| Résumé | Amongst the various ideas that have been brought forward during the latest revival of electron phase plate imaging [1,2], we have proposed the "hole-free phase plate" (HFPP) concept [3]. In this design, the phase shift between the low- and high-frequency parts of the object spectrum is achieved by the localized charging induced by the transmitted beam impinging on a continuous thin film placed in the back focal plane. The proposed design offers several advantages over conventional phase plates (PPs): i) the charged area, comparable in size with the central spot, is automatically centred; ii) charging, the primary obstacle preventing large scale implementation of PPs so far, is turned into an advantage; iii) any microscope can accommodate a HFPP; iv) replacing an exhausted HFPP is accomplished by moving the same film by a few microns. Similarly to most other PP designs, HFPP imaging provides in-focus phase contrast resulting in imaging dose reduction at constant resolution and SNR. Drawbacks of the HFPP are the difficulty in keeping the charge under control, resulting in lack of tunability, and the peculiar image intensity that appears to combine the clean Zernike phase contrast with fringes reminiscent of Fresnel imaging. The aim of this paper is to study more in detail image formation with a HFPP, and compare it with other imaging methods to illustrate the benefits and drawbacks just mentioned. |
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