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FiRE_MaN's avatar

What color is hardest to see at night?

Asked by FiRE_MaN (684points) September 29th, 2009

I have heard that in the navy you have to have red flashlights at night if you go outside and I have also heard that soldiers in the field are required to put red filters on their lights. Is this because red is the hardest to spot out at night? If not what is?

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17 Answers

dpworkin's avatar

Any. Your Rod receptors in your retina are not designed to discriminate color. That is the function of the Cones, and they operate in the light or during the day.

Les's avatar

I think there are two reasons for this. The first is red light doesn’t mess your eyes up as much as “white” light. Red has a very long wavelength which means that it has low energy (as opposed to blue, which has a short wavelength, thus high energy..). There is something called the Purkinje effect which basically says that in low light scenarios, the sensitivity of our eye shifts towards blue/green. Reds are some of the first colors we lose when it starts to get dark out. What is of interest to your question in that Wiki article is the part where they talk about rods and cones. Red lights (ie in submarine control rooms, dark rooms though, there are other reasons for red lights in dark rooms, and the flashlights) are used because they allow the person to switch between a low light scenario to a lighted scenario without that “blindness” that happens when you come out of a dark movie theatre into the sunshine.

There also may be an advantage to red flashlights in that an “enemy” far away may not be able to see the light, as the low energy of it may cause the majority of the light to dissipate before reaching the eye of the enemy. But I’m just guessing at that point.

dpworkin's avatar

During WWII, pilots used to brief in a red-light room before going on night missions so that their rod cells had time to adjust before the flight.

FiRE_MaN's avatar

thanks guys

rooeytoo's avatar

If red is the first to go at night (and I am not disputing that fact) why are brake lights and tail lights on cars red????

FiRE_MaN's avatar

@rooeytoo thats a good point. and i was thinking something the same. on towers and tall objects they put red strobe lights on them so low flying planes wont run into them… i dont get that.

dpworkin's avatar

Because they are bright enough to stimulate the correct receptors which then provide warning information to the brain. They are lamps. If they were merely reflected light under nighttime circumstances from a red object they would be visible only as black.

Darwin's avatar

I would have to say that black is the hardest color to see at night. A coworker unfortunately found that out the hard way when he was drunk and crossing a roadway at night while wearing black clothing. He was hit by a car.

However, as this guy puts it:

“When our eyes are fully night adapted, we are using our rods. The rods cannot detect red light, but our cones can. So if we use a red light flashlight we can see what is around us using our cones. The rods in our eye can’t see the red light and our night adapted vision is unaffected. Turn off the red light and you can go right back to looking at the stars in the same detail as before without having to wait another ½ hour for our rods to work again.”

To totally trash someone’s night vision he says shine a bright turquoise or cyan light at them:

“If you want to thoroughly ruin someone’s night adapted vision, shine a bright Cyan/Turquoise at them. Since the rods are responsible for our night vision and are most sensitive to this color light, they’ll immediately go “night blind” and will be unable to see in the dark at all immediately after turning off the light. Of course this will work with a bright white light too.”

Les's avatar

@pdworkin is right on. Do an experiment. Take something red and something blue/green outside when it is starting to get dark. I’m willing to bet you’ll have a hard time knowing the red object is red before the blue/green object is lost. Also, make sure it is a “true” blue, not navy blue. You have to compare colors of the same value for this to work.

Les's avatar

I also want to add: Purkinje effect is related to the luminance of an object. Luminance is the “amount” of light that is emitted from something. So a red light has luminance, but a red piece of paper also has luminance. The red lights are most likely used to be safer, because they don’t cause night blindness like a white or cyan light would. If that red light on the tower had a “white” or cyan light below it, my guess is that the light you’d see would be the white light. Yes, you can still see the red light at night. But a pilot looking for a light would not be blinded by a red light as she would if looking at a white light.

dpworkin's avatar

Without resorting to a text (so someone may have to correct me here) I believe that white light depletes rhodopsin in the rods, while, because of the Purkinje effect, red light does not tend to consume it as much.

It takes about 30 minutes to adapt to darkness from full daylight – the first 8 minutes the cones adapt, but plateau at their limits, during that 8 minutes rhodopsin has begun to build in the rods, which take over, and in the additional 22 minutes light sensitivity as measured from the first moment of dark-adaptation to the full 30-minute acclimation period is something like 10^-9! Quite an impressive difference.

SeventhSense's avatar

Red light is the most helpful in allowing your eyes to adapt to the darkness while assisting your limited night vision.

PerryDolia's avatar

Red. That is why fire engines are white and flourescent yellow now. Red is the first color lost in dim light.

FiRE_MaN's avatar

ohh haha i was wondering why they stopped painting the trucks red

krasher's avatar

A shade Pink pastel is actually the most difficult color to see at night. US Air Force was given opportunity to fly this shade of color on its F117 Nighthawk but they refused this color scheme and insisted on black. It would look silly to fly pink planes. But that didnt stop the the British SAS during World War II during the North African campaign against the German Afrika Corps. British light armored vehicles and jeeps were painted a drab hue of pink and were virtually invisible in the desert at night. See link

FiRE_MaN's avatar

oh wow! thats interesting

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