Making of master hologram
What is Holography ?
Holography is a technique which allows the recording and playback of true three-dimensional objects. The produced image is called a hologram. The playback provides an image in light that can be viewed in different angles and is an exact copy of the original 3D object.
The inventor of holography was a Hungarian-born electrical engineer Dennis Gabor, who won the Nobel Prize for Physics in 1971 for this invention. He developed the idea of holography in 1947, while working on improving the electronic microscope. The basic idea was that for perfect optical imaging, the total of all the information has to be used, which means, not only the amplitude of the light-wave as in usual optical imaging, but also the phase. In this manner a complete holo-spatial picture can be obtained. In 1947 coherent light sources were not available and the conventional light sources generally provided too little light or light that was too diffuse to obtain good results (DISPERSION of light). In May 1960 THEODORE MAIMAN at Hughes Research Laboratories built the first LASER, pulses of red light coming from a ruby cylinder that was put inside a very bright flash lamp with a helical shape. Maiman later received also a Nobel Prize for Physics, together with Townes and Schawlow, for the development of the laser theory.
LASER means: Light amplification by stimulated emission of radiation. This is a process for emitting electromagnetic radiation, typically light or visible light, via the process of stimulated emission. The emitted LASER LIGHT is usually a spatially COHERENT, narrow low-divergence beam, which can be manipulated with lenses (like in holography). In laser technology, “COHERENT LIGHT” denotes a light source that produces light waves of IDENTICAL FREQUENCY, PHASE and POLARIZATION. This also means that a laser beam has a precise mathematical property and can carry a large amount of intelligent information. When T. MAIMAN had produced the first laser, then the stage was set for the production of the first three –dimensional images and in 1960 the researchers LEITH and UPATNIEKS of the University of Michigan created the first off-axis laser transmission holograms followed around the same time by YURI DENISYUK of the Soviet Union who created reflection holograms that one could see using ordinary white light.
Characteristics of a hologram.
Holograms have unique characteristics:
1) On the same holographic (photographic) film one can record much more independent scenes, images of 3D objects, on the not yet developed film. It depends on the angle under which the reference beam hits the film and creates the interference pattern with the object beam. All these different scenes can be called up, by aiming a laser beam on the photographic film, under the same angle as the reference beam that was being used.
2) When you create a cylindrical hologram, you can see the original 3D object in a 360 degree view.
3) You can copy the Master hologram with the reference beam reflecting from the Master on a new photographic film and the result will be a positive image, exactly the same as the image produced by the original Master.
4) When a light wave hits the Master holographic film, it acts as a (diffractive) filter, allowing some light to pass through and pick up the imprint in the medium. However, unlike a photographic image, it is the interaction of the new light wave with the recorded interference pattern that is seen as a 3D image. It is 3D because the new light wave becomes modified by the wave-interference pattern recorded in the emulsion and appears thereby as the ORIGINAL light wave emitted by the object itself.
5) When the light wave hits the Master holographic medium and reflects from this film, this will contain the information originally recorded, multiplied by the intensity of the new light beam. This will be seen by the observing eye as a VIRTUAL image reconstructed in space and as a focused REAL image appearing either inside the witnessing eye or out in space just in front of it. A virtual image is the kind of image that you see when you look at yourself in the mirror, and the image appears to be behind the glass, while a real image is what you see when the image can be projected for instance on a screen, because the rays converge to focus the image.
6) Each piece of the hologram contains the interference pattern produced by adding the reference wave to the object (information) wave. Each point on the object wave acts as a tiny sender of spherically distributed information waves, and these waves hit all points on the surface of the recording medium together with the reference wave. This means that every tiny piece of the recording medium contains information from all points on the surface of the 3D object. This is where the idea of “the part contains the whole” comes from. However as the hologram size reduces, when you break it up in pieces, the result will be that the tiny pieces will show a loss of image perspective, resolution and brightness.
To understand the process of the making of a hologram, the term INTERFERENCE is very important and in the case of the holography we talk about the interference of light waves. What it basically means is the addition (superposition) of two or more waves which results in a new wave pattern. Although interference is a characteristic behavior of light, it is not solely an optical phenomenon. Interference also occurs between sound waves, as well as waves induced in a standing pool of water. A very nice and easy interference experiment can be performed, using a swimming pool and two stones. First let the water become very still, than simultaneously let two persons throw the stones into the water from opposite sides of the pool. Just as with light waves, the two stones will produce a series of waves in the water going in all directions. The waves that were formed in the area between the spaces where the stones entered the water will eventually collide. Where they collide in step, they will constructively add together to make a bigger wave and where they collide out of step they will destructively cancel each other out. The resulting wave pattern is called an INTERFERENCE PATTERN. Holography is entirely dependent on the interference of light to create its three-dimensional effects. In transmission holograms, both a reference and an object beam (reflecting from the 3D object) are reflected on a holographic film from the same sides. These beams interfere to produce light and dark interference wave field areas (See figure 3 and figure 4) and this interference pattern contains ALL the information of the original 3D object. The aim in holography is to record the complete wave field on the recording holographic film. This includes both amplitude and phase information.
-We describe the process for the reconstruction of the 3D image of Shroud data with holography. Copies of the 1931 Giuseppe Enrie originals were digitalized and were enhanced to improve the detail.-The gray levels in the Shroud data were then translated into depth data.-A sequence of up to 625 images was generated and these were combined with our Holoprinter into a 3D image of the Shroud.
In 1977 John Jackson and Eric Jumper analyzed photographs of the Shroud. In their experiments with volunteers they used a microdensitometer to follow the ridge lines to obtain a graphic record of the relationship of the image intensity of the cloth-to-body distance. They were able to conclude that Vignon’s hypothesis (that the image on the Shroud varies inversely with the cloth-to-body distance) was correct. They then subjected the image points of the Shroud photos to VP-8 image analysis. One of their findings was that their process resulted in an anatomically perfect 3D model. Since then more 2D to 3D conversion processes have been described (Tamburelli, Balossino,Italy and Galmarini in Argentina).
In our initial tests we used a commercial available tool. From the Shroud data we extracted a gray scale height image and used this to generate a 3D image of the Shroud.We made several views of this 3D model and we generated an Anaglyph image which made viewing in 3d possible using red and cyan glasses.This convinced us to continue, and more sophisticated methods were developed.A virtual camera moves in a horizontal line past the 3D model. Between the center view and the most left and right image all the other images (625) are interpolated.These sequences are used in the DFCH Holoprinter developed at the Dutch Holographic Laboratory in Eindhoven, the Netherlands.For the large size holograms (100×50 cm and 200×100 cm) we chose the one step process, because it is an easier possibility to produce large size holograms.
The theory which we have developed (1,2) is different to the existing theories. We compare making a:
1) Computer Generated Hologram (CGH) or a Multiple Photo Generated Hologram (MGPH) with making a:
2) Traditional Hologram (TH) of the same object. Because in the end we want the CGH to look the same as a Traditional Hologram (TH) of that original object.
By using a reversed reasoning we were able to develop this theory.The theory describes the exact recording geometries. Of course this way of recording is in itself astigmatic.The vertical parallax doesn’t change. The horizontal parallax does. Using a long enough H1-H2 distance takes care of this problem and the astigmatic effect is not noticeable.
We will summarize this theory (MPGH).
Theoretical principle of geometrical method for artificial holography
To perceive true perspective in an artificial hologram we made an analogy with a traditional hologram of an object. When this hologram is illuminated with its conjugated reference beam the recorded object focuses 3D in space.
If we slit a hologram, by cutting the hologram up, we could join these slits together and we would have of course the same hologram. Of course, if the slits are not adjacent to each other but a little apart the object would become distorted. Imagine we reconstructed the hologram of the object with the conjugate wave. If we only illuminate a small dot of the hologram the object will be reconstructed. It doesn’t focus in space though and we can project this image on a screen at any distance. A dot adjacent to the first dot will give a flat, slightly different perspective of the object.
Is it then possible to reverse this process, to make a 3D hologram using only 2D images (without the vertical parallax)?
Therefore we divided the hologram in 150 dots on a horizontal line. Instead of projecting the images produced by every dot on a screen, we use photographic paper instead. If we holograph every picture that is on the photographic paper in the right dot, we could record the same hologram as before and we reversed the process.
Instead of using photographic paper we can make slides of the real object at the position of the different slits and project these on a transparent screen. The slides have to be taken in such a way that the projection of these slides will give the same result on the screen as the photographic paper did. So how can we determine how to take the right perspectives and what parameters are involved?
The new approach:
We use the analogy with a traditional hologram as explained before.
We will look at two methods.
1) We can make pictures of a real 3D object.
2) We have a 3D object in a computer and a software camera makes the slides. The slides are then visualized on the computer screen. To transfer these pictures to the holographic setup we make photos of the screen (1:1).Alternatively we use a LCD screen in the setup.
We use the set-up proposed by King, de Bitteto, Benton, Newswanger, Molteni.