A method of calculating a first parameter of a first sample of a material is provided. The method includes determining a first and second wavelengths at which the material exhibits substantially no absorption; measuring a transmission of the first sample at the first wavelength; measuring a transmission of the first sample at the second wavelength; and calculating the first parameter of the first sample using a first multivariate regression model including first regression coefficients. The first parameter is a parameter which affects a total amount of radiation scattered by the first sample at the first and second wavelengths.