We know that light is electromagnetic radiation advancing in space at the speed of light (c).

Radiometry is the measure of light distribution in space and time.

Since there is a mix-up between the different definitions used to describe light, we shall summarize the main terms and the connections between them.

Radiant Energy (E) - The basic term which describe the amount of energy which is carried by light. Measured in units of Joule [J].

Radiant Power (P) - the amount of radiation energy carried at a unit of time:

(Delta t) is the time (in seconds) of illumination.

Radiant power is measured in units of :

Watt = Joule/sec [W] = [J]/[s]

The mathematical relation between radiation power and power density is:

Power Density (Irradiance) - The radiation power (P) divided by the illuminated area (Delta s).