steradian (sr)

The term solid angle, Steradian (sr), is used in lighting technology and is the reference quantity between the luminous intensity and the luminous flux.

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.