The holographic image is increasingly used today. Some even believe that over time it can 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 all know holographic stickers. Many manufacturers use them as a means of protection against counterfeiting. The photo below shows some holographic stickers. Their use is a very effective way to protect goods or documents from counterfeiting.
History of the study of holography
The volumetric image obtained as a result of the refraction of rays has begun to be studied relatively recently. However, we can already talk about the existence of a history of its study. Dennis Gabor, an English scientist, in 1948 first determined what holography is. This discovery was very important, but its great significance at that time was not yet obvious. Researchers working in the 1950s suffered from a lack of a coherent light source, 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. 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 articles in which the concept of holography was studied have since been published, and many 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 available language.
What is holography?
One can propose the following definition: holography is a volumetric photograph obtained with 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, a three-dimensional image is obtained that looks like a real object; The use of lasers has played a decisive role in its development.
Holography and its application
The study of holography allows you to clarify many issues related to conventional photography. As a fine art, a three-dimensional image can even challenge the latter, since it allows you to reflect the world around you more accurately and correctly.
Scientists sometimes distinguish epochs in the history of mankind by the means of communication that were known in certain centuries. You 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, if we talk about its use in the media, there is reason to believe that the devices based on it can replace the means of communication we know in the future, or at least expand the scope of their application.
Sci-fi literature and mass print often present holography in the wrong, distorted light. They often create a misconception about this method. The 3D image seen for the first time is fascinating. However, no less impressive is the physical explanation of the principle of its structure.
Interference pattern
The ability to see objects is based on the fact that light waves, being refracted by them or reflected from them, get into our eye. Light waves reflected from a certain object are characterized by the wavefront shape corresponding to the shape of this object. The picture of dark and light bands (or lines) is created by two groups of coherent light waves that interfere. So volumetric holography is formed. In this case, these bands in each case make up a combination that depends only on the shape of the wave fronts of the waves that 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 wave interference is observed .
Variety of holograms
A way to record (register) the wavefront reflected from an object, and then restore it so that it seems to the observer that he sees a real object, is holography. This is an effect that is explained by the fact that the resulting image is three-dimensional in the same measure as the real object.
There are many different types of holograms that are easy to get confused about. In order to uniquely identify a particular species, four or even five adjectives should be used. Of all the many, we will consider only the main classes that modern holography uses. However, first you need to talk 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 goes 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 quite difficult to explain it strictly.
Only in a very small angle does light penetrate into the shadow region, so we hardly notice it. However, if there are many small obstacles in its path, the distances between which are only a few wavelengths of light, this effect becomes quite noticeable.
If a wave front falls on a large single obstacle, its corresponding part "falls out", which practically does not affect the remaining region of this wave front. If many small obstacles are in its path, it changes as a result of diffraction so that the light propagating behind the obstacle will have a qualitatively different wavefront.
The transformation is so strong that the light even begins to propagate in a different 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 wavefront. The device forming it in the manner described above is called a diffraction grating. We will tell you more about it.
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 in its path meets a grating, which consists of several blurry dark and bright stripes? Part of it will go straight through the grate, and part will be bent. Thus, two new beams are formed, which exit the grating at a certain angle to the initial beam and are located on both sides of it. If one laser beam has, for example, a plane wave front, two new beams formed on the sides of it will also have plane wave fronts. Thus, passing a laser beam through the diffraction grating, we form two new wave fronts (plane). Apparently, the diffraction grating can be considered as the simplest example of a hologram.
Hologram Registration
Familiarity with 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 the 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 assume that one of the plane waves is A, and the second is B. Wave A is called the reference wave, and B is the object wave, that is, reflected from the object whose image is fixed. It may not differ from the reference wave. However, when creating a hologram of a three-dimensional real object, a much more complex wavefront of the light reflected from the object is formed.
The interference pattern presented on the photographic film (that is, the image of the diffraction grating) is the hologram. It can be placed in the path of the reference primary beam (a laser light beam with a plane wave front). In this case, 2 new wave fronts are formed on both sides. The first of them is an exact copy of the wave front of the object, which propagates in the same direction as wave B. The above stage is called image restoration.
Holographic process
The interference pattern created by two plane coherent waves, after it is recorded on a photographic plate, is a device that allows one to restore the other plane wave in the case of illumination of one of these waves. Thus, the holographic process has the following stages: registration and subsequent “storage” of the wave front in the form of a hologram (interference pattern), and its restoration after any time when the reference wave passes through the hologram.
The real wavefront can actually be anything. For example, it can be reflected from some real object, if it is at the same time a coherent reference wave. Formed by any two wavefronts with coherence, the interference pattern is the device that allows one of these fronts to be converted to another by diffraction. 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 a hologram image
Nowadays, a special device, a holographic projector, is beginning to be used to read holograms. 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. We will 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. Through the diffraction grating, one must look in the direction in which the plane waves (reconstructed) leave it. Since it is the plane waves that fall into the eye of the observer, the holographic image is also flat. It appears before us like a “blank wall”, which is uniformly illuminated by light having the same color as the corresponding laser radiation. Since there are no specific signs of this “wall”, it is impossible to determine how far it is located. It seems as if you are looking at an extended wall located at infinity, but at the same time you can only see a part of it that can be seen 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.
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. Laser beams can illuminate any diffraction grating. In this case, the radiation should differ in color from that used when recording it. The bending angle of the color beam depends on what color it has. If it is red (the longest wavelength), then such a beam bends at a greater angle than the blue beam, which has the shortest wavelength.
Through the diffraction grating, you can skip the mixture of all colors, that is, white. In this case, each color component of this hologram is bent at its own angle. At the output, a spectrum is formed similar to the created prism.
Grid Stroking
The strokes of the diffraction grating should be made very close to each other so that the curvature 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 ray of light begins to bend even more. To “record” this grating, you need a photographic plate that is capable of recording so thin details. In addition, it is necessary that the plate during the exposure, as well as during registration, remain completely motionless.
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 not see an interference pattern, but simply a glass plate, uniformly black or gray over its entire surface. Of course, in this case, the diffraction effects formed by the diffraction grating will not be reproduced.
Transmit and reflect holograms
The diffraction grating considered by us is called transmissive, because it acts in the light passing through it. If we apply the lines of the grating not on a transparent plate, but on the surface of the mirror, we get a diffraction grating reflective. It reflects light from various angles at different angles. Accordingly, there are two large classes of holograms - reflective and transmissive. The former are observed in reflected light, and the latter in transmitted light.