Influence of phase filter shape and size on phase contrast imaging

Phase contrast is a technique that allows the visualization of phase variations of an object by transforming them into intensity variations at the image plane when a phase filter is set at the Fourier plane of the optical system. Ideally, only the zero frequency of the object's Fourier transform must be affected by the phase filter to have an intensity image that follows the object's phase variations. The implementation of the technique in real systems requires a filter with a defined spatial size, and therefore, more Fourier frequencies can be phase shifted. In this work, we investigate the impact of the phase filter shape and size on the image contrast. Phase objects with binary and quadratic phase distribution are analyzed. The shape of the filter was modified from an abrupt binary to a smooth phase variation. We specifically examined four types of filters: disk, stepped conical, continuous conical, and Gaussian. Size variations were achieved by varying the radii of the filters, while maintaining their constant maximum phase values. The results demonstrate that the shape and size of the filter affect the obtained image, showing that filters with smooth variation maintain their capability for contrast even for radii larger than that of the main lobe of the object Fourier transform. Experimental results using a phase spatial light modulator to control the phase, form, and radius variation of the phase filter are presented. High consistency is achieved between numerical and experimental results.