The area that the solid angle cuts out of a unit sphere serves as a measure. If the solid angle is 2Pi, then the area corresponds to a hemisphere. The steradian (sr) in this example is `1/(4pi)`. The solid angle of a whole sphere surface represents with 360 ° the largest solid angle and amounts to 4Pi, which corresponds to 12.566 sr.
The solid angle plays a decisive role in lighting elements. This is the case for low- voltage lamps, light-emitting diodes and power LEDs. 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.