1.6 Definition of Radiometry

In remote sensing, electro-magnetic energy reflected or emitted from objects is measured. The measurement is based on either radiometry or photometry, with different technical terms and physical units.

Radiometry is used for physical measurement of a wide range of radiation from x-ray to radio wave, while photometry corresponds to the human perception of visible light based on the human eye's sensitivity as shown in Figure 1.6.1.

Figure 1.6.1shows the rdoiometric definitions of radiant energy, radiant fiux, radiant intensity, irradiance, raiant emittance and radiance.

Table 1.6.1 show the comparision with respect to the techical terms, symbols and units between radiometry and photometry.

One can add an adjective "Spectral" before the technical terms of radiometry when defined as per unit of wavelength. For example, one can use spectral radiant flux ( W m) or spectral radiance (Wm sr m ).

Radiant energy is defined as the energy carried by electro- magnetic radiation and expressed in the unit of joule (J).

Radiant flux is radiant energy transmitted as a radial direction per unit time and expressed in a unit of watt (W). Radiant intensity is radiant flux radiated from a point source per unit solid angle in a radiant direction and expressed in the unit of Wsr. Irradiance is radiant flux incident upon a surface per unit area and expressed in the unit of Wm. Radiant emittance is radiant flux radiated from a surface per unit area, and expressed in a unit of Wm. Radiance is radiant intensity per unit projected area in a radial direction and expressed in the unit of Wm sg.