In Compton's original experiments the wavelength shift given above was the directly measurable observable. In modern experiments it is conventional to measure the energies, not the wavelengths, of the scattered photons. For a given incident energy E γ γ = h c / λ λ , the outgoing final-state photon energy, E γ γ ′ ′ , is given by E γ γ ′ ′ = E γ γ 1 + ( E γ γ / m e c 2 ) ( 1 − − cos ⁡ ⁡ θ θ ) .