Microsecond X-ray reflectometry

We accelerate synchrotron X-ray reflectometry (XRR) by more than an order of magnitude and demonstrate the acquisition of full reflectivity curves within 213 µs over a q _z range of 0.05 to 0.35 Å^-1 at 0.001 Å^-1 resolution. This is achieved by rapidly sweeping the incidence angle with a high-speed galvanometer and recording the reflected beam on an area detector. The method preserves a monochromatic parallel-beam geometry, requires only straightforward geometric and exposure time corrections, and yields quantitative agreement with standard XRR in thickness, density and roughness. At the shortest acquisition times, the photon statistics enter the low-count Poisson regime, where conventional least-squares fitting becomes biased. We show that applying an Anscombe variance-stabilizing transform restores near-Gaussian error behavior and significantly improves fitting robustness. Our approach benefits not only ultrafast XRR but reflectometry data in general at large q _z , where count rates are intrinsically low. This advance enables time-resolved studies of thin-film growth, diffusion, photoswitching and other rapid kinetic processes.