Peculiarities have been studied of modulation of nonpolarized light in reflection from glass-plane parallel ferroelectric smectic C boundary in the vicinity of total-internal-reflection angle under the action of electric field. LC materials with optical inclination angle of the local director of the smectic layer with respect to normal to the layer 45° are discussed. Optical characteristics are studied of modulated light beam for the cases of one- and two-fold reflection from the boundary. The results of investigations makes it possible to contruct an LC-valve characterized byhigh transmission (≈ 90%) of nonpolarized light in a wide spectrum range in the open state.