience with today’s eye care professionals. As the owner of Special Eyes Optical Services, I am dedicated to supporting the optical industry. You can reach out to me at Tom@SpecialeyesOpticalServices.com.
