Temporal and spatial optical field distributions of a stepped pulse on the hohlraum wall are numerically analyzed using fast-Fourier-transform method combined with chromatographic theory. Changes in the rising edge of the stepped pulse caused by spatio-temporal effect are systematically studied. Results demonstrate that a certain time difference exists for the laser to reach the hohlraum wall because of oblique incidence, which causes distortion to the stepped pulse, especially to the rising edg
