METHOD FOR PARTICLE WAVE RECONSTRUCTION IN A PARTICLE-OPTICAL APPARATUS

In a method for the iterative formation of an image of a specimen in a particle-optical apparatus a series of experimental images (the experimental series) is recorded with each time a different setting of an imaging parameter (for example, the focal distance), and a comparable series of images is calculated on the basis of the electron wave at the specimen (the estimation). The two series are compared and on the basis thereof an electron wave is calculated (the feedback) with which a new series of images is calculated which better approximates the experimental series. This iteration step is repeated until the correspondence between the experimental series and the calculated series is sufficient, after which the associated electron wave is considered to be the desired image of the specimen. A substantial gain as regards calculation time can be achieved during the estimation step as well as during the feedback step by executing the operation of the invention by means of FFTs. This is possible by writing the correlation expressions to be calculated in both steps in accordance with the invention in such a manner that they occur as pure correlation integrals which can be calculated by means of FFTs. The calculation time required when use is made of FFTs is substantially shorter than in the case of explicit calculation of the correlation expressions in conformity with the state of the art. The calculation time can thus be reduced by a factor of the order of magnitude of 50,000.