Rainbow Hologram Overview
From the perspective of product packaging, packaging anti-counterfeiting labels should not only have a strong anti-counterfeiting function, but more importantly, when the anti-counterfeiting labels are used on the packaging, the logo not only does not destroy the overall coordination and decoration effect of the original packaging pattern, but also The original packaging decoration promotion function should be enhanced. Currently commonly used holographic anti-counterfeiting signs mainly use rainbow holograms. Therefore, this book focuses on the production process of rainbow holograms.
Rainbow hologram is a hologram recorded with laser, a holographic technology that reproduces monochrome or color images with white light. From the similarities and differences between the reproduced image and the original color, the rainbow hologram can be divided into false color rainbow hologram and true color rainbow There are two types of holography.
The basic feature of the color rainbow hologram is to add a slit at an appropriate position in the recording system. Its function is to limit the reproduction light wave to reduce the color blur of the image, thereby realizing the white light to reproduce the monochrome or color image. The rainbow hologram was first inspired by Benton by the fragmentation of the hologram to reproduce the complete image of the object. It was proposed in 1969 as a two-step recording hologram (two-step rainbow hologram).
The two-step rainbow hologram first records an off-axis Fresnel hologram (called the main hologram or mask), as shown in Figure 1 (a). Illuminate the main hologram with the conjugate light of the reverse reference direction when recording the main hologram to make it reproduce the twin real image, and place a horizontal slit S with width a close to the main hologram to limit the diffracted beam to form the twin real image with the narrow beam. As shown in Figure 1 (b). The hologram recorded in this way is a two-step rainbow hologram. When the rainbow hologram is illuminated with reproduced white light, the reproduced image of the object and the slit will record the reproduced image and slit image of the red, green, and blue (R, G, B) wavelength light in the image beam when the laser is recorded. At different positions, in this way, different color images are seen at the position of the slit images of different wavelengths, which is why the monochromatic image can be reproduced by illuminating the hologram with white light. If the human eye moves along the z-axis, so that several colors of light enter the eye, it will be observed that the color of the image is like a rainbow in the sky after the rain. This is the origin of the name of the rainbow hologram.
Figure 1 Two-step rainbow hologram recording and reproduction
(a) Main holographic recording optical path (b) Two-step rainbow holographic recording optical path (c) White light reproduction image
Because the two-step rainbow hologram proposed by Benton needs to record the secondary hologram, the procedure is more complicated, it is easy to generate noise, and the observation direction of the color of the reproduced image cannot be set. Therefore, one-step rainbow holography, one-step rainbow with field lens, astigmatism two-step and one-step rainbow, slit-free rainbow, lensless rainbow, strip-scattering screen integrated slit rainbow, coded two-step rainbow and zero optical path were developed later Various rainbow holographic technologies such as differential rainbow holography. Considering that the product packaging should have the function of using the special colors to represent the brand characteristics of the anti-counterfeiting logo, through the design of Jingte colors, Jingte has a deep sense of depth, good decoration effects, and bright and variable colors, etc., this book only Introduce the manufacturing process of rainbow hologram with color coding function.
Rainbow holographic features
Now we analyze the characteristics of the rainbow hologram. When recording the hologram H, the object beam is limited by the slit S, but only a thin beam is projected on H, so the information corresponding to an object point Ol only occupies a small part of H0 in the y direction of the hologram. For this part of the hologram, it can also be called a line hologram (Figure 2).
Since the size of a hologram of an object point is limited in the y direction (vertical direction), it is not limited in the x direction (horizontal direction). In this way, the reproduced image loses the stereoscopic sense in the y direction, and still has a stereoscopic effect in the x direction.
Figure 2 Rainbow holographic features Figure 3 Color design principles
Rainbow holographic original recording method
From the above analysis, we know that when the rainbow hologram is illuminated with white light, different colors can be observed at the angles corresponding to the slit images reproduced at different wavelengths, which indicates that if the rainbow hologram is recorded, the position of the slit is made With proper design, you can observe a specific monochrome or color image at a specific angle. In order to enable the rainbow hologram reproduced by white light to reproduce a color image at a certain viewing angle, people have made special designs for the position of the slit. This book introduces the method of false color coding design using the three-slit area division method. The so-called false color means that the color of the reproduced image does not match the color of the object, and the true color means that the color of the reproduced image is the same as the object.
1. False color coding monochrome design
To get a predetermined color combination, you must set the gap between the light path parameters. This section mainly discusses how to design the color of each layer or different parts of the original hologram, specifically, how to design the distance between the slits according to the parameters of the recording optical path, the angle between the object light waves and the recording wavelength.
Due to the limitation of the recording material used to make the relief-type rainbow hologram, the purple laser is used for recording, so in general, the slit design is often desired on both sides of the normal of the dry plate center on the optical path arrangement, so the slit position Often taken during design. For multicolor holograms with low color requirements, according to the parameters of and, the distance between the slits only needs to separate the colors of the reproduced image. When it is constant, the width of the slit is small, and the color of the reproduced image is relatively pure. Limited by the resolution of the recording holographic material, it is generally around 35 °. When the width of the slit is small, the object light for making the hologram is relatively weak. Note that when actually reproducing the hologram with white light, the individual wavelength slits are not at a depth.
2 False color coding compound color design
3 Multi-slit rainbow hologram
In the recording optical path of the two-step rainbow hologram, we see that only a narrow strip is used for the main hologram. It is easy to think of dividing a dry plate into several band-shaped areas, each of which records an object or different parts of an object. When recording in the second step, they are simultaneously reproduced and recorded on a hologram H. This is called a multi-slit rainbow hologram. When the hologram is reproduced by laser illumination, it is observed at the position of a certain slit image. You can see the image corresponding to the object. If you use white light to reproduce, you will see the recorded objects appear in different colors; if you record different parts of an object, you can see the entire object appear in different colors at a certain position. This situation is called False color coding of objects. The multi-slit rainbow hologram can also be completed in a one-step method, which mainly sets multiple slits on the entrance pupil or exit pupil of the imaging lens.
The recording method of the encrypted rainbow holographic original is mainly to produce the color sequence of the object through the design of the lateral area of ​​the main hologram and the design of the dispersion observation window to achieve the purpose of encryption and anti-counterfeiting. The encryption methods it uses include Moiré technology encryption and random phase encoding optical anti-counterfeiting methods.
Moore Technology Encryption
Encrypted holographic logo refers to placing a password on a part of or the entire layout of the original holographic logo image. The password is a physical model based on some special function transformation spectrum or optical phenomenon. The encryption method when the password finally appears in the form of an optical pattern is called "explicit encryption method". The "invisible encryption method" must use a decoder to read the designed password. The difference between an invisible password and an explicit password is that the invisible password is concealed. Observers generally do not find anything special about the holographic logo with the invisible password. But as long as you have a physical model of another function used in the recording process, that is, the decoder, paste it on the logo and rotate it by an angle, you can read the result of the two function equations designed in advance, which is a kind of New image. And this new image can be completely different from the two functions that generated it. The reason is that "invisible cryptography" is more effective than many current anti-counterfeiting methods in distinguishing authenticity and anti-counterfeiting.
1. The basic law of moiré phenomenon The optical moiré image is formed by superimposing two periodic patterns with similar frequency. We use two gratings to overlap to illustrate the formation of Moa fringes, and the characteristics of the grating are characterized by its transmission coefficient or transmission book.
2 The characteristics of the Moiré image are:
â‘ Its shape and stripe interval depend on the shape of the two periodic patterns that make up it and the placement when they overlap.
â‘¡ Its shape can be completely different from the shape of the two periodic graphs that constitute it.
Therefore, different periodic patterns and different placements can form a variety of different Moiré images. From the Moiré images you see, it is difficult to infer the shape of the graphics that constitute it, so that the hologram logo forgers cannot imitate. This is the use of The reason why Mohr Technology makes holographic ciphers. The production of the Moiré code is to record the Moiré image in a certain part or the entire layout of the hologram by holographic means. The production process usually uses a two-step method, and it can also be recorded in the layout of the holographic logo when the first Fresnel hologram is produced.
It can be seen from the formation rule of moiré fringes of the grating that the periodicity of the graphics determines the shape of the moiré and the stripe spacing, so as long as two periodic graphics are properly selected, one of them is made on the holographic logo and the other is made on a separate The holographic version (ie, the decoded version) is collected by the decoder. When the decoder puts the decoded version on the holographic logo on which the password information is recorded and rotates by a certain angle, you can see the Moir image in the sun. If the decoded version is placed on a holographic logo that is not injected with different holographic cryptographic information, the expected Moiré image will not be visible, so that it can be immediately determined that this holographic logo is forged. Because the holographic mark is recorded on the photoresist plate, and the photoresist plate is phase type, the password information recorded in the hologram mark with a single laser is not visible to the naked eye after development; if the decoded plate also uses photoresist plate, after development It is also just a whiteboard, and even the person who holds the decoded version does not know what the information in the version is; in this way, the forgery performance is even more enhanced. This method of judging the authenticity of the hologram is fast and reliable, compared with existing methods,
3 Moir holographic plate-making process First, with the help of computer drawing software, various line families are depicted to form a Moiré chart. Then input one of the two-function image models that form a satisfactory Moiré diagram into a certain part or the entire layout of the original hologram logo and refine the plates respectively, and then make a contrast film D and record it on the exposure but no The developed holographic mark on the photoresist plate is finally developed together with the hologram. Because it is only copied with a single beam during recording and is completely phase-type after development, it cannot be read out, so it does not affect the quality of the original holographic logo. When we use a contrast film recorded with another function image model and attach it to the hologram logo layout and rotate it through an angle, we can read a clear Moiré. The information on the decoder is closely related to the information on the holographic logo. These two pieces of information are indispensable when the Mortogram appears.
Random phase coding optical anti-counterfeiting
Random phase encoding optical anti-counterfeiting technology mainly includes the following two types, one is composed of a plane phase template containing a large number of pixels. The ordinary light intensity detector cannot detect the phase delay of each pixel on the encoder board, so it is difficult to read the content of the phase mask. Without knowing the phase encoding key, it is impossible to reproduce the image. The phase template is used to encrypt the protected image trademarks and the information on the card, and the optical correlator is used for decryption detection. Only the real phase template has a high correlation peak related to anti-counterfeiting information. The second is a double random phase template optical anti-counterfeiting technology with stronger anti-counterfeiting performance. This technique uses two independent random phase templates to encode the protected image in the spatial domain and the Fourier frequency domain, and encodes the original image as fixed white noise. If you do not know the two phase keys, it is impossible to recover image. This method can perform image encryption that requires a high degree of security. Although the phase encoding method has the characteristics of good confidentiality, the encrypted image is a complex number, which makes the production of anti-counterfeit cards difficult, and the inspection also requires two LCD TVs, one for phase and one for amplitude. And because the speckle noise will cause the resolution of the reconstructed image to decrease, it is even difficult to distinguish the plate making.
True color coding holographic original recording method
The purpose of color holography is to record and reproduce a three-dimensional image with a color very close to the original object. It involves problems in color optics and chromatics. From the color of holograms, there are two types of color holography and achromatic (or achromatic) holography. This section only describes the main transmission methods of true color hologram recording.
There are many recording methods for transmission true color holograms. For example, with three primary lasers, one-step or two-step rainbow hologram recording optical paths can directly record true color holograms of two-dimensional and three-dimensional objects. For the two black and white positives after the color separation of the two-dimensional object, the true color hologram can be recorded with monochromatic light. For three-dimensional objects, a three-primary laser can also be used in the first step. The second step is to record with a single wavelength to obtain a true color hologram. True color holograms recorded with monochromatic light are suitable for making embossed masters of molded holograms.
1 Single wavelength laser recording two-dimensional true color hologram 2 Two-step true color rainbow hologram 3 Single wavelength two-step true color rainbow hologram
Method for making original version of computational hologram
Computational hologram is different from optical hologram, it does not need the actual existence of the object, but the mathematical model of the object light wave is input into the computer for processing, and the computer-controlled plotter draws the coding pattern displayed by the cathode ray tube, and then remakes it on the photosensitive film. Made. The coding diagram is composed of two lattice points of transparent and opaque, so it is also called binary computing hologram. If the computer is connected to the TV and the electron beam is instructed to scan on the fluorescent screen, the coded pattern is composed of grid points with different intensities or different gray levels, which is called a gray-scale hologram after remake. Computational holograms are also divided into holograms, Fresnel holograms and Fourier transform holograms.
The production process of computational hologram (abbreviated symbol CGH) can generally be divided into four colors:
â‘ Select the mathematical expression of the object or light wave surface and sample it;
â‘¡Calculate the light field distribution of the object light wave on the holographic plane;
â‘¢ Transmittance change encoding the above light wave field into hologram;
â‘£Display the above-mentioned refractive index changes on the cathode ray tube or expose to the photographic film, or output by the plotter on the drawing paper, and reproduce the image with the optical reproduction device
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