Characterization of composite nanoparticles using an improved light scattering program for coated spheres. The objectives of this paper are twofold. First, the paper developed an improved algorithm to perform light scattering calculations by coated spheres. The improved algorithm was implemented in FORTRAN90 as a subroutine to allow flexible application of the code. Second, the new program was applied to the characterization of composite aluminum nanoparticles. In this application, multiple elements of the Mueller scattering matrix were measured at multiple angles to infer the properties of the nanoparticles, including the size distribution function and the thickness of the coating. The new program played a key role in the fitting of the measured data, and this application demonstrated the advantages of the new program in situations that demand high efficiency and reliability.