The natural objects can either emit radiation (radiance, emittance) or be "illuminated" by a source (irradiance). In the following a series of definitions for each of these terms is provided.
The first basic radiometric quantity is the radiance (Iλ) and it is defined as the ratio of the differential radiant energy (dE) to the product of effective area (dA) with the time interval (dt), wavelength interval (dλ) and differential solid angle (dΩ). Iλ can be also referred as monochromatic intensity and it is expressed in units of energy per area per time per wavelength and per steradian (W m−2 sr−1).
The monochromatic flux density (Fλ) or the monochromatic irradiance of radiant energy is defined by the normal component of Iλ integrated over the entire hemispheric solid angle. It is expressed in units of energy per area per time per wavelength (W m−2). For isotropic radiation (i.e., if the intensity is independent of the direction), the monochromatic flux density is then Fλ = π Iλ.
The total flux density of radiant energy (F), or irradiance, for all wavelengths (energy per area per time, i.e., W), can be obtained by integrating the monochromatic flux density over the entire electromagnetic spectrum.
All the above definitions refer to a point source of radiation. When the flux density or the irradiance is from an emitting surface (i.e., an extended widespread source), the quantity is called the emittance. When expressed in terms of wavelength, it is referred to as the monochromatic emittance. The intensity or the radiance is also called the brightness or luminance (photometric brightness). The total flux from an emitting surface is often called luminosity.