Ever since I was a boy toys would pitch flashlights with red and green lights as night vision lights. Having a red flashlight was cool, but I never bought into the “gimmick” that it was night vision as a child. Now that I’m older I see the same thing in all the outdoor flashlights and headlamps that I buy. Is there something behind red lights preserving night vision?
Is using red light to preserve night vision a myth? Red light preserving night vision is not a myth! The rods in your eyes that are used to see in the dark can’t pick up the long wavelengths of red light. So red red light prevents your eyes from constricting preserving night vision.
In the rest of this post I’ll explain why red light helps preserve night vision and explain a few of the many advantages/disadvantages of using a red light in the dark. There’s another great option with an extra benefit, so you may decide to use that instead.
Red Light Night Vision Myth Explained
Everybody has experienced the refractory period where your eyes have to adjust after going from bright to dim light. It takes 20-30 minutes for your eyes to transition to their dilated night vision state. Red light preserving night vision isn’t a myth! Using a red light prevents your eyes from adjusting to the increased light, helping to preserve night vision.
Red light is by far the best color for preserving night vision (green is 2nd best with more clarity). The part of your eyes that deals with dim light (the rods) can’t pick up the longer wavelengths of the red light spectrum. So the daytime sensors (cones) are the only part of your eye working to deal with the light, which leaves your night vision intact.
Since red light is invisible to the part of your eye that deals with night vision, they maintain their dilated shape that helps absorb light in dim environments. I’ll explain more about this below, but here are the main advantages/disadvantages to using a red light in the dark.
- Preserves Night Vision: As I mentioned above, red light helps preserve night vision since your eyes won’t have to transition from a constricted state (bright light) to a dilated state (dim light). That’s great for preserving night vision, but the cones in your eyes aren’t used to dealing with dim light environments. That leads to vision problems in dim light.
- Other Animals Can’t See Red Light: Red lights are the primary choice for hunters (hog and small predator hunters use green) when they’re entering/exiting camp and trying to spotlight animals at a distance. Animals have a much harder time seeing red light in the dark (just like humans). Most animals have underdeveloped color sensors so they perceive the color red as a dim washed out grey. So using a red will reduce the chances of scaring away your prey.
- Great For Emergency Lights: There’s a reason why every exit sign, stop light, etc. uses red or green lights. Red lights don’t affect your night vision so you can seamlessly transition from light to dark environments and green is used to improve clarity/contrast while maintaining night vision. Emergency vehicles add blue and white lights to purposely transition your eyes into daylight mode so you have improved clarity to avoid hitting first responders.
- Red Lights Reduce Focus and Clarity: Red lights are great for preserving night vision, but they make it harder for your eyes to focus in dim light. You have a much harder time focusing on small close up objects (maps, compasses, etc.) and seeing far away outlines with red lights. For tasks that require extreme clarity it’s better to use a green light. You get most of the night vision preservation with improved clarity, contrast, and focus.
That makes red lights great for tasks that don’t require a high level of detail, but there are better options when you need to focus close/far away. It’s better to go with a green light when you need to preserve night vision with improved clarity/contrast, and blue lights when you need to find fluids with invisible fluorescent properties (blood, body fluids, oils etc.).
You might want to check out my longer guide that explains the different flashlight colors and their many uses. It goes over how/when to use red, green, and blue lights to perform a variety of tasks.
How Does Red Light Preserve Night Vision? The Visible Light Spectrum Chart Holds The Answer
It’s hard to explain how red, green, and blue lights affect night vision without looking at the visible light spectrum chart. I’m sorry for all the science talk, but I’ll try to simplify my explanation.
There are two parts of your eyes that perceive brightness. The cones in your eyes are used to deal with bright light and the rods increase brightness in dim light.
- Cones In Your Eye: Cones are like a high definition camera that need lots of light to function. Their main job is to absorb bright light and deal with color. They’re primarily used in bright environments, but they play a secondary role dealing with color in dim light.
- Rods In Your Eye: The rods in your eyes increase the brightness in dim environments, but they get saturated in bright light. That’s why it takes 20-30 minutes for your eyes to adjust after transitioning from a bright to dark room. It takes time for the proteins in the rods to build back up to absorb light in dim environments. They only see in black and white so rods work alongside the cones in dim light to increase brightness while distinguishing color.
So why do the rods and cones matter when it comes to red light preserving night vision? It all has to do with how our eyes perceive light and color. Looking at the visible light spectrum chart above you can see that red light has the longest wavelength. There are wider gaps in the wavelength so it requires a high brightness level for our eyes to perceive red light.
Red light is too bright for the rods in our eyes to recognize (making it invisible to the rods). The cones are used to dealing with bright lights so they’re the only part of the eye that perceives red light. Since the cones are the only part of the eye interpreting the light, it leaves the rods that deal with night vision unaffected.
That’s great for preserving night vision, but the cones have a hard time dealing with dim light. There’s not enough light for the cones in your eyes to work effectively which makes it harder for your eyes to focus. This leaves you with a washed out appearance which makes it hard for your eyes to distinguish close up and far away outlines. That’s where green light comes in!
Green Light Appears Brighter and Clearer Than Red
As you move across the visible light spectrum chart the wavelengths decrease making the colors easier to detect in dim light. Green light uses an equal combination of rods/cones which is why it’s the 2nd best color for preserving night vision. The entire part of your eye can work so that’s why green light is by far the best color for clarity/sharpness and contrast.
Red light is by far the best color for preserving night vision, but green is another excellent choice. Green light falls in the center of the visual light spectrum. You get about 75% of the benefit of preserving night vision with a much clearer/brighter high contrast view. So you’ll lose a small amount of night vision after shutting off the light, but the improved clarity while using a green light more than makes up for that.
Green lights will appear 4 times brighter than red at the same lumen value or mW rating. That means you can take a much lower lumen green bulb and get a similar perceived light value to red light. So using a lower lumen flashlight will significantly increase the battery runtime on your flashlight. You can get 30-40 percent longer runtime out of a green bulb.
Being in the center of the visual light spectrum also makes green the easiest color to see at night. The workload is split evenly between the rods and cones in your eyes. The cones are used to perceive color in dim light and the rods deal with brightness in dim light. So you end up with clearer/sharper vision with much higher contrast.
That’s why military grade night vision goggles always use a green light. You get a much clearer sight picture with extra contrast while preserving most of the night vision once the light gets shut off. That allows you to see targets at a distance and allows your eyes to focus on small closeup objects. Once you shut off the night vision goggles/light your eyes will bounce back to the darkness in a minute or two (versus 20-30 minutes with white light).
What About Blue and Purple Light?
Blue and purple light, aka black lights, have the exact opposite affect on the eyes (to red lights). It has a short wavelength so it’s easy to detect blue/purple light in dark environments. So the rods are the only part of your eye that functions.
So using a blue light is comparable to a similar lumen white light. That’s why most manufacturers limit the lumen ratings on their blue lights to minimize the effect on your night vision. There is one advantage to using a blue light!
Since blue light increases the amount of work done by the rods in your eyes it amplifies the appearance of your surroundings. It makes fluorescent fluids that are invisible under normal light glow under a blue or black light.
That’s why people use blue lights to follow blood trails, spot oils/chemical leaks, and search for bodily fluids at crime scenes. The picture above shows a glowing drop of blood and a pinhole leak on a hydraulic line under a blue light. Blue lights make it easy to find any type of fluid/solid with fluorescent properties that would be nearly invisible under normal circumstances.