JP3416976B2 - Phase mask and phase mask reproducing apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a phase for preventing forgery.
The present invention relates to a mask and a reproducing device thereof.

[0002]

2. Description of the Related Art Conventionally, gratings or holograms have been used for the purpose of preventing forgery. That is, various cards or securities (including bank notes)
There was an idea to form a hologram on a part of the surface of and to discriminate the authenticity from the presence or absence of a reproduced image. For example,
The patents of US patent Nos. 4014602, 4171766, 5138468 relate to protection and guarantee technology using holograms.

[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]

The first phase mask of the present invention is formed by stacking two or more phase distribution structures.
Information for authenticity determination is recorded in one phase distribution structure, and information for hiding information for authenticity determination is recorded in another phase distribution structure.

The first phase mask reproducing apparatus of the present invention comprises a coherent light source, a spatial light modulator for controlling the optical wavefront of the light emitted from the coherent light source, and an optical wavefront passing through the spatial light modulator. The phase mask, comprising means for reproducing information recorded on a phase mask formed by stacking one or more phase distribution structures, and detecting means for detecting information contained in a reproduced wavefront from the phase mask. The information recorded in is used to hide the information for authenticity determination recorded in one phase distribution structure and the information for authenticity determination recorded in another phase distribution structure. Information, the spatial light modulator, the information included in the reproduction wavefront from the phase mask is equal to the information for authenticity determination, the information for authenticity determination. Information to hide And displaying the phase distribution and / or the complex amplitude distribution for erasing Chi.

[0007]

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

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

[0010]

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

(Embodiment 1) The concept underlying the present invention is that the complex amplitude distribution displayed on the spatial light modulator is used as a "key" and the phase formed on the first phase distribution structure in the phase mask is used. By opening the structure, that is, the "lock", the "information for authenticity determination" recorded in the second phase distribution structure is read out.

In order to simplify the explanation, a phase mask composed of two phase distribution structures will be taken up here. The phase distribution structure will be distinguished from the first and second in the order in which the light is illuminated. It is easy to extend the following discussion for a phase mask consisting of three or more phase distribution structures.

When 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 shown.
Can be written as W _LC = A exp (jφ _LC ) ... (1). However, j is an imaginary unit.

[0014] To write 'the, W _LC' wavefront W _LC when the wavefront W _LC reaches the first phase mask = D {W _LC} ···· and (2). Here, D {} represents an arbitrary optical conversion. This optical transform is, for example, a Fourier transform or a Fresnel transform, and can be easily calculated using a fast Fourier transform algorithm.

Next, the phase mask of the present invention will be described. The phase mask is composed of first and second phase distribution structures, and information for authenticity determination is recorded in the second phase distribution structure. The phase structure formed in the first phase distribution structure is for hiding the information recorded in the second phase distribution structure.

The phase distribution recorded in the first phase distribution structure is φ ₁ . When this structure is reproduced with the wavefront of the equation (2), the wavefront W ₁ immediately after the structure is W ₁ = D {W _LC } · exp (jφ ₁ ) = D {Aexp (jφ _LC )} · exp ( jφ ₁ ) ... (3) can be written.

It may be considered that when the first and second phase distribution structures are sufficiently close to each other, this wavefront W ₁ becomes the wavefront W _re for reproducing the second phase structure as it is. . That is, W _re = W ₁ ... (4).

The second phase structure has a function of converting the wavefront R into the wavefront O. The wavefront O contains information necessary for authenticity determination. That is, to satisfy the following equation,
The phase distribution φ ₂ of the second phase structure is given.

Exp (jφ2) R = O (5) When a suitable optical conversion K is applied to this wavefront O, a two-dimensional or three-dimensional intensity distribution is generated. The authenticity of the article is determined by detecting this intensity distribution. If this intensity distribution is I, then I = | K {O} | ² (6) can be written.

As can be seen from the equation (5), in order to reproduce the object information in a perfect form, the wavefront W _re represented by the equation (4) is
The amplitude distribution A and the phase distribution φ _LC displayed on the spatial light modulator may be determined so as to be substantially equal to the required wavefront R for the second phase structure. That is, the following expression is obtained from the condition of W _re = R.

Aexp (jφ _LC ) = D ⁻¹ {R · exp (−jφ ₁ )} (7) However, D ⁻¹ {} means the optical inverse transformation.

In order to create the wavefront of equation (7), 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 (7),
The wavefront W _re will be exactly equal to the required wavefront R for the hologram. That is, exp (jφ ₂ ) · W _re = O, and the information recorded in the second phase distribution structure (in other words, information necessary for authenticity determination) is completely reproduced.

When the distance from the spatial light modulator to the first phase distribution structure is almost negligible, the action of the optical conversion D {} is eliminated and the equation (7) is given by Aexp (jφ _LC ) = R · exp (−jφ ₁ ) ... (8)

Next, paying attention only to the phase component of the wavefront of the equation (7), the phase distribution given by the following equation is displayed on the spatial light modulator.

Exp (jφ _LC ) = P [D ⁻¹ {R · exp (−jφ ₁ )}] (9) 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 (9), the wavefront W _re does not completely match the required wavefront R for the second phase structure. However, this difference is extremely small and does not hinder the determination of authenticity. That is, exp (jφ ₂ ) · W _re ≈O, and information necessary for the true / false determination can be obtained.

A phase modulation type spatial light modulator is sufficient to create the wavefront of the equation (9) (see, for example, 51st Autumn Meeting of Obi, 26a-H-10). The phase modulation spatial light modulator is
It is easier to manufacture than the complex amplitude modulator cited above.

The concept underlying the present invention is shown in FIG. In the figure, 101 is a spatial light modulator, 102 is a first phase distribution structure, and 103 is a second phase distribution structure. The combination of the first and second phase distribution structures is the forgery preventing phase mask of the present invention.

Reference numeral 104 is a wavefront detecting means for converting the complex amplitude distribution of the wavefront into an intensity distribution.

FIG. 1A shows the state of the information detection space when the condition of the expression (7) is not satisfied. A random intensity distribution appears, and the intensity distribution (information) that guarantees that "the object is true" cannot be obtained. Thus, "the object is false"
Is judged.

On the other hand, FIG. 1B shows the state of the information detection space when the condition of the expression (7) is satisfied. This time, an intensity distribution that allows easy pattern recognition appears. This intensity distribution guarantees that the object is "true"
You can choose any method within the constraint of "recognizing patterns".

FIG. 2 shows two detailed structures of the phase mask of the present invention. In FIG. 2A, 201 is a first phase distribution structure, 202 is an intermediate layer, and 203 is a second phase distribution structure. Both 201 and 203 are binary surface relief structures. Here, the structure in which the relief surfaces are faced to each other is shown, but the relief surfaces may be arranged in the same direction. In FIG. 2B, 205 is a first phase distribution structure, 206 is an intermediate layer, and 207 is a second phase distribution structure. Both 205 and 207 are 2
It is a value distribution refractive index structure. 204 and 208 are support members for the phase distribution structure. In addition to these, a structure in which the surface relief structure and the distributed index structure are combined may be used.

The intermediate layers 202 and 206 in FIG. 2 have a thickness of several μm, which is sufficiently smaller than the length of one side of the cell of the phase structure. That is, the influence of diffraction can be ignored while the light travels from the first phase distribution structure to the second phase distribution structure. Although photopolymer is suitable for the medium that forms the phase distribution structure,
It is also possible to select glass as a member and form a relief structure by etching, or ion-doping to form a distributed index structure.

In this embodiment, the size of the phase mask, that is, the phase distribution structure is 5 × 5 mm ² , the cell size of the first phase distribution structure is 500 × 500 μm ² , and the cell of the second phase distribution structure is The size was set to 250 × 250 μm ² . The cells may be arranged in any manner, but here they are arranged in a rectangular grid.

For the first phase distribution structure, the phase value of each cell is set to either 0 or π. On the other hand, regarding the second phase distribution structure, the phase distribution of each cell is set to 0 or ψ so that the intensity distribution obtained by Fourier transforming the phase distribution engraved therein may be concentrated only in a specific diffraction order. Set to either. ψ is a phase value between 0 and π and is an optimum value obtained by calculation. Simulated annealing methods (Science 220, 671-679 (198
3)) and the Fourier iteration method (Opt. Eng. 19, 297-305 (198
0)) is available.

The number of cells of the first phase distribution structure is 1
Since 00 pieces are, as a phase distribution, a combination of 2 ¹⁰⁰ types can be considered. When the number of combinations increases by this amount, the reproduction wavefront R (see equation (7)) is searched for and the first
It is almost impossible to decipher the phase distribution engraved in the phase distribution structure of. Therefore, it is impossible to reproduce the information recorded in the second phase distribution structure,
There is no concern that the phase mask will be forged.

In this embodiment, the binary phase distribution is used when the first and second phase distribution structures are created. However, when selecting the phase value such as ternary or quaternary. It is also possible to increase the degree of freedom, and use a phase structure with low randomness or a fractal phase structure (J. Opt. Soc. Am. 72, 1034-1041 (1982)). Is also possible.

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 type phase mask. 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 phase mask of the present invention, 305 is a Fourier transform lens, 306 is a photodetector that receives the reproduced wavefront from the phase mask, and 307 is liquid crystal spatial light modulation. It is a driving device of a container.

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 phase mask is true", information that guarantees that can be read out as a wavefront. In the present embodiment, the second phase structure is formed so that the Fourier transform image of this object wavefront forms a 3 × 3 spot array, and this image is captured by the photodetector 306 of FIG. , The authenticity of the phase mask is determined on the monitor or the computer.

The phase-modulating spatial light modulator used in this embodiment is a liquid crystal spatial light modulator of homogeneous alignment, and can modulate only the phase of the light wave (the 51st meeting of the Autumn Meeting of Bibliography, 26a-. See H-10). The number of pixels is 50 × 50, the pixel pitch is 100 μm both horizontally and vertically, and the size of the pixel opening is 80 × 80 μm ² . The phase modulation characteristic of this liquid crystal spatial light modulator is shown in FIG. 2 required to display the phase distribution
A phase change of π or more can be obtained linearly with respect to the applied voltage.

When the spatial light modulator having the discrete apertures is used as in the present embodiment, the information (here, here) is present on the Fourier reproduction plane because the array of the apertures is periodic.
A plurality of 3 ×× spot arrays) appear. Only one piece of information (for example, the information that appears at the center) is necessary for authenticity determination, and it is difficult for a plurality of pieces of information to overlap each other.

It is necessary to determine the cell sizes of the first and second phase distribution structures and the pixel pitch of the phase modulation type spatial light modulator so that such crosstalk of information does not occur.

In consideration of this point, in the present embodiment, the cell sizes of the first and second phase distribution structures are each set to 5
The pixel pitch of 00 × 500 μm ² , 250 × 250 μm ² , and the phase modulation spatial light modulator was set to 100 × 100 μm ² . FIG. 6 shows a reproduced image plane (here, a Fourier transform plane).
The state of the intensity distribution in is shown. For the sake of explanation, only the part containing nine pieces of information near the center is shown. When the wavelength of light is 0.5 μm and the lens focal length is 500 mm, the reproduction area occupied by each periodic structure is 0.5.
5 × 0.5 mm ² , 1.0 × 1.0 mm ² (a in the figure), 2.5
× 2.5 mm ² (b in the figure).

In order to correctly read the object information even when the relative position of the phase distribution structure formed on the phase mask and the liquid crystal spatial light modulator is slightly shifted, (1) it is suitable for the space holding the phase mask. (2) The size of the pixel opening of the liquid crystal spatial light modulator is made smaller than the cell size of the phase distribution structure, and (3) the phase distribution structure is provided. It is effective to make the cell shape into a ring zone to allow rotation.

As described above, the feature of the present invention is that the phase distribution displayed on the spatial light modulator is used as a "key" to read the information for authenticity determination recorded on the phase mask. And it is almost impossible to decipher the contents of this "key" (phase distribution).

With the phase mask and the phase mask reproducing apparatus of the present invention, (1) the phase mask formed by stacking the first and second phase distribution structures is (2) the phase displayed on the spatial light modulator. The distribution is used to adjust the shape of the light wavefront that has passed through the first phase distribution structure, and (3) The authenticity determination recorded in the second phase distribution structure is performed using the shape-adjusted light wavefront. By reproducing the information for, the forgery of the phase mask was made impossible, and the reliability of the protection and guarantee of the article using the phase mask could be significantly improved.

In this embodiment, the Fourier transform lens is used as a means for extracting information for authenticity determination contained in the reproduced wavefront from the second phase structure, but in addition to this, shearing interference is also used. Wavefront measurement methods such as the method, Talbot interferometry, and two-beam interferometry can be used.

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

A major difference from the reproducing apparatus for the transmission type phase mask of the first embodiment is the rule of creating the phase distribution displayed on the liquid crystal spatial light modulator. In the configuration shown in FIG. 4, the light wavefront is divided into the first phase distribution structure and the light wavefront once in the second phase distribution structure and once after exiting from the second phase distribution structure. It will pass twice.

In the arrangement of FIG. 4, the wavefront W _re immediately after passing through the phase distribution structure in the first, second and first order can be expressed by the following equation.

W _re = W ₁ · W ₂ · W ₁ · W _LC ··· (10) However, for the sake of explanation, the distance from the liquid crystal spatial modulator to the first phase distribution structure can be ignored. did. 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.

In order to read the object information correctly, W _re
The phase distribution to be displayed on the liquid crystal spatial light modulator must be determined so as to satisfy the condition of ≈O. That is,
The following equation is used instead of equation (9).

Exp (jφ _LC ) = O · exp (j (φ ₂ −2φ ₁ )) (11) The configuration of the present embodiment includes components such as a laser and a spatial light modulator, and a phase mask. Is the same as in Example 1 except that the wavefronts reconstructed from are located on the same side of the phase mask.

According to the phase mask and the phase mask reproducing apparatus of the present invention, it is impossible to forge the phase mask for the same reason as in the first embodiment, and the reliability of the protection and guarantee of the article using the phase mask can be prevented. Sexuality is dramatically improved.

[0056]

According to the present invention, the phase distribution displayed on the spatial light modulator is used as a "key" and the information for authenticity determination recorded on the phase mask is reproduced. Compared to the situation in which a hologram is attached to a part of an article and the authenticity of the hologram is determined from the reproduced image of the hologram, that is, the authenticity of the article with the hologram is determined, the reliability of protection and guarantee of the article is Greatly improved.

The phase mask and the phase mask reproducing apparatus of the present invention can be widely applied to the protection and guarantee of various cards, 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 a structure of a phase mask.

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.

FIG. 6 is a diagram showing a state of intensity distribution on a reproduced image plane.

[Explanation of symbols]

101 spatial light modulator 102 first phase distribution structure 103 Second phase distribution structure 104 Wavefront detection means 105 reconstructed image 106 Wavefront immediately after spatial light modulator 107 Wavefront immediately after the first phase distribution structure 108 Wavefront including object information 201 First phase distribution structure 202 Middle layer 203 Second phase distribution structure 204 Support member 205 First phase distribution structure 206 Middle class 207 Second phase distribution structure 208 Support member 301 Semiconductor laser 302 Collimating lens 303 Liquid crystal spatial light modulator 304 Phase Mask of the Present Invention 305 Fourier transform lens 306 Photodetector 307 Liquid crystal spatial light modulator driving device 308 goods 401 half mirror

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-3-257419 (JP, A) JP-A-2-143201 (JP, A) JP-A-6-76059 (JP, A) JP-A-4- 110928 (JP, A) JP 61-193123 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 27/42 G02B 5/18

Claims (4)

(57) [Claims] 1. A structure in which two or more phase distribution structures are stacked, and information for authenticity determination is recorded in one phase distribution structure, and information for authenticity determination is recorded in another phase distribution structure. The phase mask is characterized in that information for hiding information for recording is recorded. 2. A coherent light source, a spatial light modulator for controlling the light wavefront of light emitted from the coherent light source, and a light wavefront that has passed through the spatial light modulator, in which two or more phase distribution structures are stacked. And a detection unit for detecting information contained in the reproduced wavefront from the phase mask, wherein the information recorded in the phase mask is one. Information for authenticity determination recorded in the phase distribution structure, and information for hiding information for authenticity determination recorded in another phase distribution structure, including the space, The optical modulator has a phase distribution for canceling the information for hiding the information for the authenticity determination so that the information included in the reproduced wavefront from the phase mask becomes equal to the information for the authenticity determination. And / or Phase mask reproducing apparatus and displaying the complex amplitude distribution. 3. The phase mask reproducing apparatus according to claim 2, wherein the spatial light modulator is a complex amplitude modulation type liquid crystal spatial light modulator. 4. The phase mask reproducing apparatus according to claim 2, wherein the spatial light modulator is a phase modulation type liquid crystal spatial light modulator.