) Here the object is at the focal length ( f = 100 units) of the lens 1 (In fact, a distance less than f is preferable, as image formation of the object is not our concern. We can call this simple technique as '6f configuration' (see the below figure) even though just two lenses are enough to do this. But it is possible to take this Fourier plane out of the objective and make it accessible for filtering process. But practically, it is not possible to access the Fourier plane of an objective to do spatial filtering, as this lies inside the objective barrel. Employing an objective will drastically increase the resolution of the image as the numerical aperture (NA) can be very high for objectives. This technique can be used to experimentally determine the dispersion relation (energy band diagram) of an unknown sample.īut when you work with objects of micrometer size, you can’t use ordinary lenses as the resolution would be very poor. By blocking all other points on the Fourier plane, it is possible to filter out the information about the rays coming at a certain angle from the object. All the light rays coming at an angle with respect to the object plane, will converge at a certain point on the Fourier plane. Another application of Fourier plane filtering is to realize angle-resolved measurements. This is an efficient method to remove noise from an image. Spatial filtering process includes blocking some part of the Fourier plane such that image formed is only due to the unblocked spatial frequencies. The 4f configuration is widely employed to achieve this. Image quality can be extensively higher after filtering only desired spatial frequencies. The Fourier plane, thus formed, is used mainly to do spatial filtering process.
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