Peculiarities have been studied of modulation of nonpolarized light in 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 degrees are discussed. Optical characteristics are studied of modulated light beam for the cases of one- end two-fold reflection from the boundary. The results of investigations makes it possible to contruct an LC-valve characterized byhigh transmission (approximate to 90%) of nonpolarized light in a wide spectrum range in the open state.