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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.^[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.

For a uniform on-axis plane wave reference, choose its constant wave phase to be zero, so $R=A_R>0$. The reconstructed cross terms are then proportional to $O$ and its complex conjugate $O^{\ast }$. This pair creates the twin image in holography. An outgoing curved wavefront and its conjugate have opposite curvature: under forward propagation, one can appear to diverge from a virtual image while the other converges to a real image. In in-line holography their depths differ, but a defocused member still overlaps the other. This is the same conjugation of spatial wave phase used in phase conjugation, although a transmission hologram here launches both components forward rather than automatically creating a returning phase-conjugate mirror.