Abstract: There is provided a transmission X-ray diffraction (XRD) apparatus, the transmission XRD apparatus including an X-ray source for generating a direct X-ray beam; sample holder for receiving the sample, the sample being positioned to receive the direct X-ray beam when held by the sample holder; a detector for receiving X-rays transmitted through the sample and outputting an X-ray diffraction pattern therefrom; and an optical element positioned between the X-ray source and the detector, the optical element including a Montel optic and a secondary pin-hole collimator collectively adapted to focus the direct X-ray beam on the detector, wherein a ratio between a dimension of the direct X-ray beam projected on the detector and a sample-to-detector distance is equal or smaller than 1/570. Related methods are also provided.