Abstract
Zernike phase contrast microscopy is a well-established method for imaging specimens with low absorption contrast. It has been successfully implemented in full-field microscopy using visible light and X-rays. In microscopy Cowley's reciprocity principle connects scanning and full-field imaging. Even though the reciprocity in Zernike phase contrast has been discussed by several authors over the past thirty years, only recently it was experimentally verified using scanning X-ray microscopy. In this paper, we investigate the image and contrast formation in scanning Zernike phase contrast microscopy with a particular and detailed focus on the origin of imaging artifacts that are typically associated with Zernike phase contrast. We demonstrate experimentally with X-rays the effect of the phase mask design on the contrast and halo artifacts and present an optimized design of the phase mask with respect to photon efficiency and artifact reduction. Similarly, due to the principle of reciprocity the observations and conclusions of this work have direct applicability to Zernike phase contrast in full-field microscopy as well.
Highlights
Scanning X-ray microscopy is a powerful tool for analyzing various types of samples due to the possibility of extracting several signals from a single measurement [1]
We present a more detailed description of the reciprocity in scanning Zernike phase contrast (SZPC) and explain qualitatively how the contrast and artifact formation can be described by exchanging the roles of the phase mask and the sample
It should be noted that the integration support for the bright-field cone images need to be defined only once for each phase mask, and for all the measurements with that particular phase mask, the same integration support can be used for calculating the Zernike phase contrast signal
Summary
Scanning X-ray microscopy is a powerful tool for analyzing various types of samples due to the possibility of extracting several signals from a single measurement [1]. These signals include e.g. X-ray absorption [2], differential phase contrast [3], fluorescence [1], and emission of photoelectrons [4]. X-ray absorption is not sensitive enough in case of light elements and differential phase contrast imaging requires reconstruction to reveal the phase of the specimen. Zernike phase contrast (ZPC) with X-rays [5] is successfully applied in full-field microscopes for contrast enhancement in the case of weakly absorbing specimens. The phase shift of the unscattered wave needs to be ±π/2 for optimal contrast enhancement
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