Holography is a technique used to record an image in the form of an optical interference pattern onto a holographic film. The recording (“hologram”) then displays part or all of the same image upon illumination. As a hologram contains both amplitude and phase information, a three-dimensional image is recorded. The three-dimensional properties of the image can be viewed from different perspectives through the holographic film.
A hologram is produced by recording the diffracted light field scattered from an object.
The first stage of holography is to record an interference pattern. A coherent laser beam is separated into two identical beams comprising plane wave fronts, the “illumination beam” and “reference beam”.
An object is placed in the illumination beam path and diffracts the light. Every point on the surface of the object then behaves as a new source of spherical wave fronts (a phenomenon known as the Huygens-Fresnel principle). The spherical waves are then superimposed with the plane waves in the reference beam.
The resulting inference pattern is imprinted on a recording medium (“holographic film”), forming a hologram. Examples of recording media include photoresists or photographic emulsions such as silver halide.
A transmission hologram displays a virtual image that can be observed through the recording medium. The reference beam is typically expanded in order to illuminate the whole holographic film. However, the beam must maintain approximately plane wave fronts for a virtual image to appear.
Holograms reconstruct the wave fronts diffracted by the original object. The reconstruction is called a “virtual image”. A virtual image can be generated using a “transmission hologram” or a “reflection hologram”.
A transmission hologram generates a virtual image when illuminated with the same laser source used to make the recording i.e. the reference beam. The hologram then transmits the virtual image. If part of the hologram is blocked, then the observer sees the virtual image through a smaller window, as each point on a transmission hologram includes information about light diffracted from every point on the object.
A reflection hologram displays a virtual image when viewed from the same side as a white light source.
A reflection hologram generates a virtual image when illuminated with white light. The virtual image appears in front of the hologram’s surface when observed from the same side as the illumination source.
Security holograms are holographic thin foils used to confirm the authenticity of identification cards/passports, credit cards, bank notes, or sealed consumer goods. A hologram is indented on the surface of a photoresist, and then used as a template to emboss the same hologram onto many thin foils. Security holograms are difficult to imitate due to their complexity and highly specialised production process, and therefore protect against counterfeiting.
Holographic interferometry is a technique that enables the displacements of objects to be measured to microscopic precision. The diffracted light field scattered from an object is superimposed with a recorded light field from the same object. If the two fields are identical then no interference occurs. However, if the object is displaced, the relative phases of the two light fields will differ, and it is possible to observe interference fringes. The fringe pattern indicates changes in surface position, signalling that the object is out of position, possibly due to vibrations or a surface deformation.