Scanning slit is one of the most important elements in step-and-scan lithographic tools for controlling exposure doses. As the technology of lithography extends to 90-nm (and lower) process nodes, the lithographic-tool illumination subsystem has presented significant requirements for the precision and repeatability of the penumbra measurement. Based on this, a blade edge's penumbra measurement technique for scanning slits of lithographic tools based on the pupil image is proposed. The corresponding relationship between the penumbra of the mask surface and pupil image is deduced by analyzing the imaging optical path of the coplanar scanning slit. A penumbra measurement system is developed, and the scanning slit edges' penumbra of a 90-nm lithographic-tool illumination system is measured. The experimental result shows that the proposed measurement method can effectively reduce the impact of light-intensity fluctuation on the penumbra measurement. The repeatability of the penumbra measurement is 0.026 mm, which is 3.46-times higher than that of the conventional scanning method. This technique can be used to measure the optical penumbra parameters of high-numerical-aperture immersion-lithographic-tool illumination subsystems.