Edwards Signaling Catalog

Candela Seconds or Candlepower Seconds
This quantity is the actual light energy contained in a pulse of
light. Candela seconds is used by the Society of Automotive
Engineers SAE) and the California Highway Patrol to specify
the minimum requirements for light output from a flashing
light because flash energy has been shown to be a relatively
accurate and fair way of comparing radically different types of
lights such as incandescent rotators and xenon strobe lights.
Candela seconds is merely a relative measure of how bright a
flash of light will appear to a human eye. A light with a higher
candela second rating will appear brighter than a light with a
lower candela second rating even if the lower rated light has
a much higher peak candela rating.
Effective Candela or Effective Candlepower
Effective Candela is based on candela seconds and attempts
to equate the brightness of a flashing light source to the
brightness of a steady burning source. If a flashing light has
a candela effective rating of 100 then it will be visible at the
same distance as a 100 candela steady burning source. The
National Bureau of Standards, the FAA, and the Illuminating
Engineering Society use effective candela in specifying
intensities of flashing light source because this rating is the
most meaningful when it becomes necessary to predict the
visible range of flashing warning lights versus steady burning
light sources.
Please note that the actual perceived light output of a visual
signal depends on a number of interdependent factors which
can vary the light output by a factor of 10 or more for a given
amount of energy per flash.
Some of these factors are:
• Viewing Distance
• Viewing Angle
• Flash Rate
• Pulse Width or Duration
• Ambient Light Conditions
• Chromaticity or Color Saturation
• Lens or LED Light Source Color
• Lens Optics
• Physical shape of Light Source and positioning
relative to Lens (optical coupling)
• Light Source Efficiency
• Voltage Variation
Linear Perspective
When selecting a visual signal it is also important to keep in
mind that as objects are viewed from a greater distance, they
appear smaller because their visual angle decreases. The
visual angle of an object is the angle subtended at the eye by
a triangle with the object at its base. The greater the distance
of the object from the eye, the greater the height of this
triangle, and the less the visual angle. This follows simply from
Euclidean geometry.
You already know this from everyday life: buildings look
smaller as they are further away. So do people. If you know
approximately how big something is (its physical size) -
for example, a person of average height is
usually around 5 or 6 feet tall) and you
observe that person to be a certain
apparent size, you are able to automatically
estimate roughly how far away they are.
The relationship between distance and
apparent height of objects is an
inverse-linear function:
h= a – d
where h is the apparent height,
d is the distance of the object, and
a is the actual size of the object.
For example: A 94PLEDMR120A
Polaris Beacon has an
“Actual Size” of 7.75” high.
One does not have to actually do the
math to realize that regardless of how
bright this beacon actually is, at a great
distance away (1000’), to the viewer, it
would appear a small point of light.
TM
www.edwardssignaling.com 25