 The electron emission intensity maps formula_5 are representative of the intrinsic distribution of the electrons in the solid formula_6 expressed in terms of their energy formula_2, and the Bloch wave vector formula_8, which is related to their crystal momentum and group velocity.The Bloch wave vector is linked to the measured electron's momentum formula_9, formula_10, of which only the component parallel to the surface is preserved.Thus, from ARPES, only formula_11 is known for certain, and its magnitude is given by formula_12.