Edwards Signaling Catalog

TM
Science of Light
Photometers
Photometric measurements are made with instruments called
photometers. The devices function by collecting light through
some kind of input optics, passing it through a spectral modifying
filter and then measuring the light with a photosensitive
detector. The filter is carefully trimmed to modify the detector
response so that it matches the CIE photopic (or scotopic)
function. The detector converts the incoming light energy into
an electrical signal, which is then amplified and displayed.
Because the filter/detector combination approximates the eye
response, the measured electrical signal is a true measure of
the light as perceived by a human observer.
Note that traditional photometers, because of their inability to
simulate the response of the human eye at the ends of the
visible spectrum generate significantly flawed data when
testing red, blue, and some styles of white LEDs.
Spectroradiometers
Spectroradiometers are another class of instruments that
can be used to perform photometric measurements. A
spectroradiometer measures the radiant flux of light at different
wavelengths and then mathematically multiplies these spectral
values with the CIE-defined photopic values at those
wavelengths. By summing these multiplications at small
wavelength intervals throughout the visible spectrum, a
spectroradiometer accurately calculates photometric quantities.
A good spectroradiometer can offer high accuracy for
measuring any kind of light source, as there are no filters
causing spectral mismatches. These instruments do not
require special calibration factors for measuring narrowband
LEDs, high-intensity discharge lamps, CRT phosphors, laser
projectors, etc.
Proper Selection of
Visual Signaling Devices
General
Selecting the “best ” visual signaling device for any particular
application can be a daunting challenge for many users.
However, the reason behind this complexity is really quite
simple: There is much more to consider than the signaling
device itself. Choosing properly begins with understanding the
reality that we are dealing with a complex system involving not
just the signaling device itself, but light and the way our eyes
“see” it as well as the way our brains process and perceive it.
When comparing two different warning lights, the first question
typically asked is which one is “brighter”? This can be a complicated
question when one is comparing very different light
sources such as rotating incandescent lights, xenon strobes
and variable flash rate LEDs of different colors.
Let’s briefly discuss three different commonly specified
“intensity” ratings:
Peak Candela or Peak Candlepower
Peak Candela can be defined as the maximum light intensity
generated by a flashing light during its light pulse. It indicates
NOTHING ABOUT HOW BRIGHT THE LIGHT APPEARS
TO THE HUMAN EYE. Peak candela alone cannot be used
to directly compare two warning lights. In addition there is no
set multiplication factor for converting peak candela, a unit of
luminous intensity, to either candela seconds or effective candela,
both units of luminous energy. Edwards discourages the
use of peak candela ratings when comparing warning lights.
Perceptual Phenomena
1. Broca-Sulzer Effect
A brief, relatively bright flash of light (optimal flash duration of
0.05 to 0.1 s) is subjectively perceived to be brighter than a
longer flash of greater luminance intensity.
2. Brücke-Bartley Effect
Below the critical flicker frequency (i.e., the frequency where
a flashing light appears constant), the apparent brightness
of a flashing light will gradually increase as the frequency is
reduced and reach a point (approximately 8 to 10 Hz) where it
appears brighter than an uninterrupted light source of
equal luminance.
3. Bloch’s Law
For sufficiently short stimulus durations, detection threshold
decreases inversely with the duration of the stimulus. (Under
about 100 milliseconds stimulus duration it is possible to
exchange the amount of light for the duration and maintain a
constant effect.)
4. Blondel Rey
The product of flash intensity times its duration is equal to the
asymptotic threshold value times the sum of the duration plus
a “visual response time constant”.
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