Considering the porous nature of the electrodeposited Cu nanoparticle film and the large dimensional difference between the nanoparticles and thermal radiation wavelength, we adopt the effective medium theory (EMT) to fit the experimental results and obtain the relationship between the emissivity and the Cu film thickness with different volume fractions (Figure ). The effective permittivity ε eff of the porous Cu film is calculated by Bruggeman's EMT (eq. 1):( )