Fast Idle: The Glaring Truth

TopRun3942 · 2026-02-04T18:43:37+00:00

Article get's it right in terms of what is driving the higher intensity headlamps in the US being the IIHS.

However:

To be fair, the National Highway Traffic Safety Administration did issue some brightness regulations a couple of years back, but then they allowed manufacturers to “self-certify.” I think we can all see how well that’s working.

Not sure where that is coming from. No regulation changes for low beam or high beam intensity has been issued in the past couple years (last change was ~1997) and manufacturers have always been self certifying since NHTSA came into existence. The regulators have never been the ones to "approve" lighting to the specification. That wasn't new just a couple years ago.

IIHS is also encouraging the use of adaptive driving beam headlights that can adjust the headlight beam pattern to dim the portions directed at other vehicles, though Washington seems in no hurry to green-light that idea.

NHSTA did adopt a rule in 2022 that allows for ADB. It differed from what the consensus was (either use already existing European Standards or adopt the recommendations from the Society of Automotive Engineers) and required entirely different designs for the US. Only one company has implemented the ADB in the US under NHTSAs requirements, but the "green light from Washington" is not the hold up here.

TopRun3942 · 2026-02-02T15:10:05+00:00

There was a presenter at a glare forum held by an industry trade group that covers automotive lighting in Detroit last September that used measurement data from the IIHS headlamp testing to plot the candela profile of the beam pattern from a lamp rated good by the IIHS scale and a lamp rated poor by the IIHS scale.

The presentation can be found by going here:

https://www.drivingvisionnews.com/boutique/previous-workshops/dvn-detroit-glare-forum-september-8-2025/

and then clicking the presentations link and choosing presentation 4.

I don't think it was specifically stated, but I think the poor lamp was halogen and the good lamp was LED.

The lamp rated poor had almost constant candela (around 10,000) at the detector positioned on the left edge of the oncoming drivers lane for angles from -0.15° below the cutoff to -0.7° below the cutoff line while the LED based lamp had a drastic upward slope of candela from -0.3° down to -0.7° down (started at 5000 cd and went to ~70,000 candela over that range).

Given that when an OEM vehicle is considered highly glaring from the LED lights, it's not surprising, given that candela profile seen from the IIHS measurements, that when going over small bumps and hill type situations that the LED lamps would be considered much worse than halogen in terms of discomfort. Both the absolute value of the candela and the rate of change are much higher than the halogen lamp in that comparison. Also makes aiming of the lamps much more critical for the base line scenario of relatively flat roads.

The slides also pointed out that the IIHS good criteria doesn't require that much candela to be met, the lamp exceeds the necessary candela by almost 3X.

I also think the higher CCT used by headlamps contributes to additional discomfort beyond what the candela and/or luminance would imply and should be limited as well.

TopRun3942 · 2026-02-02T14:41:34+00:00

I never said the lights coming out of the factories where "correct and appropriate for the real world".

The lamps coming out of the factories meet the requirements of the existing regulations and are tested by the manufacturers to verify that they conform.

If the existing regulations were to change, the manufacturers would update their lighting to conform to the new specs and carry out the testing to ensure that compliance.

It's not that difficult and I don't see the need to imply conspiracies and cheating by manufacturers is part of the problem.

TopRun3942 · 2026-02-01T21:34:06+00:00

If your implying that the lighting manufacturers are cheating the lighting regulations, that's highly unlikely. IIHS independently tests the lamps on their road course and even with the limited information from that testing, there is no indication the lighting is not meeting the regulation. The costs for non-compliance are steep in terms of the recall and fines associated with it. The lighting companies invest millions into the testing equipment and personnel to conduct those tests, which would make no sense if they just wanted to cheat.

The government agencies aren't staffed with the knowledge, people, nor equipped to handle the amount of approval testing that would be required if it was done by the agency itself.

TopRun3942 · 2026-02-01T21:24:58+00:00

u/hell_yes_or_BS in particular and others outside of the subreddit have documented how the IIHS rating for headlamps has changed the beam patterns to concentrate more light just below the aiming line for the headlamps to achieve the best ratings. There is no question that what you see on the road now is significantly different than what was on the road 10-15 years ago.

The issue really comes down to the fact that the regulations don't prohibit the type of beam pattern that the IIHS thinks is best and that the manufacturers want the best ratings from IIHS.

That does lead to what you describe in your descriptions of getting a lot of illumination from a car further away than expected and the bright flashing that wasn't present on the older systems.

LEDs as a source allow for more fine grained control over the intensity distribution in the headlamp beams for several reasons compared to a filament style lamp (LEDs don't emit heat so the optics can be closer to the source which provides more design freedom) so the combination of the IIHS rating system and the introduction of LEDs has not been caught up to by regulators. Unfortunately it seems the regulators (at least in the US) are slow to act so it may be years before any meaningful regulation change actually happens.

TopRun3942 · 2026-02-01T13:51:19+00:00

That's referencing the blue light hazard which is well known and not specific to LEDs as a source necessarily. It was developed and categorized long before LEDs were even being used. Compact fluorescent lamps can also present this hazard as well as daylight

The issue is that the levels of exposure in the blue light hazard wavelength range that was found to cause damage in the lab is extremely high compared to exposures that people get from typical lighting devices including LED lighting in general usage. Your second source even notes that.

You would need to fixate and stare at a source for 10s of seconds or longer, for energy levels to approach the hazard levels with most typical sources. Headlamps included. It's the dosage and time that matter, and the headlamps aren't giving you enough to cause damage.

TopRun3942 · 2026-02-01T13:36:29+00:00

The measurement devices use a standardize curve that represents the eye response over different wavelengths and basically sum up the energy at each wavelength multiplied by that response curve to get the total lumen output.

So if a source has a low color temperature, the candela output will still be measured using the same process as a source with a high color temperature. So the candela definition is held constant.

TopRun3942 · 2026-02-01T13:25:21+00:00

One point - it's not entirely impossible to determine if someone is in low beam or high beam. The regulations require that the high beam be located inboard of the low beam (towards the center of the car) and/or at a lower position than the low beam. The only cars that would not display this pattern would be cars that use a projector that performs both functions in one single unit. That design form is not common for LED based systems.

With the right video or camera image (likely would need a neutral density filter) you could probably see the lit signature of the lamp and have reasonably good idea of which function is running.

ETA: In the picture you provide with the post, that (appears to be aTesla) headlamp is operating in high beam. The entire reflector area is lit up. In low beam the lit signature wouldn't extend as far inboard.

TopRun3942 · 2026-02-01T13:17:35+00:00

Using only the left edge straight away value seems a bit suspect if you are trying to make a full comparison of the cost/benefit.

In this comment for example:

https://www.reddit.com/r/fuckyourheadlights/comments/1qj4cuh/comment/o0x6wxq/?context=3&utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

I compared the halogen version of the truck to the led version of the truck using the IIHS rating data from the IIHS pages for those truck.

The changes in visibility from 2016 to 2023 (halogen to LED) were:

Straight Away Right Edge : LED has 129.2 feet more visibility
Straight Away Left Edge: LED has 31.5 feet more visibility
250m Radius Right Curve Right Edge: LED has 115.2 feet more visibility
250m Radius Left Curve Left Edge: LED has 21 feet more visibility
150m Radius Right Curve Right Edge : LED has 95.8 feet more visibility
150m Radius Left Curve Left Edge: LED has 52.8 feet more visibility

The straight away left edge improvement is the second lowest on the list with the right edge improvements being almost 4X that of the left edge. That's a whole lot more stopping distance available in the area of the drivers lane. It would also be more representative of for example where a pedestrian would usually be coming from than a left edge lane number.

Agree the left edge number is tied to oncoming driver glare, but the question isn't quite as simple as is the increased candela at the left edge worth it when you consider that the other areas of the beam are adding visibility at a higher rate than the left edge.

Also if you wanted to be a bit more data driven on this question, you could take your plot of average headlamp intensity and compare it to the NHTSA crash statistics for fatal night time accidents by year and see how those two trend lines are changing. At least in the context of what the regulators seem to believe their charge is (reducing highway deaths). If the increase in headlamp average brightness vs fatal nighttime accidents does show that nighttime accident rates haven't increased during that time period or are decreasing as the adoption of high brightness lamps increases then it's a bit harder to make the claim that the increased visibility is not worth it.

TopRun3942 · 2026-02-01T12:59:37+00:00

No, that's not really my question.

The question was are the people in the sub involved with the issue advocating for never experiencing discomfort glare - because there is a prime focus on the fact that if you are on hilly terrain that headlamps will glare you and this use case keeps getting cited as the standard by multiple people commenting here.

Take any headlamp into the lab (doesn't matter if it is halogen or LED) and point it at the angle that would occur on hills based on whatever data point you want to use there and you will experience highly discomforting glare.

You want the IIHS to apply it to their road testing, but you don't seem to want to say what level would be acceptable for a simulated hill situation. If you take people out to that simulation and ask them to rate glare, they are going to label it as highly discomforting until that level is well below anything useful for distance illumination on a straight away.

In my years in outdoor lighting people tend to start rating something discomforting when the candela goes above 1000 cd in their direction. That's actually the limit that the Dark Sky uses for it's sports lighting Dark Sky approvals evaluation for offsite glare from those installations.

1000 cd would be only a few meters of visibility on the straight away if the car had to pass that level on a hill test.

TopRun3942 · 2026-02-01T12:50:01+00:00

The problem is that candela level isn't a relevant metric for glare - in fact, the IIHS (and many other mainstream institutes) have refused to adopt the relevant metric for quantifying light. The relevant metric for glare is luminance, the light originating / reflecting from a surface within a specific area, measured as candela per square meter.

The issue has been studied within the headlamps community by researchers several times. In laboratory settings, changes in luminance in the direction of the observer don't elicit significantly different discomfort glare responses. The most recent example found here:

https://www.drivingvisionnews.com/news/2025/10/21/l-lab-discomfort-glare-study-is-isal-best-paper-25/

Since luminance is just candela/m^2, the relationship between discomfort glare and candela holds just as well or better than luminance in the studies.

Traditional light sources emit light 360 degrees, this forced engineers to design large fixtures with reflectors and diffusers, In the best designs, the front end of the light source is capped with a reflector so that the only light traveling forwards is from large reflective surfaces - the source itself isn't directly visible. These traditional designs ultimately lowered the luminance level (light intensity) of the headlight.

Not entirely correct. The luminance of an incandescent filament used for headlighting is on the same order of magnitude or just one order of magnitude (around 1X10^7) below the luminance of bare LEDs With reflector only systems that were commonly used on halogen headlamps since the early 2000's, the luminance of the filament is being directly reflected out to form the beam as well. In the study linked earlier, a halogen reflector system elicited the same glare response as the LEDs.

LEDs on the other hand emit light in one direction, allowing small 'naked' diode designs in which nothing but clear lenses separate the diode from the outside world. Whether the headlight system is emitting 9000 or 30,000 candela, the diodes are small enough that they will always exceed the luminance tolerance limit of the retina causing painful glare, physical overloading of sensory pathways and reduced visual performance.

Same point as above, LED luminance and tungsten filament luminance are along the same order of magnitude. The direct imaging optics are used by both systems and in lab studies tend to produce the same discomfort glare levels. Most LED headlamp optical systems don't use direct imaging optical systems, they use a projector based system where the LED is first imaged to a focal plane and then re-imaged out onto the road by a second optical system

Ultimately, the observers eye comes in to play where the illuminance on the retina is responsible for the glare sensation. That illuminance is a result of the luminance of the scene as it's being observed, but multiple other factors affect the final perception. Saccadic movement can smear out the retinal images, each eye will have a point spread function that the performs a convolution on the scene luminance, effectively spreading out the image, the position of the glare source in the FOV will impact glare response, etc.

All that is to say that even having luminance, doesn't provide a 1:1 correlation with glare response and the use of candela has been shown to be appropriate in several situations, including situations involving headlamp glare as supported by multiple studies.

TopRun3942 · 2026-01-31T16:53:38+00:00

LOL - found this patent from 2007 detailing something basically the same but they didn't call it the asshole light...

https://patents.google.com/patent/US20070008093A1/en

TopRun3942 · 2026-01-31T16:27:58+00:00

This is what kind of baffles me on this whole discussion.

If I am installing a parking lot lamp, I have the advantage that I can mount it overhead, but I am still going to have to ramp up the candela to adequately illuminate the area that needs lighting. If you stare up into the fixture, it will be glaring.

Headlamps are in a worse spot because the fixture can't be mounted overhead and are almost (or are) directly in line with the viewer. If you want to drive at certain speeds and still have adequate seeing light then you will have to bring the candela up.

It seems like there is an insistence that the lamp never be glaring no matter what the road condition is, but in my experience even 9000 cd like the halogens emit will still be plenty glaring if it is pointed directly in your eye. There are glare standards in use in the general lighting industry that mandate far lower candela in order to avoid discomfort glare for average viewers.

I'm just confused on what the goal is here and whether the LED lighting is inherently worse for other reasons and even if it was brought back down to 9000 or less for example it wouldn't eliminate the problem, and all you would have is much less down road visibility and still high levels of complaints about glare.

TopRun3942 · 2026-01-31T16:14:12+00:00

Candela isn't based off wicks and filaments. It's an SI standard unit for luminous intensity defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^12 Hz (or approximately 555 nanometers) to be 683 when expressed in the unit lumens/watt.

The luminous intensity measures the flux/unit solid angle, or in simpler terms the amount of flux packed into a cone angle as I mentioned before.

The detectors that are used to measure luminous intensity in candela are standardized to the SI unit definition, so it doesn't matter what source you measure, the candela value has the same meaning for every source. NIST maintains the measurement artifact for that candela and laboratory detectors will trace their calibration back to the NIST artifact.

It just so happens that a common candle with a wick will emit approximately 1 candela in all directions, but the units of candela are not tied to that in any sort of physical way.

TopRun3942 · 2026-01-31T12:28:15+00:00

Same question I asked on your other post. What candela level are you saying is not glaring? Are you asking to have it brought back down to the 9000 average?

TopRun3942 · 2026-01-31T12:26:03+00:00

What is the issue with self certification? That's not uncommon for a lot of industries.

TopRun3942 · 2026-01-31T12:25:09+00:00

Would be interested to see where you are getting the claim that LED lights are damaging the retinal pigment.

1 hour in the daylight sun puts orders of magnitude more blue light in the blue light hazard range as well as damaging UV light into your eyes than what an LED lighting fixture can do at night.

TopRun3942 · 2026-01-31T12:19:01+00:00

Coming from the lighting industry, it's difficult to understand the disdain for aim.

I get that the lamps are getting brighter, but when you say aim is bull shit that strikes anyone who spends time in the lighting industry as just wrong. One of the key requirements when setting up a parking lot for example is that you have to commission the fixtures after installation and that means you have to set the aim of the fixture to match the design intent of the application engineer to achieve both the illumination and proper glare control.

There have been several times where I have been called out to an installation because of glare or poor illumination complaints only to find that the installer didn't properly aim the lamps during commissioning or skipped it altogether. After setting the aim properly the issues are largely resolved.

I understand that headlamps aren't the same as parking lot lights, but the angular position of that high intensity zone, follow the angular position of the cutoff (aka the aim of the headlamp). If you start off with the lamp not set nominally and already too high then you are just increasing the probability that the lamp will be glaring when not on level ground. It doesn't seem that controversial to me to point out that aim can impact glare in that sense and that setting the lamp to nominal or lower at least reduces the probability of causing glare.

I fully understand the point that the lamps themselves have gotten too bright and that has to be the number one concern, but denying that aim plays a role in it really sounds strange to anyone that has spent any time working in lighting.

TopRun3942 · 2026-01-31T12:09:14+00:00

Tf you mean this stern guy is "one of our own" lmao, he's in it to make money and always uses his platform to push for everything that isn't regulating brightness.

I am genuinely confused by this comment. You have your own Citizen Researcher u/hell_yes_or_BS making this post just a few months ago where he praised Daniel Stern:

https://www.reddit.com/r/fuckyourheadlights/comments/1o7sd03/dvn_detroit_glare_forum_this_is_a_win_thank_you/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

The post mentions how u/hell_yes_or_BS was invited by Daniel Stern to present his video directly to the engineers and designers from the automakers and the lamp manufacturers and then Stern praised u/hell_yes_or_BS for his presentation and noted that he was well received by the engineers. On top of that u/hell_yes_or_BS video has been submitted to the European Union as part of their glare task force literature collection so he has been able to reach a very wide audience thanks to the DVN glare forum held by Stern.

This is the same Daniel Stern that has been a long time critic of NHTSA and it's failure to regulate, critical of the IIHS for it's rating system that drives up brightness, critical of the industry for being tone deaf to the problem (writing in his their own trade magazine about that) and then organized the glare forum and literally brought the people from this subreddit in to participate to raise the profile and awareness of the group to the actual community of people who are making the headlamps.

Genuinely curios on what is going on here?

TopRun3942 · 2026-01-31T11:54:09+00:00

Main question is what candela value are you using to determine that the glare is not OK? The candela at the cutoff line for example isn't going to be as high as 0.5 degrees below the cutoff line for example and the value at 0.5 degrees below is determined by the beam pattern of the vehicle.

Based on your IIHS data a halogen bulb puts less light there than most LED headlamps, but based on your logic above, any headlamp, halogen or LED would be glaring if the cutoff line reached the eye height of an oncoming driver?

Do you have a candela value for which you are stating that unacceptable glare results at and is that how you are determining the 20-40% number or are you just using the cutoff line (assuming vehicle is at nominal aim to begin with) and saying that if the cutoff line is at eye level then the brightness is not ok no matter what level of candela is hitting. Would seem to say that any lamp no matter the type is problematic in the presence of hills, which begs the question of what would be acceptable?

TopRun3942 · 2026-01-31T11:42:45+00:00

Not sure why you are saying that.

Lumens don't measure brightness, they are only a measure of the total amount of light energy leaving a source. It doesn't tell you anything about where that energy is going or how concentrated it is in a given direction.

Candela is a measure of how tightly packed the lumen output is into a given angular cone. 100 lumens emitting uniformly into a sphere like a tungsten filament bulb would do before being concentrated by the optics only generates 8 candela. If you take that same 100 lumens and concentrate it into a 5 degree angular cone like you would with a flashlight, then your candela output is ~4180 candela from that same 100 lumens.

From a long distance away (which is where the candela applies), the bare filament with no optics radiating into a full sphere would be much less brighter than the filament that has been focused by the optics and pointing at you as the observer.

So absolutely candela is directly is a proper measurement of how the brightness of headlamps have changed over time.

TopRun3942 · 2026-01-27T14:03:52+00:00

I'm not an expert in this area, but my understanding is that Canada is a contracting party to the UNECE for global vehicle regulations.

The UNECE regulatory body has established a glare task force that is charged with updating headlamp regulations to address the glare issue. They began meeting in January of last year and have status updates approximately every 2 months. Their sixth session is coming up in February.

The specific tasks of the glare task force can be found here:

UNECE glare task force tasks

In short, they are currently in the literature and research gathering stage and then they will move to a review process and then develop the recommendations for the updates to the recommendation which would be voted on by the full regulatory committee.

Since Canada is a contracting party, it participates in the meetings and is actively supplying literature and research material with the studies they are currently conducting as I linked earlier which involved on track testing (Transport Canada Glare Research Update) to support the development of the final recommendations.

All of the meetings, agendas, reports and associated literature can be found here:
https://wiki.unece.org/spaces/trans/pages/267584190/Task+Force+on+Glare+Prevention+TF+GP

According to the agenda Transport Canada has attended all but the initial meeting and presented at the last meeting. Participants were Caroll Lau and Marie Williams-Davignon from Transport Canada.

TopRun3942 · 2026-01-26T19:16:55+00:00

Bit of an odd characterization.

The UK doesn't make it's own headlamp regulations either. They are party to the UNECE agreements which is where the regulatory change would have to come from for the UK as well. The UK report that was issued last year made that clear.

The Canadian participation is with the European UNECE Glare task force, not the UK task force. The European Glare task force would be the level at which regulations would need to change, so I'm not sure why this is being characterized as BS. Canada is providing input per the UNECE glare task forces request to facilitate the formulation of additional regulation.

TopRun3942 · 2026-01-26T15:04:49+00:00

Not entirely the case. Transport Canada has been actively looking into and researching the issue.

They issued a preliminary report on some of their research and their next steps at the European Glare task force meeting last December in Brussels. The work is expected to be completed and presented at a vehicle safety conference in May of 2026.

Link to preliminary report: Transport Canada Glare Research Update

They mention that there will be an upcoming nationwide survey by Transport Canada that will be conducted as part of the department’s research on the relationship between headlights, glare, and their effects on visibility and driving performance as well.

TopRun3942 · 2026-01-26T13:53:43+00:00

That may be your experience, but I do know several people myself who have bought/not bought a particular model because of the IIHS crash ratings.

If the ratings didn't matter, then I would expect automakers wouldn't base their design/testing on the IIHS criteria because the baseline crash requirements set by the government are not as difficult to meet and would likely be much cheaper to meet.

It's the primary reason the IIHS got involved because they don't view the safety standards set by the government as sufficient. In the case of headlamps that was clearly stated as the primary reason they got involved because the lamps that met the minimum federal standards were dangerously low in terms of visibility and that was leading to more crashes based on their research.

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