Two sky luminance distributions are given for overcast sky above snow-covered terrain, and directional luminous reflectance data are presented which were obtained under these skies. Luminous reflectances computed from Krinov spectral reflectance data for a variety of natural terrains measured under overcast are also included.
Jacqueline I. Gordon and Peggy V. Church, "Overcast Sky Luminances and Directional Luminous Reflectances of Objects and Backgrounds under Overcast Skies," Appl. Opt. 5, 919-923 (1966)