How to Evaluate Safety of LED Bulb Retrofits in Brake, Signal, Marker Lights

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Mar 26, 2004
LED bulbs, no matter what they look like, put out a different distribution of light than filament bulbs. That means unlike a filament bulb, physical fit of an LED bulb in a lamp does not necessarily mean it will work safely. Safety here means a lot more than just lighting up in the right color, so it's important to evaluate the performance in the specific lamps on your car. A test lab is really the only way to get an accurate go/no-go test, but for most people that's not practical.

To do a reasonable visual evaluation, install one of the LED bulbs and leave the standard bulb on the other side of the vehicle. Turn on the lamps with the vehicle out in bright daylight. Walk about 25 feet away from the vehicle and move in an arc (180°) from one side of the vehicle to the other, keeping both lamps in view to evaluate their relative visibility. Watch for how bright the lamps appear, the apparent size of the lit area, any "dropout" angles where the lamp appears to go dark, any "flash" angles where the lamp appears extra bright, shadows, etc. Then double your distance to the vehicle and walk back and forth again to compare the two sides.

Once it's dark out, move the vehicle about 4 feet away from a wall or garage door, at 90° (not crooked), turn on the lamps, and look at the apparent brightness and size of the patch of light shining on the wall from the two lamps (still with one standard bulb and one of the LED bulbs you're testing). A handheld light meter goes a long way here, and free light meter/luxmeter apps are widely available for Android and iPhone.

If the light patch on the test bulb's side is dimmer, if it's smaller, if there are dark streaks or shadows, or if there are viewing angles where the LED bulb is less visible than the filament bulb, that LED bulb fails in that application. Of course, if you do see any shadows or bright spots or other artifacts, check to make sure the standard bulb doesn't have a comparable artifact on the other side. Also be sure to measure and visually compare (day and night) the apparent bright/dim ratio. Often an LED retrofit will have insufficient difference between the bright (brake or turn) and dim (tail or park) mode, which is very dangerous; it basically renders the lights useless for conveying the messages they're supposed to convey.

None of this is sufficient to say for sure that the test bulb is good enough, but it's enough to reject the test bulb if it's bad enough.

Important notes
Lamps with red lenses need red LED bulbs. Do not use a white LED behind a red lens, or you will get an unsafe dim pink-brown light. This advice may change if reputable white LED bulbs with color temperature at or below 3000K become available -- this post will be updated if that ever happens, but as of this writing all reputable white LED bulbs are well above that figure and not safe or effective for use with red lenses. This means there are some applications where there is a reputable LED bulb that fits and is optically compatible with the lamp (spreads the light appropriately) but the color incompatibility makes it a dead-end. For example: there is no reputable red 921 (W16W), so center brake lights that take this kind of bulb are out of luck. A yellow/amber LED with a red lens is an iffy proposition best avoided.

Lamps with yellow/amber lenses can use yellow/amber or white LEDs. A white LED with a yellow/amber lens will produce light color more yellow and less amber. Usually this color will fall within the legal range for yellow/amber turn signals, and in rear turn signals the greater color separation from the adjacent red brake-tail lights may improve perceptibility of the turn signal.

If you decide to go ahead, put in whatever flasher (or set whatever body computer configuration) is used for trailer towing, or install a resistor in each turn signal feed wire, using reliable connectors, to achieve/restore the correct turn signal flash rate.
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