This followup post owes its existence to Frank Kingma who added another tip to my original post and led me to do a little more research into the night vision capabilities of the human eye. Frank’s insights came from his background as a pilot where mine are from studying sensory perception as part of developmental psychology a long long time ago.

A simple description of the human eye is a lens which focuses light onto a layer of photosensitive cells that cover the inside of the back of your eyeball. That layer is where electrical impulses are triggered by chemical reactions to light from the lens.

There are millions of photo receptor cells in your eye, broken into two types – cones, and rods.

Cone cells are responsible for high definition colour vision. In a normal eye they are further broken down into three different colour sensitivities allow us to build up a colour image (trichromatic vision). Cones are concentrated in a 5mm area in the centre of your eye, at the focal point of the lens, and are active when you look directly at an object. Each cone is connected directly to it’s own electrical circuit to your brain providing a very sharp image. They react very quickly to light under normal daylight conditions, but they’re kind of useless in the low light conditions at night. There are about 7 million cone cells in a human eye.

The other type of photoreceptor cells are called rods. They’re much more interesting in terms of night vision.

There are around 100 million rod cells in your eye compared to the 7 million cones. While cones are concentrated in the focal point of the eye, rods are spread across the rest of the eyes inner surface where our peripheral vision operates.

Tip #1 – Turn your head slightly off your intended viewing target to engage the part of your eye which is dense with rods.

Rods are effectively colour blind and are specialised for seeing edges and motion – both changes in light and dark.

Tip #2 – Keep your eyes moving. Peripheral vision is optimised for light moving across groups of photo receptors. When the object isn’t moving, you can get the same effect by moving your eyes.

In contrast to cones, rods are 100 times more light sensitive, but operate in clusters which are wired to your brain in small groups providing a lower resolution image at much lower light levels.

Rods are highly sensitive to blue-green wavelength light and relatively insensitive to red. Some theories suggest this is an adaptation to allow humans to operate at night when the dominant light source is starlight which is predominantly blue-green.

Tip #3 – If you use a red task light at night, it is being seen by the colour sensitive cones, and largely ignored by the rods. You can use a red light to do housekeeping tasks without obliterating your night vision.

Tip #3a – If you want to reach maximum night acuity, don’t use a red light either. Observe your surroundings while there is twilight and keep your eyes at maximum adaption.

There’s a difference in the sensitivity of rods and cones too.

Cones adapt very quickly to a change in light levels within the daylight range, but they only operate in a limited range of light conditions. The main control on light intensity for cones is via the pupil which expands and contracts almost instantaneously to control the amount of light that gets through to your eye. Typically cones adjust to extreme brightness within 9-10 minutes with most of the adjustment happening in 30-45 seconds.

In low light ranges, rods react much more slowly. The first adaptation takes about 5 minutes and continues to improve your light sensitivity for between 45 minutes to reach 80% efficiency and two hours for maximum acuity. This is because the photosensitive protein in Rods, rhodopsin, is consumed by its reaction with light and needs to be replenished by your body.

Tip #4 – It takes 45 mins to 2 hrs to reach peak night accuity. Avoid resetting this to zero by looking at any bright lights.

Once your eye is fully adapted to night vision, it can be “reset” by bright lights. Good night vision will return in the first minutes, but it will be up to 2 hours before you are operating at peak night vision level. In the last stages of adaptation, your night vision improves by several orders of magnitude.

Tip #5 – If you can’t avoid looking at a bright light, keep one eye closed until you are past the light source, then switch eyes, closing the one that just got blasted. Your eyes calibrate independently and your blasted eye will be back at 80% in 4-5 minutes while your preserved eye will still be at 100% acuity.

One of the earliest references to night blindness was in De Medicina by Roman encyclopedist Aulus Cornelius Celus in 30AD, who linked night vision with a deficiency of dietary liver (although he did suggest pouring the contents of a male goats liver over your eyes, so maybe he was just making a lucky guess). Today, we understand the linkage between vitamin-A, retinol, and the creation of the photosensitive protein rhodopsin. The Romans had it right with liver being a source of vitamin-A as well as fish oil and dairy.

Tip #5 – Vitamin-A helps keep your night vision in top shape – not carrots. You can’t make it better, but you can keep it healthy.

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Further reading

https://en.wikipedia.org/wiki/Adaptation_(eye)