Tail-light red

Hey Matt:

Why are nearly all automobile tail lights the exact same shade of red? Does it have a name?

-- Matt, on 805 southbound

Commuter-coma Red. Panic-stop Red. Screeeech! Red. Look Out! Red. Hey, You Jerk, You'd Better Have Insurance! Red. There are lots of good possibilities. But since we're dealing with engineers, not poets or lipstick manufacturers, the color is called SAE J578c. Well, actually it's called Red, but the specific red is defined in the Society of Automotive Engineers standard number J578c. It all begins with the feds, as if I had to tell you that.

All brake lights and tail lights are some shade of red because the National Highway Traffic Safety Commission sez they have to be. But rather than leave the definition of red up to our imaginations-- no, you can't put cool, flamingo-pink brake lights on your classic Thunderbird-- they give us Standard 108, Part 571 of the Federal Motor Vehicle Safety Standards and Regulations. And Part 571 sez the red brake lights must meet the requirements of SAE J578c. And SAE J578c sez the red in question is that particular shade defined as "red" on the CIE 1931 2-degree Observer Chromaticity Coordinates chart. The chart maps out the visible light spectrum by wavelength and intensity, like a deconstructed rainbow.

Basically, the feds want the brake lights to be a color we all can agree is "red" and to be bright enough to be visible for a certain distance in order to "enhance the conspicuity of motor vehicles on public roads." Well, if you're anything like me (though of course you're not) right now you'll be leaping out of your chair and yelling, "Conspicuity? There's no such word as conspicuity!" Then you'll go to the dictionary and find out you're wrong.

So anyway, where were we. Oh yeah. The chromaticity chart. It shows what that government-approved light will look like. The strength of the light source in the tail-light housing and the red acrylic resin lens must combine to meet J587c standards as measured from outside the housing. Ergo, we all end up looking pretty much the same from behind.

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