Section 2.2.4.2. Recording of zero layer

Normal-beam geometry (i.e. the X-ray beam perpendicular to the rotation axis) is used to record zero-layer photographs. The film is held in a cylindrical cassette coaxial with the rotation axis. The centre of the gap in a screen is set to coincide with the zero-layer plane. The coordinate of a spot on the film measured parallel () and perpendicular () to the rotation axis is given by where is the rotation angle of the crystal from its initial setting, f is the coupling constant, which is the ratio of the crystal rotation angle divided by the film cassette translation distance, in ° min⁻¹, and D is the camera radius. Generally, the values of f and D are 2° min⁻¹ and 28.65 mm, respectively.