1. Apparatus for imaging an object irradiated with an X-ray beam by detecting a transmitted X-ray beam transmitted through the object, the apparatus comprising: a crystal analyser for receiving the transmitted X-ray beam and emitting a first diffracted X-ray beam to a detector assembly comprising first and second X-ray detectors, wherein the first detector is a monochromating semiconductor detector which detects a first portion of the first diffracted X-ray beam to generate first image data, and which diffracts a second portion of the first diffracted beam to the second detector which generates second image data, and image processing means are provided for combining the first and second image data to derive a refraction image and an absorption image of the object.