A method is presented to accurately determine the optical constants and thickness of thin metal films when the real and imaginary parts of the dielectric constants satisfy ε_r < -1 and ε_i < |ε_r|. The method utilizes the excitation of surface plasmon waves using an inhomogeneous light wave generated by total reflection. The accuracy of this method is discussed, with an example showing the determination of the optical constants of silver foils in the wavelength range 4000–6000 Å. The paper introduces a method to determine the real and imaginary parts of the dielectric constants ε_r and ε_i of thin metal films, as well as their thickness, by exploiting the resonance behavior of surface plasmons. It is shown that in the vicinity of the plasma frequency (where ε ≈ 0), the transmission of light polarized parallel to the incidence plane at various angles provides favorable functions for determining these parameters. The method uses the phenomenon of "plasmon resonance absorption" of "radiating" surface vibrations. It also describes how the resonance absorption of the second type of surface plasmon vibrations, the "non-radiating" ones, can be used to obtain very accurate optical constants. Similarly, scattering light measurements can be used to determine optical constants through plasma-resonance emission. Non-radiating surface vibrations can occur at the interface between a metal and vacuum if the complex dielectric constant of the metal satisfies ε_r < -1 and ε_i < |ε_r|. These vibrations are characterized by the component of the wave vector perpendicular to the interface, k_⊥, being greater than the wave vector of light in vacuum (ω/c). The method excites surface waves by directing parallel polarized light at an angle greater than the critical angle for total reflection onto a prism surface with a thin metal film. At certain angles, the reflection is strongly reduced due to the absorption of the energy by the excited surface wave. The position, width, and depth of the reflection minimum, which is characteristic of surface wave excitation, depend strongly on the optical constants and film thickness. Thus, the resonance absorption of non-radiating surface vibrations at different angles provides favorable functions for determining optical constants in the ε range where surface waves exist. The method is described in detail, including the derivation of an approximate formula from known formulas of metal optics that shows the resonance character of reflection during surface wave excitation. The influence of thin protective layers on the metal is discussed, and an optical arrangement is given for precisely measuring reflection from metal films in the total reflection angle range. As an example, measurement curves for thin silver films in the wavelength range 4000 Å < λ < 6000 Å are provided. From these curves, the optical constants are determined, and the accuracy of the method is discussed.A method is presented to accurately determine the optical constants and thickness of thin metal films when the real and imaginary parts of the dielectric constants satisfy ε_r < -1 and ε_i < |ε_r|. The method utilizes the excitation of surface plasmon waves using an inhomogeneous light wave generated by total reflection. The accuracy of this method is discussed, with an example showing the determination of the optical constants of silver foils in the wavelength range 4000–6000 Å. The paper introduces a method to determine the real and imaginary parts of the dielectric constants ε_r and ε_i of thin metal films, as well as their thickness, by exploiting the resonance behavior of surface plasmons. It is shown that in the vicinity of the plasma frequency (where ε ≈ 0), the transmission of light polarized parallel to the incidence plane at various angles provides favorable functions for determining these parameters. The method uses the phenomenon of "plasmon resonance absorption" of "radiating" surface vibrations. It also describes how the resonance absorption of the second type of surface plasmon vibrations, the "non-radiating" ones, can be used to obtain very accurate optical constants. Similarly, scattering light measurements can be used to determine optical constants through plasma-resonance emission. Non-radiating surface vibrations can occur at the interface between a metal and vacuum if the complex dielectric constant of the metal satisfies ε_r < -1 and ε_i < |ε_r|. These vibrations are characterized by the component of the wave vector perpendicular to the interface, k_⊥, being greater than the wave vector of light in vacuum (ω/c). The method excites surface waves by directing parallel polarized light at an angle greater than the critical angle for total reflection onto a prism surface with a thin metal film. At certain angles, the reflection is strongly reduced due to the absorption of the energy by the excited surface wave. The position, width, and depth of the reflection minimum, which is characteristic of surface wave excitation, depend strongly on the optical constants and film thickness. Thus, the resonance absorption of non-radiating surface vibrations at different angles provides favorable functions for determining optical constants in the ε range where surface waves exist. The method is described in detail, including the derivation of an approximate formula from known formulas of metal optics that shows the resonance character of reflection during surface wave excitation. The influence of thin protective layers on the metal is discussed, and an optical arrangement is given for precisely measuring reflection from metal films in the total reflection angle range. As an example, measurement curves for thin silver films in the wavelength range 4000 Å < λ < 6000 Å are provided. From these curves, the optical constants are determined, and the accuracy of the method is discussed.