driving

Dark Adaptation Red Alert

Dark Adaptation Red Alert

I’ve wondered about the use of red for brake lights and tail lights. It’s a standardized system coded many years ago for countries to follow. But I sometimes have a hard time at night gauging where that car in front of me really is so I don’t have to slam on my brakes. Throw in some head lights and glare from oncoming cars and it’s not a pretty thing trying to stay out of trouble.

Most of you know about the rods and cones in our eyes, the light detecting photoreceptors in our retinas that is. During daylight hours when inside with normal lighting or outside during the daytime, our vision is sharp (for most of us) owing to how those cones work. They’re good for that kind of lighting. In very dark places we have the other photoreceptors, the rods, to help detect dim light but not colors. The rods don’t help at all with sharpness but they do tell us if there is a light on somewhere and those cells connect to each other unlike cones so they can add to each other’s light sensing effect. All those photoreceptors are working practically all the time but there is a light level where the rods and cones might shift to whichever is more effective for that lighting. See below for that graph.

But red light is a special thing. We detect light between 400nm and 760nm. This is the rainbow of colors where 400nm (nanometers) is violet. At the other end of the visible range of light we can detect, 760nm is red which has the least amount of energy of all the others. But violet isn’t just 400nm. It could look violet-ish at 430nm. And the same goes for red, give or take. And before all hell breaks loose here, I’ve read the visible spectrum may start a little lower than 400nm and the upper end might be a little higher than 760nm. Websites are all over the place with numbers and that’s like a lot of things by the way. Some things in life aren’t always clear or exact.

Here’s the great thing about red. If you turn off all lights and just use red light, you can see with it…of course…(even though the other cones are basically switched off). Maybe you won’t see as easily than with regular lighting but we can read (not red letters though). And why does any of that matter? If you want to be dark adapted, meaning, if you want to see in the dark but still need to read something written, you can do it with red light and it won’t negatively affect your ability to have good vision in the dark (by good I mean as good as we can possibly see in the dark). This is why red (I think) was chosen as the light color for brake and tail lights for at least night driving. This is also why you will find red lights in dark places like movie theaters or those exit signs we have everywhere. Red light won’t ruin your adapting to dark or dimly lit places.

And this is also interesting about red…..there is a point when lights get so dim that you can’t differentiate colors but if something has a red color you can still see it (not it’s red color but you can tell it’s there). When light is eventually dimmed completely, then there is no light at all for your eyes to detect (total darkness) so red is the last color standing! And…red alert means it’s getting pretty serious :), as in war time serious. Hiding in the dark still required seeing so red lights helped and pilots kept their dark adaptation by using red light or red goggles.

One more thing about red light. That long wavelength (almost twice as long as violet) actually focuses a bit “behind” our retina so those needing reading glasses or getting close to needing them…might have to bump up the reading power a bit if you’re going to read best in red light.

http://www.aoa.org/optometrists/tools-and-resources/clinical-care-publications/aviation-vision/the-eye-and-night-vision

This is a particular interesting “web book” on vision….http://www.yorku.ca/eye/thejoy.htm

light dark graph

light dark graph

 

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Specular reflection and driving at night

Specular reflection and driving at night

One night I was driving on a winding road near Golden, CO and wasn’t sure if I was in my lane or not. There were four lanes so I know I was on the right side of the road. But I could not see any markings between the two lanes going in my direction and the road was really dark for some reason. I’ve thought about that situation enough to try and figure out why that happened. It’s not just that road I was on but something on the road.

Do you know how our headlights work? Silly question. Those beams shine out along the road in front of us and the road surface reflects the light back into our eyes. That’s a particular type of reflection called diffuse reflection. You are involved in the process whether you knew what it was called or not. If light didn’t bounce back into our eyes we wouldn’t see the road. Not a spectacular deduction but that’s how we see where we’re going. And thank goodness the surface has these little “imperfections” that do that. But what if those little imperfections, those little teeny bumpy surfaces in the road, were filled in with water, maybe just a little bit like after a sprinkle of rain or from melting snow running across the road?

When moisture fills up all those little imperfections in the road this is why the road is harder to see. The light from our headlights hits less of those imperfect reflecting surfaces, we get less diffuse reflection into our eyes and it just bounces up further down the road (away from us). That’s called specular reflection. Light is bouncing off the surface of the water and heading in the same direction as our car (truck, whatever). It ain’t coming back to help us see the road like when it was dry. What are we going to do about that? Not a whole lot unfortunately.

I can make a couple of suggestions though. Clean your windshield, clean the gunk off your headlights and slow down. Don’t drive? Get those glasses out of the glove box and put them on 🙂

An example of a beneficial type of specular reflection is when photographers use the smooth surface off of water that is reflecting mountains or some landscape. That’s kinda of like a reverse specular reflection.

Here’s a little more information from this website that might help understand the optics:

http://www.physicsclassroom.com

wet road

wet road