  A relativistically invariant expression for the magnetic and electric
helicities of free electromagnetic field through the currents, which created
those field, are derived. It is demonstrated that for radiative fields (e.g.
laser pulses) the both helicities are conserved and equal to each other.
Moreover the conservation of the helicity means that in radiative
electromagnetic fields the electric and magnetic vectors are always orthogonal
to each other. Proceeding to the quantum theory, it is shown that the
corresponding quantum helicity field operator is equal to the sum of
projections of spins of all photons onto their momentums. Finally, as an
example, the expressions for the total helicity, number of photons and the
total spin momentum of the field of a dipole electromagnetic pulse are obtained
and discussed.
