Radiometrics

Radiometrics is the science of measuring electromagnetic radiation across the spectrum. It deals with the quantification of radiant energy emitted by sources, as well as energy reflected or transmitted by matter. Radiometrics covers wavelengths from radio waves to gamma rays and is foundational to remote sensing, astronomy, and various industrial and scientific fields. It differs from photometry, which is concerned with perceived brightness by the human eye, by using absolute physical units and radiometric quantities.

Key quantities include radiant flux (power, measured in watts), irradiance (watts per square meter), radiance (watts per square meter per steradian), and spectral radiance (watts per square meter per steradian per nanometer). Quantities can be resolved in space, angle, and wavelength using instruments called radiometers and spectroradiometers. Radiometric measurements require careful calibration against standards such as blackbody radiators, integrating spheres, and calibrated lamps, with on-orbit and vicarious calibration in satellite remote sensing.

Applications: In remote sensing, radiometry enables estimation of surface reflectance and thermal emissions, leading to maps of land cover, vegetation health, and temperature. In astronomy, radiometric data characterize the energy output of stars, galaxies, and other sources. In atmospheric science, radiometry aids in monitoring solar radiation, cloud properties, and atmospheric composition. Industrial process monitoring and nuclear safeguards also rely on radiometric measurements to quantify radiation fields and emitted energy.