Knowledge of wavefront amplitude is as important as the knowledge of phase for a coronagraphic high contrast imaging system. Efforts have been made to understand various contributions to the amplitude variation in Terrestrial Planet Finder's (TPF) High Contrast Imaging Testbed (HCIT). Modeling of HCIT with as-built mirror surfaces has shown an amplitude variation of 1.3% due to the phase-amplitude mixing for the testbed's front-end optics. Experimental measurements on the testbed have shown the amplitude variation is about 2.5% with the testbed's illumination pattern having a major contribution to the low order amplitude variation.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.