Irradiance (or flux density) is a term of radiometry and is defined as the radiant flux received by some surface per unit area. In the SI system, it is specified in units of W/m2 (watts per square meter). Irradiance may be applied to light or other kinds of radiation.
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 with a given orientation. The irradiance caused by a laser beam, for example, which hits a workpiece under some angle <$\theta$> against normal direction, is the beam intensity times <$\cos \theta$>. The numerical value of the irradiance is thus generally smaller than that of the beam intensity. Irradiance may be caused by a combination of multiple light sources.
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 <$\cos \theta$> factor.
Measurement of Optical Irradiance
Irradiance values can be measured with suitable photodetectors. For accurate measurements, they should have the following properties:
- Within the relevant spectral region, the responsivity should have a low wavelength dependence. For a photon detector such as a photodiode, that implies that the quantum efficiency must be inversely proportional to the optical wavelength.
- The detector should have the correct angular dependence of its sensitivity. This means that incident photons should contribute to the measurement signal with no dependence on their incidence angle.
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