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.)