In Eq. (1), $L$ describes the radiance reaching the sensor produced by a DIRSIG simulation. This radiance comprised many components that are included within the governing equation Display Formula
$L(\lambda ,\theta v,\phi v,\theta l,\phi l)=El(\lambda )\pi \u2009cos(\theta l)\tau 1(\lambda ,\theta l)\rho (\lambda ,\theta v,\phi v,\theta l,\phi l)\tau 2(\lambda ,\theta v)+Lu(\lambda ,\theta v),$(2)
with the angular orientation of the solar light source represented by zenith angle $\theta l$, and by azimuthal angle $\phi l$. The sensor viewing position is described using $(\theta v,\phi v)$. $El$ defines the irradiance that is emitted from a single light source. The spectral BRDF $\rho (\lambda ,\theta v,\phi v,\theta l,\phi l)$ depends on the directionality of both the sensor and the light source. Transmission of radiation is denoted by $\tau $. The radiation paths to and from the target are considered separately using two transmission terms. Additional upwelling path radiance is represented as $Lu$.