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The “Why” behind the lights

What is Night Vision?

Night vision is the ability to see in low light conditions. The back of the eye, called the ‘retina’ detects light and allows the eye to ‘see’. Rods and cones in the retina of the eye work together to produce night vision. Cones require more light than rods to function, so they don’t play a role in night vision.

How does the Eye work?

The retina is made of 2 types of structures, cones and rods. The cones are responsible for normal daytime vision. Cones detect both the wavelength (colour) and intensity (brightness) of light that goes into the eyes and passes that information to the brain.

The rods in the eyes increase the brightness in dim environments, but they get saturated in bright light. That’s why it takes 20-30 minutes for the 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. When the eye adjusts to low light levels, it can then see more clearly in the dark. This process can be quickly undone by exposure to bright light, which causes the eyes to readjust to the brighter conditions.

So, why do the rods and cones matter when it comes to red light and preserving night vision? It all has to do with how the eyes perceive light and colour. Looking at the visible light spectrum chart it is evident that red light has the longest wavelength. Red light works differently from other light colours because of the wavelength. It uses less energy than other light colours, which is why it’s less disruptive to the eyes. Since the red light is on the longer side of the visible light spectrum the human eye is less sensitive to it than to other colours.

Why use a red light?

One of the most significant benefits of using red light is that it helps preserve night vision. When exposing the eyes to bright white light, it takes time for them to adjust to the darkness again. Using a red light prevents the eyes from adjusting to the increased light, helping to keep night vision in tack and preserving it in low light conditions.

Red light is less visible from a distance than white light. The red light helps maintain a low profile, perfect for maintaining operational effectiveness in low light conditions. This low profile will make it ideal for those who wish to be less noticeable.

Red light can penetrate fog and smoke better than other colours of light, making it useful for search and rescue operations as well. Also, it doesn’t attract insects as much as other colours of light, which can be helpful in certain situations.

Can any other colour be used?

Red light is by far the best colour for preserving night vision, but green is another excellent option. Green is the easiest colour to see in the dark because it is part of the visible light spectrum. On the visible light spectrum chart the wavelengths decrease making the colours easier to detect in dim light. Green light uses an equal combination of rods/cones which is why it’s the 2nd best colour for preserving night vision.

The green light allows for 75% of night vision preservation, however it provides a much clearer and 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 makes up for it.

Red lights are great for preserving night vision, but it is does have its draw backs. Using red lights on small close-up objects (maps, compasses, etc.) might make it difficult to see intricate details. For tasks that require clarity, it’s better to use a green light. You have reduced night vision preservation, but improve in contrast, focus and detail.

Can I use both?

These two options are often used together which is why every exit sign and stop light use red or green lights. Red lights don’t affect night vision making it a seamless transition from light to dark environments and green is used to improve clarity/contrast while maintaining night vision.

Red or green lights are both excellent options for use at night, whether it’s to preserve night vision or to see with greater focus and clarity, the benefits have been proven to help work effectively in low light conditions.