License to learn: The dark night rises

Give your eyes time to adapt

May 1, 2013

Rod Machado

I remember the time I spent an entire day at a nearby airport filming video segments for an online aviation magazine. It was the perfect confluence of airplanes, sunlight, and even more sunlight. That’s right. For eight hours, the cameraman kept a large light reflector pointed at my face. You know you’ve been out in the sun too long when you hear the sound of bacon frying and there isn’t any bacon. Yeah, that was my face. At the end of the day I was camera crispy, but that wasn’t the worst of it.

Toward the end of the session, we filmed a few landings from inside my Bonanza A36. Everything went well, except for the landings. For a reason that I’ll reveal soon, my depth perception was off, rendering me unable to properly estimate the height for the landing flare. Thump! OK, we all have bad days but few of us have them on video. With daylight waning, we packed up and I flew off into the sunset. Literally. The sun was setting as I approached my home airport. That’s when I went blind—or nearly so.

While on a left base to Runway 19L, I glanced toward the runway. It was gone. So was the airport and coastline. Bermuda Triangle? Not this time. Unbeknownst to me, I was staring in my own reality show titled, When Night Blindness Strikes. Believe me when I say that a two-lane airport with a high-volume mix of airliner and GA traffic is the last place you want to lose sight of your sight. My instinct had me climbing away from the airport, returning 20 minutes later once my eyes had begun acclimating to the dark.

I was the victim of extreme photobleaching. It turns out that the cells in your retina that are responsible for night vision, known as rods, contain a light-sensitive chemical called rhodopsin. I’m very fond of chemicals, especially this one, because it’s responsible for our sight. When sunlight strikes the rods, the chemical rhodopsin splits apart (a process known as photobleaching) and initially forms two other chemicals, opsin and trans-retinal. As this split occurs, a small amount of electrical energy is released and sent to the brain via the optic nerve. This is what forms an image inside your noggin. Eventually these chemicals recombine to form rhodopsin (they become unbleached), which once again becomes useful for converting light into sight.

The problem occurred because I spent way too much time in direct (and directed) sunlight. Since rhodopsin can only be replenished at a certain rate, I managed to photobleach a substantial amount of my eyes’ available supply. My subsequent exposure to darkening conditions rendered me night blind.

As a general rule, it takes about 30 minutes for the eyes to replenish their supply of rhodopsin and adapt to the dark. You can, however, achieve a moderate degree of adaptation to the dark after 20 minutes if you’re using dim-red cockpit lighting.

Since then, I’ve learned to wear sunglasses when shooting videos in direct and intense sunlight. I’m even prepared to slip on an arc-welding helmet if the cameraman treats me like an ant under a magnifying glass (the Darth Vader look should eliminate any chance of being ramp checked, too).

Over the years my fondness for unsplit rhodopsin has grown even stronger, especially since it regenerates at a slower rate as we get older. Age also tends to diminish the amount of light entering the eye. The retina of a 60-year-old person receives only one-third of the light it did at 20 (even during the daytime). You don’t have to be a rhodopsin scholar to realize what this means in terms of your eyes’ ability (or lack of it) to adapt to night time conditions with age.

If that weren’t enough, your color sensitivity declines with age, making it more difficult to discern what lies beyond your airplane’s windscreen at night. Blues (taxiway lights) and greens (threshold lights and right-wing position light) are more affected than other colors and become less detectable. Of course, FAA inspectors wearing blue or green jumpsuits are harder to see at night, too. One might easily nab you for a ramp check—unless you were wearing your arc-welding helmet, of course.

Considering the eye’s limitations, a part of me wishes I were a bat, or at least carried one in my pocket during sunset flights. Echolocation has never looked so good; at least, that’s what I hear. So give your eyes time to adapt before the dark night rises, especially if you’ve spent a bright day in the sunlight.

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