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Why Do Police Cars Use Red & Blue Lights? They're Visually Confusing

Why Do Police Cars Use Red & Blue Lights? They're Visually Confusing

*Much of the text for this blog was originally found here.

When you have a policeman behind you, flashing his or her lights, it can be hard to focus, and is certainly jarring to your senses. Why did the red/blue light combination come about, why does it affect us so much, and what is the science behind it? The featured image in this blog post is difficult to stare at because of a phenomenon known as chromostereopsis, which the American National Standard Institute defines as “the perception of depth resulting from the close proximity of two colors of disparate wavelengths”.

"Because of where in our eyes the receptors for different colors are, and how our eyes focus, we perceive different colors as being at different distances. Printers and others who do graphic layout have long known that because they are at opposite ends of the spectrum, it’s not a good idea to use blue letters on red backgrounds and vice versa. Most people perceive blue as closer than red, and as a result the human eye cannot focus on both red and blue at the same time, causing the optical illusion of blurry letters in the graphic below.

In addition to chromostereopsis, as LEDs have proliferated, people have come to realize that its harder to focus on pure blue lights than on any other color. Our retinal receptors are known as rods and cones. Visual acuity comes from rods and is mostly a black and white phenomenon. Color is added by cone receptors. Rods are sensitive mostly to light in the yellow-green part of the spectrum. Pure blue light doesn’t activate rods sufficiently for clear vision.

Flashing blue lights make it hard to focus but flashing red and blue lights together is an even worse idea. To begin with it makes it hard to estimate the distance of an emergency with flashing red and blue lights. More dangerously, when your visual system is being flooded simultaneously with bright red and blue lights, the effect is almost blinding, certainly visually confusing. It’s a problem for motorists but it seems to me it would create an even more dangerous situation for police officers who have to make out shapes and distances in visually confusing lighting situations.

So why do police cars use blue lights in the first place and even worse, red and blue lights together? I suspect the reason is partly historical. In some states police used red lights and in others blue lights. It made sense for manufacturers to offer units with both colors. However, I think the main reason is exactly chromostereopsis. I think the companies selling emergency lights have to be aware of the phenomenon, and they wanted to come up with lights that would surely get your attention. Does the sign above get the attention of your visual system?"

So you can see that a bit of history, and a bit of anatomy are the reasons why red/blue lights are the prevalent notification system for a police car in this century. At the end of the day, let's just hope you never have to see them behind you, right?

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