The Commission Internationale de l’Éclairage (CIE; International Commission on Illumination) standard CIE2004rcaiman defines 15 pre-calibrated sky luminance distributions, each described by a pair of analytical functions, the gradation function \(\Phi(\theta) = 1 + a \cdot \exp\left(\frac{b}{\cos\theta}\right)\), and the indicatrix function \(f(\chi) = 1 + c \cdot \left[ \exp(d \cdot \chi) - \exp\left(d \cdot \frac{\pi}{2}\right) \right] + e \cdot \cos^2\chi\). Combined, they can predict the relative radiance \(\rho_R\) in any sky direction \((\theta, \phi)\) as: \( \hat{\rho_R}^{\circ}(\theta, \phi) = \frac{\Phi(\theta) \cdot f(\chi(\theta, \phi; \theta_\odot, \phi_\odot))} {\Phi(0) \cdot f(\chi(\theta, \phi; 0, 0))} \), where \(\theta\) is the zenith angle, \(\phi\) is the azimuth angle, and \(\theta_\odot, \phi_\odot\) are the zenith and azimuth of the sun disk.
cie_tabledata.frame with 15 rows and 8 columns:
agradation function parameter.
bgradation function parameter.
cindicatrix function parameter.
dindicatrix function parameter.
eindicatrix function parameter.
indicatrix_groupfactor with six categories and numerical tags.
general_sky_typefactor with three categories: "Overcast",
"Clear", and "Partly cloudy".
descriptionuser-friendly description of the sky type.