Equations for horizontal radiation/ illuminance under overcast sky

Hello Radiance Group,

I want to right a code which is estimating the illuminance (or ever better
radiation) in horizontal level under overcast sky. Although I have managed
to find similar equations for clear sky, I haven't managed to find any for
an overcast CIE sky.

Can you lead me to any reference that can help me on this research?

Thank you

Sotiris Papantoniou

PhD Candidate

Environmental Engineering Dept.

Technical University of Crete