Dear Salazar, you can take the help from different interpolation model used for solar radiation. Haze & Perez model theory would be help full for you for knowing the optical thickness of cloud .

http://meteonorm.com/images/uploads/downloads/mn71_theory.pdf

NREL- Solar Radiation resource assessment 

Deriving COD from broadband radiation measurements is very difficult. Using spectral data would be easier. Using broadband direct irradiance (DNI) is the most difficult because the cloud transmittance function is an extremely steep function of COD (see plot; red is for DNI, blue for GHI, with two different values of effective radius). Only thin clouds (up to a max COD of about 5) can transmit some DNI anyway. So you cannot use DNI for thick clouds.

AERONET now has a cloud mode--but again this is because of spectral measurements: https://www.researchgate.net/publication/251438524_Cloud_optical_depth_retrievals_from_the_Aerosol_Robotic_Network_%28AERONET%29_cloud_mode_observations

Other spectral methods: doi:10.3390/atmos4010048, doi:10.5194/acp-11-7235-2011, http://dx.doi.org/10.1155/2014/849279

Methods using broadband irradiance:

Development and Evaluation of a Simple Algorithm to Find Cloud Optical

Depth with Emphasis on Thin Ice Clouds, The Open Atmospheric Science Journal, 2008, 2, 46-55.

Inferring Optical Depth of Broken Clouds above Green Vegetation Using Surface Solar Radiometric Measurements, JAS 2001.

Estimating Cloud Optical Depth from Surface Radiometric Observations: Sensitivity to Instrument Noise and Aerosol Contamination, JAS 2005.

Obviously, the answer provided by Ashish is unrelated to your question.

Article Cloud optical depth retrievals from the Aerosol Robotic Netw...

Dear Sir

There are many models for calculation. The cloud optical depth usually depends on some climatological parameters. The interaction with areosol and irradiance are more important.