= In-line holography
<In-line holography> uses nearly the same direction for the <holographic reference wave> and <holographic object wave>. In a transmission geometry, the undiffracted illumination supplies $R$ while the object supplies the scattered component $O$. This makes small <optical path differences> possible, but leaves the desired image, its <twin image in holography> and the bright background sharing an axis.
<Dennis Gabor>'s electron-microscopy motivation led to optical tests: one line of a high-pressure <mercury-vapor lamp> illuminated a 3 μm <pinhole>, then approximately 1 mm <microphotographs>, recording approximately 1 cm <holograms> on sensitive <photographic emulsions> with minute-scale exposures. The roughly 0.1 mm <optical coherence length> favoured an in-line arrangement. Reillumination reconstructed images; focusing separated their depths incompletely. Distorted letters and random optical disturbances limited the results. These were optical demonstrations, not an operating <electron microscope>.<Holography, 1948-1971 by Dennis Gabor>[Gabor, Holography, 1948-1971, Nobel lecture, Figs. 2–6 and accompanying optical-experiment account; authored transcription inspected at https://studylib.net/doc/28317456/dennis-gabor-nobel-lecture]{ref}
The recorded <microphotograph> and its reconstructed lettering provide an object-to-image comparison in that account. The blocked-reference control described above is an explanatory control for the model; it is not a claim about an additional historical measurement.
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