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practice, the objective lens can only capture light over a limited angular range (see Figure 2). It turns out that the spatial resolution with which one can examine a specimen of interest is inversely proportional to the collect- able size of the diffracted light cone. If a lens can collect all of the dif- fracted radiation, then an image can be formed that resolves features down to the wavelength scale. However, if a lens situated far away (or with a small diameter) only captures a narrow cone, then the resulting image reso- lution is significantly worse. Not surprisingly, an objective
Figure 1: A standard optical microscope images a specimen across a FOV that is defined by the microscope objective lens. Left, a large FOV results in a low-resolution image. Middle, zooming in sharpens the resolution but significantly restricts the FOV. Right, an ideal microscope would offer both, which is the goal of Fourier pty- chography.
lens situated far away from the speci- men can observe across a larger area, which leads to the inherent trade- off between the resolution and FOV of images mentioned above. Fou- rier ptychography applies a simple insight to overcome this trade-off, as diagrammed in Figure 2b. If the thin specimen is illuminated with a plane wave at an angle, then the diffracted
Figure 2: (a) Simplified microscope diagram with an LED array illuminating a sample from below. Scattered light is imaged by a lens of finite width and projected to an image detector. Diffracted light from the sample, represented as the sample’s Fourier spectrum, does not fully pass through the lens aperture, limiting the detected image’s resolution. (b) Illuminating the specimen from an angle, using an off-center LED, shifts the sample’s spectrum to cause a new spectrum segment to pass through the lens and contribute to image formation. (c) Fourier ptychography collects many uniquely illuminated images and combines them in the Fourier domain to synthesize a more complete spectrum, in turn leading to an image reconstruction with higher resolution.
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