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Optically generated holograms can be recorded on CCD-arrays if the sampling theorem is obeyed. The digitized and quantized holograms are processed digitally for the reconstruction of intensity and phase of the real or virtual image. This digital reconstruction consists in a numerical realization of the diffraction integral. One approach is the Fresnel approximation employing a single Fourier transform, the other is the interpretation of the diffraction formula as a convolution integral and calculation of this convolution by a double or triple Fourier transform. In this convolution approach the impulse response of free space propagation has to be defined which is then Fourier transformed or the free space transfer function is defined immediately. Impulse response as well as transfer function can be defined exactly or in an approximated version. The main difference between the Fresnel and the convolution approach is the different size of the resulting images. Furthermore in the Fresnel case this size depends on the wavelength and the distance of the object from the CCD, in the other case it does not. In this paper consequences on the reconstructed wavefields and on the interference phase distributions of holographic interferometry are indicated and demonstrated by experimental results.

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