Due to the chromatic dispersion properties inherent in all optical materials, even the best designed multi-spectral objective will exhibit residual chromatic aberration effect. Here we show that the aberration correction ability of Fourier Ptychographic Microscopy (FPM) is well matched and well suited for post-image acquisition correction of these effects to render in-focus images. We show that an objective with significant spectral focal shift (up to 0.02 μm/nm) and spectral field curvature (up to 0.05 μm/nm drift at off-axis position of 800μm) can be computationally corrected to render images with effectively null spectral defocus and field curvature. This approach of combining optical objective design and computational microscopy provides a good strategy for high quality multi-spectral imaging over a broad spectral range, and eliminating the need for mechanical actuation solutions.
Differential phase contrast (DPC) is a non-interference quantitative phase imaging method achieved by asymmetric optical systems. Quantitative DPC images are achieved previously with asymmetric illumination systems. However, it works well for on-focus thin samples only. Considering the limitation, we develop a pupil modulation differential phase contrast (PMDPC) imaging method. Instead of modulating the illumination, we use a spatial light modulator (SLM) to modulate a 4f imaging system’s pupil plane. When half of the pupil plane is blocked by the SLM, a phase gradient image forms on the image plane. Using two such phase gradient images captured separately by applying complementary half-circle pupils on SLM, a DPC image can be constructed that carries the sample’s phase information. A quantitative phase image of the sample can be reconstructed after a deconvolution procedure. Further, we are able to combine this quantitative phase with the sample’s intensity image to obtain the complete complex object field which then allows us to post-process the image. We report experimentally that aberrations arising from the optical elements in the system can be corrected by deconvolving the reconstructed image with a pre-calibrated pupil function. We can also digitally extend the depth of field using angular spectrum propagation algorithm. With our PMDPC imaging setup where NA equals to 0.36, a quantitative phase image with periodic resolution of 1.73µm is obtained. The depth of field for a 20x, 0.4NA objective is extended digitally by 20 times to -50~50 micrometers.
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