Accurate measurements of the transmittance of large optical elements are essential to improve the energy density in inertial confinement fusion (ICF). The required complex transmittance of such optical elements used in ICF is obtained by computing the phase difference between the illuminating and transmitting fields using modulation coherent imaging (MCI). A phase plate designed as a modulator has a known transmission function in this technique. It presents a simple and quick method to measure the transmittance of large optical elements with irregular surface profiles by retrieving it from only two diffraction patterns recorded by a CCD camera. The complex transmittance of a continuous phase plate (CPP) with a large aperture used in ICF is measured experimentally, and the results are found to agree with the designed value.