The dawn of holography The famous image of Gabor’s 1948 publication; showing the original object (bottom left); the in-line hologram; having no obvious resemblance with the original object (top-center); and the amazingly legible reconstruction of the object from the hologram (bottom right).

Principle of Gabor holography (schematic) Scatter of light at the edges of a small particle creates a diffraction pattern through interference with the background illumination. White light Small particle The fringe frequency increases with the angle between scattered light and background light; while the fringe contrast decreases due to lack of temporal coherence of the white light source (see next slide). Multicolor diffraction pattern

Coherence length Decreasing fringe contrast Small particle White light Diffraction of light at the edges of a small particle demonstrates the extremely low coherence length of white light: just a few micrometers. This entails a severe drawback: Holographic recording of objects is only possible in transmission. White light Decreasing fringe contrast Small particle The path difference between diffracted light and background light increases with the diffraction angle; so that fringe contrast decreases. About 10 fringes are visible: white light coherence length ≈ 5 µm

Principle of Gabor holography (schematic) Scatter of light at the edges of a small particle creates a diffraction pattern through interference with the background illumination. The use of a red filter improves the contrast of the fringes. Red light Small particle Monochrome diffraction pattern: the fringe contrast is enhanced.

Principle of Gabor holography (schematic) A photograph of the diffraction pattern (the hologram) is illuminated with the original beam and light is diffracted into the -1st order; reconstructing a virtual image of the original object (which is no longer present). virtual reconstruction Hologram

Principle of Gabor holography (schematic) A photograph of the diffraction pattern (the hologram) is illuminated with the original beam and light is diffracted into the +1st order; reconstructing a virtual image of the original object (which is no longer present). real reconstruction Hologram

Principle of Gabor holography (schematic) Virtual image; real image and zero order non-diffracted light are in-line; hence the name “in-line holography”. The in-line position of both reconstructions (the so-called twin image) deteriorates the quality of the reconstruction because they cannot be optically separated. -1st order virtual reconstruction real reconstruction +1st order It may look trivial; but Emmett Leith; after repeating Gabor’s experiments using a mercury arc lamp in the early 1960s; observed: “I was most astonished when I saw it because it was an image without an object. Incredible!” Hologram

Gabor’s experiment repeated In-line holography with a slide projector In the following slides the results of my 1985 experiments with Gabor’s in-line holography and a slide projector are presented. I had been applying in-line holography for many years as a means to analyze particle distributions of deep volumes. In 1985 the idea struck my mind that one would neither need a laser with its coherence length of tens of centimeters, nor a low-pressure mercury-arc (as Gabor used), which has a coherence length of some 1000 wavelengths, but that a slide projector & pinhole combination might also do the job, although it’s coherence length would be at best some 10 wavelengths. In my first experiments I used a 0.3 mm pinhole and a red filter, and the results on a relatively high resolution document film were just good. Looking at the holograms, which -due to the low coherence length of the source- only showed about 7 fringes, I realized that high resolution film might not at all be required. Curiously, I proceeded with 35 mm Kodak Ektachrome color positive film in white light. I was astonished: I could record and successfully reconstruct my Gabor holograms even in white light! Gabor’s holographic process is an astonishing optical phenomenon: the reconstruction of the original object is actually the shadow of a shadow… Published in part in: R.L. van Renesse, “hologrammen, wat zit daarachter” (“Holograms, what’s behind them?”), Natuur en Techniek, Vol. 54, No. 9, pp. 676-689 (1986) (in Dutch).

Recording Reconstruction Object: insect wing; length: 14 mm. Light source: halogen bulb slide projector with red filter and 0.3 mm pinhole. Distance from pinhole to object: 3 m. Distance from object to photographic film: about 200 mm. Distance from photographic film to real image: about 200 mm. Illustrations from: R.L. van Renesse, “hologrammen, wat zit daarachter” (“Holograms, what’s behind them?”), Natuur en Techniek, Vol. 54, No. 9, pp. 676-689 (1986) (in Dutch).

The original object: an insect wing of 14 mm length.

Recording with red light The hologram of the object; or rather its far field diffraction pattern at a distance of 20 cm behind the object: the details have become completely illegible.

Reconstruction with red light: the “shadow” of the “shadow”. Holographic reconstruction of the object; or rather the far field diffraction of the hologram: the details of the original object have become legible again; although some resolution is lost at the enormous distance of 20 cm between object and hologram.

Object Hologram Reconstruction the “shadow” of the original object the “shadow” of the “shadow”

White light Gabor holography These are the results of my experiments with Gabor holography using white light. A test target of 14 x 15 mm on film was used as a transparent object and the recordings were made on Kodak Ektachrome positive color film. The object to film distance was about 50 mm. Hologram Reconstruction

Applications of Gabor holography In-line holography of gas bubbles in water In the following slides an example is given of an application of Gabor’s in-line holography to the measurement of the size-distribution of gas bubbles in a water tank. H.W.H.E. Godefroy, R.H.J. Jansen, A.P. Keller, Y. Lecoffre, D.M. Oldenziel and R.L. van Renesse, “Comparison of measuring and control methods of the water quality with respect to cavitation behaviour”, Proceedings of meeting and mutual experiments at the water tunnel of Delft Hydraulics Laboratory, Delft, The Netherlands, January 1981, pp. 44-51. For the full report, contact rvr@vanrenesse-consulting.com

Details of an in-line hologram of gas bubbles in water Typical examples of bubble diffraction patterns in the plane of an in-line hologram recorded with a ruby laser (λ = 694 nm). Size of hologram sections: 5.5 x 3.3 mm

Reconstructions from a Gabor hologram of gas bubbles in water 15 μm; 51 mm; 143.4 17 μm; 39 mm; 85.4 17 μm; 35m; 76.6 18 μm; 35 mm; 68.4 17 μm; 29 mm; 63.5 17 μm; 28 mm; 61.3 17 μm; 25 mm; 54.7 24 μm; 25 mm; 27.5 24 μm; 42 mm; 46.1 26 μm; 25 mm; 23.3 28 μm; 28 mm; 22.6 30 μm; 38 mm; 26.7 30 μm; 26 mm; 18.3 55 μm; 52 mm; 10.9 These images are microphotographs of real image reconstructions from ruby pulse laser holograms, recorded at 694 nm and reconstructed using a 633 nm helium-neon laser. The numbers with the reconstructions respectively indicate bubble diameter, reconstruction distance behind the hologram and the related far-field number Nf. For an explanation of the relevance of Nf reference is made to the report: by Godefroy et al.

Reconstructions from a Gabor hologram of gas bubbles in water 240 μm; 18 mm; 0.20 240 μm; 59 mm; 0.56 200 μm; 81 mm; 1.28 145 μm; 38 mm; 1.14 120 μm; 25 mm; 1.10 120 μm; 50 mm; 2.20 95 μm; 37 mm; 2.59 55 μm; 47 mm; 9.83 These images are microphotographs of real image reconstructions from ruby pulse laser holograms, recorded at 694 nm and reconstructed using a 633 nm helium-neon laser. The numbers with the reconstructions respectively indicate bubble diameter, reconstruction distance behind the hologram and the related far-field number.

Reconstructions from a Gabor hologram of gas bubbles in water 560 μm; 25 mm; 0.05 420 μm; 112 mm; 0.40 417 μm; 39 mm; 0.014 These images are microphotographs of real image reconstructions from ruby pulse laser holograms, recorded at 694 nm and reconstructed using a 633 nm helium-neon laser. The numbers with the reconstructions respectively indicate bubble diameter, reconstruction distance behind the hologram and the related far-field number. 463 μm; 85 mm; 0.25 470 μm; 62 mm; 0.18

Comparison of measuring and control methods of the water quality with respect to cavitation behavior; 1981 In-line holographic recording set-up In-line holographic reconstruction set-up

Applications of Gabor holography In the period 1980 – 1993; Dr. Jan van der Meulen of the Netherlands Ship Model Basin in Wagenigen; The Netherlands (Now MARIN) conducted many experiments in a high speed water tunnel, while I assisted in recording numerous Gabor holograms of the occurring cavitation and boundary flow phenomena. Dr. van der Meulen was always kind enough to mention me as a co-author of his publications of these holographic investigations. We used a 30 ns ruby pulse laser (694 nm; 30 mJ single mode) to capture the Gabor holograms. Agfa-Gevaert 8E75 Holotest plates with a resolution > 5000 l/mm were used as a recording medium. In the reconstruction set-up a 2 mW HeNe laser (633 nm) served as a light source and photographs of the holographic reconstructions were made via a microscope. The following slide shows a beautiful result of our first paper: R.L. van Renesse and J.H.J. van der Meulen; In-line holography for flow and cavitation visualization on hydrofoils and for nuclei measurements; Proceedings of the Second International Symposium on Flow Visualization; September 9-12; 1980; Bochem; West- Germany; pp. 527-533.

Schematic diagram of in-line holographic system for making holograms of cavitation and flow phenomena on hydrofoils (top). Separated shear layer for NACA 16-012 hydrofoil at 12° angle of attack. The flow is from left to right; V0=0.9 m/s (bottom)

After the invention of the laser; various holographic portraits were made of Dennis Gabor. This portrait is derived of a pulsed laser hologram by MacDonnel Douglas Corporation (http://www.jrholocollection.com/collection/spindler.html).

In 1971Dennis Gabor was at his desk explaining holography and being holographed by means of Denisyuk holography on a 50 cm x 60 cm holographic plate. The plate holder is facing Gabor; just in the center of the arrangement. Inset: Image of Gabor seen through the hologram. Setup and photo: McDonnell Douglas Electronics; Missouri Hologram image: Klaus Biedermann at the "Laser Grotto;" KTH; Stockholm. Source: http://nobelprize.org/nobel_prizes/physics/articles/biedermann/

End of presentation Goto 1st slide Dennis Gabor; “A new microscopic principle”; Nature; Vol. 161; 15 May 1948; pp. 777-778. Dennis Gabor; “Microscopy by Reconstructed Wavefronts”; Proc. Roy. Soc.; Vol. A197; London; England; 1949; pp. 454 – 487.