One desirable present-day hologram recording technique is the Fourier transform hologram recording process wherein an object light beam is directed upon an object or data source, the hologram of which is to be recorded, and then the Fourier-transform of the light waves emanating from the object or data source is focused by a lens onto a hologram storage medium such as a photographic film. This focussed beam is combined with the light from a reference beam, also directed onto the film, to create a Fourier-transform hologram of the data. Data storage and retrieval techniques also frequently make use of one-dimensional records, wherein each Fourier-transform hologram is recorded substantially in the form of a line extending across the storage medium. Reconstruction or read-out of the recorded hologram may proceed by directing a reconstructing beam of light upon the previously recorded hologram-containing film and selectively focussing light diffracted through the film onto a viewing medium or photo conversion medium, such as an opto-electronic read-out system. Normally, the diffracted light of interest is the first order diffracted beam which corresponds to the signal or data band, higher order beams being neglected. Non-diffracted light is contained within the zero order beam.
Unfortunately, there are a considerable number of factors which adversely influence the reconstruction of the original data from the hologram. In addition to degradation imparted by phase, scaling and detector positioning errors, scattering by large phase defects adds ten times more noise to the signal in some types of films at low spatial frequencies than would otherwise be expected from only film grain contributions.
Physical handling of the film may also affect the data. For example, if the film becomes scratched, some of the information is lost due to the removal of the photo-sensitive chemical medium, such as silver halide. Moreover, scratches also affect the amount of light scattered from the zeroth order to the data or signal band. Also, while fingerprints disturb the phase uniformity of the film base in a way that adds no significant scattering or zero order to signal band noise, they do contribute signal band to signal band data crosstalk due to defocussing and other aberrations of all data bits. Film thickness non-uniformities are usually slow-varying so that only the first order effect (wedge effect) which produces a displacement of the Fourier transform (signal band shift) is significant.
Scaling errors of the signal band, generated by the film's dimensional instability, as caused by aging or due to temperature and humidity fluctuations, can usually be corrected by simple mechanical adjustment (zoom lens). Still, the strict positioning requirement of the read-out detector array (opto-electronic array) may be a very difficult problem in reconstruction systems that are subjected to vibration or a wide range of the above-described environmental conditions.