LED Insights
Brightness Factors
Understanding the three core factors that affect the brightness of your LED signage
B y J . B r ya n V i n c e n t, P h . D .
Shown here is the V-180
Series LED for shallow
signs. (Photo courtesy
of SloanLED)
J . Bryan Vincent is an
expert in the field of solid
state lighting and electronic
materials and has a Ph.D.
in Chemistry and Materials
Science. He has dedicated the
past 10 years to developing LED solutions specific to the sign industry and is a partner at
Principal LED (www.p-led.com).
W
hen you look at it, there are three
core factors that affect the brightness of your LED signage:
• Sign Depth
• Beam Angle and Sign Dimension
• Signage Materials
And just how bright should it be?
Well, this is a question every sign maker
asks when trying to understand which
LED to use for a particular electric sign
application. The answer is deep, actually.
How deep? Well, it turns out that light as a
function of distance follows what is called
an inverse square relationship (1/R 2).
Let’s put this into sign language. If
you have a single LED module that is
evenly lighting the face of a sign at 1,000
LUX (lumens/m2), and you begin increasing the depth of the sign, the light intensity will drop (see Figure 1) according
to the following relationship or pattern:
• 1" from the sign face = 1,000 LUX
• 2" = 250 LUX
• 3" = 111 LUX
• 4" = 62.5 LUX
Sign Depth
Figure 1: This diagram illustrates the
relationship between
light on the face of a
sign and the distance
from the LED source.
As you can see—depth matters. As
depth increases, the light at the face
begins falling off exponentially. For
example, if you are happy with the light
output in a five-inch deep channel letter,
and your customer says he wants it to
be eight inches instead of five, you will
lose about 2.5 times the amount of LUX
available at the sign face just by increasing the return by three inches. To put it
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However, there is an upside. If you go
from a five-inch deep sign to one that is
three inches, you theoretically need 2.8
times fewer LED s to achieve the same
LUX at the surface. The illustration in
Figure 1 demonstrates this principal
empirically. An LED product backlighting a 7328 White acrylic face was measured to be 1700 LUX at 3.75" depth.
The same product measured 1185 LUX
at five inches from the face. This represents an actual light reduction of 40
percent versus a predicted value of 56
percent according to the inverse square
relationship. The additional 16 percent
difference can be attributed to another
brightness factor based on geometry—
specifically the beam angle of the LED and
the stroke and depth of the can.
Effects of
sign depth on
surface LUX.
(Photo courtesy
of Principal LED)
Beam Angle and Sign Dimensions
Figure 2: Typical
Lambertian profile
of a 120 degree surface mount LED.
Here is an example of a low profile illuminated channel letter. (photo
courtesy of Sign Source, Lima, Ohio)
This is good example of a halo-lit LED sign.
(Photo courtesy of Ad Art Sign Company,
Flower Mound, Texas)
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another way, you would have to put 2.5
times as many LED s in the sign just to
achieve the same light output you had at
the five-inch depth.
I have seen many a sign maker who is
“a penny wise and pound foolish” when
it comes to this situation. They want to
buy the lowest-cost LED s possible, but
by the time the project is over they end
up using four times as many LED s plus
labor costs versus having used just the
right LED with the right output for the
job.
Most surface-mount LED s that are on
many of the LED modules for sign applications have about a 120o Lambertian
beam profile (see Figure 2). This makes
sense, as this gives a pretty even coverage without directing light away from the
face. As a rule of thumb, a white 100-150
lumen-per-foot product that has a 120o
beam angle will provide bright, even
illumination for most channel letter and
single-sided cabinet applications at fiveto six-inch depths.
At the same time, wider beam angle
products can provide more even illumination, but in practical terms this can
be very inefficient due to the increased
distance the light must travel to get to
the face. This is even more dramatic in
systems that use special “batwing” optics
that force even more light laterally. In a
shallow sign, however, light intensity is
not nearly as important.
Remember the relationship between
distance and light output at the face. The
take-home message from this factor is
threefold:
There are options available for shallow cabinets, and many of these products
are worth the extra per-foot cost. They
can reduce the frustration and time trying to make the sign illuminate evenly
and provide good illumination due to
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To help make sure your sign is
bright, evenly lit and within
regulatory guidelines, keep
a hand-held LUX meter in
your shop. (Photo courtesy
of Kimo Industries)
the short distance
to the face.
Using 120o
beam angle products in a shallow application by pushing modules really close together can cause the
sign to be too bright, making the sign
blurry and difficult to read.
Wide beam angle products in standard depth (>5") and larger signs results
in throwing away a lot of light that never
gets to the face, making for a less-efficient sign.
Remember: Don’t pay for light that you
don’t use. Obviously, these are general
rules and there are exceptions where a
wide beam angle product may be needed
for larger letters (i.e. larger box signs
with minimal brightness requirements)
and cases where narrow beam angle
products work for shallower signs (i.e.
single stroke letters with a dark or perforated vinyl face). This brings us to our
third and final brightness factor.
Materials
LED lighting is simply a component
in the overall sign. The type of face and
reflectivity of the inside of the sign can
dramatically impact overall brightness.
White acrylic is the most commonly
used face material in signs. Different
acrylics have very different transmission
properties. For example: 2447 acrylic
lets about 50 percent of the light pass
through, while 7328 blocks about twothirds of the light. This means that a sign
with a 2447 face will be about 50 percent
brighter using the same LED s. However,
a 2447 face can show more hot spots and
looks more “dull” during the day. The
2447 is ideal when a colored vinyl or
graphic is applied over all or part of the
sign face.
Colored faces and vinyl overlays work
like a filter. They only let light through
at a certain wavelength, so it is important
to match the color of the LED to the
face material. Many LED providers, like
SloanLED , have comprehensive compatibility guides that recommend the color
and wavelength of the LED for various
colored acrylics and vinyl.
Perforated vinyl allows
for the sign to be one color
during the day and another
at night. Perforated vinyl
typically blocks an additional
50 percent or more of the total light.
It often makes sense to go to a higherbrightness LED option with this material.
Light diffusing materials are sometimes applied to the backs of faces to
reduce hot spots. These materials have
small particles that cause the light to
bounce around internally. They often
reduce the total light output by as much
as 30 percent.
Internal paint finish will definitely
have an impact on the overall light output. Using a high-reflectance white paint
or prefinished gloss-white aluminum
material is the least expensive way to
maximize light output.
The reflective color of the wall is
very important for halo-lit applications.
Darker surfaces absorb more light and
require brighter LED s to achieve the
desired effect.
Other Factors
Other factors that must be considered when deciding how much LUX is
required relate more to location. These
might include the ambient light around
the building and the distance of the sign
from the observer. Some cities have
codes (although often ill-written) regulating the brightness of illuminated signs,
so being able to measure brightness is
important.
I recommend that every sign shop
keep a hand-held LUX meter handy.
They can be purchased online for about
$40 and are useful tools to make sure
your sign is bright, evenly lit and within
regulatory guidelines. Finally, knowing
your materials and keeping in mind the
relationship between distance and light
output are important factors in creating
an illuminated sign that “wows” your
customer while using the available light
to your maximum benefit. SDG
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