Fraunhofer lines in the context of Joseph von Fraunhofer

Diffuse sky radiation, is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, the determinative process for changing the colors of the sky. It is normally measured on a horizontal surface, thus frequently termed diffuse horizontal irradiance (DHI), often in the unit of watts per square meter (W/m). Approximately 23% of direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere; of this amount (of incident radiation) about two-thirds ultimately reaches the earth as photon diffused skylight radiation.

The dominant radiative scattering processes in the atmosphere are Rayleigh scattering and Mie scattering; they are elastic, meaning that a photon of light can be deviated from its path without being absorbed and without changing wavelength.

Lidar (/ˈlaɪdɑːr/, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranges by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. Lidar may operate in a fixed direction (e.g., vertical) or it may scan directions, in a special combination of 3D scanning and laser scanning.

Lidar has terrestrial, airborne, and mobile uses. It is commonly used to make high-resolution maps, with applications in surveying, geodesy, geomatics, archaeology, geography, geology, geomorphology, seismology, forestry, atmospheric physics, laser guidance, airborne laser swathe mapping (ALSM), and laser altimetry. It is used to make digital 3-D representations of areas on the Earth's surface and ocean bottom of the intertidal and near coastal zone by varying the wavelength of light. It has also been increasingly used in control and navigation for autonomous cars and for the helicopter Ingenuity on its record-setting flights over the terrain of Mars. Lidar has since been used extensively for atmospheric research and meteorology. Lidar instruments fitted to aircraft and satellites carry out surveying and mapping – a recent example being the U.S. Geological Survey Experimental Advanced Airborne Research Lidar. NASA has identified lidar as a key technology for enabling autonomous precision safe landing of future robotic and crewed lunar-landing vehicles.

Starfire Optical Range (SOR - Pronounced as an initialism) is a United States Air Force research laboratory on the Kirtland Air Force Base in Albuquerque, New Mexico. Its primary duty, according to the official website, is to "develop and demonstrate optical wavefront control technologies." The range is a secure lab facility and is a division of the Directed Energy Directorate of the Air Force Research Laboratory.

SOR's optical equipment includes a 3.5 meter telescope which is "one of the largest telescopes in the world equipped with adaptive optics designed for satellite tracking" according to the Air Force, a 1.5 meter telescope, and a 1-meter beam director.

The sodium layer is a layer of neutral atoms of sodium within Earth's mesosphere. This layer usually lies within an altitude range of 80–105 km (50–65 mi) above sea level and has a depth of about 5 km (3.1 mi). The sodium comes from the ablation of meteors. Atmospheric sodium below this layer is normally chemically bound in compounds such as sodium oxide, while the sodium atoms above the layer tend to be ionized.

The density varies with season; the average column density (the number of atoms per unit area above any point on the Earth's surface) is roughly 4 billion sodium atoms/cm. For a typical thickness of 5 km this corresponds to volume density of roughly 8000 sodium atoms/cm.