Box M A, Deepak A
Appl Opt. 1979 Jun 15;18(12):1941-9. doi: 10.1364/AO.18.001941.
Whenever a solar radiometer is used to measure direct solar radiation, some diffuse sky radiation invariably enters the detector's field of view along with the direct beam. Therefore, the atmospheric optical depth obtained by the use of Bouguer's transmission law (also called Beer-Lambert's law), that is valid only for direct radiation, needs to be corrected by taking account of the scattered radiation. In this paper we shall discuss the correction factors needed to account for the diffuse (i.e., singly and multiply scattered) radiation and the algorithms developed for retrieving aerosol size distribution from such measurements. For a radiometer with a small field of view (half-cone angle < 5 degrees ) and relatively clear skies (optical depths < 0.4), it is shown that the total diffuse contribution represents approximately 1% of the total intensity. It is assumed here that the main contributions to the diffuse radiation within the detector's view cone are due to single scattering by molecules and aerosols and multiple scattering by molecules alone, aerosol multiple scattering contributions being treated as negligibly small. The theory and the numerical results discussed in this paper will be helpful not only in making corrections to the measured optical depth data but also in designing improved solar radiometers.
每当使用太阳辐射计测量太阳直接辐射时，总会有一些漫射天空辐射与直接光束一起进入探测器的视场。因此，利用仅适用于直接辐射的布格传输定律（也称为比尔-朗伯定律）获得的大气光学厚度，需要通过考虑散射辐射来进行校正。在本文中，我们将讨论考虑漫射（即单次和多次散射）辐射所需的校正因子，以及为从此类测量中反演气溶胶粒径分布而开发的算法。对于视场较小（半锥角<5度）且天空相对晴朗（光学厚度<0.4）的辐射计，结果表明，总漫射贡献约占总强度的1%。这里假设，探测器视锥内漫射辐射的主要贡献来自分子和气溶胶的单次散射以及仅分子的多次散射，气溶胶多次散射贡献被视为可忽略不计。本文讨论的理论和数值结果不仅有助于对测量的光学厚度数据进行校正，也有助于设计改进的太阳辐射计。
