ESSENTIAL FEATURES Of The PRINCIPAL GEOMETRIES For HOLOGRAPHY

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Essential features of the principal geometries for holography are shown in figure below. The Fresnel transform techniques use planar reference waves and have resolution limited by grain size of the recording ,medium. The on_axis (Gabor)
form is inherently simple but suffers from overlap of the real and virtual images. The off_ axis (Leigh_upatniek) modification reduces the image overlap problem but requires a ,mirror and a broadened beam for system illumination; both forms may be difficult at x-ray wavelengths. The Fourier (stroke) geometries,using curved wavefronts,achieve large fringe spacings and are therefore less sensitive to grain size.
Coherence is characterized by the effective finite length of a photon wave train in the transverse direction (spatial) . Both coherence length and geometry limit the holographable volume of a specimen. For most specimens of biological interest,spatial and temporal coherence lengths of 10 micrometer to 1 nm are adequate.