Headlight
modulators; the Federal Law authorizing them; the technical
details
(and, California Law)
©
hdlite.htm
24-C
The human eye
and brain; how light bursts and modulated light affects us:
We humans have some characteristics that are common with other
primates. Some of our responses, particularly to color and
light bursts, are relatively unique. Very
simplified, here are some things you might like to know.
Since you likely already 'know' that a headlight modulator causes
'attention' by other humans, and I WANT you to READ this...I will
put the section on bursts, first:
It has been known for a very long time that our eye/brain
response to light bursts are very peculiar. Some of today's
knowledge about this subject was known by such as Augustin
Fresnel, who invented the Fresnel Lens, which, rotated, was used
in Lighthouses, up until perhaps the mid-20th century.
Fresnel found that using a lot of prisms in such position to
result in a collimated radial light beam would produce a beam of
light that could be seen by mariners at a great distance.
UNfortunately, Fresnel did NOT know about the Broca-Sulzer
Effect. Broca and Sulzer determined that short pulses
of light of a CERTAIN DURATION will appear BRIGHTER than if the
pulses are longer in time (or, for that matter, continuous!).
They developed a chart. The Broca-Sulzer Effect
requires that the light intensity be at least 25 lux to be
relatively noticeable....and as the light intensity increases,
the Broca-Sulzer effect becomes very pronounced. Further,
as light intensity increases above 25 lux, the peak effect is at
more and more reduced light duration.
On a practical basis, here is what this all means:
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.
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.
Now, I will guess here that some of you will immediately say something like: "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 change size". You'd be only partially correct.
Broca and Sulzer's work was published in 1902. Similar work was done by Blondel and Rey in 1911.
The bottom-line here is that if you wanted 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 yet of considerably higher power.
This is NOT the only strange effect of light on us (besides resetting our Circadian clocks and so on..). If the light is in continuous pulses, a certain and narrow range of pulse RATES will 'attract' even more attention from the human brain. Some ascribe this sensitivity to such as the brain's alpha rhythms....but this is only one explanation of several. The Federal Law that allows headlight modulators on motorcycles is a mixture of mostly science, and some practicality....in that INCANDESCENT headlight lamps can NOT have their actual LIGHT OUTPUT decrease to anywhere's near zero, at the modulating RATE that is best to attract attention by the human brain. Thus, the Federal requirements are a compromise, although a good one. I note here that HID headlights and likely also LED lights, will eventually be very common. I anticipate these will bring about their own special effects on us, as well as bringing some solvable problems.
FEDERAL LAW:
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]
TITLE 49--TRANSPORTATION
CHAPTER V--NATIONAL HIGHWAY TRAFFIC
SAFETY ADMINISTRATION, DEPARTMENT
OF TRANSPORTATION
PART 571--FEDERAL MOTOR VEHICLE SAFETY STANDARDS--Table of
Contents
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 that is located at the sensor and 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.
S7.9.4.2(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 and a headlamp of the minimum rated power, and 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 including location on the motorcycle, distance above the
road surface,
and orientation with respect to the light.
CALIFORNIA LAW:
California law is a bit different, and is very brief.
Federal Law will supercede California law on Federal Highways,
and probably on highways/roads that were paid for in part or
wholly by federal funds.
However, at least in California, knowing the broad brush law in
this State may be of some help, if ever a question on modulators:
California Vehicle Code 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, 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 and sunset is, at least in this section.
Rev:
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.
© Copyright, 2010, R. Fleischer