When talking about light, intensity is the power per unit area when the light is transmitted through a (real or imaginary) flat surface that is perpendicular to the direction of the beam. It is usually represented by the symbol Iand common units are W/m2 or W/cm2. For example, if a laser beam is circular with a radius of 1 mm and has a power of 100 mW, the intensity is:

According to SI standards, the power per unit area is called the irradiance and intensity is a different quantity. However, colloquially, intensity and absolute intensity is also used for irradiance. StellarNet uses the two words interchangeably to mean power per unit area. Because power is a type of flux, this means irradiance is also a flux density because it is divided by unit area.

Note: Whenever a quantity is modified by the word “spectral,” this implies the quantity per wavelength/frequency. For example, the spectral irradiance is the irradiance at each wavelength.