JP3264491B2 - Method of forming diffraction grating - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to diffraction gratings and their manufacture, and in particular, comprises a diffraction grating applied, for example, as a protection device for banknotes and credit cards.

Technical Background The 1988 plastic Australian $ 10 bill uses Captain Cook's grating image as an anti-counterfeiting device, which consists of a regular pixel with each curve containing one or more grid lines. It consists of a matrix. These pixels, when illuminated, each have a two-dimensional optical catastrophe image.
The diffraction pattern of e) results, whereby the diffraction pattern of the whole image of the grating is optically variable but structurally stable. The image of Captain Cook is an application of the inventors' theory for a generalized curved grating to optical diffraction catastrophes. This theory is based on Optica Acta
Acta), Vol. 30, Nos. 3 and 4, and its application to optical diffraction catastrophe is described in Vol. 30, No. 4
Pages 449-464. In essence, the Captain Cook image is a diffraction catastrophe pixel onto a regular diffraction grating.
xels) requires the imposition of a matrix.

In the context of this specification, "image diffraction pattern"
By "fraction pattern" is meant the optical image observed by the naked eye focused on a grid when illuminated by a light source of finitely divergent diffuse light, such as a fluorescent tube. Used "diffraction grating (diff
The term "racting grating" refers to a grating consisting of either reflective or transmissive lines. The pattern here is "optically variable" where it varies with the viewing position.
And is "structurally stable" if its wide shape at any given viewing position is not substantially altered by slight distortion of the grating plane.

Kupten Cook's pixelated diffraction grating image (pixell
ated diffraction grating image) is substantially more secure than conventional metal foil insertion and is an improvement over protection devices such as multi-film devices, conventional linear gratings, and image holograms because of these. This is because, as compared to the device, as the banknote becomes severely wrinkled in daily use, an acceptable level of structural stability can be maintained. However, for wider commercial applications, such as credit cards, it would be desirable to be able to easily convert a recognizable image, in fact all selected image areas, to a pixelated grating. This has not been disclosed before and is the object of the first aspect of the invention.

SUMMARY OF THE INVENTION In a first aspect, the invention creates individual corresponding diffraction grating pixels for successive pixels of an optically invariant image, and in a preferred device, the grating pixels (gr
It essentially involves the concept of placing ating pixels to reflect the observed saturation or color value of the image pixels. In a second aspect,
The present invention relates to reflection contact printin.
g) the reproduction of the diffraction grating is impaired by the moire or the interference fringe effect of Talbot,
And with the realization that pixelated diffraction gratings can be formed to amplify the visual impact of these tall bats to the extent that counterfeit reproduction of the grating is very clearly impaired. . This moiré, or tall bat interference fringe, is a function of the unavoidable spacing between the photographic plate and the grating, and the change in local groove spacing across the grating plane.

Accordingly, the present invention, in a first aspect, is a method of forming a diffraction grating that provides an optically variable image, comprising manufacturing a corresponding pixelated diffraction grating of a substantially optically invariant image. Wherein each pixel of the optically invariant image is mapped to each pixel of a pixelated diffraction grating, wherein the pixels of the grating are individual optical diffraction gratings of relevant pixels in the optically invariant image; As a result, when illuminated, a pixelated diffraction grating provides a method of forming a diffraction grating that results in an optically variable reproduction of the optically invariant image.

Preferably, the pixels of each grating should also be a function of the given saturation or color value of the associated pixel in the optically invariant image, and the illuminated grating when illuminated A visually perceptible, but optically variable reproduction of the optically invariant image is provided. More preferably, the pixels of each grating are such that the pixelated diffraction grating also provides a structurally stable reproduction of the optically invariant image.

“Optical diffraction gratin
g) "means a grating that produces a diffraction pattern of a two-dimensional optical image when illuminated.

In a first aspect, the present invention extends to a diffraction grating created by the above method, and further provides a diffraction grating that provides an optically variable image, wherein each pixel of the image is an individual optical diffraction grating. And the pixelated diffraction grating when illuminated produces a pixelated diffraction grating that produces an optically variable image. The pixels of each grating are preferably a function of saturation or color value, and the pixelated diffraction grating is also preferably a structurally stable image.

In the simplest case of a black and white image, the saturation and color value are for example a grayness factor on a scale of 1 to 7, or 1 to 16.

The invention further provides, in a second aspect, a diffraction grating formed by a regular matrix of pixels that when illuminated each create a diffraction pattern of a two-dimensional optical image, wherein:
The grating in some or all of the pixels may be due to the reproduction of the grating by reflective exposure, by moiré or Tallbat interference fringe effects, and preferably by a zero order pattern of the reconstruction. Are arranged to create a zero-order pattern that is significantly impaired to a range intermediate between the negative and positive images of the zero-order pattern of the original grid.

Expressed mathematically, the reflective / transmissive line of the grid of each pixel according to the first aspect of the present invention is represented by the formula S _ij (x, y) = kN in x, y coordinates in the grid plane. Is preferably defined,
Where k is a scaling factor,
N is an integer, and the function S _ij (x, y) is represented by the following equation.

S _ij (x, y) = W _ij (x, y) + β _ij C _ij (x, y) (1) where S _ij (x, y) is vertically illuminated by the collimated monochromatic light wave. Where W _ij (x, y) is the non-zero order carrier wave and C _ij (x, y) is the x, y The function changes abruptly with respect to x and y, and the Hessian is not zero, ie, does not vanish, and β _ij is the given saturation of pixel ij, or Is the color intensity (colo
i, j are the coordinates of each pixel.

The Hessian of C _ij (x, y) is a standard complex derivative expressed as

δ ² C _ij (x, y) / δ x ² .δ ² C _ij (x, y) / δ y ^2- [δ ² C _ij (x, y) / δxδy] ^{2 In} one embodiment, the pixels of each grid are , The individual optical catastrophic gratings of the relevant pixels in the optically invariant image. This is because the Hessian is the Fresnel of the grating or the surface of the Fraunhofer diffraction pattern (caus
(1) except when along a certain characteristic line corresponding to (tics), when the value is not equal to zero.

A particularly useful set of functions suitable for C _ij (x, y) is
It consists of a product of sinusoidal terms, a sum, or a combination thereof. In general, convenient functions are those that are easily scalable to saturation or color value, are smooth, and are easily programmed.

For the purposes of the second aspect of the present invention, the right hand side of equation (1) is the sum term Sd (x, x), which is a periodic shield function that is substantially larger than the size of the pixel. y) may be included. The blocking function may also be the sum and / or the product of periodic sinusoidal terms greater than C _ij (x, y). Alternatively, a second aspect of the present invention provides for C _ij (x,
This is implemented by including different period terms in the sinusoidal component of y). In general, moiré or Tollbat fringe effects in reproduction are amplified by choosing a function term that will create a very curved wavefront.

In any of the embodiments of the present invention, the pixel has an area of 1 mm ²
Preferably smaller, most preferably square. The reflective / transmissive line may be a reflecting groove, for example a groove cut square in cross section on a metal-coated surface.

The present invention naturally extends to a combination of the above two aspects of the present invention in one lattice image.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described, by way of example only, with reference to the accompanying drawings, in which FIGS. 1 and 2 each show the original optically invariant structurally stable black and white A photographic print of the statue,
FIG. 3 is a photographic print of the diffraction image of a corresponding pixelated diffraction grating made in accordance with both aspects of the invention, wherein each pixel is an individual optical diffraction grating; FIG. FIG. 4 is a greatly enlarged view of each pixel from FIG. 2; FIG. 5A is a further enlarged view of the upper left corner of the pixel of FIG. 4; FIG. 5B is an extreme close-up of a typical pixel cross section, and FIG. 6 is an enlarged view of the center (10 × 10 cells) of FIG.

BEST MODE FOR CARRYING OUT THE INVENTION An exemplary embodiment of the first aspect of the present invention is described herein. For illustration, reference is made to the optically invariant structurally stable black and white image of FIG. To create a corresponding pixelated diffraction grating, the image of FIG. 1 is first divided into 0.125 mm square pixels, and the saturation or color value, in this case, the gray level, of each pixel is determined in turn. This process typically causes the image to be captured by a video camera connected to a suitable computer system, and the gray level of each pixel is stored in computer memory. Using a predetermined function S _ij (x, y), a pixelated grid is created, for example, by electron beam lithography as described below. In this grid, each pixel is an individual optical diffraction grating consisting of the corresponding pixel in the original image and is a function of the defined gray level of the corresponding pixel in the original image. It will be appreciated that each pixel of the original image is mapped to each pixel of the pixilated grid.

FIG. 2 is a diffraction image at a given viewing angle created when a pixilated grating is illuminated. The actual grid dimensions are 20.75mm x 26mm, 1
There are four gray-level rating scales. This pattern is visually recognizable but is an optically variable reproduction of the original optically invariant image shown in FIG.

 A suitable pixel grid function is given by:

Here, z = m / n is the groove index parameter of the cell (cell
groove index parameter), the pixel phase function S
_ij (x, y), n is the total number of grooves in the pixel (= 100 in this case), and m is a value from 1 to n.

α _ij is a preset variable that reflects color and determines line density, β _ij is a parameter proportional to the determined gray level, 0.8 ≦ α _ij ≦ 1.2 0.004 ≦ B _ij ≦ 0.064 0.25 ≦ x .Ltoreq.0.5 0.75.ltoreq.y.ltoreq.1.0, and i and j are coordinates of each pixel.

This form of the grating function embodies both aspects of the invention. The product of two sinusoidal terms with different periods gives rise to moire,
Alternatively, it is effective to significantly impair the interference fringes of the tall bat. Reconstruction of such a pixilated grid for FIG.
Produces a diffraction pattern of the zero order image shown in FIG. 3, which is visually, substantially halfway between the negative and positive images of the diffraction pattern of the original grating image, for any observer. Very obviously impaired. In creating this regeneration, copy plate separation (copy plate
The separation was 10 microns and the exposure was equivalent to five toll bat fringes.

An enlarged view of a typical pixel grid is shown in FIG. This is actually a computer plot of one of the pixel grids produced from FIG. 1, where the optical effects resulting from slight discontinuities in the lines were produced by the printer. These lines are shallow parabolas. The effect of the decreasing gray level parameter β is to increase the line-to-line spacing, being more pronounced at the center than at the edges, thereby increasing the curvature of each parabola. This, of course, has the effect of brightening the gray level of the grating image diffraction pattern, so that the overall effect is that the grating image diffraction pattern is a pixelated visually recognizable reproduction of the original image. It is becoming.
The pixel in FIG. 4 is the brightest of the 14 gray levels, ie, β _ij = 0.064, α _ij in the above equation (2).
= 1.0. Each curved grid line,
That is, it can be seen from FIG. 4 that the groove is approximated by four straight portions having different angles connecting the ends. Each linear portion is, as shown in FIG. 5A, actually a reduced miniature polygon in the shape of a rhombus with a well-defined width. FIG. 5B is an extreme close-up of a portion of a typical pixel, showing the abutment of polygonal grooves and changes in groove width. Sixth
The figure shows a small group of 10 × 10 pixels from the center of the grid of FIG. 2, and for clarity only the differences in the curvature of the grooves from pixel to pixel for each of the pixels for a few grooves. Plotted.

In general, S _ij (x, y) is the interval at which the diffraction image has the highest clarity, in order to choose the brightness and stability of each pixel, to choose the color and orientation of each pixel, It will be appreciated that adjustments are made to determine the angle and angle, and finally to select the light source that has the greatest effect on the diffraction portrait image.

Because the pixels diffract light over a much larger solid angle range than the conventional grid, the observed energy density reaching the retina of the eye at a particular viewing angle is much smaller than with the conventional grid. It should be noted. This means that the requirement for diffraction efficiency is more important for the grating according to the invention than for any other type of grating. In particular, if the grating lines are grooves, the depth of the grooves should be optimized for the highest diffraction efficiency. Square wave groove profile (a square groov
For e profile), this generally means that the groove depth should be about 40% of the average groove spacing. For the grating of FIG. 4, the groove depth should be between 0.5 and 0.6 microns, and preferably about 0.56 microns.

The actual grating in the form of FIG. 2 is constructed by programming an electron beam lithography system. The grating is written on a PMMA electronic resist that is spin coated on a chrome-coated glass substrate, and it is processed to make a gold-coated master on which a plastic film replica grating is pressed.

A practical process is to convert a given photograph of a portrait, or any view, into a set of data files used for controlling exposure, and that the machine has a corresponding master grid according to the invention. Involves writing the features of the electron beam lithography system to allow for making

First, the portrait is scanned into a computer graphics system by a high quality color scanner such as the Sharp JX-300, and has a special purpose created based on the inventor's above-mentioned theory of generalized gratings. Processed by a software package correlated with The software package provides the grid designer with a set of options for converting portrait data into diffraction patterns and a corresponding data file for electron beam production of the grid by an electron beam lithographic printing system. provide.

The first part of the program was digitized (digitise
d) "Screens" the portrait onto a selected array of NXM pixels. The resolution of the screen or the size of the pixel is within the range of the designer's choice. Clearly, the smaller the pixel, the larger the synthesized data file.
The second part of the program converts each pixel of the projected portrait into a corresponding diffraction grating as described above.

The last part of the design program consists of a subroutine for displaying the range of the light source and the observed diffraction pattern of the grating under viewing conditions. If the original design is not satisfactory in some respects, the designer can go back and modify the design before creating the final data file for grating electron beam formation.

A durable metal master with an optimized grid, 9
Vacuum coat the photoresist master with 9.99% gold wavelength 200 Å light and support (supp
(ort) by electrodeposition of a thin layer of nickel.

After separation from the glass master, the gold-coated nickel master is bonded to a brass block and used as a die for hot pressing a plastic film replica grid. To maintain the optimized diffraction efficiency of the master die for plastic replicas, the combination of temperature and pressure associated with the hot embossing process ensures that the depth of the replicated grooves is It should be as close as possible to the groove depth of 0.56 microns.

After being metallized with aluminum and plastic coated for protection, the plastic replica is affixed to banknotes or credit cards.

The representation of image information by a matrix of optically variable pixels is currently used as a security counterfeit label on credit cards for this type of diffraction grating due to line shape limitations inherent in the hologram formation process. Provides some unique properties that image holograms do not have.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to manufacture the diffraction grating which produces a realistic original and a portrait image near the quality of a photograph. This image is sharply contoured and optically variable under most lighting conditions, including very diffuse light sources. This is in contrast to holograms, which create very blurred images and lose optical variability under dark lighting conditions. The kinegram used in Australian 5,000 shilling banknotes
The optically variable device maintains a high degree of optical variability under a diffuse light source, but only produces a schematic type of image.

The optical variability of the present invention is also easily manifested, as the viewing angle changes, the image simply changes from negative to positive. The images produced by the present invention exhibit a high degree of structural stability with respect to bending undulations of the grating surface. They are therefore suitable protective devices for banknotes, stock certificates and the like. When the grating according to the preferred embodiment is viewed directly under an extended light source such as a fluorescent tube,
The intensity of the pixel changes smoothly with the change of the viewing angle. A pixel can be said to have structural stability because any disturbance of the initial wavefront due to wrinkling of the lattice plane near the pixel will only cause a change in intensity at the pixel. This means that the local line pattern in the area equivalent to the size of the pixel is linear, and therefore any local wrinkle disturbance will "switch off" the points of the observation image completely. In contrast to the conventional generalized grating of the resulting image hologram. Conventional generalized gratings of image holograms are therefore very unstable structurally, because for a given wavelength, the small area of the grating is the spread beam created by the image of the grating. This is because it diffracts a narrow and narrow linear beam that is much more sensitive to disturbances.

Preferred gratings of the type described herein are formed in accordance with the second aspect of the present invention and resist copying by well-known reflective exposure. The zero order portrait image of the copy grating is covered by many dark spoiler bands, which makes the zero order portrait image unrecognizable. In contrast, holograms and kinegrams are easily forged by this method.

An additional related feature of the present invention is that the depth of the pixel grooves can be increased by altering the electron beam flow during the grating formation process so that the visible textual or graphic image appears in the zero order diffraction. Ability to selectively change This static image is not apparent in any counterfeiting of the reflective exposure grating and is thus an added protective feature.

Finally, these gratings are binary phase devices.
are easily manufactured in large quantities by the same embossing method used for holograms.

Another approach to the second aspect of the invention is to use C _ij (x, y) with sinusoidal terms of similar period and to sum up longer periods spanning several pixels. Is to provide the shielding function Sd (x, y). For example, a suitable function would be:

Sd (x, y) = αy + β [Cos2πQxCos2πQy−2 / 3 (Cos2πQx + Cos2πQx)] Here, α and β are the parameters described above, and Q is a further periodic parameter.

Continuation of the front page (56) References JP-A-62-232616 (JP, A) JP-A-52-90959 (JP, A) JP-A-2-72320 (JP, A) JP-A-2-165987 (JP, A) JP-A-62-223215 (JP, A) JP-A-2-72319 (JP, A) JP-A-50-108934 (JP, A) US Patent 4,568,141 (US, A) US Patent 4,450,561 (US, A) WO 90/7133 (WO, A1) WO 87/7034 (WO, A1) A. Lee, Generalized ed curvilinear dif fraction gratings. Diffracton cat strophes, OPTICA ACTA, Vol. 30, No. 4, p. 449 −464 (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 5/18 G02B 5/32 G03H 1/00-5/00

Claims (33)

(57) [Claims] 1. A method of forming a diffraction grating that produces an optically variable image that can be changed optically according to an observation position, comprising: dividing an optically invariant image into a plurality of pixels; For each of the pixels of the invariant image, determine a grating pixel that is one optical catastrophic grating corresponding to the pixel of the optically invariant image, and from the set of grating pixels A pixelated grating that, when illuminated, results in the reconstruction of an optically variable image of the optically invariant image, for each of the pixels in the image whose gray level is optically invariant. A respective set of reflective or transmissive lines, wherein the diffraction grating pixels are determined between the pixels and have a curvature related to the determined gray level of the pixels of the optically invariant image. Diffraction grating manufactured to consist of Forming method. 2. The method of claim 1, wherein the curvature of the set of reflective or transmissive lines of the grating pixel is inversely proportional to gray level. 3. Each of the individual optical diffraction gratings consists of reflective or transmissive lines defined by the equation S _ij (x, y) = kN in x, y coordinates in the plane of the grating. , K is a scale factor, N is an integer, and a function S _ij (x, y) is given by the following equation: S _ij (x, y) = W _ij (x, y) + β _ij C _ij (x, y ) (1) S _ij (x, y) is the initial phase function created by the pixel ij of the grid when vertically illuminated by the collimated monochromatic light wave, and W _ij (x, y) is In the zero-order carrier wave, C _ij (x, y) is a function of x, y and changes abruptly with respect to x, y, and the Hessian is not zero, ie, vanishes The diffraction grating according to claim 1 or 2, wherein β _ij is a factor proportional to the determined gray level of the pixel ij of the optically invariant image, and i, j are the coordinates of each pixel. Method. 4. The diffraction grating according to claim 3, wherein the function C _ij (x, y) is selected from a set of functions consisting of a product or a sum of sinusoidal terms, or a combination thereof. Formation method. 5. The method according to claim 4, wherein the sinusoidal components of C _ij (x, y) include terms having different periods. 6. The method of claim 4, wherein the right side of equation (1) further includes a summed shielding function term with a period substantially greater than the size of the pixel. 7. The method of claim 6, wherein the summed term is a sum and / or a product of sinusoidal terms having a period greater than C _ij (x, y). 8. The method of forming a diffraction grating according to claim 1, wherein the pixels of the grating are squares having an area larger than 1 mm ² . 9. The method according to claim 1, wherein the pixelated diffraction grating is formed by forming a reflection type groove, for example, a groove having a square cross section in a metal-coated surface. . 10. The pixelated diffraction grating is a permanent master grating formed by electron beam lithography capable of producing a variety of replica gratings.
The method for forming a diffraction grating according to any one of the above. 11. The method according to claim 6, wherein the shielding function term comprises a sum and / or a product of sinusoidal functions having a period larger than C _ij (x, y). 12. The reflective or transmissive line is formed as a groove, and the groove depth of the grid pixel is selectively changed so that a visible original or graphic image can be seen as a zero-order diffraction. Item 12. The method for forming a diffraction grating according to any one of Items 1 to 11. 13. The method according to claim 12, wherein the groove depth is changed by changing an electron beam flow in the electron beam lithographic printing. 14. Diffraction according to claim 1, wherein the gray level of the pixels of the optically invariant image is selected from a measure comprising a predetermined finite number of gray levels. How to form the grid. 15. The method according to claim 1, wherein the grid is a master grid and includes the step of hot pressing a plastic film in a die having the master grid to produce one or more replica grids. 15. The method for forming a diffraction grating according to any one of the above items. 16. The method according to claim 10, wherein the master grating is a nickel master coated with gold. 17. A diffraction grating according to claim 1, wherein said reflection type or transmission type line is formed as a groove formed in a polygon having a variable depth, width and interval. Method. 18. A diffraction grating for producing an optically variable image corresponding to an optically invariant image, comprising a set of diffraction grating pixels, said pixel producing an optically variable image when illuminated. This image is optically variable and changes depending on the observation position.
Each of the grating pixels is an individual optical catastrophic grating, wherein the grating pixels vary between pixels;
And a diffraction grating comprising a respective set of reflective or transmissive lines having a curvature associated with a predetermined gray level of a pixel of the optically invariant corresponding image of the optically variable image. 19. The diffraction grating according to claim 18, wherein each grid pixel is also a function of saturation or color value. 20. The line of claim 18, wherein said line has a linear density that varies between pixels, said linear density being related to a predetermined saturation or color value of said optically invariant image pixel. Or a diffraction grating according to 19. 21. The curvature of the set of reflective or transmissive lines of the diffraction grating pixel wherein the curvature is inversely proportional to gray level.
21. The diffraction grating according to any one of 18 to 20. 22. Each of the individual optical diffraction gratings consists of a reflective or transmissive line defined by the equation S _ij (x, y) = kN at the x, y coordinates in the plane of the grating. , K is a scale factor, N is an integer, and a function S _ij (x, y) is given by the following equation: S _ij (x, y) = W _ij (x, y) + β _ij C _ij (x, y ) (1) S _ij (x, y) is the initial phase function created by the pixel ij of the grid when vertically illuminated by the collimated monochromatic light wave, and W _ij (x, y) is In the zero-order carrier wave, C _ij (x, y) is a function of x, y and changes abruptly with respect to x, y, and the Hessian is not zero, ie, vanishes ₂₃ , wherein β _ij is a factor proportional to the determined gray level of the pixel ij of the optically invariant image, and i, j are the coordinates of each pixel. lattice. 23. The diffraction grating according to claim 22, wherein the function C _ij (x, y) is selected from a set of functions consisting of a product or a sum of sinusoidal terms or a combination thereof. . 24. The diffraction grating according to claim 23, wherein the sinusoidal components of C _ij (x, y) include terms of different periods. 25. The right hand side of equation (1) includes a summed shielding function term with a period substantially greater than the size of the pixel.
25. The diffraction grating according to any one of to 24. 26. The diffraction grating according to claim 18, wherein the pixels of the grating are squares having an area larger than 1 mm ² . 27. The pixilated grating comprises reflective or transmissive lines of varying depth so that a static visible original or graphic image appears to be of zero order diffraction. 27. The diffraction grating according to any one of 18 to 26. 28. A diffraction grating according to claim 18, comprising a permanent master grating formed by electron beam lithography capable of producing a variety of replica gratings. 29. The diffraction grating according to claim 25, wherein the shielding function term comprises a sum and / or a product of sinusoidal functions having a period greater than C _ij (x, y). 30. The reflective or transmissive line is formed as a groove, and the groove depth of the grid pixel is changed so that a visible original or graphic image can be seen as a zero-order diffraction. 30. The diffraction grating according to any one of to 29. 31. A diffraction grating according to any one of claims 18 to 30, wherein the diffraction grating is provided on a banknote, credit card, stock certificate, or other security document to protect the document against counterfeiting. 32. Diffraction according to claim 18, wherein the gray level of the pixels of the optically invariant image is selected from a scale comprising a predetermined finite number of gray levels. lattice. 33. A diffraction grating according to claim 18, wherein said reflection type or transmission type line is formed as a groove formed in a polygon having a variable depth, width and interval.