A general thermodynamic description of the photoferroelectric effect, where the spontaneous polarization of a ferroelectric material is changed by interaction with light, is proposed. This description leads to a distinction between primary and secondary photoferroelectricity. At a certain reduced temperature, the primary photoferroelectric effect originates from the light-induced change in the polar ferroelectric order, whereas the secondary photoferroelectric effect is a purely thermodynamic consequence of the light-induced change in the ferroelectric transition temperature T-c. Using simple approximations, the general theory was applied to photoferroelectric Sm-C* liquid crystals where the photoresponse is induced by the Z,E photoisomerization of azobenzene dopant molecules. The results agree with experimental data obtained for a photoferroelectric Sm-C* mixture containing 5 mol % of the photoisomerizable dopant 4,4'-bis-[(2-methyl)-butyloxy]azobenzene dissolved in the commercial ferroelectric Sm-C* host phase abbreviated FLC 6430.