Radiant intensity

The radiation intensity ( irradiance also ) is the proportion of total radiation power which is emitted from a light source in a given direction in space in the desired small solid angle:

The power is measured in watts, the solid angle in steradian (sr ), the SI unit of radiation intensity is therefore

The radiation intensity is measured at a long distance to the light source and is obtained as the product of the total radiation output, and the direction of the distribution function of the light source.

In the case of light is called the light intensity as a differential size in space or angle of light intensity as a weighted differential size in the solid angle. The weighting is done with the standard sensitivity curve of the human eye.