Chapter 2 : Holographic methods

2.1 What is a Hologram ?

A hologram is a record of the interaction of two beams of coherent light, in the form of microscopic pattern of interference fringes. It is a photographic registration of the interference pattern formed by two laser beams of coherent light. One beam goes straight from the light source and the other is scattered from a physical object. The photographic film or plate is exposed by two laser beams and is processed in such way that when illuminated appropriately a three-dimensional image is produced.

People often seem to think of a hologram simply as some sort of a three-dimensional photograph. Certainly, both photography and holography make use of photographic film, but that is about all they have in common.

The most important difference is the way the image is produced. A photographic image produced by a camera lens can be described fairly accurately using a simple geometric or ray model for the behaviour of light, whereas the holographic image cannot be described by this simple ray model. Its existence depends on diffraction and interference, which are wave phenomena.

2.2 Applications of holograms

Holography represents one of the most fascinating examples of recombining of scattered radiation to produce pictures. It has been a well used method to produce image, and a important tool in science and technology.

Holography is now spreading from the research laboratory to industry, and finds wider employment in communication and other engineering problems. A hologram can store numerous quantities of information. In the computer technique one can make a memories which are much larger and faster than in today's computers, but this has still not been realised even if the improvement are fast.

The use of small holograms in credit cards, which are made to prevent falsification, has made holograms a well known concept. Holograms show up more and more often on tickets and on original covers on software computer programs.

An example of an important area of application is bar-code readers in shops, warehouses, libraries and so on. A code reader like this is based on the application of holographic components like optical gratings. This large important industry has contributed to make holography an industrial success.

In the aircraft industry head-up displays (HUD) are an impotant example of holographic technology. HUD helps the pilots so they do not need to look down onto the instrument panels, because the instruments are projected onto the windscreen with help of holographic technology, and thus make flying easier.

Holography is also in use for making holographic optical elements (HOE), based on interference. The HOE are optical diffraction gratings, mirror, lenses and so on. This technique is used in bar-code readers.

2.3 The most common types of holograms 38

2.3.1 Transmission hologram 6

In the figure 2-1 is depicted the set-up for recording transmission hologram. To make a hologram we need two coherent light waves, laser light. One beam is reflected from the object and carries information about the object. This wave is called the object beam. The other one is a plane wave without information, which is called reference beam. The object beam (OB) and the reference beam (RB), generate an interference pattern which is recorded in the form of a hologram on film emulsion.

Absolutely stable conditions are required during the exposure of the film. If we have an instability of one tenth or more of a wavelength (633 nm), the result will be low diffraction efficiency and a weak image reconstruction.

This type of hologram is called transmission hologram because the light passes through the holographic plate. An other characteristic of transmission holograms is that the object beam and the reference beam come in from the same side of the holographic film plate during the exposure.

Figure 2-1 Recording transmission hologram

Figure 2-2 Transmission hologram reconstruction

To reconstruct the holographic image, we develop the hologram and place it in its original position in the reference beam as during its recording. If we look along the reconstructed object beam we see a replica of the object, and as we shift viewpoints we see object from different perspectives. Thus the object appears to be three-dimensional (3D). The light does not actually pass through the image, but only generates a wavefront that makes it appear as though the light had been generated in the position of the object. This image is called virtual image.

In contrast to the virtual image, an image that light has actually passed through is called a real image. The difference between the real image and the virtual image is that the real image can be caught on a screen placed in its plane without additional lenses. The real image is used in the two-step process which really is a hologram of a hologram. The real image is focused just in front of the recorded filmplate and so a reflection hologram can be produced.

Figure 2-3 and figure 2-4 shows us the virtual and the real image of a transmission hologram.

Figure 2-3 Virtual image in a transmission hologram

In figure 2-4 the hologram is turned 180 degrees.

Figure 2-4 Real image of a transmission hologram

To get a 3 dimensional image of the object, we have to recreate the original wavefront. That means that the hologram must be illuminated by a wave like one of the original waves which was used during the exposure.

When the developed film is illuminated, diffraction and interference will give rise to a new wavefront which is quite like the original wavefront. The result is that, it is difficult to see the difference between the object and the image. The image appears to us as though it is formed at a distance behind the filmplate as shown in figure 2-3. The plane of the image is called the holographic window. This image is the virtual image.

2.3.2 Reflection hologram 6 8

During the recording a reflection hologram, the reference beam and the object beam illuminate the filmplate on opposite sides as shown in figure 2-5. As a consequence of this, the resolution of film emulsion must be very high. The recording of a reflection holograms needs 10 to 100 times as much power as for a transmission hologram. The result is that the exposure time will be long, and we need an optical arrangement which is multi-stabile.

Figure 2-5 Recording reflection hologram

The interference fringes are formed by standing waves generated when two beams of coherent light travelling in opposite directions interact. The fringes formed are in layers more or less

parallel to the surface of the emulsion, and these sheets are roughly one half-wavelength apart. Under these circumstances, Bragg diffraction is the controlling phenomenon in image formation. The diffraction efficiency can be very high, in certain types of hologram it can approach 100 %. In addition, we can replay the hologram using white light. A reflection hologram reflects light only within a narrow band of wavelength, so if we illuminate it with a highly directed beam of white light such as is given by a spotlight or light from the sun, the hologram will select the appropriate band of wavelengths to reconstruct the image, the remainder of the light passing straight through. In the work with the 3-D printer we concluded, however, that this one step method is not practical.

Figure 2-6 Reflection hologram reconstruction

As already mentioned, another common method to make a reflection hologram is to use two steps in the production, what we call 2-step reflection hologram. First we make a transmission hologram called H1, because it is the first hologram or a master hologram. Sometimes the H1 is the master hologram from which we make multiple copies . A high quality transmission hologram is often used as a master hologram. Transfer copies (making another hologram using the image on the master as the subject) can be made in quantity from the master. These transfer holograms can either be other laser-visible transmission holograms or reflection holograms H2.

Figure 2-7 Recording 2-step reflection hologram

Historically one of the big problems that holographers used to have was placing the object to be holographed exactly where they wanted it.

For example, we want the object in the final hologram to appear half in front and half behind the recording plate. The way in which we have to do this is to first make a transmission hologram. We call this H1 because it is our first hologram. Now, since we can make a hologram of the H1's image, we take time to move the image around to wherever we want it positioned. In this case, we adjust the H2 recording plate so that the image of the object is half in front and half behind the plate and then make our H2. The problem of getting half the object in front of the plate, and half behind, is solved.

2.3.3 Rainbow hologram 31

The rainbow hologram separates out components wavelengths of white light and sends them in different directions, so that the viewer sees the image by light of only one wavelength, the actual wavelength being determined by the viewpoint. In order to achieve this, the hologram contains a plain diffraction grating which disperses the light into a vertical spectrum with red at the top and violet at the bottom. This diffraction grating is produced in the transfer process, and takes the place of the vertical parallax. So when we view a rainbow hologram at average height the image appears yellow-green. If we stand a little higher, it changes to orange or red, and if we dip, it becomes blue or violet.

In the horizontal plane the image has full parallax, and appears in three dimensions, as does any other type of hologram.

We may mention that the concept of two-steps rainbow hologram is practical in the 3-D printer recording process.

Figure 2-8 Recording rainbow hologram

Figure 2-9 Rainbow hologram reconstruction

2.3.4 Thick and thin holograms

Another broad classification of holograms is made when differentiating between thick (called volume 39 ) holograms or thin holograms. One of the reasons the words thick and thin are used in conversation is that they allow one the instantly get an idea of some of the properties of the hologram. Very thin holograms provide little depth to their object upon reconstruction. Embossed holograms, such as the images on bank cards, are examples of thin holograms. Thick holograms have the ability to replay or reconstruct the image with considerable depth or projection.

A hologram is considered to be thick if the thickness of the recording medium is greater than the spacing between the interference fringes. Otherwise the hologram is considered a thin hologram.

The distance between interference fringes recorded on the film will depend on a number of things, such as the wavelength of light being used, and the density of particles in the emulsion of the film plate.

These interference fringes are called Bragg planes, and actually go all the way through the medium, but are visible to our eye only where they meet the surface.

In a reflection hologram, the reference beam and the object beam strike the plate from opposite sides, the Bragg planes slice through the medium at very shallow angles.

Figure 2-10 Bragg planes in a reflection hologram

Conversely, in a transmission hologram, where the reference beam and the object beam strike the plate from the same side, the Bragg planes cut the emulsion at much sharper angles and thus are further apart.

Figure 2-11 Bragg planes in a transmission hologram

2.4 Embossed holograms 31

Embossed holograms are holograms which are mass-produced by taking a shim, or metal negative of the holographic image, and making impressions of the image onto a desired substrate. Foil is probably the most popular due to its low cost.

The major drawback of embossed holograms is that they lack depth. It is difficult to obtain a depth of more than 1 inch.

There are great advantages with embossed holograms, and there are tricks one can use to get around the problem of depth. For example, since a photograph is 2D and has no depth, it is an ideal subject. Furthermore, there is no reason why can not take several photographs, splice them together in extremely small strips and produce a three-dimensional effect for the viewer.

2.5 Holographic optical elements (HOE)

Holographic optical elements (HOEs) are lenses, mirrors, gratings, prisms and beam splitters made by holographic methods. Although they work by diffraction rather than by reflection or refraction, they obey all the rules of geometrical optics, and can be used for any purpose that conventional optical elements can be used for, with only one provision: they operate efficiently only over a narrow band of wavelengths. However, this is not of importance in holography, as careful processing can ensure the diffraction efficiency is a maximum at the wavelength of the laser that was used to make the HOE.

2.6 Head-Up Display (HUD)

A Head-Up Display projects the display image onto a partially transparent screen called a combiner that reflects the display to the viewer while allowing the viewer to see through to the outside world.

The advantage of a HUD is that it allows the viewer to see the projected display information while still looking at the scene beyond. An example of how useful this can be is in allowing a pilot to see both the runway and his instruments simultaneously during landings.

Another advantage is that the distant display image saves the time needed to refocus the eyes between nearby instruments and the world outside.

Figure 2-12 Aircraft Head-Up Display

One of the advantages of a holographic HUD combiner is its ability to reflect only a very narrow wavelength spectrum. This means that the reflectivity can be very high for the wavelength for the display while still remaining very low for all other wavelengths in the field of view.

By using a narrow band display source such as a phosphor cathode ray tube the HUD display can be both very bright and very transparent, with minimum coloration of the see-through scene.

opp LOGO opp LOGO neste LOGO