The holographic image is increasingly used today. Some even believe that it may eventually replace the means of communication known to us. Like it or not, but now it is actively used in a variety of industries. For example, we are all familiar with holographic stickers. Many manufacturers use them as a means of protection against counterfeiting. The photo below shows some of the holographic stickers. Their use is a very effective way to protect goods or documents from forgery.
History of the study of holography
The three-dimensional image resulting from the refraction of rays began to be studied relatively recently. However, we can already talk about the existence of a history of its study. Dennis Gabor, an English scientist, first defined holography in 1948. This discovery was very important, but its great significance at that time was not yet obvious. Researchers working in the 1950s suffered from the lack of a coherent light source, a very important property for the development of holography. First laserwas made in 1960. With this device it is possible to obtain light having sufficient coherence. Juris Upatnieks and Immet Leith, American scientists, used it to create the first holograms. With their help, three-dimensional images of objects were obtained.
In subsequent years, research continued. Hundreds of scientific papers exploring the concept of holography have since been published, and many books have been published on the method. However, these works are addressed to specialists, not to the general reader. In this article we will try to tell about everything in an accessible language.
What is holography
The following definition can be proposed: holography is a three-dimensional photograph obtained using a laser. However, this definition is not entirely satisfactory, as there are many other types of three-dimensional photography. Nevertheless, it reflects the most significant: holography is a technical method that allows you to "record" the appearance of an object; with its help, a three-dimensional image is obtained that looks like a real object; the use of lasers played a decisive role in its development.
Holography and its applications
The study of holography allows us to clarify many issues related to conventional photography. As a visual art, three-dimensional imaging can even challenge the latter, as it allows you to reflect the world around you more accurately and correctly.
Scientists sometimes single out eras in the history of mankind by meansconnections that were known in certain centuries. We can talk, for example, about the hieroglyphs that existed in ancient Egypt, about the invention of the printing press in 1450. In connection with the technological progress observed in our time, new means of communication, such as television and telephone, have taken a dominant position. Although the holographic principle is still in its infancy when it comes to its use in the media, there are reasons to believe that devices based on it in the future will be able to replace the means of communication known to us, or at least expand their scope.
Sci-fi literature and mainstream print often portray holography in the wrong, distorted light. They often create a misconception about this method. The volumetric image, seen for the first time, fascinates. However, no less impressive is the physical explanation of the principle of its device.
Interference pattern
The ability to see objects is based on the fact that light waves, refracted by them or reflected from them, enter our eye. Light waves reflected from some object are characterized by the shape of the wave front corresponding to the shape of this object. The pattern of dark and light bands (or lines) is created by two groups of coherent light waves that interfere. This is how a volumetric holography is formed. In this case, these bands in each particular case constitute a combination that depends only on the shape of the wave fronts of the waves that interact with each other. Suchthe picture is called interference. It can be fixed, for example, on a photographic plate, if placed in a place where wave interference is observed.
Variety of holograms
The method that allows you to record (register) the wave front reflected from the object, and then restore it so that it seems to the observer that he sees a real object, and is holography. This is an effect due to the fact that the resulting image is three-dimensional in the same way as the real object.
There are many different types of holograms that are easy to get confused about. To unambiguously define a particular species, four or even five adjectives should be used. Of all their set, we will consider only the main classes that are used by modern holography. However, first you need to tell a little about such a wave phenomenon as diffraction. It is she who allows us to construct (or rather, reconstruct) the wave front.
Diffraction
If any object is in the path of light, it casts a shadow. Light bends around this object, partially entering the shadow area. This effect is called diffraction. It is explained by the wave nature of light, but it is rather difficult to explain it strictly.
Only in a very small angle does light penetrate the shadow area, so we hardly notice it. However, if there are many small obstacles in its path, the distance between which is only a few wavelengths of light, this effect becomes quite noticeable.
If the fall of the wave front falls on a large single obstacle, the corresponding part of it "falls out", which practically does not affect the remaining area of this wave front. If there are many small obstacles in its path, it changes as a result of diffraction so that the light propagating behind the obstacle will have a qualitatively different wave front.
The transformation is so strong that the light even begins to spread in the other direction. It turns out that diffraction allows us to transform the original wavefront into a completely different one. Thus, diffraction is the mechanism by which we obtain a new wave front. The device that forms it in the above way is called a diffraction grating. Let's talk about it in more detail.
Diffraction grating
This is a small plate with thin straight parallel strokes (lines) applied on it. They are separated from each other by a hundredth or even a thousandth of a millimeter. What happens if a laser beam meets a grating on its way, which consists of several blurry dark and bright stripes? Part of it will go straight through the grate, and part will bend. Thus, two new beams are formed, which exit the grating at a certain angle to the original beam and are located on both sides of it. If one laser beam has, for example, a flat wave front, two new beams formed on the sides of it will also have flat wave fronts. Thus, passing throughdiffraction grating laser beam, we form two new wavefronts (flat). Apparently, a diffraction grating can be considered as the simplest example of a hologram.
Hologram Registration
Introduction to the basic principles of holography should begin with the study of two plane wave fronts. Interacting, they form an interference pattern, which is recorded on a photographic plate placed in the same place as the screen. This stage of the process (the first) in holography is called the recording (or registration) of the hologram.
Image restoration
We will assume that one of the plane waves is A, and the second is B. Wave A is called the reference wave, and B is called the object wave, that is, reflected from the object whose image is fixed. It may not differ in any way from the reference wave. However, when creating a hologram of a three-dimensional real object, a much more complex wavefront of light reflected from the object is formed.
The interference pattern presented on photographic film (that is, the image of a diffraction grating) is a hologram. It can be placed in the path of the reference primary beam (a beam of laser light with a flat wave front). In this case, 2 new wave fronts are formed on both sides. The first of these is an exact copy of the object wave front, which propagates in the same direction as wave B. The above stage is called image reconstruction.
Holographic process
The interference pattern created by twoplane coherent waves, after its recording on a photographic plate, it is a device that allows, in the case of illumination of one of these waves, to restore another plane wave. The holographic process, therefore, has the following stages: registration and subsequent "storage" of the wave object front in the form of a hologram (interference pattern), and its restoration after any time when the reference wave passes through the hologram.
The objective wave front can actually be anything. For example, it can be reflected from some real object, if at the same time it is coherent to the reference wave. Formed by any two wave fronts with coherence, the interference pattern is a device that allows, due to diffraction, to transform one of these fronts into another. It is here that the key to such a phenomenon as holography is hidden. Dennis Gabor was the first to discover this property.
Observation of the image formed by the hologram
In our time, a special device, a holographic projector, is beginning to be used to read holograms. It allows you to convert an image from 2D to 3D. However, in order to view simple holograms, a holographic projector is not required at all. Let's briefly talk about how to view such images.
To observe the image formed by the simplest hologram, you need to place it at a distance of about 1 meter from the eye. You need to look through the diffraction grating in the direction in which the plane waves (reconstructed) come out of it. Since it is plane waves that enter the observer's eye, the holographic image is also flat. It appears to us like a "blind wall", which is evenly illuminated by light that has the same color as the corresponding laser radiation. Since this "wall" is devoid of specific features, it is impossible to determine how far it is. It seems as if you are looking at an extended wall located at infinity, but at the same time you see only a part of it, which you can see through a small "window", that is, a hologram. Therefore, a hologram is a uniformly luminous surface on which we do not notice anything worthy of attention.
Diffraction grating (hologram) allows us to observe several simple effects. They can also be demonstrated using other types of holograms. Passing through the diffraction grating, the light beam is split, two new beams are formed. Laser beams can be used to illuminate any diffraction grating. In this case, the radiation should differ in color from that used during its recording. The bending angle of a color beam depends on what color it has. If it is red (the longest wavelength), then such a beam is bent at a greater angle than the blue beam, which has the shortest wavelength.
Through the diffraction grating, you can skip a mixture of all colors, that is, white. In this case, each color component of this hologram is bent at its own angle. The output is a spectrumsimilar to that created by a prism.
Diffraction grating stroke placement
The strokes of the diffraction grating should be made very close to each other so that the bending of the rays is noticeable. For example, to bend the red beam by 20°, it is necessary that the distance between the strokes does not exceed 0.002 mm. If they are placed more closely, the light beam begins to bend even more. To "record" this grating, a photographic plate is needed, which is capable of registering such fine details. In addition, it is necessary that the plate remains completely motionless during exposure, as well as during registration.
The picture can be significantly blurred even with the slightest movement, and so much so that it will be completely indistinguishable. In this case, we will see not an interference pattern, but simply a glass plate, uniformly black or gray over its entire surface. Of course, in this case, the diffraction effects generated by the diffraction grating will not be reproduced.
Transmission and reflective holograms
The diffraction grating we have considered is called transmissive, since it acts in the light passing through it. If we apply the grating lines not on a transparent plate, but on the surface of a mirror, we will get a reflective diffraction grating. It reflects different colors of light from different angles. Accordingly, there are two large classes of holograms - reflective and transmissive. The former are observed in reflected light, while the latter are observed in transmitted light.
