The term solid angle, Steradian (sr), is used in lighting technology and is the reference quantity between luminous intensity and luminous fl
ux. The area that the solid angle cuts out of a unit sphere is used as a measure. If the solid angle is 2Pi, then the area corresponds to a hemisphere. The steradian (sr) in this example is 1/(4 x pi).
The solidangle of an entire spherical surface is 360 °, the largest solid angle, and is 4Pi, which corresponds to 12.566 sr.
The solid angle plays a decisive role in lighting elements. A solid angle of one steradian (1sr) corresponds to an aperture angle of 65.54° and with a sphere radius of 1 m to a sphere surface area of 1 sqm. With a beam angle of 15° the solid angle is 0.54 sr.