Nils Abramson

Light-in-flight recording by holography

A flat object surface and a hologram plate are both illuminated at an oblique angle by laser light of short pulse duration or short coherence length. Only those parts of the object surface are holographically recorded that correspond to a small-pathlength difference between object beam and reference beam. The holographic plate therefore corresponds to an infinite set of gated viewing systems triggered by the traversing reference beam. Scanning along the processed plate produces a continuous-motion picture of the light in flight. This new technique probably represents the ultimate in high-speed photographic recording, as no mechanical or electrical inertia is involved.

Light-in-flight recording: high-speed holographic motion pictures of ultrafast phenomena.

Holographic recordings have been made using lasers of short coherence and pulse length. Continuous frameless moving pictures show the wave front (pulse front) of light reflected by a mirror and focused by a lens. Light passing through interferometers has also been studied using this new method of dynamic observation. Cross sections between a thin sheet of light and a 3-D object have been recorded to demonstrate the possibilities of contouring. Finally a number of future experiments are proposed ranging from the measurements of industrial products to the study of relativistic effects.

Single pulse light-in-flight recording by holography

We have succeeded we believe for the first time, in making a light-in-flight (LIF) recording using one single pulse of a mode-locked frequency doubled Nd:YAG laser. Many experiments made earlier with multiple picosecond pulses have been repeated and the results were the same. New experiments have become possible such as the contouring of a fast moving object and recording through a nonrigid scattering medium. We demonstrate a LIF transmission method for rejecting scattered light which can be developed into a method for diagnostic imaging inside living tissue.

Sandwich Hologram Interferometry: a New Dimension in Holographic Comparison

New methods are introduced that utilize interferometric comparison of images of diffusely reflecting objects from different hologram plates that are sandwiched together. To make the plate positions optically and mechanically identical during reconstruction and exposure, they are also exposed in sandwiches. If the two sandwiched hologram plates are separated by a small distance a new method of fringe evaluation can be used. Fringes caused by object tilt between two exposures can be eliminated by an analogous, but much larger, tilt of the sandwich hologram during reconstruction. Even the direction of tilt, forward or backward, is found this way.

The Holo-Diagram: a Practical Device for Making and Evaluating Holograms

The engineer is interested in the measurement of small deformations of large machine parts, for instance the deformation caused by force and temperature of large slideways in machine tools. For this purpose it was thought that hologram interferometry would be particularly suitable. However, no general method appeared to be available either for the making or for the evaluation of holograms recording large objects; therefore, the method described in this paper was worked out. It is based on the use of a special diagram which the author has named the holo-diagram. This can be used to make both ordinary holograms and Lippman holograms. We have found that it simplifies the evaluation of interference holograms for measuring dimension, deformation, interferometers. and vibration. This work also inspired new ideas on the design of interferometers.

Sandwich hologram interferometry. 3:Contouring

This new holographic contouring method produces interference fringes formed by illumination shearing during recording in combination with observation shearing during reconstruction. One holographic plate is exposed after which the point source of light illuminating the object is displaced laterally to produce a shear of the illumination beam. A second exposure is made on another plate, whereupon the two plates are processed and bonded together with their emulsions outward. When this sandwich hologram is reconstructed, contouring fringes are formed that represent the intersection of the object by a set of interference surfaces. A tilt of the sandwich hologram produces a shear of the observation that results in a corresponding rotation of the intersecting surfaces of up to 180 degrees in every direction of the three-dimensional reconstructed object space. Thus, sign, direction, and magnitude of object slant can be evaluated by measuring the analogous sandwich tilt. The possibility of studying cross sections in any random orientation from a single sandwich hologram could be very useful for measuring and demonstration purposes.

Holographic contouring by translation

Holographic interference fringes caused by rigid object motion have been explained as intersections of the object by surfaces in space. These interference surfaces can be visualized as the result of a three-dimensional moiré effect between two sets of the ellipsoids of the holodiagram. The angles and the curvatures of the surfaces vary rapidly in the vicinity of the foci of the ellipsoids. It is graphically explained and experimentally verified how some translations of an object placed at a certain distance from the focal points cause contouring fringes that represent intersections by surfaces that are perpendicular to the line of sight. It is also demonstrated how this angle can be changed during reconstruction by moving the eye away from the hologram plate.

Sandwich Hologram Interferometry. 2: Some Practical Calculations

Two hologram plates representing different object situations are bonded together with their emulsions separated. Fringes are formed that can be manipulated during reconstruction by moving the sandwich hologram in relation to the reconstruction beam. We have studied practical mathematical relations between sandwich motions, fringe changes, and object motion between the two exposures. Four dimensionless equations have been found that can be used for fringe evaluation by measuring sandwich motions. The wavelength of light is not involved in this new analogous compensating method. In contrast to ordinary double-exposed holograms, the sandwich hologram can directly reveal signs of displacements and maximal object strain.

The Holo-Diagram. II: a Practical Device for Information Retrieval in Hologram Interferometry.

The holo-diagram (hologram-diagram) is a device designed to simplify the making and the evaluation of holograms, e.g., double exposed holograms made for interferometric measurements. It has been demonstrated in an earlier paper how this holo-diagram can be used to find the direction of maximum sensitivity in a holographic arrangement and how the amplitude of displacement in this direction can be evaluated. The aim of this paper is to show how the evaluation of real amplitude and direction of displacement in all three dimensions can be made in a rather simple way by the use of the holo-diagram, a slide rule, and a ruler. No trigonometric calculations have to be made because they are already built into the diagram. An example is given where an approximately vertical movement of the eye behind the photographic plate used to reconstruct the hologram gives the vertical component of the displacement, while a horizontal movement discloses the other two components. A method to calculate a displacement even when the fixed point is out of sight in the finished hologram is also described.

The Holo-Diagram. IV: A Practical Device for Simulating Fringe Patterns in Hologram Interferometry.

A new simple moiré equivalence to hologram interferometry has been investigated. We have found this equivalence to be very useful as an educational tool. The analogy to any interference pattern can be projected before a large auditorium using an ordinary overhead projector, and the fringes can be studied directly during formation. The equivalence is rather easily understood as it is only based on the loci of constant pathlength for the light traveling from the light source to the observer via the object. No, or very little, mathematical, optical, or linguistic preknowledge is needed, because the ellipsoids onto which the method is based can be demonstrated using just two nails and a string. The new equivalence method is also being tested in a simple analog computer, which, we hope, will be used as the holographers sliderule.