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Definition of direct and diffuse radiation

Description of direct and diffuse parts of global radiation

Direct horizontal radiation (Bh)

Shortwave radiation (0.2 - 3.0μm) arising from a narrow solid angle (5° aperture) centered around the sun's disk impinging on a horizontal surface.

Direct normal radiation (beam, Bn, DNI)

Shortwave radiation (0.2 - 3.0μm) arising from a narrow solid angle (5° aperture) centered around the sun's disk and impinging on a surface normal to the direction of the radiation.

Diffuse horizontal radiation (Dh)

Shortwave isotropic radiation (0.2 - 3.0μm) arising from the upper hemisphere reduced by the direct solar radiation from the sun's disk and its surroundings (5° aperture)

Equations

  • Gh = Dh + Bh
  • Gh = Dh + Bn * sin(hs), where hs = height of sun (above horizon)