Power Per Unit Solid Angle

The power (flux) per unit solid angle (sometimes called pointance) is the nearest precise terminology to the common term intensity. It expresses the directionality of the radiated energy and is appropriate for the description of point sources. In the case of radiant power, it is expressed in watts per steradian. For visible light it is expressed in lumens per steradian = candela.

If the intensity ( I = dF/dw ) of a source is the same in all directions, the source is called isotropic. Otherwise, for a flat radiating surface, known as a lambertian, the intensity falls off as the cosine of the observation angle with respect to the surface normal.

Power Per Unit Area Per Unit Solid Angle

The power per unit area per unit solid angle is sometimes called sterance. In the radiant case it is measured in watts/m² steradian and is also called radiance. In the luminous case it is measured in lumens/m² steradian which is equivalent to candela/m² = nit. This quantity is also called luminance.

Point Source vs Extended Source

The Candela

The candela is the foundation unit for the measurement of visible light. It is one of the seven foundation SI units. It's formal definition is:

The candela is abbreviated cd and its standard symbol is I_v. The candela is then used to define the lumen and other quantities used in the measurement of visible light. It is approximately equal to the old unit "candlepower" and is generally taken to be equivalent.

For an isotropic source, the relationship between the candela and lumens is 1 cd = 4p lm and the unit relationships is 1 cd = lm/sr.

Candela per square meter