This bachelor thesis concentrates on a correction of background in the phase image of cells from a transmitted-light digital
holographic microscope (TDHM). Optical aberrations in the optical systems of the object and reference arms cause "deformation" of the phase image. The cell is not displayed on a background with a constant value of phase, it is displayed on a background with phase values continuously changing between different places in the image. A method for removing the consequence of this imperfection from the image is described in this thesis. The principle lies in partial convolution of the phase image with a convolution kernel determined by a two-dimensional Gaussian function with the same variance in the both axes. To eliminate loss of information about the cells, the convolution is computed only from those parts of the phase image which contain no cells. The result of the convolution is considered to be an approximation of the background. This approximation is then subtracted from the phase image. The resulting phase image has a constant background, containing only noise. The necessary mathematical concepts are resumed in the introductory part of the thesis. The basic principle of the TDHM image reconstruction is also described. As a part of the thesis, a computer programme Odečet pozadí (Background subtraction) was created, which processes the phase images from the TDHM using this method.