Classical RF (Radio Frequency) imaging techniques, including diffraction tomography, are subject to a limit in resolution known as the Rayleigh limit or Rayleigh criterion. This limit states that the minimum resolvable distance or spatial resolution of an imaging system is approximately equal to half the wavelength of the radiation being used.
The Rayleigh limit arises due to the phenomenon of diffraction, which occurs when waves encounter obstacles or pass through small apertures. When an electromagnetic wave, such as an RF wave, passes through an aperture or interacts with an object, it diffracts, causing the wavefront to spread out and create interference patterns. The diffraction phenomenon leads to a blurring or spreading of the image, limiting the ability to resolve fine details.
In the case of RF imaging, where the wavelength is relatively large compared to other imaging modalities like optical imaging, the diffraction effects are more pronounced. The larger wavelength of RF waves makes it challenging to achieve high-resolution imaging because the diffraction patterns are more significant, leading to a broader point spread function (PSF) and reduced ability to distinguish closely spaced features.
To overcome the Rayleigh limit and achieve higher resolution, several techniques have been developed, such as using shorter-wavelength electromagnetic radiation (e.g., X-rays or higher-frequency microwaves), employing advanced algorithms for image reconstruction and deconvolution, and utilizing specialized hardware configurations, such as phased array antennas or multiple-input multiple-output (MIMO) systems.
These advanced techniques can help improve the resolution beyond the classical Rayleigh limit, but they often come with their own set of challenges, costs, and limitations. Therefore, the Rayleigh limit serves as a fundamental constraint in classical RF imaging techniques and sets a practical limit on the achievable resolution.