The paper presents a method for accurately determining the optical constants and thickness of thin metal films when the real and imaginary parts of the dielectric constants satisfy the conditions $\varepsilon_r < -1$ and $\varepsilon_i < |\varepsilon_r|$. This method utilizes the excitation of surface plasmons using inhomogeneous light waves obtained by total reflection. The accuracy of the method is highlighted, and it is demonstrated through an example where the optical constants of silver foils in the wavelength interval 4000 to 6000 Å are determined. In the introduction, the author discusses the necessity of a sensitive measurement method that is influenced by the optical constants and film thickness. The method leverages the "plasma resonance absorption" of excited surface plasmons, particularly the non-radiating type, to achieve precise optical constant measurements. The non-radiating surface waves occur at the metal-vacuum interface when the complex dielectric constant of the metal meets the conditions $\varepsilon_r < -1$ and $\varepsilon_i < |\varepsilon_r|$. These waves are characterized by a wave vector parallel to the interface, $k_{\perp}$, which is larger than the vacuum wave vector $(\omega/c)$. The method involves exciting surface waves by incident parallel polarized light on a prism with a thin metal film. The reflection is strongly reduced at certain angles due to the absorption of incident energy by the excited surface wave. By varying the incidence angle, the position, width, and depth of the reflection minimum become sensitive functions of the optical constants and film thickness. The plasma resonance absorption of non-radiating surface waves at different angles is used to determine the optical constants in the $\varepsilon$-range where surface waves occur. The paper then describes the detailed method, including an approximation formula derived from known metal optics formulas, the influence of thin films on the metal, and an optical setup for measuring reflection at angles of total reflection. Example measurement curves for thin silver films in the wavelength range 4000 Å to 6000 Å are provided, and the accuracy of the method is discussed.The paper presents a method for accurately determining the optical constants and thickness of thin metal films when the real and imaginary parts of the dielectric constants satisfy the conditions $\varepsilon_r < -1$ and $\varepsilon_i < |\varepsilon_r|$. This method utilizes the excitation of surface plasmons using inhomogeneous light waves obtained by total reflection. The accuracy of the method is highlighted, and it is demonstrated through an example where the optical constants of silver foils in the wavelength interval 4000 to 6000 Å are determined. In the introduction, the author discusses the necessity of a sensitive measurement method that is influenced by the optical constants and film thickness. The method leverages the "plasma resonance absorption" of excited surface plasmons, particularly the non-radiating type, to achieve precise optical constant measurements. The non-radiating surface waves occur at the metal-vacuum interface when the complex dielectric constant of the metal meets the conditions $\varepsilon_r < -1$ and $\varepsilon_i < |\varepsilon_r|$. These waves are characterized by a wave vector parallel to the interface, $k_{\perp}$, which is larger than the vacuum wave vector $(\omega/c)$. The method involves exciting surface waves by incident parallel polarized light on a prism with a thin metal film. The reflection is strongly reduced at certain angles due to the absorption of incident energy by the excited surface wave. By varying the incidence angle, the position, width, and depth of the reflection minimum become sensitive functions of the optical constants and film thickness. The plasma resonance absorption of non-radiating surface waves at different angles is used to determine the optical constants in the $\varepsilon$-range where surface waves occur. The paper then describes the detailed method, including an approximation formula derived from known metal optics formulas, the influence of thin films on the metal, and an optical setup for measuring reflection at angles of total reflection. Example measurement curves for thin silver films in the wavelength range 4000 Å to 6000 Å are provided, and the accuracy of the method is discussed.