Intensity

With regards to light, intensity is the energy moving through a region of area per unit of time. Intensity is proportional to the light's photon density, meaning the number of photons contained in a square meter. For example, if a beam of light has 10 photons passing through a square centimeter every second, that is twice the intensity of a beam with 5 photons passing through the same area every second.

The amount of energy of any radiation incident upon (or flowing through) unit area, perpendicular to the radiation beam, in unit time. The intensity of thermal radiation is generally expressed in calories per square centimeter per second falling on a given surface at any specific instant. As applied to nuclear radiation, the term intensity is sometimes used, rather loosely, to express the exposure (or dose) rate at a given location.

The square of the electric field strength of an electromagnetic wave. Note: Intensity is proportional to irradiance and may be used in place of the term "irradiance" when only relative values are important.