A new dual-channel method for determining cloud optical thickness and cloud particle size is presented. The method is applied to both the experimental measurements of cloud transmittance and also to a synthetic data set derived from the numerical solution of the radiative transfer equation. The results of the validation show that the technique can be, indeed, applied to optically thick clouds. The technique is superior with respect to its speed and flexibility and with respect to existing up-to-date cloud retrieval methods based on the measurements of the transmitted solar light.