An overview of the current state of the art in LED Light Bars, focusing on lumens, lux, candella, beam patterns, as well as an in depth analysis of the ideal beam pattern for rally racing.
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What is the goal of auxiliary lighting?
Lumens, Candela, and Lux
How are optics designed?
What is a good light source?
Types of Light Bar Optics
Diode Dynamics Stage Series
Rallying Beam Pattern Analysis
Suggested System Designs
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Auxiliary lighting is any light source used to complement or supplement existing factory-
installed forward lighting.
Auxiliary lighting can be used while racing, exploring back roads, or even driving on
highways (as allowed by applicable regulations).
The goal of auxiliary lighting is to adequately illuminate the forward vision in such a way
that driver confidence and comfort is increased during night-time driving, while reducing
driver stress and uncertainty.
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Driver confidence is increased by providing a
higher amount of directed light in areas ahead
of the vehicle, increasing time for driver
analysis.
Driver comfort can be increased if distribution
of light is smooth, and if distractions are
removed:
◦ by avoiding hot spots, or intense
concentrations of light;
◦ by reducing the amount of glare, or
unwanted light illuminating areas
unimportant to the task at hand: the vehicle
hood, treetops, and the ground immediately
in front of the vehicle.
Example of a system with no additional glare where needed (extreme left and
right of vehicle)
Noticeable hot spots of intense light
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What is a lumen?
◦ A lumen is the unit of luminous flux, or total
quantity of light emitted by a source in all
directions.
◦ In an automotive forward lighting application, it
is desirable to have focused light – not just
omnidirectional illumination provided by a
filament or LED.
◦ Lumens are a useful measurement for a
standard Edison light bulb in your home,
radiating in all directions, but in automotive
terms, this is only the potential amount of
useful light from a light source, since it must be
directed and focused to be useful in an
automotive setting.
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Why are lumen ratings used for auxiliary lamps?
◦ LED chip manufacturers rate their LEDs on a datasheet, and include a theoretical
maximum output, in lumens. It is practically impossible to reach these theoretical
maximums in prolonged use, but these datapoints are easily accessible.
◦ Most manufacturers of LED lamps do not possess the costly test equipment
necessary to directly measure lumen output. So instead, this “maximum lumen”
figure, multiplied by total LEDs used in the lamp, serves as a convenient datapoint to
provide to customers for reference.
◦ Because of thermal and electrical limitations, as well as optical inefficiencies, this
datapoint is rarely the actual measured output from the device – this is why you may
see terms such as “raw lumens” or “rated lumens.”
LED datasheet examples
Image: cree.com
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What’s the problem with lumens?
◦ No automotive testing procedure calls for lumens – because it should not be used as
a metric for evaluation! Whether it’s a headlamp, tail light, or auxiliary lamp, it doesn’t
matter how much total light is being emitted, if the light is not directed where it is
needed.
◦ Overall, this is easily the most misunderstood concept in aftermarket lighting. More
lumens DOES NOT equal a better product.
◦ However, again, it’s a convenient term to use, and easy to sell because “bigger =
better.” But the biggest tool is not always the best one, especially when it is not used
with a clear goal and focus.
Equal lumens, but not equal output
Image: dmlights.com
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The candela (cd) is the unit of luminous intensity. Intensity refers to the total light emitting
from a source in a specific direction.
Candela is the unit most often used in lighting design, test standards, and regulations,
since it indicates total light required, shining in a specific direction, independent of
distance. This is very difficult to directly measure easily by the average consumer in the
real world. Instead we use lux.
Candela is not dependent on distance.Candela is total light shining at
a specific angle.
Images: dmlights.com
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Lux is the unit of illuminance. It is mathematically related to Candela as follows:
1 lux = 1 lumen distributed over one square meter
For our purposes, lux = how much light is hitting a specific spot down the road.
Normally, lux measurements are supplemented by a distance. For example, one might
measure “500 lux @ 10 meters.” This indicates that the light source 10 meters away
shines 500 lux on a specific point where it was measured.
Lux = Candela(Cd) /distance(m)
2
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The relationship of distance and lux is not linear. It is exponential.
Double the lux at one point does not mean double the distance that you can see.
◦ Therefore a light that is rated “twice as bright” (let’s say 800 lux vs 400 lux) does NOT mean that
you’ll see 100% farther, in reality you’ll only see 40% further.
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Luxvs Distance
Real World Exponential Relationship Natural Assumption
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Lux is easily measured through inexpensive handheld tools (a light meter).
However, hand tools should only be used for quick comparisons.
Stray light and other variables make it impractical to compare two lux
readings taken by two different people in two different locations, unless
using extremely high-cost and highly-calibrated equipment.
Lux measurements are the best and easiest way for a consumer to test a
lighting system’s performance.
Again, any lux measurement should be accompanied by the distance from
the light source, and where the measurement is in the pattern (usually
“max” or “peak”).
How much is needed?
◦ 1 Lux is generally defined as the
amount of light required to spot an
obstacle at night. To give a better
sense of scale, here are some
common levels of illuminance:
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An isolux chart attempts to provide a bird’s-eye view of the expected distribution of light.
The distribution area of specific minimum lux values are plotted against distance.
Some manufacturers include 0.25 lux plots to provide extraordinary distance figures. However, this level
of light is barely perceivable by the human eye while driving – imagine driving down the highway at
night by just the moonlight!
We include these numbers in our data solely as a gauge against the data provided by other
manufacturers. We strongly recommend focusing on 1 lux readings at minimum.
Isolux chart source: Grote.com
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Each lux measurement is only for one specific point. It’s possible to have an extremely high-lux
“peak intensity” figure at one point, but the beam pattern may not be useful if it is a tightly-focused
“spot” pattern, creating a distracting “hot spot” where the intensity is concentrated. Conversely, you
may have a very low lux measurement, but it fills the entire viewing area completely- this may be
useful as a flood lamp when not driving. Therefore, it’s not about “peak lux,” either. The consumer
must weigh their needs vs. the total lux distribution.
In the example below, a simulation is run with the exact same source and total lumen potential, with
a spot pattern optic. On the right, a widening secondary optic was added, to spread the original
concentrated output pattern. The result indicates almost a direct relationship: the spot is almost
exactly 2.4 times higher peak lux than the widened beam pattern. The consumer must decide: is this
more useful?
24° Beam Pattern
Peak Lux = 490
10° Beam Pattern
Peak Lux = 1192
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Almost all modern, high-performance optical systems are designed using computer-aided
engineering software such as OPTIS, Lucidshape, TracePro, and others. Even with knowledge and
understanding of lighting and optical design theory, these software tools provide an optical designer
the ability to explore new designs without lengthy and costly physical prototyping processes.
Designing a new optic is an extremely iterative process. A light source is set, and “rayfiles” are
created to simulate light rays coming from that source. Geometry is designed to develop the optic,
the system is simulated by “ray-tracing,” and geometry is tweaked over and over until the simulations
provide the desired result in output.
One single LED may have a simulated source of over 5-10 million individual light rays.
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When designing optics, the perfect light source is a “point.” The goal is to focus the light –emission
from a small point makes it easier to do so. For example, the filament from an incandescent bulb or
the arc from a gas discharge bulb are not ideal sources.
The closer the source is to a pinpoint, the easier it is to control and redirect the light. This is why
lasers are considered extremely good light sources, and are being used as auxiliary long-distance
high beams in some vehicles today.
When selecting a light source, designers focus on “lumen density”, or how many lumens per square
millimeter is emitted from a source. The higher the density, the more intense the source is, and the
lumens emitting from that small area can be better controlled.
Luxeon ZES
CREE XHP35 Luxeon TX
Luxeon M
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On modern LED auxiliary lights known as light bars, there are three
common types of optics in current use:
◦ Parabolic Reflector (“2D”)
◦ Lens (“4D”)
◦ Parabolic Reflector with Lens (“5D”)
◦ Segmented Reflector
◦ Total Internal Reflector (TIR)
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This is arguably the most common type of optic used. It is simple to
design and simple to manufacture. It is a piece of inject-molded
plastic that is metalized to create a mirror surface. The depth of the
reflector is critical to its optical efficiency.
The deeper a reflector, the more light that can be captured and
redirected. The shallower it is, the more light escapes and is lost as
either a designed flood pattern, or as unwanted glare.
The deeper a reflector, the more powerful the beam will be.
25mm Deep Reflector, CREE LED source
12mm Deep Reflector, CREE LED source
Peak of 17,000 Cd Peak of 8,000 Cd
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This style of optic was introduced circa 2013. It allows for less
glare, a “tighter” more focused beam pattern, and is
cosmetically attractive. The downside, however, is a loss of
optical efficiency since a majority of the light is lost within the
housing, and not redirected through the lens.
25mm Diameter, 12.5mm standoff,
CREE LED source
Peak of 8,115 Cd
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The most recent light bar design, widely available from Asian
manufacturers. Combination of a parabolic reflector and lens.
Light that is bouncing off the parabolic reflector is redirected
through the lens, thus scattered in an unintentional direction,
creating significant excess glare.
In theory this can be executed successfully, but requires
careful engineering of the size and location of the lens – not
simple placement on the top of the reflector.
25mm Diameter, 12mm standoff w/
CREE chip (Auxbeam 5D)
Peak of 8,500 Cd
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This style of light bar is not as common, although a few high end
brands do use this method (Lazer Lamps, PIAA LED). It is also
employed in a number of factory-installed fog lamps.
Reflector designs offer flexibility in design and beam pattern shaping,
but there are significant optical efficiency problems. Many rays from
the source that are lost, since they cannot all be collected and
redirected. There are also limitations in size, similar to the “2D”
reflector design. In order for the reflector to be useful, it must be quite
large.
In order to match a TIR-style optic, the design must either become
very large, which comes with a weight penalty, or reflector optics
must be reduced in size, making it optically inefficient compared to
other methods.
Peak of 27,420 Cd
Reflector size: 50mm x 50mm x 52mm
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TIR optics use the optical phenomenon of Total Internal Reflection, in
which light is internally reflected off an optically polished surface at such
an angle that light does not escape, and instead bounces away at the
same angle it entered.
This type of reflector is highly efficient, and like the reflector type optic, is heavily dependent on the
size of the optic: the larger, the better.
The key advantage of a TIR optic design is that all of the light rays are collected and redirected.
There is extremely little light lost and unused.
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A small, intense point source is critical for creating an optimal TIR optic. The reason for this can be
visualized in the diagrams shown. When a TIR is designed, it is developed around a point source.
The reason for this is that the surface must be designed so that all the rays hit it at less than the
“critical angle” (as shown on the previous page).
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If a larger source is used, rays from corners of the emitting surface won’t be exactly where the
theoretical focal center is, so rays hitting the surface will not go in the exact direction as intended.
Blue rays in this diagram represent the rays coming from the outer edge of a large source surface.
In one analogy – you can line up a mirror perfectly to view something on the other side of a room.
But take one step to the side, and you’ll be completely unable to see that object. You must stay
perfectly in focus, with all angles perfectly aligned to a single point.
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Small point sources make for a very intense spot pattern with a TIR optic. These can become so
intense that a secondary optic or optical surface is needed to create a useable beam pattern. This
allows designers the flexibility to shape a beam pattern exactly as desired, instead of trying to adjust
surfaces on a reflector, or combinations of lenses.
Note below: nearly half the potential lumen output translates into over three times the peak intensity.
30mm Diameter
TIR, CREE LED
@ 600 lumens
(Large source)
Peak of 17,250 Cd
30mm Diameter
TIR, Lumileds
LED source
@ 350 lumens
(Small source)
Peak of 58,616 Cd
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After reviewing potential solutions for optics, we elected to use a TIR optic in our design.
TIRs offer high optical efficiency, compact packaging, and no wasted light.
Universal TIRs are available through catalogs, but they are not system-optimized.
The only way to achieve the highest possible optical efficiency is to design a custom TIR around the
source point and application. As mentioned earlier, when designing an optical system it is a very
iterative process. Hundreds of combinations of inner diameter, thickness, draft angle, radius,
sources, outer diameter, and several other variables were attempted and simulated.
The result is our patent-pending TIR lens, with a driving beam pattern built into the optic.
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Currently, only a handful of optic patterns are offered on the market. These include a very tight and
intense Spot (or hyperspot), a wide Flood pattern, or a “combo” of both spot and flood.
In our informed opinion, basic spot and flood patterns are not ideal solutions. Flood optics produce
excess glare. Tight spot patterns create distracting hotspots, and do not provide adequate coverage.
OEM forward lighting solutions found on most vehicles are generally adequate in providing
foreground illumination. A combination of SAE-standard fog lamps, low beams, and high beam lamp
patterns can fill the foreground.
To an average consumer, the greatest potential benefit of light is enhancing the high beam function.
Consumers gain comfort and confidence by being able to see objects from a distance, providing
more time to react.
Rather than a Spot or Flood, the Stage Series Light Bar has a “Driving” pattern.
Diode Dynamics, “Driving”
Rigid Industries, “Spot”
Fixed camera settings, 10 meters
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We define our Driving pattern as 20° wide, and 8° tall.
A wider pattern was chosen specifically to meet realistic needs. Almost no roads in the world are
dead straight and level for mile.
Even with the slightest turn, a very gradual bend in the road, a typical 5° “spot” beam pattern means
that the driver cannot see the road clearly in the distance – it will only illuminate the side of the road,
becoming a distraction.
With the Stage Series Driving pattern, the illuminance is strong enough off-center, across the Driving
pattern, to continue illuminating the roadway.
Stage Series
Beam Pattern
Bird’s Eye View of a Roadway Road centerline with 1000m radius
Typical 5° Spot
Pattern
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Lighting is very dynamic, especially in racing. There may be racetracks or rally stages where a driver
needs extreme long distance output, with no tight corners. Conversely, there may be the opposite -
where a driver needs lots of light to fill tight corners. This is especially true in staged rally racing:
needs may change from stage to stage! To meet this need, we also developed clip-on secondary
lenses, to shape the beam pattern exactly as needed for each stage. They are designed to work with
the primary Driving optic.
No tools are required or adjustment is needed. Lenses can be stored in the vehicle and kept handy
for when the need arises. Simply snap a lens on to achieve the desired result.
Diffuse
Wide
Vertical Mount
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Rally racing has unique situations where varying terrain, turns, and elevation are encountered.
Rarely are there extremely long straights that require a high-power spot lamp, nor are there often
such slow speeds that a full flood lamp is needed.
We hope to show that “combo” light bars, or combinations of spot and flood, are not ideal for rally
racing.
We will focus on developing a complete light system that allows for the driver to feel comfortable
driving at speed, as well as reduce the amount of fatigue and stress that comes with driving at night.
Ideal Rally Beam Pattern Isolux (1 lux)
(USA Jemba Note System)
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The beam pattern can be split into several segments that correspond with the traditional Jemba Note system used in
North American Rallying.
Through CAD analysis, we determined the horizontal angle that corresponds with a turn degree (see previous page).
“Right 6,” for example, can be illuminated easily from 0° to 6°, a “Right 5” from 6° to 15°, etc.
The ideal beam distance for the fastest portions of a rally race (straights, 6’s, and 5’s) was determined based on the
distance required for the driver to have 10 seconds time from recognizing a object (1 lux), to arriving at it while
traveling 135mph on a straightaway.
It was then determined that 600 meters was the ideal peak distance needed for a top-level car. Cars that struggle to
hit 100mph will only need a peak distance of about 425 meters.
R5 R4 R3 R2L5L4L3L2 L6 R6
6° 15° 24° 30° 40°-6°-15°-24°-30°-40°
CAD Simulation of a
Right 6 at eye level
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Another important topic for rally racing applications is avoiding distraction. Drivers can easily overpower their vision
with too much foreground light, up close. It is important to have tight corners and hairpins illuminated, however this
illumination cannot be too strong, or else a driver will effectively ruin their “contrast ratio” of vision.
Human eyesight is similar to a camera, where the brain is constant re-adjusting total exposure, to “normalize” vision
depending on the surrounding light. For example, when leaving a movie theater during the day, the daylight is
incredibly intense since the eyes have adjusted to low light levels inside a dark room.
So what does this have to do with racing and lighting? Essentially, we want to develop a lighting system so that the
absolute brightest section of the beam pattern is filling the extreme fast sections, while the peripheral vision has just
enough light that a driver can pick up details – but, not so bright that your eye is having to compensate and “re-adjust”
overall exposure, which gives the sensation that you cannot see as far.
When the lighting system is a smooth gradient of light from center to the edge of your vision, your mental workload is
reduced, and it makes driving at night less stressful, which results in faster stage speeds!
R5 R4 R3 R2L5L4L3L2 L6 R6
6° 15° 24° 30° 40°-6°-15°-24°-30°-40°
CAD Simulation of a Right 4 at eye level
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The following slides include several suggestions for potential system designs.
All systems are designed around an 80° Field Of View (FOV) horizontally, and 15° vertically. This is
essentially what a driver can see when strapped down inside a car. Anything beyond that is wasted
light.
These are complete systems that will allow a driver to attack rally stages with confidence, being able
to slide through fast 4’s and 5’s without losing sight of their apex, but also not lighting up the side of
the road too much.
Balance is the key with a well developed lighting system. Peak distance isn’t everything. A solid and
even band of light is ideal, as the road is never perfectly straight.
These designs have clip-on lenses in mind where applicable (available April 2017)
There are several variations:
◦ Entry Level ($340)
◦ Entry Level Flexible ($500)
◦ Entry Level Flexible for TMIC ($580)
◦ Clubman ($700)
◦ Clubman Alternative ($660)
◦ Professional ($980)
NOTE: All Intensity Graphs are normalized to 200,000 candela for easier comparison.
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One SS30 White Light Bar
Two “Wide” Clip-on Lenses
$340 retail price
Peak Lux at 10m = 1770
Distance @ 1 lux = 421m or 1380 ft
Weight: 5.3 pounds
The Extreme Wide diffusing lenses offer a wide beam to light up tight corners
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One SS30 White Light Bar
Two SS6 White Light bar (aimed 12.5° outwards)
Two “Wide” Clip-on Lenses
$500 retail price
Peak Lux at 10m = 1910
Distance @ 1 lux = 437m or 1,433 ft
Weight: 7.9 pounds
Added SS6 light bars help fill the “mid-range” to make the light gradient better, as well as adding to
the peak intensity in the middle.
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Four SS12 White Light Bars
Two “Wide Diffusing” Clip-on Lenses
$580 retail price
Lower lamps aimed 2.5° outwards
Upper lamps aimed 12.5° outwards
Weight: 8.8 pounds
Peak Lux at 10m = 2211
Distance @ 1 lux = 470m or 1,542 ft
Leaves center of the hood open for cars with Top Mount Intercooler (WRX & STI)
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Two SS12 White Light Bars
Two SS18 White Light Bars
Two “Wide Diffusing” Clip-on Lenses
$700 retail price
Lower lamps aimed 5.5° outwards
Upper lamps aimed 20° outwards
Weight: 10.8 pounds
Peak Lux at 10m = 2600
Distance @ 1 lux = 510m or 1,672 ft
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Two SS18 White Light Bars
One SS30 White Light Bar
Two “Wide” Clip-on Lenses
$760 retail price
Upper lamps aimed 12° outwards
Weight: 11.7 pounds
Peak Lux at 10m = 3150
Distance @ 1 lux = 561m or 1,841 ft
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Four SS18 White Light Bars
One SS12 White Light Bar
Two “Wide” Clip-on Lenses
$980 retail price
Upper lamps aimed 12° outwards
Lower lamps aimed 5.5° outward
Weight: 15.0 pounds
Peak Lux at 10m = 3989
Distance @ 1 lux = 632m or 2,074 ft
