JPH06222705A - Hologram and hologram reconstructing device - Google Patents

Abstract

PURPOSE:To provide the hologram which makes forgery difficult by a simple means and its reconstructing device by providing a part of a hologram supporting member with a phase distribution structure so that the hologram is reconstructed by the light wave front past the phase distribution structure. CONSTITUTION:This hologram has the phase distribution structure 102 in a part of the hologram supporting member and is reconstructed by the light wave front 106 past the phase distribution structure 102. The hologram reconstructing device has a coherent light source and a spatial optical modulator 101 for controlling the wave front shape of the light from the coherent light source and reconstructs the hologram via the phase distribution structure 102 formed in a part of the hologram supporting member by the light wave front past the spatial optical modulator 101. The formation of such wave front is sufficient with the spatial optical modulator of a phase modulation type which can be easily produced. The hologram which makes forgery difficult by the simple means and its reconstructing device are thus obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hologram for preventing forgery and its reproducing device.

[0002]

2. Description of the Related Art Conventionally, gratings or holograms have been used for the purpose of preventing forgery. That is, there has been the idea of forming a hologram on a part of the surface of articles or securities (including banknotes) and discriminating the authenticity from the presence or absence of a reproduced image. For example, US p
The patents of atent No. 4014602, 4171766, 5138468 relate to protection and guarantee technology using hologram.

[0003]

However, as the hologram manufacturing technology has advanced, the hologram forgery technology has become more sophisticated, and it has become difficult to use the hologram for the purpose of preventing forgery.

The present invention solves such a problem, and an object thereof is to provide a hologram which is difficult to forge by a simple means and a reproducing apparatus thereof.

[0005]

A first hologram of the present invention is characterized in that it has a phase distribution structure in a part of a hologram support member and is reproduced by an optical wavefront that has passed through the phase distribution structure. .

The second hologram of the present invention is characterized in that, in the first hologram, the phase distribution structure is a surface relief structure having a binary phase value.

A third hologram of the present invention is characterized in that, in the first hologram, the phase distribution structure is a distributed refractive index structure having a binary phase value.

A first hologram reproducing apparatus of the present invention comprises a coherent light source and a spatial light modulator for controlling a wavefront shape of light from the coherent light source, and the optical wavefront passing through the spatial light modulator is used to It is characterized in that the hologram is reproduced through a phase distribution structure formed in a part of the first hologram supporting member.

The second hologram reproducing apparatus of the present invention is the same as the first hologram reproducing apparatus, wherein the wavefront that has passed through the phase distribution structure formed in a part of the hologram supporting member is the required reproduction light wavefront for the hologram. The phase distribution to be displayed on the spatial light modulator is determined so as to be substantially equal.

A third hologram reproducing apparatus of the present invention is characterized in that, in the first hologram reproducing apparatus, the spatial light modulator is a complex amplitude modulation type liquid crystal spatial light modulator.

A fourth hologram reproducing apparatus of the present invention is characterized in that, in the first hologram reproducing apparatus, the spatial light modulator is a phase modulation type liquid crystal spatial light modulator.

[0012]

EXAMPLES The contents of the present invention will be described in detail below based on examples.

(Embodiment 1) First, the concept underlying the present invention will be described.

The interference fringe I recorded on the hologram will be expressed by the following equation.

I = | R | ² + | O | ² + RO ^* + R ^* O (1) Here, O and R are the object light wavefront and the reference light wavefront at the time of recording the hologram, respectively. Represents a complex conjugate. (1)
In the equation, the object information is contained in the third and fourth terms. Then, by selecting the exposure and process conditions, only one of the information can be reproduced as a wavefront. Therefore, in the following, in order to simplify the explanation, only the term of R ^* O will be focused on as the object information.

In the present invention, the phase distribution formed on the hologram supporting member and the phase distribution displayed on the spatial light modulator (the phase of the light wave front can be two-dimensionally modulated) are defined as follows.

First, assuming that the amplitude distribution displayed on the spatial light modulator is A and the phase distribution is φ ^LC , the wavefront W ^LC immediately after the spatial light modulator is W ^LC = Aexp (jφ ^LC ) ... ( 2) can be written. However, j is an imaginary unit.

[0018] Next, to write 'the, W ^LC' wavefront W ^LC when the wavefront W ^LC reaches the hologram supporting member ^{= D {W LC} ···· (} 3) and. Here, D {} represents an arbitrary optical conversion.

[0019] The phase distribution formed on the hologram supporting member and phi ^sub, wavefront W ^sub immediately after the support ^{member, W sub = exp (jφ sub} ) · D {W LC} = exp (jφ sub) · D It can be written as {Aexp (jφ ^LC )} (4). The wavefront W ^sub becomes the reproduction wavefront W ^{re of the} hologram.

Focusing on the term of R ^* O including the object information of the equation (1), only when the hologram is reproduced on the wavefront R,
It can be seen that the object information is reproduced in perfect form. In order to satisfy this condition, the amplitude distribution A and the phase distribution φ ^LC displayed on the spatial light modulator are determined so that the wavefront represented by the equation (4) is almost equal to the required wavefront R for the hologram. Good. The following expression is obtained from the condition of W ^re = R.

Aexp (jφ ^LC ) = D ⁻¹ {R · exp (−jφ ^sub )} (5) where D ⁻¹ {} means optical inverse transformation.

In order to create the wavefront of equation (5), a spatial light modulator capable of independently modulating both the amplitude and the phase of the light wave is required. Such spatial light modulator (complex amplitude modulator)
Can be configured by connecting two types of liquid crystal panels with corresponding pixels aligned (52nd Autumn Meeting, 10a-
See ZK-2).

If φ ^LC is selected so as to satisfy the equation (5),
The wavefront W ^re will be exactly equal to the required wavefront R for the hologram. That is, W ^re · R ^* O = O, and the object information (in other words, information necessary for authenticity determination) is completely reproduced. However, for the sake of explanation, here, | R ² | =
1 (this condition is easily satisfied by considering how to form the reference light wavefront used during hologram exposure).

When the distance from the spatial light modulator to the phase distribution formed on the support member is almost negligible, the action of the optical conversion D {} is removed, and the equation (5) is expressed as exp (jφ ^LC ) = R · exp (−jφ ^sub ) ... (6) At this time, if a plane wave is used as the reference light wavefront for hologram exposure, R = 1. Then, as can be seen from the equation (6), the phase distribution φ ^LC displayed on the spatial light modulator and the phase distribution φ ^sub formed on the hologram support member are in a complex conjugate relationship with each other.

Next, paying attention only to the phase component of the wavefront of the equation (5), the phase distribution given by the following equation is displayed on the spatial light modulator.

Exp (jφ ^LC ) = P [D ⁻¹ {R · exp (−jφ ^sub )}] (7) where D ⁻¹ {} is the optical inverse transformation and P [] is the phase. Each operation means extracting only the component.

When φ ^LC is selected according to the equation (7), the wavefront W ^re does not completely match the required wavefront R for the hologram. However, this difference is extremely small, and there is no problem in reproducing the object information from the hologram and performing the authenticity determination. That is, W ^re · R ^* O≈O
Therefore, as much object information as necessary can be obtained. However, for the sake of description, | R ² | = 1 is set here (this condition is easily satisfied by considering how to form the reference light wavefront used during hologram exposure).

A phase modulation type spatial light modulator is sufficient to create the wavefront of the equation (7). The phase modulation type spatial light modulator is easier to manufacture than the complex amplitude modulator cited above.

The above concept is shown in FIG. 10 in the figure
Reference numeral 1 is a spatial light modulator, 102 is a phase distribution structure formed on a hologram supporting member, and 103 is a hologram layer. 1
The combination of the phase distribution structure 02 and the hologram layer 103 is the forgery prevention hologram of the present invention. Figure 1
(A) is a state of the image reproduction space when the condition of expression (6) is not satisfied, and object information is not obtained. On the other hand,
FIG. 1B shows a state of the image reproduction space when the condition of the expression (6) is satisfied, and the object information is obtained as a three-dimensional real image.

FIG. 2 shows two detailed structures of the hologram of the present invention. In FIG. 2A, 201 is a hologram supporting member having a phase distribution structure, and 202 is a hologram layer. The phase distribution structure of 201 is a surface relief type. On the other hand, in FIG. 2B, 205 is a hologram supporting member having a phase distribution structure, and 206 is a hologram layer.
The phase distribution structure 203 is of a distributed index type. (A)
Both of the phase distribution structures (b) are binary (0, π) random phase distributions.

For the support member having the phase distribution structure, plastic or glass is properly used according to the purpose of use. Although a photopolymer is suitable as a medium for forming the phase distribution structure, if the support member is glass, a relief structure can be formed by etching or a distributed refractive index structure can be formed by ion doping.

In this embodiment, the hologram size is 20 ×
The cell size of the phase distribution structure is set to 20 mm ² by 2 × 2 mm ²
Stipulated in. The cells can be arranged in any order, but in this case, 10
Zero cells were arranged in a rectangular grid. Phase value of each cell is 0
There are 2 ¹⁰⁰ combinations, or 2π combinations as the phase distribution. With such a large number of combinations, it is almost impossible to decipher the phase distribution created in the supporting member of the hologram. Therefore, it is impossible to know the object information recorded in the hologram, and there is no concern that the hologram will be forged.

In this embodiment, the binary random phase is used as the phase distribution structure formed on the hologram support member, but the degree of freedom in selecting the phase value such as ternary or quaternary is increased. It is possible to go, and also
It is also possible to use a phase structure with low randomness.

In order to correctly read the object information even when the phase distribution structure formed on the hologram support member and the relative position of the liquid crystal spatial light modulator are slightly deviated, (1) an appropriate guide is provided in the space for holding the hologram. (2) The size of the pixel aperture of the liquid crystal spatial light modulator is made smaller than the cell size of the phase distribution structure, and (3) the cell shape of the phase distribution structure is circular. Means such as allowing the belt to rotate are effective.

In any case, the pixel size of the spatial light modulator needs to be smaller than the cell size of the phase distribution structure. In this situation, with discrete wavefronts,
It will illuminate the hologram area. Therefore, when recording the object information on the hologram, it is necessary to consider the redundancy as high as possible.

As described above, the feature of the present invention is that the phase distribution displayed on the spatial light modulator is used as the secret code key to read the object information recorded in the hologram, and the secret key (phase It is almost impossible to decipher the contents of (distribution).

Next, the structure of the hologram reproducing apparatus of the present invention will be described. FIG. 3 is an example of a device configuration for a transmission hologram. 301 is a semiconductor laser (wavelength 0.78
μm), 302 is a collimating lens, 303 is a liquid crystal spatial light modulator, 304 is a hologram of the present invention, 305 is a photodetector that receives a reproduced wavefront from the hologram, and 306 is a drive device of the liquid crystal spatial light modulator.

The beam emitted from the semiconductor laser 301 is collimated by the collimator lens 302 to illuminate the liquid crystal spatial light modulator 303. The liquid crystal spatial light modulator 303 displays a known phase distribution. According to the principle of wavefront reproduction described above, if the hologram is a real hologram, the object information can be read out as a wavefront. In this embodiment, a hologram is recorded so that this object wavefront forms a real image, and this image is captured by the photodetector of FIG. 3 so that the authenticity of the hologram can be judged on the monitor or on the computer. I chose Of course, depending on the purpose of use,
The authenticity may be determined by visually observing the reproduced image from the hologram.

The phase modulation type spatial light modulator used in this embodiment is a homogeneously aligned liquid crystal spatial light modulator, which can modulate only the phase of a light wave (the 51st meeting on autumn fall, 26a-. See H-10). The number of pixels is 10 × 10, the pixel pitch is 2 mm both horizontally and vertically, and the size of the pixel opening is 1 × 1 mm ² . The phase modulation characteristic of this liquid crystal spatial light modulator is shown in FIG. The phase change of 2π or more required to display the phase distribution is linearly obtained with respect to the applied voltage.

According to the hologram and the hologram reproducing apparatus of the present invention, (1) a phase distribution structure is formed in a part of the hologram supporting member in advance, and (2) a phase distribution to be displayed on the spatial light modulator is recited. By using the key as a key to adjust the shape of the light wavefront that has passed through the phase distribution structure, and (3) by reproducing the information for authenticity determination recorded in the hologram by the light wavefront having the shape adjusted, The hologram cannot be forged, and the accuracy of the protection and guarantee of the article using the hologram is significantly improved.

(Embodiment 2) FIG. 4 shows another structure of the hologram reproducing apparatus of the present invention. This is an example of a device configuration for a reflection hologram.

A major difference from the reproducing apparatus for the transmission hologram of the first embodiment is the rule of creating the phase distribution displayed on the liquid crystal spatial light modulator. In the configuration of FIG. 4, once before entering the hologram and once after exiting the hologram,
The light wavefront passes through the phase distribution structure formed on the hologram supporting member twice in total.

In the arrangement of FIG. 4, the wavefront W ^ref immediately after passing through the hologram and the phase distribution structure formed in the hologram support member can be expressed by the following equation.

W ^ref = W ^sub · R ^* O · W ^sub · W ^LC ··· (8) However, for the sake of explanation, the distance from the liquid crystal spatial modulator to the phase distribution formed in the indicating member is ignored. I thought I could. When considering the distance, it suffices to execute the optical conversion in the same manner as the equation (7) and extract only the phase component from the obtained complex amplitude distribution.

To read object information correctly, W
^It is necessary to determine the phase distribution displayed on the liquid crystal spatial light modulator so as to satisfy the condition of ^ref = O (| R ² | = 1).
That is, the following equation is used instead of the equation (7).

Exp (jφ ^LC ) = R · exp (−j2φ ^sub ) ... (9) The configuration of this example is such that the laser, the spatial light modulator, and other components and the object wavefront reproduced from the hologram are used. Are the same as in the first embodiment except that they are arranged on the same side with respect to the hologram. Therefore, detailed description is omitted.

According to the hologram and the hologram reproducing apparatus of the present invention, the hologram cannot be forged for the same reason as in the first embodiment, and the accuracy of the protection and guarantee of the article using the hologram is remarkably high. improves.

[0048]

According to the present invention, the phase distribution displayed on the spatial light modulator is used as a recitation key, and the information for authenticity determination recorded in the hologram is reproduced. Compared to the situation where the hologram is simply reproduced with a spherical wave or a light wavefront corresponding to these, and the authenticity of the hologram (the authenticity of the article with the hologram attached) is determined from the reproduced image, The accuracy of protection and guarantee of goods will be greatly improved.

INDUSTRIAL APPLICABILITY The hologram and hologram reproducing apparatus of the present invention can be widely applied to the protection and guarantee of articles, securities and bank notes.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention.

FIG. 2 is a sectional view showing the structure of a hologram.

FIG. 3 is a plan view showing the configuration of the first embodiment of the present invention.

FIG. 4 is a plan view showing a configuration of a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a phase modulation characteristic of a spatial light modulator.

[Explanation of symbols]

 101 spatial light modulator 102 phase distribution structure of hologram support member 103 hologram layer 104 wavefront immediately after spatial light modulator 105 wavefront immediately after hologram support member 106 wavefront 107 containing object information 107 reproduced image 201 phase distribution structure 202 hologram layer 203 intermediate layer 204 protective layer 205 phase distribution structure 206 hologram layer 207 intermediate layer 208 protective layer 301 semiconductor laser 302 collimating lens 303 liquid crystal spatial light modulator 304 hologram of the present invention 305 photodetector 306 drive device for liquid crystal spatial light modulator 307 article

Claims (7)

[Claims] 1. A hologram, wherein a part of a hologram supporting member has a phase distribution structure, and is reproduced by an optical wave front that has passed through the phase distribution structure. 2. The hologram according to claim 1, wherein the phase distribution structure is a surface relief structure having a binary phase value. 3. The hologram according to claim 1, wherein the phase distribution structure is a distributed index structure having a binary phase value. 4. A coherent light source and a spatial light modulator for controlling a wavefront shape of light from the coherent light source, wherein the optical wavefront passed through the spatial light modulator.
A hologram reproducing apparatus characterized in that the hologram is reproduced through a phase distribution structure formed on a part of the hologram supporting member. 5. A phase distribution to be displayed on the spatial light modulator so that the wavefront that has passed through a phase distribution structure formed in a part of the hologram supporting member becomes substantially equal to the required reproduction light wavefront for the hologram. The hologram reproducing apparatus according to claim 4, wherein 6. The hologram reproducing apparatus according to claim 4, wherein the spatial light modulator is a complex amplitude modulation type liquid crystal spatial light modulator. 7. The hologram reproducing apparatus according to claim 4, wherein the spatial light modulator is a phase modulation type liquid crystal spatial light modulator.