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Unformatted text preview: r
rather than a two radii (the apex of the cone
is at the center). Instead of part of a circumference, the cone subtends part of the
surface area of the sphere. A solid angle
(measured in steradians) is defined as the
ratio of the subtended surface area of the of
sphere to the square of its radius. This
definition applies even if the subtended area
is not circular. There are 4 π steradians in a
sphere since S = 4 π r 2. angle =
1 radian arc length = 1 unit radius
= 1 unit Luminous Intensity
Recall that illuminance is flux per unit area. A related quantity is luminous
intensity, I, which is defined as flux per unit of solid angle. Thus, I = Ø / 4 π,
since there are 4 π steradians in a sphere. You can think of luminous intensity
as the amount of light contained within a cone whose apex is at the source.
The same amount of light confined to a skinnier cone would mean a greater
intensity. Just as the “flat” angle is independent of the size of the circle, the
solid angle is independent of the s...
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- Fall '11