What we were taught in military aviation is that the average person loses half of their night vision every 10-15 years.
Hopefully Dave our eye Doc can chime in with more recent data.
I’m not aware of an specific statistic Scott. I’m also not sure that it would hold true today. There are several reasons why our night vision is worse as we age.
The first is that our pupils get smaller over the years. At 61 years old, there is a lot less light entering my eye because of a smaller pupil than when I was 20 years old.
Secondly, the cornea and lens of the eye become less clear over time (cataracts anyone?). This causes increased glare. That increased light scatter spreads the bleaching affect of a point light source around the eye. That is one reason we have more trouble with night driving as we age.
Finally our ability to regenerate the photopigments in the eye that actually receive the light decreases over time. Part of this is simply a slowing of metabolism with age. Part of it is decrease blood flow to the retina and decreased O2 saturation. That is why smoking is so detrimental to night vision. With any type of bright light, the rods in the retina are bleached out very rapidly. This means that all of the photopigment is depleted in the photo-receptor. This has to be replaced by the outer layer of the retina and takes a while. The cones do the same thing but their photopigments are replaced much faster as they have two ways two biochemical routes to replace the pigments.
Bottom line is that the rods are our most sensitive low light receptors. Back in the day, the navy would have lookouts posted above deck, away from any light source and the ship would run dark. This allowed the sailors to fully dark adapt and they had incredibly sensitive night vision as a result. Sailors were warned however than even striking a match would mean a half hour of recovery time for the night vision.
We don’t operate in scotopic conditions (ie rods only) but we operate in low light which gives us a combination of rods and cones. However, any bright light, even brief, which take out the rods and therefore our low light sensitivity.
If you are ever looking at something at night and cant tell what color it is, you are experiencing mostly scotopic vision, as your cones are inactive with not enough light to stimulate them.
Most of what has been said already I agree with. It takes only a second to ruin your night vision and then can take 30-40 minutes to be fully dark adapted. I avoid using my spotlight. I also have all illumination extremely dim in the PH during night operation. I will use a spotlight when coming into an unfamiliar marina at night if I need to identified a dock or slip number. However, I don’t use it while cruising. The problem with the spotlight is it will allow you to see what it illuminates, but will prevent you from seeing anything else. The use of a spotlight makes it all but impossible to see the navigation lights of other boats at a distance. My feeling is, if you don’t feel you can safely run at night without lights, you shouldn’t be running at night.
As has been mentioned, it also is dangerous for other boaters who have their own vision bleached out by your spotlight.
