Reflective Fourier ptychographic microscopy using a parabolic mirror.

Fourier ptychography uses a phase retrieval algorithm to reconstruct a high-resolution image with a wide field-of-view. Reflective-type Fourier ptychographic microscopy (FPM) is expected to be very useful for surface inspection, but the reported methods have several limitations. We propose a darkfield illuminator for reflective FPM consisting of a parabolic mirror and a flat LED panel. This increases the signal-to-noise ratio of the acquired images because the normal beam of each LED is directed toward the object. Furthermore, the LEDs do not have to be far from the object because they are collimated by the parabolic surface before illumination. Based on this, a reflective FPM with a synthesized numerical aperture (NA) of 1.06 was achieved, which is the highest value by reflective FPM as far as we know. To validate this experimentally, we measured a USAF reflective resolution target and reconstructed a high-resolution image. This resolved up to the period of 488 nm, which corresponds to the synthesized NA. Additionally, an integrated circuit was measured to demonstrate the effectiveness of surface inspection of the proposed system.