Upgrading a microscope with a spiral phase plate.

We present the implementation of a spiral phase plate in a standard bright-field microscope to enhance the contrast of phase and amplitude samples. The method can be employed in all types of microscopy where standard phase contrast methods are applicable, for example, in bright-field transmission or reflection microscopy using an illumination source with partial spatial coherence. The spiral phase filter is placed into an accessible Fourier plane of the imaging path of the microscope. It is shown that this produces not only a strong contrast enhancement but in theory also improves the spatial resolution of the microscope for white light. A series of different set-ups for transmissive or reflective samples, including epi-illumination, are presented to demonstrate the practical range of applications of this contrasting method. A minute shift of the spiral phase plate out of the centre results in relief-like images that are similar to those obtained by differential interference contrast microscopy. A series of such relief-like images can be numerically processed to obtain quantitative phase and amplitude information of the sample.