DENSITY CORRECTION OF PHOTOGRAPHIC MATERIAL FOR FURTHER IMAGE-PROCESSING IN ELECTRON-MICROSCOPY

Digitizing images recorded on photographic material yields images diff erent from those recorded using a CCD camera directly on the electron microscope. This is due to the non-linear density curve of the photogr aphic material, the ''Callier'' effect, and the non-linear relation be tween optical density and transmitted intensity. Using a coherent elec tron source and a Mollenstedt biprism in the microscope, it is possibl e to compensate those non-linear effects after digitization if one add itional ''reference'' image is taken. All photographically recorded im ages must be developed in one and the same process. Measuring carefull y the illumination intensity in the digitizer allows to compensate for the non-linear transmitted intensity. Evaluation of the reference ima ge then allows to compensate the effect of the optical density curve o f the photographic material if the reference image has been digitized under the same conditions as the image of interest. The efficiency of the procedure is demonstrated for off-axis electron holography. With r espect to linearity the images thus obtained are close to the images o btained from a CCD camera on the microscope.