Irradiance (or flux density) is a term of radiometry and is defined as the radiant flux (optical power = energy per unit time) received by some surface per unit area. In the SI system, it is specified in units of W/m2 (watts per square meter).
In the context of laser technology, the common term optical intensity has the same units as the irradiance. It is not the same quantity, however. It is important to realize that the intensity is defined as the amount of energy going through an area perpendicular to the beam, while irradiance refers to what amount of energy arrives on a certain surface. The irradiance caused by a laser beam, for example, which hits a workpiece under some angle α against normal direction, is the beam intensity times cos α. The irradiance is thus generally smaller than the beam intensity.
One should also avoid confusion of optical intensity with the term radiant intensity, which has a different meaning: the radiant flux per unit solid angle.
A related quantity is the spectral irradiance, which is the irradiance per unit frequency or wavelength interval. It has units of W / (m2 Hz) or W / (m2 nm), for example.
The related term radiance essentially means irradiance per unit solid angle, apart from a cos θ factor.
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