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Headlight modulators.
Lighting regulations, front and rear.
Federal Law authorizing modulators.
California & other States.
>>> How pulsing light affects humans <<<

Also see Part 6 of:

Copyright 2020, R. Fleischer

Headlight modulators are, or can be, controversial, with differing viewpoints & arguments for & against.   Some find modulators annoying.  I believe that is, perhaps not always, folks seeing OTHER folks' modulators, not their own.  Some are concerned about oncoming drivers fixating on one's modulating headlight & aiming for them, this seems to come from the idea that a motorcyclist tends to ride to wherever they fixate on.  Studies have shown that adding lights at the rear (& in some instances the sides) of motorcycles makes them more visible and recognized earlier.  For the rear, the lights should be red & as differentiated as possible between Run & Brake.  Bright clothing, greenish-yellowish helmets, etc.  ...all have been shown to reduce accidents.  I see no one arguing about 'fixating' on THEM.

Whether or not to install a headlight modulator on YOUR bike, is a personal choice; the USA government has not made them mandatory.  My comments in this article apply to the USA, as headlight modulators may not be legal in some other Countries.

There ARE reasons that motorcycles are specified, since 1978 in California, & specified in many other States, to automatically, upon ignition turn-on, to power headlights ON, without having to manually turn the headlight(s) on.  State law & Federal law are not necessarily the same thing, but, typically, Federal law supersedes.   In this instance, always on headlights after a certain year model does apply.  There is also an argument about certain cars, such as late models, having constant-on headlights ...the argument usually is negative, as motorcyclists want only themselves to have such lights, to differentiate between them and cars.   That's a decent argument.

Many safety agencies ...and other studies,  ...have proven that safety is enhanced for motorcyclists having modulated headlights the pulsing light attracts attention, particularly important to a motorcyclist, who wants, or should want, an oncoming driver to notice & recognize that a motorcycle is there.  This has been shown to reduce left-turn accidents, & in general, reduce most biking accidents.  Studies have also proven that bright clothing and certain colors of helmets help reduce accidents considerably.  I  conducted experiments with various lights.  I did a lot of experiments with various reflective stripes on my sidecar rig.  I did experiments while riding my two-wheeler too (LOTS of experiments).  I additionally did experiments while wearing my white helmet and also with my fluorescent yellow-greenish intensely colored helmet, & have put an article on this subject here:

California has long had law and DMV regulations allowing modulators for motorcycles.  Now, with the Federal law and details on headlight modulators, all States must conform to the Federal Law permitting motorcycles to have headlight modulators.

I suppose an argument could be made that Federal law would only have to be applied to roads that had Federal money paying for all or some of those roads, but I have not dug into that, it being, in my opinion, rather a moot and likely not to be tested idea.  For practical purposes, a properly configured with proper specifications lamp control/lamp, installed & operating modulated headlight, is legally allowed, everyplace, in the USA.

No manufacturer that I know of supplies modulating headlights as standard, IMO because the manufacturer would want the rider/owner to have the choice; and, manufacturer's ship to all over the world, where various Country's have their own rules and regulations.  My understanding is that there is a long term study underway now, about safety increases with motorcyclists and modulators.  If this results in modulators reducing accidents, or otherwise shown to be beneficial, we might end up with a Federal Rule/Regulation/Law, that all new motorcycles must have daytime modulating headlights (probably meeting the specifications shown later in the article you are reading.

Studies have definitively shown that adding lights at the rear (& in some instances the sides) of motorcycles makes them more visible (and recognized earlier).  For the rear, the lights should be red & as differentiated and distinguishable as far as possible between Run & Brake.  Bright clothing, greenish-yellowish helmets, etc. ...all reduce accidents.  I see no one arguing about oncoming auto drivers from a motorcylists rear 'fixating' on them, but have no test results. Haven't looked.

This article concerns itself mostly with headlights, but a lot of the same science applies to lights in general...and, reflections from lights.

The human eye and brain; how light bursts & modulated light affects us:
We humans have some characteristics that are common with other primates.  Some of our responses, particularly to color & light bursts, are relatively unique.  It has been known for a very long time that human's eye/brain responses to light bursts are peculiar.

Some of today's knowledge about this subject was known by Augustin Fresnel, who invented the Fresnel Lens, which, when rotated, was very commonly used in seaside lighthouses.  Fresnel found that using a lot of prisms in such a position as to result in a collimated radial light beam, would produce a beam of light that could be seen by mariners at a great distance.  Lighthouses had been in use for a very long time, long before there were electric powered lamps.  Unfortunately, Fresnel did not know about the Broca-Sulzer Effect.

Broca & Sulzer determined that short pulses of light of a certain duration will appear brighter than if the pulses lasted longer in time (or, for that matter, a steady un-pulsed beam). There is much more to Broca-Sulzer, but here is what is important on a practical basis, assuming a quite modest or brighter light source:  If the light duration is over 0.150 second, the light burst only marginally appears brighter than a continuous light.  The effect is more if the light is quite bright.  BUT ...if the light duration is around 0.044 second, a light of maybe 150 to a few hundred Lux, will appear very much brighter than a continuous light.

I will guess that some of you will immediately think: "Well, bright light causes the eye's pupil to contract to protect the eye, so a short burst will not last long enough for the pupil to become smaller", so, yeah, sure... the light appears brighter. NOPE!    If you think about it, & the above paragraph, note CAREFULLY that the 0.150+ second duration is only marginally brighter-appearing, than a SHORTER duration effect, comparatively ...etc.  So, the pupil effect is NOT pertinent, and the effect noted is quite prominent at very very low light levels, where the pupil is already wide-open.

Broca & Sulzer's work was published in 1902.  Similar work was done by Blondel & Rey in 1911.

The bottom-line here:  if you want a light to attract attention, without having it monstrously bright, one method is to have the light come in short bursts, lasting on the order of  0.030 second to maybe 0.200 second.   Thus, a headlight with a short burst of light, will be more visible to others, than one running continuously of very considerably higher power!!!

Think about that.  On a practical basis, you should always want your headlight to be visible and noticeable by oncoming traffic, in particular so they do not turn left in front of you; or, really, cause a collision no matter the directions.  You can use a modulated headlight beam of the 'correct' characteristics, and it will be more attention-getting than a very substantially more powerful headlight!  We are not, yet, directly even considering the use of a higher power lamp, which many use to light up the road better!!

What has been discussed here, so far, are NOT the only strange effects of light on humans (besides resetting our Circadian clocks, particularly BLUE light effects, etc.). If the light is in continuous pulses of a certain width (time) & within a certain narrow range of pulse RATES, it will 'attract' even more attention by the human brain.   Some ascribe this sensitivity to such as the brain's alpha rhythms ....but this is only one explanation of several.  There are some very scholarly articles regarding our ancestors & danger, & how the brain & eyesight & hearing evolved, etc.  I will not get into those things here.

Incandescent lamps:
The Federal Law that allows headlight modulators on motorcycles was a mixture of science & some practicality.  This is because incandescent lamps, which includes Halogen's like the H4 used on Airheads, can not physically have their actual light output decrease to near actual zero at the modulating rate that is best to attract attention by the human brain. The hot filament is unable to cool fast enough during the electricity off period, & heat up fast enough for the electricity on period.   The Federal specifications are a compromise, although a reasonably good one.   For a fuller explanation, yet not overly complicated to understand, read the section titled FEDERAL LAW, located several paragraphs below.

HID headlights (High Intensity Discharge):
HID headlights are common for separate high beam use. They can not turn on to full output quickly which can be a drawback for such as using them for the high beam manual-flash-to-pass function.  HID lights are difficult ...and complex modulate, due to arc re-establishment. They are relatively electrically efficient, & if properly focused & shaped, the beams can be very good for bikers where the function will be, more or less, continuous high beam use.  The simple bulb change kits on the market for motorcyclists (and cars) are big compromises due to them not having specially matched reflectors, such as the stock lights do.  Often, these HID lights also don't play well with the stock outer lens.    My guess is that we won't see modulated HID headlights on motorcycles without some sort of new developments.  I think that such will not happen because the expensive and complex HID is going to be be replaced by LED, which can be turned on and off instantaneously, even considering that LED also will require special reflectors. LED high efficiency and minimal amount of $$ peripherals points towards near universal acceptance, including by all vehicle manufacturers.

LED headlights (Light Emitting Diodes):
LED headlights started appearing in higher-priced cars in ~2012.   LED lights can be turned on & off just about instantaneously.   An LED modulated headlight, with a properly designed modulator & reflector/lens might well be perfect for bikers.  I have experimented with some modulators & LED lamps, & find that the modulators worked wonderfully with the LED lamps, better than with Halogen H4 lamps.  The visual light output appeared to be crisp, and powerful.  I was not so much pleased by the Kelvin temperature (color is too bluish and glare is increased for many).  LED lamps are widely installed as original equipment in most cars & trucks now, near universally for all lamps except the headlight.   Some are in the headlights, some only on one beam (often as a separate assembly), but LED headlights are probably going to be the universal standard rather soon.   There are many reasons for this, including (possibly) lower cost over-all & longer lamp life (assuming cleverly designed and effective heat-sinking).

Nerdy section:
The way the Federal Law is written, I believe that modulated LED headlights will comply with the Law as written IF they can comply with this portion:   "The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity measured at the same point".  This specification is attainable on incandescent lamps due to the slow speed of temperature reduction when the power is shut off. It is much more difficult to do exactly and properly on an LED lamp. On a practical basis my guess is that it matters little, if that specification is complied with ...or not.  Just how the manufacturers can comply with the 17% requirement with LED lamps, is going to be interesting (at least to me).  What will perhaps happen, is that the LED modulators will somehow be built to do this, perhaps by dual modulation ...or, the law will be changed ...or both, or the modulators that do not comply will simply get away with not complying, as no one will much care.  Section (f) of the regulations will be changed or assumed to mean that it is only for incandescent lamps.  Since modulated headlights are only allowed for motorcyclists (we won't get into left-right-left-right taxi driver in trouble indications), I suspect no one will be much concerned about the actual modulation.  Motorcyclists have, for decades, had lighting added by owners that are quasi legal, or actually illegal.  Somewhat popular are devices that cause the rear braking light (or, added lights) to flash when the brakes are applied. ALL are illegal (well, quasi-illegal), under Federal law, which states that the brake light must be brighter than running lights.  This is a simplification of the laws.

Some preliminary tests have been done using various 'colors' (temperature of the 'white' light) of LED light.  They show an increased recognition effect.

Since 1979, most motorcycles sold in the USA ...but not all ...have had a headlight ON function when the ignition key is on.   This was done by most manufacturer's due to some States having laws that said that the headlight must be on. The federal government has its input on this, of course.  If interested in what the federal laws really say, look up CFR Title 49, section 571. There is a very large amount to read, but it is informative, over-all. You can find out about all sorts of things on the Internet, EH?....perhaps you want to know the details and problems manufacturer's had by putting TWO headlights into the fairings of motorcycles, etc.  How about having to put yellow reflectors on the front fork lowers, at 15" of height from the ground?  ETC...lots of ETC!  Manufacturers can have very pretty bikes at various world shows, and then the practicalities of producing them for specific markets result in them being garbaged-up with do-dads, and other changes...etc.


(1) To the best of my knowledge Federal Motor Vehicle Safety Standards do not 'require' daytime-on headlamps for motorcycles.  Reference:    This is the Federal Motor Vehicle Safety Standard 108, usually just shown as FMVSS 108. That long hard-to-read article has a chart section, well-buried, that is just for motorcycles.  In the chart the Federal Government has specified maximum brightness of your headlight, in Candelas.  I mention this, as many of you have installed higher wattage headlight lamps.  Of course, few are ever stopped by the police for such.  As a side note, both States and the Fed's, have had, in the past, restrictions on the WATTAGE of headlight lamps, even on brake lamps, etc.   Since 'governments' move ever so slowly, the government rules and regulations are usually far behind the times, but manufacturer's have to deal with what laws or rules actually exist.   For legal LED's, the Wattage will have to be modified.

The most pertinent part (for this article) of the federal document is at section S7.9, and you might be interested in S7.9.4, that allows either the high or low beam to be modulated (but read the rest of what must be complied-with...).  Note that the GROUNDING side of the lamp can not be modulated, by Law, which negates use of  modulators (home-made?) that do such for standard H4 9003 dual incandescent filament, hi/lo lamps.  That Regulation also prevents, by inference, the use of grounding side modulation on those motorcycles that have separate high and low beam lamps.    Note also that just what is a motorcycle is certainly not clearly stated in THAT document.  Trikes? Sidecars?  The States have their own descriptions.  The feds do too.  BELOW I have a capture of SOME of the sections....but NOT ALL, applicable to motorcycles.

In Canada, there is a similar document, but it is referred to as CMVSS 108, and it DOES require daytime lamps-on, from 1/1/90 (If I remember correctly).

Some States do require daytime running headlamps, & new production, from 1/1/90, at least was encouraged.  Federally, motorcycle manufacturers began equipping all motorcycles built for sale in the U.S. with them.  Here's another website describing current federal (USA and Canada) regulations, regarding motorcycle lighting, including headlamp modulating devices.  THIS IS A REALLY GOOD SUMMARY, AND WORTH YOUR TIME TO READ IT!  Read the article carefully, noting the description of what is a motorcycle, and lots more such things!  What about the 4 wheel Spyder?  How has YOUR State handled what is a motorcycle?  What about updated Federal rules ...really? A later version of some details is below, and it too, is worth your reading time here.

(2)  WAIS Document Retrieval [Code of Federal Regulations].
[Title 49, Volume 5].
[Revised as of October 1, 2001].
From the U.S. Government Printing Office via GPO Access.
[CITE: 49CFR571.108].
Subpart B--Federal Motor Vehicle Safety Standards.
Sec. 571.108 Standard No. 108; Lamps, reflective devices, and associated equipment.

S7.9.4 Motorcycle headlamp modulation system.
S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the upper beam or the lower beam from its maximum intensity to a lesser intensity,provided that:
(a) The rate of modulation shall be 240 <plus-minus> 40 cycles per minute.
(b) The headlamp shall be operated at maximum power for 50 to 70 percent of each cycle.
(c) The lowest intensity at any test point shall be not less than 17 percent of the maximum intensity measured at the same point.
(d) The modulator switch shall be wired in the power lead of the beam filament being modulated and not in the ground side of the circuit.
(e) Means shall be provided so that both the lower beam and upper beam remain operable in the event of a modulator failure.
(f) The system shall include a sensor mounted with the axis of its sensing element perpendicular to a horizontal plane.  Headlamp modulation shall cease whenever the level of light emitted by a tungsten filament light operating at 3000 deg. Kelvin is either less than 270 lux (25 foot-candles) of direct light for upward pointing sensors or less than 60 lux (5.6 foot-candles) of reflected light for downward pointing sensors. The light is measured by a silicon cell type light meter located at the sensor & pointing in the same direction as the sensor. A Kodak Gray Card (Kodak R-27) is placed at ground level to simulate the road surface in testing downward pointing sensors.
(g) When tested in accordance with the test profile shown in Figure 9, the voltage drop across the modulator when the lamp is on at all test conditions for 12 volt systems and 6 volt systems shall not be greater than .45 volt. The modulator shall meet all the provisions of the standard after completion of the test profile shown in Figure 9.
(h) Means shall be provided so that both the lower and upper beam function at design voltage when the headlamp control switch is in either the lower or upper beam position when the modulator is off.

(a) Each motorcycle headlamp modulator not intended as original equipment, or its container, shall be labeled with the maximum wattage, and the minimum wattage appropriate for its use. Additionally, each such modulator shall comply with S7.9.4.1 (a) through (g) when connected to a headlamp of the maximum rated power & a headlamp of the minimum rated power, & shall provide means so that the modulated beam functions at design voltage when the modulator is off.
(b) Instructions, with a diagram, shall be provided for mounting the light sensor sensor including location on the motorcycle, distance above the road surface, and orientation with respect to the light.

(3)  The following website used to have much more technical information; but, go ahead and read the various topics.

Note that NHTSA says that the federal law overrides any State law.   Article IV, section 2, of the U.S. Constitution is held to say that Federal Law is superior to and overrides State Law, when they conflict.

Laws for various states for most everything concerned with motorcycles, particularly headlights and modulators:

Knowing the broad brush law in YOUR home State may be of some help, if ever a question on modulators.  You can check State's laws, but I will bet that any Court will hold that the Federal law applies.

CALIFORNIA LAW (as regards the headlight and modulators):

California law is brief.   Federal Law will supersede California law on Federal Highways, and on highways/roads that were paid for in part or wholly by federal funds.  I believe that Federal Law will supersede any State's Law on a practical basis, if not an absolute basis, on any road in the State, that has had Federal money involved, or not.  California Vehicle Code that specifies headlight-on is section 25650.5, & the section that allows motorcycles to have modulators is 25251.2.   It specifies that the UPPER beam is to be modulated (note that Federal law allows either).  California Law provides only that the modulation is between a higher and a lower brightness, at a 200-280 rate, and not to be used in darkness.   This is basically like the federal law ...except that California has NONE of the fancy specifications from (a) onwards, as in the federal law, in (2), above.

OFFICIAL California Vehicle Code definition of DARKNESS is in section 280 of the California Vehicle Code.  Darkness is defined as 1/2 hour after sunset to 1/2 hour before sunrise; or at any other time visibility is not sufficient to clearly discern any person or vehicle on the highway (yes, they do say highway), at 1000 feet.   They do NOT specify what sunrise, sunset and highway are (in that section).

04/12/2010:  Add California Vehicle Law information.
05/22/2010:  Add technical information on human eye sensitivity to light bursts and modulated light, and clean up article somewhat.
10/08/2010:  Split the Federal Law section into 2, and add (1).
06/08/2011:  Clean up article.
06/25/2011:  Clear up...somewhat!....Federal and California Law.
10/02/2012:  Add QR code; add language button; update Google Ad-Sense code.
2013:  remove language button as its scripting code had problems with some browsers.
09/15/2014:  Clean up for clarity.
01/29/2015:  Rechecked.  Add a bit on conspicuity, etc...clarifying some statements. a tad more in Sept. 2015.
12/10/2015:  Update meta-codes, justify article to left, and clean it up considerably. NO technical changes at all.
03/05/2016:  Final version of above.
07/15/2016:  Update metacodes, scripts, H.L., layout.
11/26/2016:  Add a few comments about the Federal Laws.
01/12/2018:  Reduce html, font, & color excesses.  Nicer layout.  Add 10pxl margins. Clarifications and details updated. Some links checked.
04/07/2019:  Editing for clarity and layout.

Copyright 2020, R. Fleischer

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Last check/edit: Wednesday, December 16, 2020