Actinic dose: Quantity obtained by weighing spectrally the
radiation dose using the action spectrum.
Actinic (radiation): The radiation that produces a specified
effect.
Action spectrum (actinic): Efficiency of monochromatic
radiations for producing a specified actinic phenomenon in a
specified system.
Air mass (relative optical): Ratio of the slant optical
thickness, to the vertical optical thickness of the standard
atmosphere.
Albedo: (Definition limited to solar radiation) Reflectance of
solar radiation by the surroundings. This applies to the full
integrated spectrum; the reflectance may depend strongly on
the spectral region.
Blackbody: A body that absorbs all radiant energy incident
on it.
Collimation and angle of terrestrial solar irradiation: The
terrestrial irradiance from the sun is composed of a direct
beam with a collimation angle of approximately 0.5° and a
diffuse component. The spectra and magnitude of each
component changes through the day. Measurement of direct
radiation requires limiting the field of view (FOV). (The
recommended aperturing system limits the input to a slope
half angle of 0.5°, an opening half angle of 2.65°, and a limit
half angle of 4.65°. Measuring the total radiation requires an
instrument with a 180° FOV.)
Daylight: Visible part of global solar radiation.
Diffuse sky radiation: The part of solar radiation which
reaches the earth as a result of being scattered by the air
molecules, aerosol particles, cloud particles, or other
particles.
Direct solar radiation: The part of extraterrestrial solar
radiation which, as a collimated beam, reaches the earth's
surface after selective attenuation by the atmosphere