night

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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iphone iwatch idunno

iphone iwatch idunno

I know. I was just trying to tag along with the news. What I really think though is this new wearable computing stuff like the watches that are now coming out….are truly something to watch. What I mean by that is something that you look at. Why was a watch called a watch in the first place? Maybe our smartphones should have been called watches.

Apparently the word watch was not the first word used to describe that time piece on the wrist. It was wristlet. Wikipedia has a couple of different theories on why the word watch came about though. One of them makes more sense to me and it has to do with people who were on watch. See how I can tie this into vision ­čÖé They were watchmen and used their wrist time pieces to keep track of their shift. Maybe they looked at them a lot like people do their smartphones these days.

I kept reading through the wikipedia article and found out something interesting about analog watches. You can use them to figure out which way is north or south….during the day of course. ┬áThere’s an easy way to do that at night. I hope you all know about the north star (polaris). It’s faint but always there and two points on the big dipper always line up with that north star and they are always there because they are circumpolar stars (they are always there all day and night, rotating). You mostly need to be in the northern hemisphere to see them however.

Back to how to figure north and south on an analog wristwatch. Aim the hour hand at the sun. Then look for 12 on your watch. Halfway between the 12 and where you lined up the hour hand will point south. Use 1 instead of 12 during daylight savings. Now I know which way is generally south. The sun is never really directly overhead even though it comes close sometimes. It’s usually in the southern half (hah! always I should have said) of the sky for us northerners. But you can’t point directly south that way so using an analog phone (that has the right time on it when you’re lost in the ocean) will point you south and opposite that is north.

One more thing about time. If you forgot all your watches and your smartphone battery died, you can tell how much daylight you have left before sunset. If you hold you arm out and count how many fingers in between the sun and the horizon, multiply how many fingers that takes by 15. Basically, each finger is worth approximately 15 minutes. Keep your thumb out of the way…it’s too short!

Let’s see what kinds of technology people will start watching and wearing on their wrists besides fitbits.

http://lifehacker.com/5932126/estimate-the-time-of-sunset-with-your-hand

http://en.wikipedia.org/wiki/Watch

hand and sun

hand and sun

analog watch

analog watch

 

 

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