Laser Holographic Image Technology

Ordinary photography uses the principle of lens imaging to record the light intensity distribution of the diffuse reflection light of the subject on the "negative" (today's negative can be digitized) to obtain a two-dimensional plane image. The relationship between objects in ordinary photography is point-by-point correspondence, and the image cannot be reproduced as long as the negative is damaged. Ordinary photography on the external environment is not high, the general conditions can be photographed.

The laser can also take pictures, and in addition to recording the intensity distribution of the diffuse reflection light of the subject, it can also record the interference information of the diffuse reflection light. The "holographic image" obtained by these two types of information is more "fine", more "rich" and more "comprehensive" than the general image ". This is the laser holography technology, it has many wonderful applications.

1 The principle of holography

Laser holograms should be taken in a dark room. The recording optical path of the hologram is shown in the figure below.

After passing through the shutter, the laser beam is divided into two beams by the beam splitting plate:

A beam called the reference light, reflected by a plane mirror, beam expander, directly projected onto the holographic plate. Another beam is called object light, which is reflected and expanded by a plane mirror to illuminate the subject, and its diffuse reflected light is also accepted on the holographic dry plate. The two beams of light are on the same plane, and the angle between the object light and the reference light is about 45 °. Interference fringes are formed on the holographic dry plate and recorded.

Here are a few questions that need to be explained:

Why use a "beam expander"? Because the beam emitted by the laser is too concentrated and the irradiated area is small, the beam expander can expand the irradiated area.

What is "holographic dry plate" is made of glass, one side is coated with a layer of viscous photosensitive agent, in the red or green laser irradiation, attached to the photosensitive agent will be exposed, so as to record the dry plate received, and fine and dense interference fringes, light intensity distribution.

What is the information of the interference fringes recorded by the "holographic dry plate? If there is only diffuse reflection of object light, the dry plate records the general plane photo of the object. If the dry plate receives both reflected light and reference laser, the dry plate records interference fringes. The interference fringes include both diffuse reflection intensity of object light and phase information of diffuse reflection light (interference fringes will not be generated because there is no phase difference), as well as size, shape, brightness and contrast of the recorded object. Therefore, the interference fringes recorded on the "holographic dry plate" are the holograms of the subject.

Of course, the "holographic dry plate" can now be digitized without photosensitive materials, just like the recording method of digital cameras.

II. Advantages and disadvantages of holography

Holographic technology has the following outstanding features and advantages:

(1) three-dimensional sense of strong. Because the information on the surface of the object related to the light, not only the intensity of the light but also the phase of the light are recorded in the hologram, the information on the surface of the object is reproduced in great detail.

(2) The stereoscopic image of the object can be reproduced. If the object light is illuminated from multiple angles, and the holograms taken for multiple times are concentrated in one hologram, then the hologram is illuminated with reference light from different angles, and the images taken for each time can be displayed separately without interfering with each other, thus the 3-dimensional image of the object can be reproduced.

(3) The fragments of the hologram can still reflect the image of the whole object.? Because all parts of the holographic image contain the expression of the overall interference, a small part of the hologram can store and reproduce the image of the entire object.

(4) Large information capacity. Holograms are used to store information, which is much larger than the storage capacity of magnetic disks and optical disks.

The disadvantage of holographic technology is that the shooting environment is demanding and the shooting cost is high.

Three. Application of Laser Holography

1. Laser Holographic Image

The observer does not move, you can see the three-dimensional effect of the object, you can see the visual effect that does not belong to their own. Photographing the museum's collection with holographic technology, its observation effect is excellent.

Using holographic technology to make images, using water vapor as a curtain wall, the image can be reproduced on the water mist curtain wall.

Using holographic technology to make virtual images, combined with solid images, the observer will have a dreamy feeling of being in a three-dimensional situation.

2. Laser holographic anti-counterfeiting

Several regular holographic anti-counterfeiting technologies are listed below.

Multi-channel holographic anti-counterfeiting technology. When you turn the holographic anti-counterfeiting identification, you will see different patterns.

Stealth encryption technology. Place the encryption pattern on any position of the logo, and the encryption pattern can be seen with a laser.

Double-layer holographic technology. The first security layer is uncovered, and the second security layer is uncovered.

Fluorescence encryption holography. The same as the principle of RMB fluorescence encryption.

Dynamic encoding anti-counterfeiting technology. Slowly rotating the logo will show a pattern of continuous action.

Code holographic anti-counterfeiting technology. Put the manufacturer's phone number in the holographic anti-counterfeiting logo.

Early laser holograms can only be observed by laser irradiation at a certain angle, which is complicated in process, high in cost and low in efficiency.

Now, the use of mold laser holographic method, first with a special photosensitive material coated on the dry plate, holographic exposure, light corrosion photosensitive material, after processing can be like a relief hologram, and then the production of mold hologram master. The surface of the master is filled with uneven interference fringes, and its density can reach more than one thousand per millimeter. The metal master is installed on the molding machine and pressed on the film material with a certain pressure to obtain the holographic film material for anti-counterfeiting identification. This method can copy a large number of laser holographic anti-counterfeiting marks, which brings a leap to the application of laser holographic technology, the method also solves the problem of secondary transfer of anti-counterfeiting trademarks, and the integration of hot stamping anti-counterfeiting marks and packaging makes laser anti-counterfeiting technology develop commercially.

Laser holographic image anti-counterfeiting technology is still developing.

1) Application of computer image processing technology to improve laser holographic images

One technique is to use a computer to synthesize a two-dimensional image to form a general optical image, and then do holographic imaging to make an anti-counterfeiting mark. The advantage of this method is that both graphics and colors have an unusually flexible and dynamic effect.

Another technique is to use a computer to directly control two coherent light beams to form a holographic pattern. This method does not need to photograph the subject, does not need to control the exposure, the required graphics are completely computer generated.

2) Transparent laser holographic image

The holographic image is embossed on a transparent material. For example, in 1996, the Ministry of Public Security of my country applied a transparent laser holographic image to the resident ID card, covering the ID card with a transparent film as a whole. When observing the front of the ID card under light, not only the content of the ID can be seen clearly, but also the transparent film. Two-dimensional and three-dimensional holographic images (Chinese and English characters of "Great Wall" and "China") appeared on it.

3) laser holographic reading anti-counterfeiting

The text and image information are stored together in the holographic image.

4) optical miniature laser holographic anti-counterfeiting

The text information is recorded on the hologram in an optical miniature manner, which is usually difficult to identify with the naked eye, and the specific content can only be observed under a 10-fold or even 100-fold magnifying glass.

5) Low frequency lithography laser holographic anti-counterfeiting

Additional interference fringe patterns are printed on the hologram, and the density of the pattern fringes is lower. The pattern stripe can be a general diffraction stripe to increase the external observation effect; the pattern stripe can also be a computer-generated holographic stripe, so that there are two sets of anti-counterfeiting stripes in the hologram.

6) random laser interference fringe anti-counterfeiting

When making holograms, a random mechanism is introduced to record random interference patterns on holograms. This pattern has obvious characteristics and cannot be repeated. Even if the same person uses the same process at different times, the patterns produced are different. Therefore, it is a good anti-counterfeiting method. In addition to static plane interference fringes, the current has been developed to dynamic, three-dimensional interference fringes, counterfeits can not be copied.

7) key laser interference anti-counterfeiting

The two sets of holograms cannot be distinguished by ordinary means, and only when they are overlapped can the desired image be revealed.

8) Combined laser holographic anti-counterfeiting

Holograms of dozens of two-dimensional images are combined together. The combination can be static, resulting in a static hologram; the combination can also be dynamic, resulting in a dynamic holographic video.

In short, all kinds of anti-counterfeiting technologies can be combined with holographic images, and anti-counterfeiting means are constantly improving, which can be said to be endless.