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Hopefully this is a suitable stack for this question. I cannot think of any other place to ask.

This is about OEM tail lights with LEDs under translucent red plastic covers. I have noticed on several cars where the plastic cover is broken that the LEDs underneath are actually white LEDs. This baffles me. Obviously, red LEDs are used under clear plastic covers so there is no problem using red LEDs. I also know that white LEDs are generally more expensive than red LEDs (white LEDs are actually purple-to-UV LEDs activating a phosphor that glows white or off-white).

I just cannot understand why an automobile manufacturer would spend extra for the white LEDs when they could use red LEDs instead to save the part cost. It seems like even if they have to change the dye in the plastic to better match the color of the LEDs, it would still be cheaper in the long run.

Anyone have any insight into the manufacturing that could explain this?

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    Do the LEDs light up white, or are the LEDs merely clear? There are red LEDs with clear lenses.
    – JRE
    Commented Jan 25, 2022 at 10:16
  • @JRE The cars I’ve seen have red lights but where the plastic cover is broken, I see white light. This is as if it were a white incandescent light under a red lens, except the shape of the light, as will as how the light seems to turn off instantly instead of visually fading out show that the lights are LED.
    – DoxyLover
    Commented Jan 25, 2022 at 23:57

6 Answers 6

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My thoughts -

  • A white LED can be used in multiple lamps in the same car, brakes, side, indicators, reverse lamps, so limiting the need for different stock.
  • The assembly line will be simpler, so more cost effective.
  • Purchasing a larger quantity of white LEDs will likely be cheaper than a smaller quantity of several different colours.
  • Possibly a more consistent brightness of lamp using the same LEDs in different coloured lamps
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Quality tail lights use red LEDs for the red parts. White LEDs behind a color filter are very inefficient and typically used by cheap OEM parts (and fake OEM parts for that matter) which look good on paper because of their crazy lumen ratings, until you realize most of those lumens never make it past the color filter:

enter image description here

Having said that, a lot of what's been said about production cost advantages of using a single type of LEDs in a light block is true, so you do find while LEDs behind a red filter in some OEM or even OE parts.

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One part of the answer might be spectrum or gamut. There are govt specifications for the color of tail lights, SAE standards etc. It may be that it's easier to get the exact color needed in plastic than in LED emission lines, especially over various lots.

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    With all due respect, I don't think this is true. Could you name one actual standard which would prevent red LEDs to be used in a tail light?
    – Dmitry Grigoryev
    Commented Jan 24, 2022 at 12:30
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    – Community Bot
    Commented Jan 24, 2022 at 14:21
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    @DmitryGrigoryev Actually, Jim Mack is correct about govt and SAE standards. 49 CFR Part 393 is the federal law that governs, and the U.S. DOT defers to the SAE to set and publish the standards. See SAE standard J578C "Chromacity requirements for ground vehicle lamps and lighting equipment." All the SAE standards I could find are behind a paywall, but see this sample link for the exquisitely specific standard for blue (police) lights: sae.org/standards/content/j578_202004/preview
    – MTA
    Commented Jan 24, 2022 at 20:44
  • @MTA Yes there are standards for light colour, and it's possible that some red LEDs don't meet some of them, e.g. if they require non-monochromatic light, or a different wavelength than normal red LEDs produce. So the 2nd sentence of this answer is true, that there are standards. But the rest is pure guesswork, unless anyone can quote a specific standard and find an incompatibility; their existence doesn't necessarily prevent red LEDs. Other users have said that taillights with red LEDs do exist (at least with clear plastic), but that doesn't prove they're compliant, apparently.
    – Peter Cordes
    Commented Jan 26, 2022 at 1:32
  • "there are standards for light colour, and it's possible that some red LEDs don't meet some of them" - this is undeniably true, for instance, you could pick orange LEDs for tail lights, or green-blue LEDs for police lights (check out the linked SAE paper), and these would be non-compliant. This doesn't mean that using colored LEDs of the right color is forbidden though.
    – Dmitry Grigoryev
    Commented Jan 26, 2022 at 8:22
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Definitely red LEDs under a red (or white, for that matter) cover. This is how a (good) factory tail lights are made.

Modern illuminating LEDs are more efficient when they are narrow-band color. E.g. red or blue ones are better than white ones.

On the other hand, by covering a white LED with red cover, one loses about 2/3 of the light (this is how the red pigment works - by absorbing the photons that are not "red").

In total, by using the same efficiency class LEDs, one loses 60-85% of the brightness with the combination of white LEDs and red cover. Loses in yellow are somewhat less, but still pretty much significant.

There are quite a few reasons why one should not use LED lamps in a setup made for incandescent ones. This is why in most parts of the world this is simply not road-legal.

The reason why you see white LEDs is because the aftermarket trash manufacturers are not really eager to follow neither the code nor some brightness or efficiency standards. They make whatever they can sell.

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    This is not what I’ve seen. These are recent model BMWs, Hondas, etc. with integrated and obviously factory original LED tail lights with either a defuse light bar or visible dots through red plastic. Where the plastic is broken, I see white light. The configuration makes it obvious that they are LEDs, not incandescent. As I noted, I was very surprised to see that.
    – DoxyLover
    Commented Jan 23, 2022 at 18:42
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    "There are quite a few reasons why one should not use LED lamps in a setup made for incandescent ones." - Could you name some? For headlights, I can imagine the different light pattern wouldn't be properly formed by the reflector, potentially blinding oncoming traffic. But for taillights and brake lights?
    – marcelm
    Commented Jan 23, 2022 at 22:15
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    @marcelm rear lights have light pattern, too. It is less important, but pretty much enough to stain an insurance claim. Brightness is almost never what is supposed to be and can vary a lot with temperature. LEDs also tend to overheat in such places. LEDs in brake light also sometimes glow when brake is not applied because of leak currents (sometimes enough to be mistaken for actual brake light). The glow is sometimes triggered by weather or vertical acceleration.
    – fraxinus
    Commented Jan 24, 2022 at 7:54
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    @StianYttervik it depends on the design goals and the quality of the color filter. Filters with sharp spectral cut-off are expensive, if you allow some more orange you will get more brightness but also more yellow and green. And on the other hand, if you allow too much orange-yellow-green then someone will mistake it for the blinker yellow. In this regard, narrow-band LEDs have one more advantage - a great deal of people with mild red-green color blindness will recognize them properly.
    – fraxinus
    Commented Jan 24, 2022 at 13:57
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    "There are quite a few reasons why one should not use LED lamps in a setup made for incandescent ones." As an additional resource, here's a Technology Connections video about how incandescent bulb lenses are designed specifically to handle filaments.
    – jrh
    Commented Jan 25, 2022 at 12:48
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I'd sure like to see the numbers here. My guess is the simple answer I often use here to design related questions.

Which design is the least expensive to produce?

My guess is it's the systems cost which is driving things here.

Speaking of LEDs... its not clear if what we see in generic terms holds true for current manufacturing concepts. For example, a SMD (Surface Mount Device) LED deployed in a large matrix flat pack. If you are a manufacturing facility, how would you produce that? There is large demand for white LED's in that format. What about red LEDs? Would you dare introduce that into your manufacturing process, knowing full well that at a glance these LED's look alike? Ugh.

The other piece of the conversation I'm not entirely sure of is the driver circuitry differences between White LED's and Red LEDs. I believe those different LEDs work with different current / voltage drops. That could well lead to differences in the driver chips in use. And that leads to complexity (if introduced within the White LED manufacturing plant...) No problem, just build and tool up a brand new Red LED plant; mixing problem solved (oops. That's expensive)

You never want to introduce a potential quality problem with mixed stock How do you fix that? Well, you would introduce a whole bunch of computer checks and inspections so that never happens. And guess what happens to the prices you would have to charge? That would be ugly.

What we don't know is the prices (cost) here. It would not surprise me that the purchasing groups have leveraged huge piece cost reductions for long range committments for quanity and time. (If we would guarentee a 50 million piece sustained order, would the supplier amortize tooling and reduce piece price, etc...) If you go outside of those bounds (by say introducing red LED's into the mix) that would result with a huge price increase from the supplier. Been there, done that and it is always an ugly conversation.

And yeah, I apologize for going a bit off script here. The actual question above is why not red led's under red lenses. My answer is still the same root response.

For many elements of automotive design, the answer to "Why did they do it that way?" is because of price. When you make millions of vehicles per year, when you save even a few pennies on component piece pricing, those dollars add up.

Follow the money, follow the money, follow the money. (Don't remotely jeapordize quality, but definitely follow the money...)

And its not just the LED / drivers / heatsink costs. A additional piece of the conversation is plastic raw material and tooling costs.

Red tail lamps use red acyrlic. Clear tail lamps use clear polycarbonate (with a clear coat top spray finish). Acyrlic is a lot cheaper then polycarbonate.

Note too, there are a whole bunch of legal requirements for rear and side marker reflectivity. That stuff is easy to integrate in red molded lenses... but you have to add extra red bits to meet the requirements on a clear taillamp system. And that adds more cost.

With that said, headlamps / LED accents / tail lamps are often a "surprise and delight" visual feature. They add a lot to the visual appearance of a vehicle, and for that the manufacturer may be willing to spend more on piece costs.

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  • These points about plastic cost justify using red plastic, but don't address the question of whether the LED under the plastic emits white or red light. Red plastic makes it possible to use a white LED, but doesn't stop you from using red. The question isn't asking about tail-lights that use clear plastic.
    – Peter Cordes
    Commented Jan 25, 2022 at 0:10
  • To my knowledge, nobody is using LED's with red lenses. I believe you totally missed my point. The answer to may things in Automotive Design is "Follow the Money, Follow the Money, Follow the Money". I'd edit my response.
    – zipzit
    Commented Jan 25, 2022 at 3:05
  • Sure, but "follow the money" plus the cost info you provide would (from my naive PoV) suggest that vendors would use red LEDs under cheap red plastic as the cheap option. Unless they want to show off the inside of the fixture by making it clear. (Or is incandescent still enough cheaper to get used in cheap fixtures?)
    – Peter Cordes
    Commented Jan 25, 2022 at 3:11
  • no.. the way to reduce costs, without effecting quality is to REDUCE complexity. Less is better. Way better. Keep it simple, make it easy.
    – zipzit
    Commented Jan 25, 2022 at 3:30
  • Thanks for updating your answer, it makes sense to me now, since it actually presents a complete argument in terms of cost. I assume this is what you had in mind the whole time, but parts of it weren't clear (to me) from the text since you didn't actually mention the supply-chain / BOM issues other answers brought up with having another type of LED as part of your answer until now. (I thought you were making a different argument / point entirely since you were talking about plastic costs, but now I see you're arguing why white LED under red plastic is the cheapest overall to make.)
    – Peter Cordes
    Commented Jan 25, 2022 at 3:37
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I assume the main reason for this is that white LEDs are brighter and more efficient.

There was not much incentive to improve the efficiency and brightness of red LEDs. When car tail lights were incandescent, about the only use where red LEDs should be bright and efficient was bicycle tail lights. That's not a market where lot of money is available for improving efficiency and brightness. In most cases, red LEDs were for example power indicator lights where brightness and efficiency didn't matter.

White LEDs, on the other hand, are used for general purpose lighting, flashlights, and vehicle headlights. In these applications, brightness and efficiency matter a lot. Hence there was enough money to improve both parameters, and today we have bright and efficient white LEDs.

Of course the red cover reduces the efficiency and brightness because it only passes red light through, but it could very well be that the efficiency and brightness of white LEDs are still superior.

And besides, you said about "spending extra". I think you are comparing low power red LEDs with high power white LEDs. That's not a good comparison. Comparing high power red LEDs with high power white LEDs would be more fair. I think you would find that high power red LEDs are not as easily available and cost maybe even more than high power white LEDs.

It probably helps that stock for only one kind of LED needs to be managed, but I believe the main reason is that white LEDs today are easily available in bright and efficient configurations.

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    The cost doesn't add up. Looking at surface mount LEDs in my regular supplier, yes, white are cheaper in bulk, about £0.012 each compare to £0.060 ea for red. The white is 31 lumens while the red is 27 lumens. But filtering that white down to red you'll be throwing away more than 4/5 the output so the white won't be any cheaper. And there are loads of high-power LEDs at all sorts of wavelengths, used in things like architectural and theatrical lighting, and have been for years
    – Chris H
    Commented Jan 23, 2022 at 16:19

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