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"Blue Dot" tail lights seem to be illegal in most US states nowadays (and probably in most other countries, too), but they still seem very popular in the Hot Rod scene. And you can still buy them as 3rd-party parts, even equipped with LEDs.

But I wonder how that fashion started. This article about Blue Dot Tail Lights on MG MGAs says there are many theories about their origin, and that they were never available ex factory.

In contrary to that two posters in this forum thread claim that they were available as option on the Ford Model A and one poster even cited several models from multiple brands that were said to be equipped ex factory with blue dot tail lights while most other posts confirm that they're illegal in most US states since the '60s and that you might get a ticket if they're on your (non-vintage) car nowadays. (What doesn't seem to make them less popular…)

There are also some theories that the blue dot makes red lights better visible in fog or red lights better visible at all. The latter doesn't sound that far-fetched, given that most emergency vehicles have either blue flashlights or blue and red flashlights (depending on the country). But then again those blue dots were tiny compared to the rest of the tail light. So was this really the initial idea behind them? (If so, I assume they became forbidden later primarily to not confuse them with the blue flashlights for emergency vehicles.)

Another theory is that those gem-like blue dots were invented just "because it looks cool". My gut feeling says that "being cool" seldomly sparks an invention. So I doubt that for now.

Most of those blue dots look like a cut gem, with quite some plain surfaces. But if that refraction was the relevant part of the idea behind them, why do you see them nearly only in blue and why didn't they survive in red color instead?

Oh, and JFTR: While Wikipedia indeed has an entry about blue dot tail lights it redirects into a list of common terms for custom cars because "blue dots" seem to be one. No explanation where they come from either. sigh

P.S.: This is more an automotive history question than a question about maintenance and repair, but I found no better StackExchange site for this question and there is at least a "history" tag here (with so far only four questions). I also didn't find any site proposal on Area51 for a site covering automotive history. So I nevertheless post it here. Suggestions for better fitting sites for this question are of course welcome. :-)

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    I'd give you 10:1 the initial idea of the blue dot was just for aesthetics. You've used a lot of GoogleFu in your searches, so I know you'd have found it by now if was out on the internet. Lots of conjecture from what I've seen. Lots of differing opinions. If my Father-in-law was still alive, I'd ask him as I'm sure he'd know. Dec 14 '17 at 0:27
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    From a distance they give the lights a purplish hue. I think that distinction was enough of an incentive to the hot rod community of yesteryear.
    – SteveRacer
    Dec 14 '17 at 5:10
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I had cars then and never remember anyone claiming blue dots were anything but "cool". I never had the money to spare for them , but I did worry some that some one might "borrow' the tail lights of my '49 Lincoln ( Cosmopolitan convertible) because they were the ideal shape for blue dots.

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Having personally applied these to a few vehicles when I was younger, Steve's comment is accurate. In taillights with internal parabolic reflectors, the blue dots gave a very vivid magenta color to the taillight when viewed from directly behind. It's actually pretty neat.

On vehicles with no reflector inside the taillight housing, the effect was much less pronounced and not worth doing.

I suspect they were blue because the resulting purplish magenta looked attractive, and that other color combinations were less impressive.

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  • Interesting, I wasn't aware that there is such a big difference with and without reflector, but also didn't think about the fact that earlier taillights usually just had a plain metal base plate which kinda was used as very simple reflector — at least here in Europe. (Being from Europe, I have not much knowledge of US oldtimers or the US custom car and hot-rod scene. But I assume that such things as reflectors had the same evolution on both sides of the atlantic. :-) May 27 '20 at 14:39
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Just a thought. The most common form of color blindness is Red / Green. So, to a color blind person, red and green appear nearly the same. Adding the blue light would still be seen as a color by a person with red / green colorblindness.

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  • People with such colour blindness still see red and green as colours. There is no need to add blue to enable them to interpret the lights as coloured.
    – Chenmunka
    May 19 at 14:32
  • @RogerSTopping: Thanks for that interesting thought. Nevertheless I think it goes into the wrong direction as there's usually no green light at the rear of a car which would need to be distinguished from the red of the tail lights. Additionally, it's usually the green which gets some blue added, not the red. E.g. traffic lights in Europe have a slight amount of blue in the green light for that reason and AFAIK Japan even uses blue instead of green for their "green" lights. May 22 at 23:51
  • @Chenmunka: Having a friend who has deuteranopia I think in that aspect Roger's answer is right: While they still see red and green as colour, they usually see it as the same or a single colour, e.g. they see yellow, they see blue and they see "redgreen". That's also the reason why traffic lights in Europe (well, at least in some European countries) have a slight amount of blue in the green light and AFAIK Japan even uses blue instead of green for their "green" lights. May 22 at 23:56
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Blue dots I've read were meant for doctors so the cops knew not to pull them over for speeding and once the hotrodders found out they started putting them on their rides so cops wouldn't give chase at night... sounds legit

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  • Now that's an interesting new theory I haven't heard so far! I must admit, it sounds indeed kinda legit, but in the end I can't really imagine that it's true: This would have needed an immense amount of education on the police side while still keeping it a secret under a huge amount of doctors and car mechanics (who were surely need to install these drops). So I doubt that anyone would have started to go down that path. (Then again, that were different times back then and I might think too much based on our times.) Additionally others might have noticed these dots and started to ask questions. May 30 '20 at 15:00
  • Then again, the longer I think about, the more this sounds like a first real answer. And it gave me new keywords to search for. Found noslocars.blogspot.com/2012/01/installing-blue-dots.html that way. Actually one of my initially found links contained this theory, too, but I seem to have overseen it: jalopyjournal.com/forum/threads/… — both sides though use exactly the same phrase (and your's is similar, too :-), so I guess this only "counts" as one source. :-) May 30 '20 at 15:07
  • One more fact pointing towards this direction: There were also head lamps with blue dots, e.g. this one: baronvonkronken.com/period_blueSpot.jpg (Found it on baronvonkronken.com/busguide=accessories.htm, unfortunately without additional information.) Jun 15 '20 at 21:28
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Human perception: A red light seems farther away than it is. A blue light seems closer than it really is. It seems obvious to combine the two to give a better idea where the light actually is. Of course, that doesn't mean Emergency vehicles use red and blue for that reason, nor that blue dots were invented to get those idiots from yesterday-year to start stomping on those poor substitutes they had for brakes a bit sooner, but it sure makes for a sensible theory. It's also one reason I wish I could put one in my Harley's taillight. Plus, that would be bitch'en!

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  • Interesting theory. I though would expect from these backgrounds that the combining of the two colors would rather confuse the eye's distance calculation because it has to deal with two different values at the same spot. On the other hand, this confusion actually could cause more attention and then we are at that point again. :-) Oct 24 '20 at 6:35
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As an old street rodder, I can say the bluedots are just a cool add on, like fuzzy dice or your best girls scarfs hanging on your rearview mirror, the smaller the taillight's the better 3"or 4" round are perfect for effect true glass bluedots are a must not those mickey mouse plastic things from Amazon or eBay, ps:i still use bluedots on a 08 Chevy HHR and never have been pulled over for them.

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  • Thanks for caring and for sharing your experience with blue dots. But it doesn't really answer the question what the initial idea behind this modification was. (I guess that's also why someone already downvoted your answer.) Mar 22 at 23:52

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