Features of representing the information content of holograms in terms of optical degrees of freedom

Features of evaluating the number $N$ of the optical degrees of freedom in the case of multiplex holographic data recording are analyzed. The necessity of taking into account three types of spatial degrees of freedom in the recording medium is emphasized. It is pointed out that the properties of the volume recording medium are of primary importance in determining the limiting $N$ value. It is possible to create a recording medium and develop methods of recording such that $N$ will be greater than the value given by the van Heerden formula for the limiting density of data recording. In evaluating $N$, it is also necessary to take into account that, in holographic recording, it is impossible to obtain values of signals in one interval, which would be fully independent of the values in some other interval.