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Why should emergency exit signs be green, not red?

Emergency exit signs should be green and not red

Why should emergency exit signs be green, not red?

In most countries, viz. Europe, United Kingdom, India, China, Japan, Australia, New Zealand and South Korea, emergency exit signs are green according to Directive 92/58 / EEC, BS EN 1838 and BS 5266 while in the United States, red and green are allowed both but mostly are in red.
It is still questionable that if the red colour is more visible from a distance, then why the emergency exit signs should not be red. There may be two main reasons behind the most acceptable green colour for the emergency sings.
1. Scattering and diffraction effect of light in smoke: green causes less distortion and is more visible.
2. Human psychology: Green represent ‘safe’ while Red is ‘danger’

1. Scattering and diffraction of light in smoke

What is scattering of light?
When light interacts with the particles, it is partially absorbed by the particles and then emitted at a slightly different energy / wavelength is called light scattering. When light travel from one medium to another Eg. air, part of the light is absorbed by the particles of the medium preceded by its subsequent radiation in a particular direction. This phenomenon is called light scattering. The intensity of the scattered light differs on the size of the particles and the wavelength of the light.
Scattering occurs when a light is absorbed by particles and then emitted, while reflection occurs when light is simply reflected on the surface without interacting.

Scattering of light
Scattering of light

Blue / purple light has the shortest wavelength, therefore, most scattered, while red has the longest and least scattered wavelength, so it can be seen from a distance.
The green is in the middle and, therefore, is not very scattered, but it can also be seen from a distance.
What is diffraction of light?
Diffraction is the slight bending of light as it travels across the edge of an object. The extent of bending depends on the comparative size of the light wavelength in relation to the size of the opening. If the aperture is much greater than the wavelength of the light, the bending will be almost imperceptible. However, if the two are closer the amount of bending is considerable and easily visible to the naked eye.
The longer the wavelength, the greater the diffraction effect. Red light has a longer wavelength, while blue / purple light has the shortest wavelength and green is intermediate.

Diffraction of light
Diffraction of light
Therefore, the red light has more diffraction, while the green one has less diffraction.
In the image above, the light diffraction pattern shows a maximum point of light in the centre surrounded by secondary maxima of much lower intensity. Green light has an average wavelength, or we can say less compared to red light, therefore, a narrow diffraction, while red light has a longer wavelength, the wider the diffraction pattern. The slot light at the bottom of the light box scattered in the vicinity and increased background noise, which reduced visibility.
Red light diffract more
Red light diffract more

Because different colours of light travel at different wavelengths, some are easier to see than others, however, green is the most visible colour from a distance because the wavelength of green light is medium and it is insignificantly dispersed from the direct path to the observer. Therefore, the remaining non-scattered light is mainly of longer wavelengths and looks redder but looks like a scattered spot.
Another reason is that fluorescent green can be seen better than other distant colours in a dense smoke-filled environment due to a lower diffraction and dispersion effect. Simplifying the scattering phenomena, it clearly illustrates that the green colour cannot be easily suppressed for flashing lights in smoke-filled environments.

Diffraction of red light in dense smoke
Diffraction of red light in dense smoke

The red colour has the longest wavelength, which means that it is less scattered by small particles than many other visible colours with shorter wavelengths, therefore, visible from distance, but due to its greater diffraction effect, It looks like a red spot in a dense area full of smoke. This red spot can confuse people in a panic / crisis scenario, and they may think there is a fire and prevent them from approaching.
Perhaps most importantly, it is likely that the materials used for the faces of the green signs transmit more visible radiant energy to all occupants than the materials used for the red signs. After all, it is probably better not to have red exit signs.

2. Human science and psychology: green is ‘safe’ while red is ‘danger’

Human Sciences – Colour Sensitivity of human eyes
Human eyes are more sensitive to light with a wavelength of approximately 555 nm, which corresponds to a green colour. Human rod cells are more sensitive to the wavelength of green than red.
Our eyes have three types of photoreceptor cells called cones, which contain photographic pigments, which are designed to detect wavelengths. Together, the cones work to communicate to the brain the colours we see. During the day, our eyes can more easily capture green light, followed by yellow and blue. Along with the cones, photoreceptor cells called rods help the eye see during periods of low light. When it is dark, yellowish green becomes the most visible colour from a distance.
We can detect visible light due to cone-shaped cells in our eyes that are sensitive to the wavelengths of certain forms of light. Other forms of light are invisible to humans because their wavelengths are too small or too large to be detected by our eyes. Humans can only detect wavelengths between 380 and 700 nanometers.

Protans - colour blindness

Also, if we see, there are three main classes of colour deficiency in humans: protanopia, deuteranopia and tritanopia. Protan defects (most of the human) are characterized by the loss or alteration of the long wavelength photopigment (red); Deutans defects (less peoples) are characterized by the loss or alteration of the medium wavelength photopigment (green); and tritan defects (very rare) are characterized by the loss or deterioration of the short wavelength photopigment (blue).
In addition, an abnormal vision of colours has proven to be an important factor. Most people suffer from protan colour blindness, which means they are less sensitive to red. The Protans had more problems with the red signs as expected for their loss of sensitivity at long wavelengths. Although smoke may disperse red wavelengths less, these long wavelengths are less detectable by protons and, therefore, it would be preferable for the output signals to be green.
Green signs are more detectable/ visible in dense smoke and, therefore, more reliable.
Human psychology about colours
Red Exit sign confusing

The red colour is associated with danger, alerts people about the crisis. From childhood, people are trained to recognize red as "Danger or" Stop, so in an emergency this can cause confusion. While the colour green is related to safety, it means "safe" and "go," making it clear to the public that they should remain calm and follow the signs. This means that in case of a fire emergency, people will run to the green sign and avoid running to the red one. Therefore, a red light could discourage people from using a red exit sign, while green could be encouraging.


The red colour sign has a greater diffraction effect and, therefore, looks like a red spot in densely filled smoke spaces, it is less sensitive to normal human eyes as well as to the protans, although it has a longer wavelength.
Against red, the green colour signs has average wavelength, less diffraction effect, less dispersion and, therefore, more visible from a distance, even in densely filled smoke spaces, is more sensitive to normal human eyes and protans too.
Psychologically also, green represents a safe "GO", while red represents a danger "STOP" and, therefore, in an emergency, people prefer to exit from green.
Therefore, emergency exit signs should be green, not red.

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