1. A method for determining a property of a material, the method comprising: illuminating the material with a light beam of controlled spectral and coherence properties, wherein the light beam of controlled spectral and coherence properties originates from a collection of one or more light sources which illuminates the material with multiple wavelengths; recording a stack of speckle field images from speckle fields reflected from the illuminated material in multiple spectral channels, wherein the stack of speckle field images comprises multiple speckle field images each being recorded in a respectively different spectral channel; analyzing statistical properties of the speckle field images in the stack of speckle field images to determine at least one property of the illuminated material, wherein the analysis comprises: estimating a monochromatic speckle field for each of the multiple wavelengths from the stack of speckle field images; adding multiple monochromatic speckle fields to form multiple new speckle fields, each new speckle field having a respectively different coherence length; deriving a polychromatic speckle contrast value for each of the new speckle fields; and estimating a root-mean square (RMS) surface roughness value of the illuminated material by using the polychromatic speckle contrast values corresponding to coherent lengths that are commensurate with the surface roughness.