Edwards Signaling Catalog

the peak intensity of the beam. This measurement is important
in the world of signaling devices where beam intensity is an
important application factor.
Suppose that two LED’s each emit 0.1 lm total in a narrow beam:
One has a 10° solid angle and the other a 5° angle. The 10° LED
has an intensity of 4.2 cd, and the 5° LED an intensity of 16.7 cd.
They both output the same total amount of light, however - 0.1 lm.
A flashlight with a million candela beam may be very bright, but
only within its extremely narrowly focused beam.
Illuminance
Illuminance is the total amount of visible light illuminating, (or
incident upon), a point on a surface from all directions above
the surface. Illuminance is equivalent to irradiance weighted
with the response curve of the human eye. Standard unit for
illuminance is Lux (lx), or lumens per square meter (lm/m 2 ). A
surface will receive one Luxx of illuminance from a point light
source that emits one candela of luminous intensity in its
direction from a distance of 1 m. When using the non-standard
US units, this translates into one footcandle received from a
one candela source one foot away.
Illuminance is measured whenever the light level of a particular
surface has to be specified. For example, these measurements
are required to characterize the light falling on a
projector screen or to design light fixtures in a building.
Two laws of physics that affect illuminance measurements are
the inverse square law and the cosine law. The inverse square
law states that the intensity per unit area of a surface varies
inversely with the square of the distance between the light
source and the detector. Therefore, if illuminance is measured
at a particular distance from a source, it is possible to calculate
the illuminance at other distances.
The cosine law states that the illumination of a surface
decreases as a function of the cosine of the incident angle of
illumination. This happens because, as the angle of illumination
is moved away from the perpendicular to the surface, the
area of illumination increases and the flux density per area
decreases. Shining a flashlight on a piece of paper at different
angles will clearly illustrate this. Illuminance meters use a
cosine diffuser that lights and weighs each ambient source’s
flux density by the cosine of the angle at which it illuminates
the surface, therefore providing cosine corrected results.
Usually, a luminance meter has a lens to restrict the field of
view of the detector. The human eye is the best-known example
of a luminance meter. The unit of luminance is the candela
per square meter (cd/m 2 ) in metric units or the footlambert (fl)
in English units. The conversion factor is 1 cd/m 2 = 0.2919 fl.
A perfectly diffuse source has what is known as a “lambertian”
surface and reflects light in all directions following the
cosine law.
Because of the ways in which light propagates through threedimensional
space– spreading out, becoming concentrated,
reflecting off shiny or matte surfaces– and because light
consists of many different wavelengths, the number of
fundamentally different kinds of light measurement is large,
and so are the units that represent them.
For example, offices are typically "brightly" illuminated by an
array of many recessed fluorescent lights for a combined high
luminous flux. A laser pointer has very low luminous flux (it
could not illuminate a room) but is blindingly bright in one
direction (high luminous intensity in that direction).
Luminance
Luminance, the most commonly measured photometric
quantity, is required whenever it is necessary to know the
apparent brightness of an object or source. Luminance is the
luminous flux emitted from a source per unit solid angle per
unit area in a given direction and is, therefore, the luminous
intensity per unit area. Luminance measurements are
constant, regardless of the distance between the source and
the detector because, as the intensity measured by a detector
decreases with distance, the area of the measuring field
increases proportionately.
TM
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