  Dark-field contrast imaging with grating interferometers has proven to hold
huge potential for numerous applications with X-rays and with neutrons
conveying biology and medicine as well as engineering and magnetism,
respectively. However, a concept to extract quantitative information is still
missing. Here a general theory as well as a measurement strategy is introduced,
allowing extraction of quantitative small-angle scattering information such as
structure sizes and scattering cross sections. The validity of the description
is demonstrated by a specific example from literature.
