Holography is ... The concept, the principle of action, the application

The holographic image is now increasingly used. Some even believe that it can eventually replace the known means of communication. So it or not, but already now it is actively used in a wide variety of industries. For example, we all know holographic stickers. Many manufacturers use them as a means of protection against forgery. The photo below shows some holographic stickers. Their use is a very effective way of protecting goods or documents from forgery.

History of studying holography

The three-dimensional image obtained as a result of the refraction of the rays began to be studied relatively recently. However, we can already talk about the existence of a history of his study. Dennis Gabor, an English scientist, in 1948 for the first time determined what holography is. This discovery was very important, but its great significance at that time was not yet obvious. The researchers who worked in the 1950s suffered from the absence of a light source with coherence, a very important property for the development of holography. The first laser was manufactured in 1960. With this device, it is possible to obtain light having sufficient coherence. Юрис Упатниекс and Иммет Лейт, the American scientists, used it for creation of the first holograms. With their help, three-dimensional images of objects were obtained.

In subsequent years, research continued. Hundreds of scientific articles in which the concept of holography has been studied have since been published, and a number of books devoted to this method have been published. However, these works are addressed to specialists, and not to the general reader. In this article, we will try to talk about all the available language.

What is holography?

We can propose the following definition: holography is a volumetric photograph obtained by means of a laser. However, this definition is not entirely satisfactory, since 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 we get a three-dimensional image that looks like a real object; The use of lasers played a decisive role in its development.

Holography and its application

The study of holography makes it possible to clarify many questions connected with the ordinary photograph. As a visual art, a 3D image can even challenge the latter, because it allows you to reflect the surrounding world more accurately and correctly.

Scientists sometimes distinguish epochs in the history of mankind by means of communication, which were known in those or other centuries. You can say, for example, about the hieroglyphics that existed in Ancient Egypt, about the invention of a printing press in 1450 . In connection with the technical progress observed in our time, new means of communication, such as television and telephones, have assumed a dominant position. Although the holographic principle is still in its infancy, if we talk about its use in the media, there are reasons to believe that the devices based on it in the future will be able to replace the known means of communication or at least extend the scope of their use.

Sci-fi literature and the mass press often present a holography in the wrong, distorted light. They often create a misconception about this method. The volumetric image seen for the first time, bewitches. However, the physical explanation of the principle of its structure is no less impressive.

Interference pattern

The ability to see objects is based on the fact that light waves, refracted by them or reflected from them, fall into our eyes. The light waves reflected from an object are characterized by the shape of the wave front corresponding to the shape of this object. A picture of dark and light bands (or lines) creates two groups of light coherent waves that interfere. This is how volume holography is formed. In this case, these bands in each specific case constitute a combination that depends only on the shape of the wave fronts of the waves, which interact with each other. This picture is called interference. It can be fixed, for example, on a photographic plate, if you place it in a place where interference of waves is observed .

The variety of holograms

In a way that allows you to record (record) the wave front reflected from the object, then restore it so that the observer thinks he sees the real object, and is holography. This is an effect, which is explained by the fact that the resulting image is three-dimensional to the same extent as the real object.

There are many different types of holograms in which it is easy to get confused. To uniquely identify a particular species, you must use four or even five adjectives. Of all their many, we will consider only the main classes that modern holography uses. However, first we need to tell a little about such a wave phenomenon as diffraction. It allows us to design (or rather, reconstruct) the wave front.

Diffraction

If an object is in the path of light, it casts a shadow. The light goes around this object, going partially into the shadow area. This effect is called diffraction. It is explained by the wave nature of light, but it is strictly difficult to explain it strictly.

Only in a very small angle does light penetrate into the shadow region, so we almost do not notice it. However, if on its way there are many small obstacles, the distances between which make up only a few wavelengths of the light wave, this effect becomes quite noticeable.

If the wave front falls on a large single obstacle, the corresponding part of it falls off, which practically does not affect the remaining region of the given wavefront. If a lot of small obstacles are in its way, it changes as a result of diffraction so that the light propagating beyond the obstacle will have a qualitatively different wave front.

Transformation is so strong that light begins to spread even in the other direction. It follows that diffraction allows us to transform the original wave front into a completely different one from it. Thus, diffraction is the mechanism by which we obtain a new wave front. The device forming it in the manner described above is referred to as a diffraction grating. Let's talk about it in more detail.

Diffraction grating

It is a small plate with thin straight parallel parallel lines (lines) applied on it. They are spaced apart by a hundredth or even a thousandth of a millimeter. What happens if the laser beam on its way meets a grating that consists of several blurry dark and bright bands? Its part will go straight through the lattice, and part - bend. Thus, two new beams are formed which leave the grating at a certain angle to the original ray and are on either side of it. In the event that one laser beam has, for example, a plane wave front, two new beams formed laterally from it will also have plane wavefronts. Thus, by passing a laser beam through the diffraction grating, we form two new wavefront (flat). Apparently, the diffraction grating can be considered as the simplest example of a hologram.

Registration of the hologram

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 (first) in holography is called the recording (or registration) of the hologram.

Image recovery

We will assume that one of the plane waves is A, and the second is B. The wave A is called the reference wave, and B is the object wave, that is, reflected from the object whose image is fixed. It can not differ anything from the reference wave. However, when creating a hologram of a three-dimensional real object, a much more complex wave front of light reflected from the object is formed.

The interference pattern presented on the photographic film (that is, the image of the diffraction grating) - this is the hologram. It can be placed in the path of the reference primary beam (a beam of laser light having a plane wave front). In this case, two new wave fronts are formed on both sides. The first of them is an exact copy of the wave subject front, which propagates in the same direction as wave B. The above described stage is called image reconstruction.

Holographic process

The interference pattern produced by two plane coherent waves, after recording it on a photographic plate, is a device that, in the case of illumination of one of these waves, can reconstruct another plane wave. Thus, the holographic process has the following stages: registration and subsequent "storage" of the wave subject front in the form of a hologram (interference pattern), and its restoration after any time as the reference wave passes through the hologram.

The objective wave front can in fact be any. For example, it can be reflected from some real object, if it is a coherent reference wave. Formed by any two wave fronts with coherence, the interference pattern is a device that allows one of these fronts to be transformed into another by diffraction. It is here that the key to the phenomenon, like 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 is being used to read holograms - a holographic projector. It allows you to convert a picture from two-to three-dimensional. However, in order to view simple holograms, a holographic projector is not required at all. Let's briefly describe how to view such images.

To observe the image formed by the simplest hologram, it is necessary to place it approximately at a distance of 1 meter from the eye. Through the diffraction grating, one must look in the direction in which the plane waves (reconstructed) emerge from it. Since it is the plane waves that fall into the observer's eye, the holographic image is also flat. It appears before us like a "blind wall", which is uniformly illuminated by light having 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 located. It seems as if you are looking at an extended wall located in infinity, but you only see a part of it that you can see through a small "window", that is, a hologram. Consequently, a hologram is a uniformly luminous surface on which we do not notice anything worthy of attention.

The diffraction grating (hologram) allows us to observe several simple effects. They can also be demonstrated using holograms of a different type. Passing through the diffraction grating, the light beam splits, two new beams are formed. With the help of laser radiation beams, it is possible to illuminate any diffraction grating. In this case, the radiation should differ from the color used when it was recorded. The angle of bending the beam of color depends on what color it has. If it is red (the longest wave), then such a beam bends at a greater angle than a blue beam that has the smallest 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 curved under its own angle. At the output, a spectrum similar to that created by the prism is formed.

Placing of the grating strokes

The gratings of the diffraction grating should be made very close to each other so that the curvature of the rays is noticeable. For example, to curl a 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 ray of light begins to bend even more. To "record" this grid, you need a photographic plate that is capable of recording so thin details. In addition, it is necessary that the plate remains perfectly still during the exposure, as well as during registration.

The picture can be significantly smeared, even with the slightest movement, and so much that it will be completely indistinguishable. In this case we will see not an interference pattern, but a glass plate, uniformly black or gray throughout its surface. Of course, in this case, the diffraction effects produced by the diffraction grating will not be reproduced.

Transmission and reflection holograms

The diffraction grating considered by us is referred to as a transmission grating, since it acts in the light passing through it. If we apply the grid lines not to the transparent plate, but to the surface of the mirror, we get a diffraction grating reflective. It reflects the light of different colors from different angles. Accordingly, there are two large classes of holograms - reflective and transmissive. The former are observed in reflected light, and the latter in the transmitted light.