Drive on High Beams Until You Can’t
by
Jim Sobek, Clearly Visible Presentations, LLC
Driving a motor vehicle on today’s highways is primarily a visual task. The National Safety
Council has estimated that about 90% of a driver’s information about the situation around them
is visual. Under darkness conditions, vision is severely limited just by the relative lack of light
reaching the scene ahead of the driver. In addition, glare and visually distracting light sources
are often within the driver’s field of view, often overriding the information that is available.
Adding to the danger of night driving is fatigue. Reaction processes, already hampered by relative lack of information in darkness, are further worsened when the driver is tired and/or under
the influence of drugs and/or alcohol. The net effect is that traffic death rates are three times
greater at night than during the day.
Automobiles have been fitted with headlamps from their very earliest days. The earliest headlamps were acetylene or oil lamps. These lamps were of limited intensity and had the associated
risk of fire. The lamp beam pattern flickered badly due to wind patterns and vehicle jounce.
Starting in about 1908, electric headlamps came on the scene. Improvements in optical and electrical designs shepherded in better and better headlamps as the decades rolled by. Those improvement processes continue to the present day.
In 1915, as more and more vehicles began to show up on the roads, Guide Lamp Company produced a headlamp design that allowed the beam to be “dipped”, so as to reduce the intensity of
the beam presented to oncoming drivers. In the succeeding years, various schemes were introduced to allow the driver to raise or lower the beam pattern from inside the car while driving.
(Early designs required the driver to stop and get out of the vehicle to dip his lights.) Cadillac in
1954 offered its Autronic Eye system which automatically detected approaching vehicles and
dropped the beam pattern until those cars had passed. That automatic system was not popular
and the concept went away.
In the past couple of decades, advances in computing power and improvements in manufacturing
technology have enabled headlamp manufacturers to design and produce reflectors that put light
exactly where desired within the beam pattern. Bulb technology has seen further improvements
with the advent of the tungsten-halogen lamps. Today, the science of modern optics has brought
us light emitting diodes (LEDs). LEDs are flooding the automotive market, as interior lamps,
displays and exterior marker lamps have become quite popular, even on lower end car lines.
However, only a few car manufacturers have produced actual LED headlamps for sale, but only
in the overseas markets. So far, as of the date of this paper, no car or truck manufacturer is offering LED headlamps for sale within the United States. That will likely change quite soon.
With these improved technologies, lamp manufacturers are now able to produce headlamp beam
patterns that can put more light where they want it and less light where they don’t want it. The
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High Intensity Discharge (HID) lamps with their very small but extremely powerful light output
allows the optical designer to now “sculpt” nearly any pattern desired. The most recent standard
that controls the design and production of headlamps installed on vehicles sold in the United
States1 (May 2010) version of the SAE Standard J1383 allows peak intensity values almost three
times higher than lamps installed as recently as 1970.2
Headlamp intensities are specified in candela at specific aiming angles within the beam. At one time there was such a
thing as a British Standard Candle. But, since the recipe for
making one required you to first get your hands on a sperm
whale, improvements were made. Up until a few decades
ago, lamp intensity was described in candlepower. Today,
scientists around the world specify a lamp’s intensity in
candela, a term with the same Latin origin. To simplify
understanding, you may think of a one candela lamp as being
a simple candle.
Figure 1 A simple candle, representing
one candela
If you conceive of packing ten candles all into one small group, the intensity of light issuing
from that group would be 10 candela. Now, consider a single car high beam headlamp where the
intensity of the beam in the “hottest” spot in the beam is 40,000 candela. Not something you
could do with beeswax and a cotton wick on your best day!
In the May 2010 revision of J1383, Figures 5 & 6 respectively promulgate low beam and high
beam intensity patterns as a function of beam angle with reference to the optical axis. It must be
emphasized that the Standard addresses the performance requirements of a single lamp. Thus,
with two functional high beam lamps, the net beam intensity must be as at least 80,000 straight
ahead of the vehicle.
Figure 2 Rays diverging according to the inverse square law.
Now, in order to understand what this means to a driver,
we need to understand another aspect of how light performs. Imagine a single point source of light, say…a
birthday candle. In Figure 2 (left), the source is represented at S. We also imagine that the source radiates
equally in all directions, not strictly true for a candle, but
for this discussion we will accept this proposal. This is
our example candela. Now, hold a piece of paper one
foot away from the burning candle, at range, r. By definition, a one candela source will cast one foot-candle of illuminance onto an object one foot away. Now, imagine
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Society of Automotive Engineers Standard J1383 Revised May 2010 Performance Requirements for Motor Vehicle Headlamps
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The applicable Revision is that revision that was in place as of the date of manufacture of the vehicle.
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moving the piece of paper to a distance of two feet, at range 2r. We see that the rays diverge and
cover four times as much area. But, since there is the same amount of luminous flux as before, it
must be four times dimmer when it reaches distance 2r. Moving the paper out to 3r, the illumination is spread over nine times the area as at distance r. As the distance increases, the illuminance (denoted by the variable, E) falls off as the square of the distance. This is called the inverse square law. All radiant energy obeys this law. The equation describing the law is: 𝑬 =
𝑰/𝒓𝟐 , where E is the illuminance, I is the intensity and r is the range to the target. For our purposes we will measure the illuminance in foot-candles, the intensity in candela and the distance
in feet.3
Now, we need to know one other thing. The human eye needs to have about 0.3 foot-candles of
illumination falling onto an object in order to perceive it in full color and fine detail (think HD
TV). This is because below this value, the color sensors on the retina (called cones) do not function well. A viewer may still be able to see at lower levels, but will only be using the rods on the
retina. Rods are not color sensors and their resolution is far lower than that of cones. (Think
standard definition black and white television as compared to color HD. In reality, the differences are far greater, but the analogy is valid.)
So, how do we determine how much light is reaching an object from a car’s headlights? Let’s
start with a pair of headlamps each providing 140,000 candela operating on high beam on a flat
road. Right in the center of the beam, our lamps (the pair operating together) provide a total of
80,000 candela. We set up a light meter at 100 feet and measure the received illuminance. According to the inverse square law, we should measure 280,000 candela/(100 ft)2 or 28.0 footcandles. This is far more than the required 0.3 foot-candles to stimulate full color fine detail vision. Now, these are some high performing lamps, but they are legal and you might find them on
some upper end cars if the lamps were clean and well-aligned.
Now, let’s go to the same angular location in low beam lamps. While that location is not specifically called out in the J1383 low beam pattern, a reasonable value might be 3,000 candela per
lamp. So, at the same 100 foot distance with only 6,000 candela on that same ray, the target
would now only receive 0.6 foot-candles. This would still be enough to see a target in color and
fine detail, but we see that it is 47 times less illumination.
Now, let’s point out one additional fact. The whole purpose of having a low beam pattern is to
avoid dazzling the opposing drivers with very intense beam patterns. J1383 lamps have a sharp
cutoff just below the horizontal and in the upper left quadrant of the beam for that exact purpose.
The most intense portion of a low beam lamps intensity pattern is 0.6 degrees down and 1.3 degrees to the right. So, any objects that are above the horizontal and/or to the left are not going to
be brightly illuminated and, therefore, they will be much harder to see.
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These are the Imperial or English units of measure. In the Systeme Internationale, or S.I. (metric) system, illuminance will be measured in lux and distance in meters.
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Now, let’s see how this works out in the real world. During a recent test in Seattle, we took a
pair of photographs of a roadway scene. The car was in a downhill portion of a sag curve, commonly referred to as a “dip”. Three pedestrians were ahead of the car taking illumination measurements. Two photographs were taken at the
same camera settings, one on low beam and one
on high beam. The same three pedestrians were
present on the roadway in both photographs. In
the low beam photograph, you can barely make
out the light-colored pants in the left side of the
lane ahead. There are two white lights ahead
that appear to be in the opposing lane of travel.
These are actually retro-reflective patches on an
officer’s uniform. At this point, a driver would
Figure 3 Low beam pattern
not realize that there was a hazard on the road
ahead of him.
Now, switching to high beam, the full nature
of the scene becomes apparent. There are
three persons standing in the road about 200
feet ahead of the vehicle. It was the sharp
cutoff in the low beam headlamp pattern that
prevented the driver from seeing the situation
ahead. You will also notice that the high
beam pattern seems to have produced a lower
brightness scene ahead. That is exactly the
situation because the illumination has been
spread over a much larger scene area. But, it
becomes clear that using high beam has
reduced the hazard substantially. There was
no reason not to be using high beam in this
circumstance. There are no oncoming
vehiclea and no fog.
Figure 4 High beam pattern
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Low beams most always illuminate less of the road ahead of the vehicle than high beams.
Vertical curves (elevation changes) create the most problems although horizontal curves can also
create issues. In vertical sag curves, all rays from the headlamps strike the road closer to the
vehicle than would be the case if the road were flat, limiting sight distance for the driver.
Figure 5 Sag curve influence
Vertical crest curves (hill crests) may cause issues when facing approaching traffic. Each driver
is in the more intense portion of the illumination pattern from the other car. This increases the
potential for each driver to be blinded by the glare from the other car’s lamps. Some car
designers are now working on integrating GPS with headlamps to “steer” the headlamp beams
up/down and right/left to compensate for road profile changes.
Figure 6 Crest curve influence
HID lamps allow the designer to produce an extremely sharp cutoff above the low beam
horizontal plane. In many cases, this cutoff can be so sharp as to severely limit the driver’s
ability to see ahead of his vehicle under those conditions when he must use low beams.
While a few states actually require drivers to use high beam under all circumstances where they
can legally do so, there are no states that prevent drivers from using high beam when they may
legally do so, either.
Over years of driving, I have come to the conclusion that one should drive on high beam until
you cannot. My experience, however, based on personal observations and based on
examinations of thousands of headlamps is that nearly all drivers drivers drive on low beam until
they can’t. In fact, my experience is that very few drivers use their high beam lamps at all.
Readers who have examined lamps for “hot shock” will most likely concur with this observation.
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Unfortunately, pedestrians often choose to wear dark-colored clothing when they walk in the
roadway. Rarely do they wear retro-reflective materials or active lighting. By operating on low
beam, the driver further exacerbates the situation. By driving on high beam, the driver can
counter many of the errors made by the pedestrian.
Switching from high beam to low beam and back requires paying close attention to the situation
ahead to avoid blinding other drivers, but never let it be said that paying attention to the driving
task is a bad thing.
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