A scalar distribution function ( ) is called a spectral function for the Fourier transform̂( ) = ∫ R ( ) (with respect to anWe show that in the case I = R there exists a unique spectral function ( ) = (1/2 ) , in which case the above Parseval identity turns into the classical one. On the contrary, in the case of a finite interval I = (0, ), there exist infinitely many spectral functions (with respect to I). We introduce also the concept of the matrix-valued spectral function ( ) (with respect to a system of intervals {I 1 , I 2 , . . . , I }) for the vector-valued Fourier transform of a vector-function ( ) = { 1 ( ), 2 ( ), . . . , ( )} ∈ 2 (I, C ), such that support of lies in I . The main result is a parametrization of all matrix (in particular scalar) spectral functions ( ) for various systems of intervals {I 1 , I 2 , . . . , I }.