Enhanced grating characterization method for absorbing x-ray gratings by bidirectional angular x-ray transmission

High-quality x-ray absorption gratings are essential for grating-based x-ray phase-contrast imaging. Bidirectional angular x-ray transmission (AXT) measurements, as proposed in this work, allow for large-area and nondestructive characterization of these gratings. A custom setup consisting of two rotational axes is used to measure the transmission of x-ray absorption gratings under rotation. From the transmission values and rotation angles, pixelwise transmission profiles are reconstructed. From these transmission profiles, parameter maps that carry information about the microstructure of the grating are retrieved via a fitting procedure. Compared to unidirectional AXT measurements, more detailed and complementary parameter maps are obtained, revealing more defects and parameter variations. Apart from duty cycle, absorber height, and absorber inclination, the bidirectional measurements also provide access to grating lamella rotation on the substrate and to auxiliary structures introduced to increase grating stability. Furthermore, a novel correction technique is introduced to address projective distortions in individual radiographs caused by the rotations. This method relies on calibration phantom measurements to track the movement of registration points under the same rotations as the gratings. Based on the movement of the registration points, the distortions are reconstructed, and appropriate deformation vector fields are calculated and applied to measurement data, achieving a correction with a precision of about two pixels.