Off-specular data representations in neutron reflectivity

The different methods of data acquisition and representation in neutron reflectometry measurements are discussed. The different representations of diffuse scattering are compared and the off-specular features that can be observed in neutron reflectivity are described. The representation of diffuse data in the `natural' reciprocal-space coordinates (Q_x, Q_z) leads to a loss of information for small Q_z scattering vector. It is suggested that an intermediate representation (Q_x/Q_z, Q_z) allows the unification of data measured on different types of spectrometers and permits a straightforward comparison and understanding while keeping all the interesting features of the off-specular scattering. The discussion is illustrated by diffuse scattering data measured on neutron waveguides obtained on both fixed-wavelength and time-of-flight spectrometers. A simple procedure allowing for dense remapping between different representations is described.