Calculating the angle between crystal planes from measurements on the diffraction pattern

Two Kikuchi bands are defined by the pairs of points,  and,.  The vector  joins the point of incidence of the electron beam on the sample to the point.  The unit vector  is perpendicular to the crystal plane generating the first Kikuchi band and the vector is to the crystal plane generating the second Kikuchi band.    The angle between the planes forming the two Kikuchi bands is