The optical filter under development employing frustrated total internal reflection is a micro-mechanical device containing two silicon rectangular prisms. There is a flat silicon plane among hypotenuse edges of the prisms. The silicon plane is a Fabry-Perot resonator, with clearances h, among the plate and prism edges being its mirrors. Theoretical resolution of the optical filter would be R approximate to 1.5.10(3) if h(1) = 50 mu m and R approximate to 3.10(5) if h(1) = 100 mu m with the thickness of the silicon plane being h = 65 mu m; wavelength lambda = 100 mu m; free spectral region Delta lambda/lambda approximate to 0,3...0,4, transmission in the maximum of spectral characteristics 0,6 (provided input and output silicon prism legs bloom).