How is the 'STERADIAN' defined and how is it used?
Asked by: Undisclosed Visitor
Steradians are a measure of the angular 'area' subtended by a two dimensional surface about the
origin in three dimensional space, just as a radian is a measure of the angle subtended by a one
dimensional line about the origin in two dimensional (plane) space. Steradians are equivalently
referred to as 'square radians.'
A sphere subtends 4 pi square radians (steradians) about the origin. By analogy, a circle subtends
2 pi radians about the origin. Numerically, the number of steradians in a sphere is equal to the
surface area of a sphere of unit radius. I.e., area of sphere = 4 pi r^2, but with r = 1, area = 4
pi. Likewise, numerically, the number of radians in a circle is equal to the circumference of a
circle of unit radius. I.e., circumference = 2 pi r, but with r = 1, circumference = 2 pi.
As one would expect, steradians (square radians) can be converted to square degrees by multiplying
by the square of the number of degrees in a radian = 57.2957795... degrees. For example, the
number of square degrees in a sphere is equal to 4 pi x (57.2957795)^2 = 41,253 square degrees
(rounded to the nearest square degree). For those who prefer to work in square degrees, it is
helpful to remember that the number of square degrees in a sphere contains the digits 1 through 5,
with no repeats.
Steradians occur virtually anywhere in physics where a flux through a three dimensional surface is
involved. For example, the ubiquitous factors of 4 pi that keep popping up in formulas derived in
electromagnetics really just represent the scaling, or normalizing, of whatever is being described
to the angular area subtended by a sphere. Not surprisingly, steradians find heavy use in antenna
engineering to characterize such properties as the 'directivity' of an antenna relative to an
'isotropic' radiator (one that radiates uniformly in all directions through the surface of an
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA
'The important thing is not to stop questioning. Curiosity has its own reason for existing. Never lose a holy curiosity.'