FR3068495A1 - METHOD FOR CALCULATING THE MASK IMAGE FOR MANUFACTURING DIFFRACTIVE OPTICAL COMPONENTS WITH FIELD DEPTH VISUAL EFFECTS - Google Patents

Abstract

The invention relates to a method for calculating a mask image enabling the fabrication of a diffractive optical component capable of forming a visual effect corresponding to at least one continuous, discernable, distinguishable image at one or more observation distances and comparable. to all or part of a reference image, said method being characterized in that it comprises the following steps: Step 1: Generate a source image in binary digital format from said reference image and determine the size of the elementary pixel said source image as a function of the desired size of the diffractive optical component; Step 2: Trim said source image into a plurality of Super Pixels (SP); Step 3: Calculate by computer, from the Maxwell equations, the equivalent network of each Super Pixel; Step 4: Replace the contents of each Super Pixel by its equivalent network in digital binary format, to form the image of the mask.

Description

Mask image calculation method for manufacturing diffractive optical components with visual effects at depth of field

The present invention relates to diffractive optical components capable of forming continuous, discernible and differentiable images at determined distances. More particularly, it relates to a method for calculating the image of the mask making it possible to manufacture optical components with visual effects at depth of field.

STATE OF THE ART. 10

Many technologies are known which make it possible to produce optical components with visual effects.

In the rest of the document, we define:

- A point source as being a source whose dimension is small enough to consider that the light emission is concentrated at a point. On Earth, the Sun is considered to be a point source. In common use, an LED is considered a point source.

- An observation distance as being the distance between the object to be observed and the image capture device, which can be for example the eye, a camera, an optical sensor, etc.

- A mask as being a device making it possible to generate a modulation of the light in amplitude and / or in phase. This mask is known to a person skilled in the art under the acronym SLM (“Spatial Light Modulator”) which means in French spatial light modulator. Said mask can be a chromium mask as used in conventional lithography methods. It can be a mask reconfigurable in real time such as for example a micro-mirror matrix or an LCD screen (acronym of the term "Liquid Crystal Display"). The mask is used either to manufacture the diffractive optical element as defined by the invention, but in some cases it can also be the diffractive element itself when its resolutions allow it. These types of masks are well known to those skilled in the art.

In the field of entertainment, specific glasses allow you to modify the perception of fireworks or a concert stage lit by a multitude of light sources. The lenses of these glasses are structured by periodic diffractive gratings. When the user wears this type of glasses and observes a point source, the glasses of said glasses modify the light distribution by diffraction and form specific colored shapes within the retina. The shape observed depends directly on the type of diffractive grating. For example, a hexagonal network generates a 6-pointed star. The image is observed in transmission through the diffractive grating. The colors observed depend for their part, both on the parameters of the diffractive grating (material, material index, thickness of the structures, shape of the structures) and on the spectrum of the observed source. This type of periodic diffractive grating does not make it possible to form any type of image from the point source. For example, it does not allow to form a simplified diagram of a house (Fig. 5C).

In the field of document authentication or product security, these so-called optical security components generate optical effects as a function of the observation parameters: orientation relative to the observation axis, position and dimension of the light source, etc. The general aim of these optical components is to provide new differentiated effects of physical configuration which are difficult to reproduce. Among these optical components, we call DOVID for "Diffractive Optical Variable Image Device", the optical components producing a diffractive and variable image which is commonly called by abuse of hologram language. These optical components are generally observed in reflection, like those present on bank cards or banknotes, but they can also be observed in transmission. The visual effects observed depend on the angle of observation and / or the position of the light source. These components do not allow different images to be observed at different observation distances.

Another type of optical safety component makes it possible to generate visual effects whose characteristics are variable as a function of the viewing distance. These components are described in detail in the international patent application published under the reference WO2013 / 026747. These diffractive optical components can be used in reflection or in transmission. They are composed of a matrix of elementary circular diffractive optics. When the optical component is illuminated by a point source, each elementary optical element generates a chromatic voxel whose color or extinction (absence of light) depends on the distance of observation. The elementary optical elements are independent of each other. Their color or extinction is determined according to their intrinsic characteristics. It is therefore possible to form a discreet colored image at a determined distance. Due to the intrinsic peculiarity of these elementary circular diffractive optics arranged in a matrix, these optical safety components do not allow continuous images to be formed.

There are diffractive optical components in reflection or in transmission forming continuous images from point sources, but the images formed have forms limited by the periodic nature of their networks. These components cannot be used in the field of product and document authentication because the architecture of their networks is known and easily reproducible.

Finally, the optical safety components composed of arrays of circular diffractive optical elements do not allow the generation of continuous images. They do not allow, for example, the writing of a message in the form of text as an authentication key.

The invention proposes a means of designing a particular diffractive optic, such that when said optic is illuminated by a point source, it generates a continuous predetermined colored image at a given observation distance. The diffractive gratings used are complex due to the structure of their array and the variations in thickness of said array. They are therefore more difficult to reproduce by counterfeiters than the components known from the prior art.

PURPOSE OF THE INVENTION

The invention aims to provide a diffractive optical component capable of generating visual effects of the continuous image type at a determined distance when said component is illuminated by a point or quasi-point source.

The invention also aims to provide a calculation method for the manufacture of such an optical component, as well as an authentication device comprising said component.

OBJECTS OF THE INVENTION

In the rest of the document, a diffractive optical device is defined as any diffractive optical component making it possible to achieve the diffractive effects for which it is designed. The desired diffractive effects represent a predefined reference image. A diffractive optical device can for example be a spatial light modulator such as an LCD screen (acronym for Liquid Crystal Display) or a diffractive optical element, the structures of which are etched in a material previously deposited on a substrate.

The image of the mask defines the digital image of a diffractive optical component. The term “observation device” designates the eye or any type of device making it possible to capture an image such as cameras, photographic devices, arrays of sensors. The observation distance is defined as the distance between the observation device and the diffractive element. The image formed by the diffractive optical component within the observation device is defined by observed image. A quasi-point source is defined as a slightly extended source or a source which is sufficiently distant from said diffractive element. When a user holds said diffractive element in his hand, a light source located at the ceiling of the room in which the user is located can be considered to be quasi-punctual. An elementary pixel of an image is called an elementary pixel. In contrast, a Super Pixel has a number of elementary pixels strictly greater than 1.

In its basic principle, the invention relates to a method for calculating the image of the mask allowing the manufacture of a diffractive optical component capable of forming a visual effect corresponding to one or more continuous image (s) , discernible (s), differentiable (s) and comparable to all or part of a reference image at a determined observation distance when it is illuminated by a point or quasi-point source.

The subject of the invention is therefore a method of calculating a mask image allowing the manufacture of a diffractive optical component capable of forming a visual effect corresponding to at least one continuous, discernible image, differentiable at one or more distances d observation and comparable to all or part of a reference image, said method being characterized in that it comprises the following steps:

Step 1: Generate a source image in binary digital format from said reference image and determine the size of the elementary pixel of said source image as a function of the desired size of the diffractive optical component;

Step 2: Cut said source image into a plurality of Super Pixels (SP);

Step 3: Calculate by computer, using Maxwell's equations, the equivalent network of each Super Pixel;

Step 4: Replace the content of each Super Pixel with its equivalent network in binary digital format, to form the image of said mask.

According to an embodiment of the invention, the method comprises a step consisting in introducing Pixel Distances between Super Pixels of the mask image to transform the image of the mask of step 4 into an image of the size mask larger, and thus modify the viewing distance or the size of the visual effect generated by the diffractive optical component.

According to another embodiment of the invention, the method comprises a step consisting in transforming the image of the mask into a multi-gray level image and assigning a gray level to the level 1 pixel of each equivalent array, so as to generate one or more specific color (s) in the observation plane.

The mask image is calculated according to steps 1 to 4 described in more detail in the following paragraphs.

Step 1 consists in generating a source image in binary digital format from a reference image and in imposing the size of the elementary pixel of said source image as a function of the desired size of the diffractive optical component. The reference image may or may not be in digital format. It may have chromatic characteristics. The size of the source image is sized according to the complexity of the reference image. A reference image having a simple pattern such as a cross does not for example require a source image with a size of 2000 × 2000 pixels but a size of 500 × 500 may be suitable. The source image is an image composed of elementary pixels, the set of white pixels of which generates a shape representing the reference image, for example the diagram of a house.

Step 2 consists of cutting the source image into a plurality of Super Pixels (SP). Advantageously, this division is a function of the complexity of the content of the source image. It can be regular or irregular. Advantageously, it will be irregular in the case of large areas containing only black pixels since these areas contain no information on the shape of the reference image. On the other hand, when the shapes are complex, a division into small Super Pixels of the order of ten elementary pixels may be considered. With current computer means for detecting areas in an image, those skilled in the art will be able to implement the cutting algorithms in line with the particularities of the source images to be processed.

Step 3 consists in calculating by computer, from Maxwell's equations, the equivalent network of each Super Pixel. The equivalent network is defined as the network which makes it possible to reconstruct the shape contained in the Super Pixel when this network is illuminated by a point or quasi-point source by diffractive effect. For example, a network of white lines with constant orientation pitch (Φ) makes it possible to reconstruct by diffractive effect a white line having for normal (Φ). Alternatively, a network of squares makes it possible to reconstruct a cross or a network of hexagons generates a star-shaped motif with 6 branches.

Step 4 consists of replacing the content of each Super Pixel with its equivalent network in binary digital format, to form the image of the mask.

The equations for generating a simulation algorithm for diffractive optics are known to those skilled in the art. An example of such an algorithm is proposed in appendix A of the thesis of Emilie Bialic entitled "study of a new axiconic optical element: the multiple annular diffractive linear axicon" published on January 10, 2011 at the school of Telecom Bretagne engineer (Brest, France). This algorithm simulates the effects of a mask image. It associates each elementary pixel with a surface of the diffractive optical component. For example, an elementary pixel corresponds to an area of 1 pm ² . It takes into account the parameters of the material chosen. In the case of a monolayer material, the parameters retained are the thickness of the diffractive grating and the optical index of the material. This algorithm makes it possible to visualize the visual effect by specifying the distance of observation of this effect according to the spectral and energy parameters of the point source considered.

If the observation distance obtained is not the desired distance, two solutions to correct this dimension are possible:

- The first solution (the simplest) consists in artificially increasing or reducing the surface corresponding to an elementary pixel;

- The second solution consists in modifying the arrangement of the Super Pixels of the source image by introducing Pixel Distances within this image.

In the second solution, Pixel Distances are introduced between each Super Pixel of the mask image to transform the mask image from step 4 into a larger mask image, and thus modify the observation distance and / or the size of the visual effect. These Distance Pixels are Super Pixels containing no information. They are therefore Super Pixels only composed of elementary black pixels. Their dimensions can for example be calculated from known geometric rules for the propagation of diffractive effects.

Finally, it is possible to control the colorimetry of the visual effect. The previously presented method must be completed by a step consisting in transforming the image of the binary mask into a multi-gray level image and assigning a gray level to the white pixel of each equivalent array, in order to generate one or more colors. specific in the observation plan. The colorimetry of the images observed depends on the materials making up said diffractive optical component, on the nature of the structures (shape and thickness) and on the spectral nature of the source which illuminates said diffractive element. The mathematical relationships used to calculate the efficiency of a diffraction grating, as a function of wavelengths, of the optical index of the material and of the thickness of the grating, are well known to those skilled in the art. As an example, consider a circular binary phase network. The thickness (e) of the network is given by the following formula:

___ X -___ ^e 2 (η (Λ) - 1; i

Where Àw represents the working wavelength and η (λ) the index of the material as a function of the wavelength. When the network is illuminated by another wavelength λ, a different phase apparaît appears and expresses by:

# = -τλ

The diffraction efficiency of this grating is directly dependent on the order considered (m), in our case the order +1 and on this phase difference Φ. It is expressed by:

As an indication, for a network thickness of 565 nm, a resin index of 1.64, and a working wavelength of 723 nm, the efficiency of the red wavelength (640 nm) will be 39% , that of the green color (520 nm) by 27% and that of the blue color (460 nm) by 16%. It then suffices to use the geometric relations of wave propagation according to the different orders of diffraction known to those skilled in the art, to determine the color generated by said grating at a given distance. By way of example, in the case of a diffractive circular grating of binary phase, the focusing distance z _m of the wavelength λ for the order considered m is written (the radial distribution is directly deduced from this relationship) :

mA where d represents the period of the network and R the radius of the opening.

Another object of the invention relates to the diffractive optical component produced from the mask image obtained according to the calculation method previously described. Said component can be made, for example, from optical projection lithography techniques. In this case, the image of the binary mask that one wishes to engrave is projected through a mask by an optical system on a photosensitive material. In order to correct the colorimetric parameters, said mask can be binary or coded in gray level. The grayscale coding can be carried out from a spatial filtering of light intensity. The part allowing the light to pass is no longer uniform but is composed of a periodic structure of holes whose size varies according to the desired gray level. This technique can be performed for example from a mask aligner. We then speak of contact photolithography. This method consists in using a deposit of photosensitive material (for example a photosensitive resin deposited on a substrate). This photosensitive resin is exposed to light (for example Ultra-Violet (UV) light in the case where the resin is sensitive to UV radiation) by direct contact with the mask (for example a chrome mask having opaque and transparent zones of the binary mask image pattern). The resin is then developed to obtain the diffractive optical component. This technique has the disadvantage of being limited in resolution by the diffraction of light around the patterns. To overcome this problem, other etching techniques can be used, such as direct writing single or multi laser beams, electrons or focused ions.

Concretely, the diffractive optical component according to the invention can be used as a security optic allowing the authentication of products or documents.

The diffractive optical component can also be used to generate visual effects making it possible to display an intrinsic characteristic of the product, for example a personalized message. For example, the diffractive optical component can be engraved in the glass of a watch. In this specific application, a point source inserted into the watch hand holding system illuminates the diffractive optical component which then forms the personalized message on the user's retina. However, this etching of the watch glass can generate unwanted visual effects when the system is not in operation. It can result in a local reduction in transparency.

Advantageously, the watch glass can be replaced by a semi-transparent photovoltaic module. The diffractive optical component is then etched in the photoactive layers of the photovoltaic module during the process for producing the semi-transparency of the photovoltaic module. The diffractive optical component is then invisible to the eye and does not generate a local reduction in transparency. It is revealed in transmission by the point LED source inserted in the back of the watch. It can also be revealed in reflection by a point source external to the application.

The invention also relates to an object authentication system, characterized in that it comprises at least one diffractive optical component as described above, combined with a spatially polychromatic light source comprising one or more point sources or almost one-off. As a variant, the point light source can be infrared or ultra-violet, or constituted by any type of point source which is not visible.

DETAILED DESCRIPTION

The invention is now described in more detail using the description of Figures 1 to 7.

FIG. 1A is an example of a source image composed only of black pixels and of white pixels.

FIG. 1B represents a possible matrix division of the image of FIG. 1A in Super Pixels.

FIG. 2A is an example of a binary source image containing a simple form.

FIG. 2B represents a Super Pixel division of the source image of FIG. 2A.

Figure 3A shows the content of a Super Pixel and the content of its equivalent network.

FIG. 3B is an image of the binary mask originating from the source image of FIG. 2A.

FIG. 3C is an example of inserting Pixel Distance into the image of the bit mask of FIG. 3B.

FIG. 4A represents the image of a diagram of a house in white pixels under a background of black pixels.

FIG. 4B corresponds to the image of the binary mask of FIG. 4A making it possible to form the image observed at a distance of 14 cm.

FIG. 4C is the result of a simulation of the propagation of light through the binary phase mask resulting from the image of the binary mask at an observation distance of 14 cm.

FIGS. 5A to 5C are diagrams showing partial sectional views of examples of diffractive optical components according to the invention intended to be applied or engraved on an object to be authenticated.

FIG. 6A describes an example of the use of a visual effects device according to the invention.

FIG. 6B shows an example of an object capable of integrating the optical security component as well as the source making it possible to visualize the visual effects.

FIG. 1A is a source image (1), that is to say an image in binary digital format composed only of black pixels (3) and of white pixels conventionally having the respective values 0 and 1 (2). FIG. 1B represents a matrix breakdown of the source image of FIG. 1A in Super Pixels (SP). Figure IB is made up of 28 rectangular Super Pixels of the same size. The Super Pixels (11, 12, 21, 31) are indexed by their coordinates corresponding to their row number then their column number. Arbitrarily, the uppermost line of the image bears line number 1 and the leftmost column carries column number 1. Thus, the Super Pixel (11) being the uppermost and the most left of the image will be indexed SP (1,1). Generally, the Super Pixel in row i and column j is indexed SP (i, j).

In order to facilitate the description of the method for calculating the image of the binary mask according to the invention, a simplified source image (1) is proposed in FIG. 2A. The source image is composed of white pixels (2) surrounded by black pixels (3). The white pixels (2) form a cross, while the black pixels (3) represent four squares. FIG. 2B represents a matrix breakdown of the source image of FIG. 2A in Super Pixels (SP). Figure 2B is made up of 9 square Super Pixels of the same dimensions. Consider the Super Pixel (21) in row 2 and column 1 of the source image (1). It is made up of a horizontal white line. It is known to those skilled in the art that its equivalent network is a network of lines of direction perpendicular (Φ) to the white line. FIG. 3A is a diagram which represents the replacement of the content of the Super Pixel (21) by its equivalent network (21A). The content of each Super Pixel in the source image (1) of FIG. 2B is replaced by its equivalent network to form the image of the binary mask presented in FIG. 3B. The dimension of the elementary pixel is then fixed as a function of the size of the visual effect that one wishes to observe. When the dimensions of the elementary pixel are defined, the observation distance is directly determined by the image of the binary mask of FIG. 3B. To increase this distance or modify the size of the image, it is possible to insert pixel distances between the Super Pixels. These distance pixels can be square or rectangular. FIG. 3C shows the image of the binary mask of FIG. 3B in which Pixels are placed Square distances between each Super Pixel and between the edge and each Super Pixel.

The diffractive optical component according to the invention can for example be produced from optical lithography methods by projection or by laser single-beam writing. With reference to FIG. 5A, the diffractive optical component according to the invention comprises a structured layer (8) over part or all of its thickness (e) to produce at least one diffractive structure (7). This structured layer (8) can be previously deposited on a substrate (9), for example made of glass. FIG. 5A is a cross-sectional view of part of said diffractive optical component according to the invention.

As illustrated in FIG. 5B, the structured layer (8) can be covered for example with a metallic layer (10) deposited directly on the diffractive structure (7), and also comprise a protective layer (15) of the structure diffractive (7), and / or an adhesive layer (13) to allow assembly of the component with the object to be authenticated. The substrate (9) can also be replaced by a functional surface (14), for example photovoltaic.

The diffractive optical component according to the invention can also comprise several structured layers (8) and (8A) as illustrated in FIG. 5C. The diffractive structures (7) and (7A) can then be independent of each other or correlated in order to form several visual effects at different observation distances. The complexity of such a configuration makes counterfeiting more difficult and therefore makes it possible to increase the robustness of authentication.

According to a variant illustrated for example in FIGS. 5A and 5C, the diffractive optical component according to the invention is transmissive in the spectral band of the source used to illuminate the component. It will then be placed on a transparent part of the object to be authenticated.

According to another variant, the diffractive optical component is reflective. The structured layer (8) is then, for example, covered with a metal layer or a material with a high refractive index (10). If the device to be identified is the photovoltaic surface (14), the diffractive optical component is directly integrated into the photovoltaic device.

According to another variant, the different layers structuring the diffractive optical component with visual effects, are chosen so that it is transparent or semi-transparent.

Concretely, the authentication of products using the diffractive optical security component according to the invention can be carried out for example according to three distinct methods described below.

The first method concerns authentication by a human operator. FIG. 6A describes an example of use. The human operator has the object to be authenticated on which the diffractive optical component (52) with visual effects according to the invention is inserted. The human operator places himself in a light environment, for example under a bulb emitting white light (50). He holds the object at a distance, for example, 40 cm from his eye. The light rays (51) from the light bulb (50) are reflected on the visual effects device (52). The rays from the reflection (53) form the image observed on the retina of the observer. The observer verifies that the image he has just viewed corresponds to the image he received from an authorized security operator on a control device such as his mobile phone, and this for the viewing distance of 40 cm. If this is the case, the object is authenticated by the optical device with visual effect, otherwise the observer can conclude that a counterfeit object.

The second method (not illustrated) refers to an authentication system from an image capture means allowing the user to take a video of the optical effect observed by moving the acquisition system along of the optical axis of the optical component. The deduction of authentication is then carried out through the analysis of the images constituting the video. For example, a user uses the camera function of their mobile phone. He uses the lit mode of the LED used as a flash for the camera as a light source and observes through the screen of his phone the film resulting from the observation. An image is formed at a distance of 20 cm between the camera of the telephone and said diffractive optical component. The user then continuously films a video sequence during which the user gradually changes the distance between said device and the camera, for example from 20 cm to 40 cm. Different images were then formed successively and are recorded in the video sequence. For example, 10 different colored images formed between the distance of 20 cm and 40 cm. They are analyzed to verify the authenticity of the object.

The third method (not illustrated) is based on the principle of the second method but is automated thanks to an image capture system having an autofocus system, which allows the user to place the acquisition system in a position fixed, take a series of images at different depths of field to authenticate the item.

Alternatively, the diffractive optical component can be designed for wavelengths not visible in the infrared ranges. The authentication will then be revealed thanks to an optical system adapted to these wavelength ranges.

The diffractive optical component in its semi-transparent and photovoltaic version (101) according to the invention can be integrated into a watch glass as shown in FIG. 6B. Advantageously, it will be associated with an LED microchip (100) integrated for example into the device for rotating the needles. When the user actuates the LED microchip and it lights up, he can directly view the desired visual effects at defined distances. These effects can be aesthetic, make it possible to display a personalized message or even allow authentication of the watch.

EXAMPLE OF IMPLEMENTATION

Figure 4A shows a simplified diagram of a house. The source image is composed of white pixels (2) surrounded by black pixels (3). The white pixels (2) form the outline of the house. After having followed the steps of the method according to the invention, the image of the binary mask (5) presented in FIG. 4B has been produced. A binary phase mask is made from this image of the binary mask (5) to simulate the propagation of light through the diffractive optical element according to the invention. The simulated material is a resin of index n = 1.64. The thickness of the simulated network is 365 nm.

A simulation of the propagation of a point white light having passed through said phase mask was carried out using the IDL software in which a diffracted field propagation algorithm was implemented according to the approximation of the scalar theory of diffraction. The image of the binary mask (5) of FIG. 4B used for the simulation represents a diffractive optic of dimension 0.75 cm in width and 1 cm in height. The image obtained presented in FIG. 4C was simulated 10 for a distance of 14 cm downstream from the diffractive optics. The image of FIG. 4C well represents the image of FIG. 4A.

Claims (7)

1 - Method for calculating a mask image allowing the manufacture of a diffractive optical component capable of forming a visual effect corresponding to at least one continuous image, discernible, differentiable at one or more observation distances and comparable to all or part of a reference image, said method being characterized in that it comprises the following steps: Step 1: Generate a source image in binary digital format from said reference image and determine the size of the elementary pixel of said source image as a function of the desired size of the diffractive optical component; Step 2: Cut said source image into a plurality of Super Pixels (SP); Step 3: Calculate by computer, using Maxwell's equations, the equivalent network of each Super Pixel, this equivalent network being defined as the network which allows the shape contained in the Super Pixel (SP) to be reconstructed when this network is lit by a point or quasi-point source by diffractive effect. Step 4: Replace the content of each Super Pixel with its equivalent network in binary digital format, to form the image of said mask. 2 - Calculation method according to claim 1, comprising at the end of the process a step consisting in introducing Pixel Distances between Super Pixels of the mask image to transform the image of the mask of step 4 into an image of the mask larger. 3 - Method of calculation according to claim 1 or claim 2, comprising at the end of the process a step consisting in transforming the image of the mask into a multi-gray level image and assigning a gray level to the level 1 pixel of each array equivalent. 4 - Diffractive optical component, characterized in that it is manufactured from the image of the mask obtained by the calculation method according to one of claims 1 to 3. 5 5 - Diffractive optical component according to claim 4, characterized in that it comprises a photovoltaic module. 6 - System for authenticating an object, characterized in that it comprises at least one diffractive optical component according to one of claims 4 or 5, combined 10 to a spatially polychromatic light source comprising one or more point or quasi-point sources. 7 - system for authenticating an object, characterized in that it comprises at least one diffractive optical component according to one of claims 4 or 5, combined 15 to an infrared or ultraviolet point light source, or any type of invisible point source.