International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 6.2, p. 596

In experiments where the crystal is rotated or oscillated, reflection of Xrays takes place as a reciprocallattice point moves through the surface of the sphere of reflection. The intensity is thus proportional to the time required for the transit of the point through the surface, and so is inversely proportional to the component of the velocity perpendicular to the surface. In most experimental arrangements – the precession camera (Buerger, 1944) is an exception – the crystals move with a constant angular velocity, and the perpendicular component of the velocity varies in an easily calculable way with the `latitude' of the reciprocallattice point referred to the axis of rotation. If the reciprocallattice point lies in the equatorial plane and the radiation is monochromatic – the most important case in practice – the angularvelocity factor is If the latitude of the reciprocallattice point is , a somewhat more complex calculation shows that the factor becomes For , the expression (6.2.3.2) reduces to (6.2.3.1). In some texts, is used for the colatitude; this and various trigonometric identities can give superficially very different appearances to (6.2.3.2).
References
Buerger, M. J. (1944). The photography of the reciprocal lattice. American Society for Xray and Electron Diffraction, Monograph No. 1.