An optical spike is sometimes observed prior to the main flash in high-velocity impact experiments. The spikes are particularly noticeable in the case of Nylon66 projectiles. In this study, we conducted experiments in which Nylon66 spheres impacted the flat surfaces of Nylon66 blocks perpendicularly at 7 km s -1 . We observed the impact phenomena by using an ultra-high-speed camera and high-temporalresolution photometers to identify the cause of the spikes. High-speed photographs show that the

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... ntire projectile was shining while it was penetrating a target. Glaring light from the shock front propagating in the projectile is assumed to become diffused within the translucent projectile and then radiated from its surface. The blackbody radiation from the shock front at 3600 K, which is calculated based on a onedimensional shock model, accounts for the radiative intensities measured by the photometers. A sub-spike was observed just after the main spike in all the experiments conducted, the cause of which was not ascertained.