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This paper compares two methods used to visualize low-density hypersonic separated flow fields: resonantly enhanced shearing interferometry (RESI) and planar laser-induced fluorescence (PLIF) of nitric oxide. The two methods are both used on a leading-edge separated flow configuration, and the values of the recompression neck position and thickness are compared between the two methods. It is seen that the lithium seeding in the RESI technique has a discernible effect on the flow behavior, changing the motion of the forebody shock wave during the experiment compared with unseeded flow, and the seeding also appears to prevent the recompression neck from moving further downstream during the tunnel run compared with the unseeded flow. The seeded lithium itself is also found to only enhance the density gradients in the flow during the first 550/s of flow time. The PLIF images are consistent with the …