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Holographic technology originated in the 1940s. The British scientist Gabor obtained the hologram and its reproduced image for the first time, laying a foundation for the development of holography. More than ten years later, the emergence of laser provides a holographic light source with good coherence. Laser holography technology has been rapidly developed and widely used. Since the laser holography technology was introduced into China in the 1980s and developed in the field of anti-counterfeiting, the 1990s was the heyday of laser holography anti-counterfeiting. The most widely used laser holographic image is the laser rainbow molded hologram. Let's talk about the related technical principles of laser rainbow molded hologram.
1. Laser holography
The most widely used laser holography technology is used to make rainbow holograms. The production process is completed in three stages, namely, laser hologram master production, electroformed metal stamping plate, and rainbow hologram stamping replication. Here, we briefly introduce the three stages.
1. The principle of laser holography
Laser holography refers to the process of recording the image information of the object we need on the photosensitive carrier by the method of laser interference, and then processing the technology such as light diffraction to form a rainbow hologram that can be reproduced under visible light. Simply put, laser holography is interference recording and diffraction reproduction.
The coherent laser beam emitted by the interference recording laser is divided into two beams by a beam splitter. One beam is called a reference beam, which is irradiated on a photosensitive carrier after passing through a mirror and a beam expander (generally photoresist Photoresist), another beam of light is called object light, which is irradiated on the object after passing through the optical lens group. The object light reflected by the object carries the optical information of the object and meets the reference light on the photosensitive carrier. And form interference fringes on the photosensitive carrier. This interference fringe records all the information of the object we photographed, including light intensity information and phase information. This photosensitive carrier is developed and fixed, which is the laser hologram we took. This kind of photo can't see the image under ordinary illumination, only the holographic image can be seen under the illumination of the laser reference beam. If you want to enjoy the exquisite hologram under the condition of ordinary light source, you must carry out the second step of the shooting process, that is, diffraction reproduction.
The light emitted by the diffraction reproduction laser is divided into two, and a reproduction beam (that is, the object beam of the second shot) is irradiated on the laser hologram obtained in the first step, and a horizontal slit is placed in front of the laser hologram The baffle of the laser hologram through the slit passes through the slit, irradiates another new photosensitive carrier, and records the optical information of the laser hologram; the other reference light and the reproduced light meet in the new Interference fringes containing optical information are also formed on the photosensitive carrier. The hologram obtained by developing and fixing the exposed photosensitive carrier is a rainbow hologram that can be directly appreciated under ordinary light sources.
In order to get a high-quality rainbow hologram, we must meet three conditions when we make a hologram: first, use a light source with good coherence, and laser is such a coherent light source; second, the object light and reference light when taking pictures The smaller the optical path difference, the better. The light intensity ratio of the two beams is generally controlled between 1: 1 and 1:10, and the angle between the two beams is generally controlled between 30 ° and 60 °. Angle, you can get different color images; third, the camera system and the environment must be stable, the laser, optical lens, the object being shot, and the photosensitive carrier should be fixed on a shockproof platform, if the air cushion shockproof platform is better, take pictures During this period, the air flow and temperature difference in the photo room should be avoided. A certain depth of anti-vibration trench should be dug around the photo room to prevent the vibration of the external environment from affecting the change of the optical path.
The rainbow hologram obtained in this way cannot be copied in large quantities, and a molding process is introduced in industrial production for mass copying. Before molding, we need to prepare more molds. These molds are called embossed plates. The process of making embossed plates is called electroforming.
2. The process of electroforming metal plate
What the laser hologram obtained is a photoresist plate with a rainbow hologram engraved on the surface (photosensitive carrier on the glass plate base). From the microscopic view, it is a relief-type concave and convex groove, which contains the light intensity information and phase information of the object. To copy it is to copy the same or opposite concave and convex grooves completely. This process is completed in two stages:
First, copy the graphic information (concave and concave grooves) on the photoresist plate to the nickel metal plate. This process is divided into four steps: (1) The photoresist surface cleaning treatment. In order to successfully complete the reaction of the silver mirror and the solidity of the silver layer, the oil stains and impurities on the surface of the photoresist layer must be removed. Since the photoresist (photoresist) is easily soluble in an alkaline environment, it must be neutral. Wash with detergent and rinse with deionized water. (2) Sensitization of photoresist surface. The surface of the photoresist is treated with a stannous chloride solution to form uniformly distributed stannous ions on the surface. These stannous ions are the reaction centers in the silver mirror reaction. (3) The silver mirror reaction on the surface of the photoresist plate, that is, a fine and uniform silver layer (3 ~ 5μm) is chemically plated on the surface of the photoresist plate to metallize its surface, which can be used as the next step in electroforming Electrode. (4) Copying by electroforming. Place the metallized photoresist plate in the electroforming bath as the cathode in the electrochemical reaction, the metal nickel block as the anode, the electroforming solution is generally nickel sulfamate, after a certain period of reaction, a piece of metal is formed on the surface of the cathode Nickel plate. This nickel plate is tightly coupled with the photoresist plate. The concave and convex grooves are completely opposite. After stripping, this is a molded master that is completely opposite to the photoresist plate.
Second, copy more molded working plates from the molded master. This process is also completed in the electroforming tank. Before the master enters the tank, it needs to be surface passivated, that is, the surface of the master is drenched or soaked in concentrated sulfuric acid or potassium dichromate solution for a certain time to form a layer Oxide, easy to peel off the daughter and master after electroforming. In this way, you can continuously copy the stamped version. We refer to the metal version that is directly used for molding as the molded working version, and other metal versions prepared for preparing the molded working version are collectively called the molded master. The copy-moulded master is performed in a soft tank, and the copy-moulded working version is performed in a hard tank with added hardness agent. [next]
3. The process of molding and copying
The main production equipment of the molding process is a round high-speed holographic molding machine. The molded work plate is fixed on the plate roller of the molding machine through a jig, and the relief hologram on the molded work plate is transferred to the PET film by means of heat and pressure, which completes the mass reproduction of the rainbow hologram. In order to enhance the diffraction effect of the rainbow hologram, a layer of aluminum film can be pre-plated or post-plated. After molding, it generally needs to be processed after gluing, compounding, and die cutting.
2. Application of laser holography
In our country, the biggest use of laser holography technology is to use laser hologram anti-counterfeiting marks in the field of anti-counterfeiting. The laser holographic anti-counterfeiting logo has played a great role in protecting consumers' health and interests during its continuous development from scratch. There are three different types of laser holographic anti-counterfeit signs.
1. Permanent type This type of laser holographic anti-counterfeiting label uses permanent PET material as the carrier of the anti-counterfeiting label. When using it, the label is directly attached to the surface of the object to be attached. It is convenient to use and suitable for automatic mechanical labeling. The main disadvantage is that this kind of logo can be used repeatedly, and can be peeled off from the surface of the object to be pasted on the surface of other objects, which affects the anti-counterfeiting effect. In response to this problem, Hubei Chenguang Industrial Co., Ltd. took the lead in developing a new type of laser anti-counterfeiting composite packaging process in China, which is to use a self-developed automatic labeling system for permanent laser holographic anti-counterfeiting marks to accurately affix it to the interlayer of commodity multilayer packaging materials At a specific location in the center, and the multi-layer packaging and laser holographic anti-counterfeiting signs are closely combined into a whole. This technology effectively improves the anti-counterfeiting performance of laser holographic anti-counterfeiting marks.
2. The one-time type laser hologram anti-counterfeiting mark is similar to the permanent type. The difference is that it uses a disposable PET material as a carrier. After it is peeled off from the surface of the object, the laser hologram image is destroyed and cannot be Used again, this kind of destruction can be random, or it can be destroyed according to the rules designed first.
3. This type of laser holographic anti-counterfeiting mark transfers the image information on the carrier PET to the surface of the object by hot pressing, so it cannot be peeled off; moreover, the production process of this mark needs to be It is much more complicated and has stricter requirements. Enterprises without certain economic and technological strength cannot produce it. Hot stamping laser holographic anti-counterfeiting marks can also be divided into three types: general-purpose chaotic hot-type, special-purpose chaotic hot-type, special-purpose positioning hot-type.
3. The development trend of laser holography technology
It is undeniable that the laser holographic anti-counterfeiting logo has also been widely copied in the process of use, and can even be faked, which has caused huge losses to consumers, users and production companies. For our practitioners, this is both pressure and motivation. In the process of combating counterfeit and shoddy products, we have also developed many new technologies, new equipment, and new processes. In particular, the new plate-making technology has made great progress, from the initial platform photography technology to the later computer holographic dot matrix technology, and now there is the computer holographic technology of Hubei Chenguang. More and more laser holographic anti-counterfeiting technologies have been developed one after another, such as laser encryption technology, computer code encryption technology, text and image miniaturization technology, true color 3D technology, computer OVD optical variable technology, combined with other directions of anti-counterfeiting technology Anti-counterfeiting technology and so on. At the same time, the development direction of continuous innovation of laser holographic anti-counterfeiting technology combined with paper printing and packaging materials is also our way forward.
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