How does it work
The word "holography" is used to describe the process, since the prefix "holo" means "total" or "complete". Holography captures the image much more completely than a standard camera.
We can understand the basic principle of holography by comparing it with the recording and reproduction of sound. Think, for example, of a symphony orchestra playing a piece of classical music. The musical notes and tones produced by different instruments result in a complex sound pattern coming out of the orchestra. Obviously, this pattern can be recorded with the disc that "stores" the sound in a coded form (actually through variations in its tracks). When the recording is played, a sound pattern is created that duplicates the original orchestras. Identical sound waves have been recreated.
Holography also registers light waves for later reconstruction. Let's see how that is possible.
First of all, what does it mean to see another person, scene or other object? Since we can not see in the dark, sunlight or any other source is necessary. In fact, every little part of an object we look at reflects light, but in different amounts and colors. This creates a complex light pattern that emerges from the object as the orchestra's sound. We see the object when this pattern reaches our eyes and is interpreted by the brain.
Suppose the pattern of light waves coming from a friend sitting opposite you is interrupted and recorded, or "saved" to "memorize" the recording sound. Your friend gets up and leaves. When this "light registration" is "read", the same light pattern can be regenerated and thus the person seems to be visible in the eyes and brain again. Because the regenerated light duplicates the original (as in the case of sound reproduction), the displayed image is just like the person in full 3D form.
This is the biggest difference between photography and holography. Photography means that you get a flat image of a scene or person, as an artist would do, but holography reconstructs the original light pattern.
Creating the hologram
The recording where the light waves are "stored" is called a "hologram". It is essentially similar to film for a standard camera, but is of superior quality and is generally shaped like a photographic glass plate.
A light beam magnified by a laser is first divided into two parts by a special mirror. One part (called the "reference beam") moves directly towards the photographic plate, while the other part illuminates the object to be holographed. The intricate light pattern reflected from the object also travels on the photographic plate. The light then hits the two-way sign and creates a very detailed registration of the pattern on the sign.
The plate is developed first (as in a normal photograph) and the object is removed. Now a single beam of light is directed at the plate. The light passes through the plate, but is modified by the pattern embedded in the plate. The result is that the emerging light exactly mimics the object's original light and therefore the object appears to reappear. To the viewer, the photographic plate is like a window through which the object can be seen deeply. If you look through the "window" in different directions, the object will be seen from different angles. The picture shows such a living realism that the viewer may be tempted to touch it, but of course there is nothing!
Interesting features
Holograms and the images they produce have many strange and fascinating properties. The hologram plate corresponds in holography to the negatives obtained from normal film. In some ways, however, it's completely different. For example, if you have black and white negatives, hold them up to the light and you will find that they contain the image (on the contrary, dark areas are light and light areas are dark). Hold the holographic plate up to the light and you will see that it does not look like a picture. You can only see the relevant information under a microscope, but always in the form of a very irregular and incomprehensible pattern of lines, bumps and swirls.
Obviously, if any part of a normal negative is damaged or cut off, that part of the image will be destroyed or missing on prints made with negatives. However, break the glass plate of the hologram and you will be surprised. The entire image can be reconstructed from all sides! The quality degrades slightly depending on the size of the part. Nevertheless, the image always remains complete!
The three-dimensional realism of the image produced by the holograms is evident in several ways. Changing the viewing position through the “window” (holographic glass plate) changes the perspective of the image as if you were watching the original scene. If something in the foreground of the image is obstructing an object behind, you can look to move your head to the side and see the hidden object. You will also find that the focus of your eyes will change as you look at the near and long points in the scene, and if you are nearsighted, your glasses will help!
An interesting effect occurs when we talk about a diamond ring with holography. In the holographic image, the diamond reflects the bright light of its aspects and these appear and disappear when the viewer moves their head, just like the real diamond!
In short, the reconstruction has all the visual properties of the real thing.