An apparatus includes: an interferometer configured to produce white light fringes with measuring light reflected or scattered by an object; an image sensor configured to generate an image signal for each pixel; and a controller. The interferometer splits the measuring light into two luminous fluxes and reflects them on reflecting mirrors having different curvatures. A white light fringe signal is obtained by varying the optical path difference between the two luminous fluxes. The controller is configured to perform frequency conversion on the white light fringe signal, with respect to the optical path difference, to determine a cross spectral density representing spectral information of each point on the object. The controller is configured to perform back-propagation computation based on Fresnel diffraction integral on the cross spectral density to determine a wavefront of light from each point on the object.
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