<Dd> A is the surface area of the spherical cap, 2πrh, </Dd> <Dd> r is the radius of the sphere, and </Dd> <Dd> sr is the steradian . </Dd> <P> For example, a measurement of the width of a visible object would be given in radians . At the same time its visible area over one's visible field would be given in steradians . Just as the area of a circle is quadratically related to its diameter or radius, so is the area of the piece of a spherical surface cut out by a circular cone related to the angle across the piece . That area, or equivalently the proportion of a sphere its cone takes up, is a measure of three dimensional proportional volume, analogous to the two dimensional measure over the circle represented by an angle . </P>

How many steradian does it take to create a hemisphere