X-ray reflectivity curves from a 15.4 nm-thick gold film on a silicon single-crystal substrate. Curve a was obtained with a data collection time of 1000 s. Curves b, c and d were obtained with data collection times of 1.0, 0.1 and 0.01 s, respectively. Curves b, c and d are shifted vertically to avoid overlapping each other. The curve e was obtained with an angle-scan X-ray (E = 15 keV) reflectometer on an undulator beamline at SPring-8 in 644 s. The origin of its vertical axis is taken to be the same as that of the curve a. The used X-ray energy ranges are given in the figure.