Simple approximate relations are given for the reflectance of a weakly absorbing dielectric layer deposited as a quarter wave or a half-wave upon transparent and metallic substrates. These approximated relations are used in the infrared to calculate the optical constants n and k and the inhomogeneity ∂n of the index n of a material deposited as a thin film. A high degree of accuracy is sought for k. Two examples are given for ZnS and ThF4 in the 3–11-μm spectral range.
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Measured and calculated values of extrema of reflectances obtained with a 3λ/4 layer at λ = 10.6 μm deposited on molybdenum (Rλ/2e) and ZnSe (Rλ/4e and Rinhe). The calculations were obtained from values of n, k, and ∂n given by relation (17). The reflectance of the bare metallic substrate is supposed to be R0 = 0.9820 whatever the wavelength, and the accuracy in the measurement of R is estimated to be 0.05%.
Table II
Optical Constants of a ZnS Coating Deposited by Evaporation in Vacuuma
Measured and calculated values of extrema of reflectances obtained with a λ/2 coating at λ = 10.6 μm on molybdenum and ZnSe (Rλ/2e and Rinhe) and a 3 λ/4 coating on ZnSe (Rλ/4e). The calculations were obtained from values of n, k, and ∂n given by relation (18). The thicknesses of the layers are, respectively, 3.39, 3.76, and 5.80 μm for molybdenum, ZnSe (λ/4), and ZnSe (λ/2).
Table 4
Optical Constants of ThF4 Coating Deposited by Evaporation in Vacuum
Measured and calculated values of extrema of reflectances obtained with a 3λ/4 layer at λ = 10.6 μm deposited on molybdenum (Rλ/2e) and ZnSe (Rλ/4e and Rinhe). The calculations were obtained from values of n, k, and ∂n given by relation (17). The reflectance of the bare metallic substrate is supposed to be R0 = 0.9820 whatever the wavelength, and the accuracy in the measurement of R is estimated to be 0.05%.
Table II
Optical Constants of a ZnS Coating Deposited by Evaporation in Vacuuma
Measured and calculated values of extrema of reflectances obtained with a λ/2 coating at λ = 10.6 μm on molybdenum and ZnSe (Rλ/2e and Rinhe) and a 3 λ/4 coating on ZnSe (Rλ/4e). The calculations were obtained from values of n, k, and ∂n given by relation (18). The thicknesses of the layers are, respectively, 3.39, 3.76, and 5.80 μm for molybdenum, ZnSe (λ/4), and ZnSe (λ/2).
Table 4
Optical Constants of ThF4 Coating Deposited by Evaporation in Vacuum