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| The Fovea centralis is responsible for your night blind spot. This can be somewhat compensated for by scanning. The optic nerve entering the back of the eyeballs is responsible for your day blind spot, which is overcome by using both eyes to view an object in your field of view. |
Within the last week or so, a 28-foot twin-engined boat ran into an unlit aid-to-navigation in the Intracoastal Waterway near Causton's Bluff. Both the ATON and the boat were significantly damaged. And it's just plain dumb-luck that the operator wasn't killed or injured. If you are going to operate a boat at night, you need to learn about the hazards involved, and your own capabilities and limitations...
When I was 13, I lived in Marathon and ran a small skiff at night. While speeding along in the darkness toward the opening in the breakwater that protected our dock, I learned to roll off the throttle as soon as I had the first hint of seeing rocks in front of me. You are much closer to an object at night than you think you are, and you might not see it at all until you are on top of it. That's what happened to PT-109 and a young John F. Kennedy.
While beginning the "Nighthawk" phase of the Army's helicopter flight-training, (without night vision goggles) I was making an approach to land at a stage-field and I was startled when the landing skids touched down on the runway well before I thought they would. I knew I was near the ground because of the dim glow from our position lights, but my distance estimation and depth perception were off.
Way off.
A couple of weeks ago, I was conducting training on a boat for a married couple and their son, and the time change and shortened period of daylight had us still on the water as it got dark. Now, if you had asked me, "are all aids to navigation lit?" I would of course have told you "No."
But learning occurs on 4 levels, (rote, understanding, application, and correlation) and it's one thing to know something and something else entirely to apply what you know to your behavior. As we got ready to accelerate to planing speed, the youngest person on the boat, who, thankfully, had listened to my pre-departure safety briefing asking everyone to keep an eye out for hazards, asked if we saw the unlit daymark on its pole in front of the boat.
He was the only one aboard who saw it, and we could have been startled or worse, but for him. Lesson learned? At night, everyone is a lookout, and you shouldn't overdrive your ability to see objects in front of you.
During the day, we use binocular vision for distance estimation and depth perception, and to help us see things in our path. At night we are using monocular vision, and we need other skills to help with distance estimation and depth perception. With practice, we can use monocular visual cues to greatly increase our ability to "see" at night.
Our eyeballs have limitations that we need to understand, or we can tear up our boat or hurt someone. In the collision mentioned above, a local boater collided with a "daymark," a lateral aid to navigation with no light; the crash damaged the mark and pretty much destroyed the boat. Those daymarks, the ones with the green rectangles and red triangles on top, are essentially signs on telephone poles, and what you don't see CAN hurt you.
About your eyeballs: You have an area on the inside of your eyeballs--your retinas-- with a large concentration of cone cells. This is the fovea centralis and this area is why you can see colors during the day. It takes a relatively large amount of light to stimulate these cells.
As it grows dark at night, you lose the ability to see colors, and your "night blind spot" becomes a factor in you seeing an obstacle in your path. Luckily for you, there is an area around the fovea centralis called the parafovea that is populated with rod cells. Rod cells do not see color, and provide less precise vision than cone cells, BUT they are about 1000 times more light sensitive. Night vision is monochromatic, or grayscale and this is why there are shapes on those lateral aids--the triangles and rectangles.
If you look directly at a dimly lit object in the dark (like an unlit daymark on a pole) it will disappear, because your lenses are then focusing the image on your cone cells. The ones that don't work at night.
The answer? Scanning and off-center viewing. When you are running on the water at night, you need to divide your "field of view" into 10 degree slices, or zones of the horizon, and move your eyes progressively across those zones. Only look at each area for a few seconds before moving your scan. You should also move your scan up and down the vertical field, scanning both near and far sections of the water to your front. You may find yourself acquiring an object in the distance, off to one side or the other of where you are looking. And when you look at it, it disappears. That's normal, there's something there. Don't hit it.
Night visual scanning takes discipline and is hard work. Don't be flirting or fooling around while running a boat at night. Post every available lookout. Slowing down helps too.
Remember the Titanic.
Brian McCarthy told me this story when we talked about running at night. "I was returning to Savannah after having dinner and enjoying the sunset with my wife and friends at Cafe Loco on Tybee. We had to run out to the ship channel and come back up the river due to the tides. As we cruised up the river under the stars, in dark night conditions, I noticed an area in front of us with no stars at all. The area above the horizon was pitch black in front of us. Nothing but a shadow."
As Brian was telling me this, I was reminded of a lesson from my night helicopter flight training at Fort Rucker. "Treat all shadows as solid obstacles" Brian's six-sense, the nagging feeling that something wasn't right, saved his life and the lives of his passengers.
"I turned the helm as hard and fast as I could, and we missed getting run over by a huge cargo ship with only a few feet to spare. I learned something that night!"
I was explaining all this night-vision stuff to a student-captain yesterday and he exclaimed to his wife, "that's why the comet we were looking at the other night kept disappearing! We'd see it while scanning and lose it when we looked at it!" He then added that during his service aboard a Nuclear Attack Submarine, he was assigned watch duty while on the surface at night, and the Navy never taught him about the features of the human eye. Score one for Army Aviation.
If you would like to experience this phenomenon for yourself some night, step outside in the dark and observe a jetliner crossing the sky at high altitude. When you look right at it, you will probably see the colored nav lights and a white strobe or two. As you slide your eyes off of the aircraft to one side or the other, the colors will disappear and the strobes will flash noticeably brighter. That's your rod cells on the job.
You can enhance your night vision by eating well (vitamin A), and protecting your levels of visual purple or rhodopsin with good sunglasses. When running at night dim your GPS and instrument lights to the lowest level at which you can still read them. And speaking of your GPS, those unlit daymarks are on that display. Take note of where they are as you cruise along. As you pass by each one, note where the next one ahead is plotted, and choose a heading to avoid it.
Don't get bumped in the night.
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