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Beam Angle
Field Angle
Beam Lumens
Beam Spread
(Term of photometry)
The Beam Angle is the angle between
the two directions opposed to each
other over the beam axis for which the luminous
intensity is half that of the maximum luminous intensity.
The luminous intensities are measured in a plane
normal to the nominal beam centerline. If the beam
is not rotationally symmetric, then the beam angle
is usually given in two planes at 90° of each other,
possibly the maximum and minimum angles. Other angles
(eg. at 45°) may also be given.
The Field Angle is the angle between
the two directions opposed to each
other over the beam axis for which the luminous
intensity is 10% that of the maximum luminous intensity.
Note that in certain fields of applications the
field angle was formerly called beam angle.
The Beam Lumens are the total luminous
flux that is emitted within the region of space, where
the luminous intensity is more than half the maximum
luminous intensity. With most light source, this region
can be described as a cone shaped by all beam angles.
The Beam Spread is a general term, describing
the angle between the two directions opposed to each
other over the beam axis for which the luminous
intensity is a certain fraction of that of the maximum
luminous intensity. The amount of that fraction needs
to be given in each specific case.
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