Defocused illumination with beam-shaping condenser for uniform wide-field X-ray imaging

The beam-shaping condenser (BSC) has become a key optical component in X-ray microscopy, providing a wide field of view with stable and uniform illumination. However, alternating bright and dark fringes often appear at the focal spot, compromising illumination uniformity. To elucidate the formation mechanism of these diffraction fringes, this study conducted theoretical analyses and numerical simulations. Based on this, a defocused illumination approach was developed to alter the focal positions and spatial relative phases of the sub-gratings, thereby enhancing illumination uniformity. The influence of defocused illumination on fringe patterns and intensity distribution was systematically investigated. To validate this approach, a BSC with a Fresnel number of 20 and a 60 µm × 60 µm field of view was fabricated and tested at the soft X-ray imaging beamline of the Hefei Light Source. Experimental results demonstrate that the device achieves uniform top-hat illumination across the full field, effectively suppressing diffraction-induced artifacts. This work offers an efficient and practical approach for achieving wide-field uniform illumination in X-ray microscopy.