Why Most LED Headlight Upgrades Don't Really Work？

Plug-and-play LED replacements for halogen headlight bulbs are a popular car mod. LEDs often look brighter than incandescent lights, but “looks brighter” and “illuminates better” are not the same thing. I got a stern talking-to from an actual lighting expert about LED retrofits, and science says: Putting LEDs where halogen bulbs are supposed to be is generally not actually an upgrade.


Why would anyone want LED headlights? 

LEDs, when housed and aimed correctly, can translate minimal input power into a lot of light, which is what makes the technology appealing in general.
Everything else being equal, it seems like swapping power-hungry incandescent headlight bulbs for brighter lower-draw LEDs would be an upgrade on two fronts. Plus the “instant-on” effect and visual crispness of light that comes from LEDs is sharp and fresh. LEDs can give older cars modern styling.
In the simplest terms: LED headlights are easily installed and readily available things that can make cars look cooler. So, people get them.

So what’s wrong with that?

Most car headlights are a lot more than just a bulb in a socket. A cradle of reflectors is shaped and angled so that light emitted from the filament of an incandescent bulb will be thrown down the road in a way that maximizes driver visibility without blinding oncoming traffic.
Most LEDs don’t emit light from the same space in the headlight housing as incandescents and from that point, they’re doomed to mediocrity.

What about LEDs that do emit from the right spot?

A few of the better-known companies selling and testing LED headlight replacements in 2020 have taken pains to mimic the positioning of incandescent bulbs to address this problem but it’s only a small part of the lighting equation.
In fact, I did a blog about this, pointing out why some LEDs seemed better than others when used as headlight retrofits. And that blog is why lighting expert Daniel Stern felt compelled to reach out to me explain how grossly underinformed I was.

Why not just use an LED that is exactly the same size and shape as a regular bulb?

Longitudinal position of the light source (where the light source starts and ends, as measured from the base plane of the bulb) is only one critical aspect. But it’s not the only thing that matters. “Others include shape, size, orientation, and luminance distribution. Getting one out of five right is better than zero out of five, but it’s still 20 percent, a badly failing grade.”
“If we could wave a magic wand and come up with a cylindrical LED emitter of the same dimensions as a filament, with the necessary luminance and flux, then the incompatibility would vanish. That is not technically possible for the foreseeable future, so we have basically two-dimensional flat LEDs in place of a three-dimensional cylindrical filament.”
“There is significant space between the two back-to-back flat LEDs (there has to be, otherwise no material to carry away their heat), so now our light source is radically different from a filament in shape, size, position, and light distribution even if we’ve taken great care to put the emitters at exactly the same longitudinal position as the original filaments.”

And why does that matter again?

The problem is that light reflectors designed for halogen bulbs are inherently incompatible with the light output of LEDs.
Stern wrote: “...the near- and far-field light distribution is quite different to what the lamp’s optics were engineered for.” And as a result, the headlight’s beam pattern isn’t what it’s supposed to be, doesn’t line up with the way the vehicle’s engineered, and is all-around suboptimal.

Why do my lights have to keep the factory beam pattern?

“I wear eyeglasses, and so does my next-door neighbor. It would be injurious and counterproductive for us to swap because even if they fit my face and look awesome, the optics don’t match my eyes (even if I think I can see OK with them).”
“And it’s not because I picked the neighbor to the left instead of the neighbor to the right. The same holds true for both neighbors’ glasses even though the one pair has glass lenses and the others are plastic, the one set has round lenses and the others are rectangular, the one set is photochromic and the other isn’t, the one neighbor is more farsighted than I am and the other is more nearsighted, etc.”
“The details are different, but the basic problem is still optical incompatibility, and the scale of the relevant differences is much smaller than ‘these lenses look the same to me!’”
To say it again, slightly more sciencey: LEDs in housings designed for halogen bulb replacements put the wrong amount of light in the wrong places.

So do LED bulb replacements ever work?

Not all incandescent-type halogen bulbs are equal, and as we’ve touched on, there’s quite a variety of LED bulb replacements on the market now too.
The problem here is “a lack of predictability.”
More specifically: “...Occasionally it’s possible to happen upon a combination of one of these ‘LED bulbs’ and a particular headlamp that works acceptably.” (For example, DiodeDynamics has a particular ‘LED H11’ Stern recalled being successful in a particular Ford truck headlamp housing.)
“But there’s no predictability to it; it’s nothing at all like ‘oh, as long as you have projectors you’re fine,’ or ‘reflectors are fine if they have a bulb shield,’[a “bulb shield” is a piece that blocks unwanted light coming from certain angles of a bulb] or anything like that. And ‘Oh, no problem, on our bulb the LEDs can be rotated relative to the base so you can focus them’ is, if anything, worse, not better.”
The whole point of standardization of bulb types, Stern explains, is “so any headlamp designed and built to take [for example] an H11 will work safely with any bulb designed and built to the H11 specifications. That doesn’t mean all H11s are alike... but the standardization ensures at least adequate safety.”
“And that really is the way it has to be, because think of a bulb that fits in any H11 headlamp but works safely in only a few of them.” That’d be pretty crap.

So what really makes a good headlight?

“Headlight beam safety performance involves multiple interdependent variables. For example, an amount of foreground light that might be just fine with a strong, well-focused hot spot, will limit the driver’s seeing to a totally inadequate 50 or 60 feet if the beam’s hot spot is weak or nonexistent. So just saying ‘Yep, the cutoff looks good’ doesn’t even begin to be close to good enough.”
“What matters much more is the amount and distribution of light under the cutoff, and that gets pretty scrambled (randomized) with most ‘LED bulbs’ in most halogen headlamps. To give just one of many examples of how easy it is to get tripped up on this point: Sometimes you’ll get a reasonably sharp cutoff with an “LED bulb”, but the hot spot (assuming it still exists) is moved.”
“Every last little bit of downward and/or rightward movement of the hot spot decreases the seeing distance for the driver, but the beam on the wall looks like it has a nice cutoff and hot spot.”
“Another example: say we’re dealing with a headlamp that didn’t start out with a very sharp cutoff. Put in an ‘LED bulb’ and the hot spot moves upward/leftward. The typical advice: ‘Re-aim the lamps to keep the bright part out of other drivers’ eyes.’ But by doing that, we’ve shifted the entire beam pattern so a bunch of other stuff no longer has appropriate amounts of light on it.”
“What’s the scale of these effects on seeing distance with different effective lamp aim (whether it’s by dint of how the lamp is adjusted or how the lamp is distributing its light)? Well, if you’re using the shine-on-a-wall method, aiming a low beam just 2.3 cm (0.9 inches) lower than it should be cuts 26 meters (85 feet) off your seeing distance at night!”

Is there anywhere I can see this for myself?

If you’ve read this far, you might be realizing what Stern spelled out to me: Most LED reviews on the internet are not helpful, nor are the criteria they often cite (sharp cutoffs, color temperature.)
As far as amateur, layperson field testing of LED headlights, Stern pointed to this thread on the Tacoma World forum as a pretty good real-world explanation of why “thoughtfully selected” halogen bulbs beat even name-brand LED retrofits.
“That’s an amateur instrumented test set up to be as favorable as possible to the LED bulb... It’s a major brand product, not some $20 no-name trinket, and it’s tried in a projector lamp, which enforces a sharp cutoff with just about any light source crammed in,” Stern wrote me.

What about LED upgrades that change out the whole housing?

Older cars and Jeeps with sealed-beam lights might, somewhat ironically, be in a better position when it comes to LED headlight retrofits in the current technological ecosystem.
If you’re replacing the bulb and housing, I asked Stern, wouldn’t you be able to get a good LED retrofit with today’s tech? His response:
“The concept is correct—this is the right way to do it, an LED headlamp engineered, designed, constructed, tested, and certified/approved as such. There are excellent ones on the market, and of course also a great deal of junk. The king daddy of them all [at the time of publication] is the JW Speaker 8700 Evolution-J3, which brings almost Star Trek-level technology to the old seven-inch round headlamp format.”
Stern also shouted out the “JWS 8700 Evo 2” as a good option one step down, and “another step or two down from there, [is] the Peterson Manufacturing 701C (in Peterson or Sylvania Zevo packaging — same lamp) and the Truck-Lite units are reasonably good with or without heated lens.”
JW Speaker was cited as the optimal choice right now for rectangular sealed-beam retrofits too and “Truck-Lite also makes respectable lamps in this size.”

Any other important features on those lights?

Stern made a good point I’d heard before: Since LEDs don’t heat headlight lenses the way halogen bulbs do, you might want a light with a heated lens to melt snow.
“Wise to get heated lenses if one does a lot of wintertime driving in heavy wet snow and slush; without a lens heater, the LED headlamp lenses run cold so slush can build up on them and freeze/occlude the lamp instead of melting off like they do from a warm halogen or BiXenon lamp lens. Short of that, though, there’s little reason to fret about it; less severe winter conditions with lower-volume, drier snow won’t make problems as the cold snowflakes will just glance off the cold lenses.”
“The situation with small round lamps is different and more difficult. Here yet again, the JW Speaker entries are terrific with or without integral LED parking light, daytime running light, and turn signal — all functions safety-approved; this is not the greasy kid stuff one finds all over the motorcycle/hot rod/chrome shop sites. Expensive, though, and most headlamp mount cups would need their central holes enlarged to clear the relatively large rear body of these lamps. No entry in this size from any of the other major makers, because of relatively minimal demand. However, I am now testing some rather promising ones very nicely made in Korea. I hope they pan out; they’d be an easier fit in most mount cups.”
“Sometimes one runs into electrical incompatibilities in trying to install LED sealed beams. Toyota-made vehicles, for example, have very unusually-configured headlamp circuits, some of which won’t play nicely with LEDs. There are workarounds that don’t involve hacking the vehicle.”

Will LED Headlights Ever Be A Worthy Upgrade For More Cars In The Future?

“There are technical working groups worldwide (SAE in America, GTB in Europe/Asia) actively working to develop a technical standard for LED retrofit bulbs to replace halogen bulbs in headlamps, fog lamps, and other such,” Stern told me, as member of such groups himself.
As for the current state of technology, Stern says: “Unlike ‘HID kits’ where there is no possibility of optical compatibility, that possibility does theoretically exist with LEDs. The products presently on the market are not close to acceptable; there are still some very substantial technical hurdles to overcome... but eventually, there will be legitimate products of this kind.”
“It’s hard to wait (believe me, I know!) but the ones on the market now just don’t cut it, no matter whose name is on the box and what promises and claims are made.”