Edwards Signaling Catalog

TM
Science of Light
Luminous flux
Typically, manufacturers measure the total lumen output by
placing a photometer at the exit port of an integrating sphere.
The light source is placed inside the integrating sphere – a
highly reflective sphere with a white, spectrally nonselective
coating. These devices also are used to measure directional
sources such as laser beams and light-emitting diodes (LEDs).
Figure 2. The steradian is the solid angle at the center
of a sphere that subtends a surface area of r 2 .
The fundamental quantity used in photometry is luminous flux,
which is the visible light energy per second radiated by a
source or amount of photometric energy traveling through
space in a given time interval. The lumen is the unit of
luminous flux, and 1 W of radiant flux at the peak photopic
wavelength of 555 nm is equivalent to a luminous flux of 683
lumens. If the radiant flux (power) of a polychromatic light
source at every wavelength of the visible spectrum from the
blue to the red end is known, the luminous flux can be calculated
by mathematically integrating the power values with the
CIE-defined photopic value for each wavelength.
For historical reasons, the candela – the unit of luminous intensity
– takes precedence over the lumen. The candela originally
was derived from the light emitted by a candle; its current definition
is the luminous intensity, in a given direction, of a source
that emits monochromatic radiation of frequency 540 × 10 12 Hz
and that has a radiant intensity in that direction of 1/683 W per
steradian (540 × 10 12 Hz is equivalent to 555 nm). The lumen
is the luminous flux, emitted per unit solid angle from a point
source whose luminous intensity is one candela. The unit of
solid angle is the steradian (Figure 2), and because the
surface area subtended by a steradian is r 2 , a sphere with a
surface area of 4πr must have 4π steradians. Therefore, the
total luminous flux emitted from a one-candela point source is
4π lumens.
Luminous intensity
Luminous intensity, or candlepower, is the luminous flux per
unit solid angle emitted by a uniform point source of light. This
is the quantity used to measure the output of point (small)
sources such as an LED. The unit of luminous intensity is the
candela, and it is given in lumens per steradian.
It is important to note that in many practical applications, point
sources are directional and do not radiate uniformly. Therefore,
candlepower is measured in a specific direction over a small
collection angle along the axis of radiation. LED receptors,
which use the technique to measure LEDs in millicandelas,
consist of tubes with baffles that have a specific collections
geometry (2° to 15° are common). The CIE has recommended
in CIE 127 that LEDs be measured using 2° and 6.5°
geometry, and that these two readings are averaged to yield
"Averaged LED Intensity." A typical LED receptor is a tube in
which one end is attached to the light measuring device and
the other end receives the LED. The collection geometry is
defined by a field stop in the tube. The receptor is then calibrated
using a source of known luminous intensity.
Flashing lights are also commonly used in many signaling
applications. The visibility or conspicuity of flashing lights
varies depending on the duration and waveform of flashes for
the same physical energy and spectrum of the flashes. To
take into account such visual effects, the term, effective
intensity, is used to specify the intensity of flashing lights for
signaling applications. Effective intensity is defined as the
luminous intensity (cd) of a steady light source that would have
the same luminous range (or visual range) as the flashing
lights in question.
An additional source of potential confusion regarding intensity
measurements involves the difference between Mean
Spherical Candela and Beam Candela, both of which use the
candela unit (lumens per steradian). Mean spherical measurements
are made in an integrating sphere, and represent the
total output in lumens divided by 4πsr in a sphere. Thus, a one
candela isotropic lamp produces one lumen per steradian.
Beam candela, on the other hand, samples a very narrow
angle and is only representative of the lumens per steradian at
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