EP3554846B1 - Method for producing a security element with a lens grid image - Google Patents

Description

The invention relates to a method for producing a security element with a lenticular pattern image for displaying one or more target images that are only visible from predetermined viewing directions, the motifs of which are formed by visually recognizable, contrasting metallic and demetallized partial areas of a motif layer.

Data carriers, such as value or ID documents, but also other valuables, such as branded items, are often provided with security elements for protection, which allow the authenticity of the data carrier to be checked and which also serve as protection against unauthorized reproduction.

Security elements with effects that depend on the viewing angle play a special role in securing authenticity, as these cannot be reproduced even with the most modern copiers. The security elements are equipped with optically variable elements that give the viewer a different image impression from different viewing angles and, for example, show a different color or brightness impression and/or a different graphic motif depending on the viewing angle.

It has been known for a long time to personalize identity cards, such as credit cards or identity cards, by means of laser engraving. In the case of personalization by laser engraving, the optical properties of the substrate material of the identification cards are irreversibly changed in the form of a desired identification by suitably guiding a laser beam.

The pamphlet EP 0 219 012 A1 describes an identity card with a partial lenticular structure, through which desired information is written into the card with a laser at different angles. This information can subsequently only be recognized from this angle when viewed, so that the different information appears when the map is tilted.

If a lenticular pattern image contains a metallic motif layer, the motifs shown can be formed by local demetallization of the metallic motif layer. Various possibilities are known for introducing a design into a metallization with a laser by demetallization. The demetallization can be carried out, for example, by direct inscription, in that a laser beam is guided over the metallic motif layer using a suitable scanning device, or also by a larger-area laser impingement using a mask. In both cases, a particular challenge is the generation of demetallized lines of a desired width in the motif layer.

That's from the pamphlet EP 3 015 279 A1 a method for producing a security element with a lenticular pattern is known, in which a marking laser source for generating pulsed laser radiation is provided and the pulsed laser radiation in the beam path between the laser source and the lenticular pattern is provided with a motif-shaped beam cross section by a beam shaper or a switchable mask. In this case, a plurality of microlenses of the lens raster is simultaneously exposed to the laser beam with the pattern-shaped beam cross-section in order to simultaneously produce a plurality of sub-pattern-shaped demetallized partial areas in the underlying metallic pattern layer.

The pamphlet DE 10 2013 007 484 A1 describes a security element with an arrangement of similar microlenses arranged on the first main surface of a central body, a laser-sensitive recording layer arranged on the second main surface of the central body with a large number of micro-characteristics generated by the action of laser radiation, and a mask layer between the arrangement of microlenses and the laser-sensitive recording layer and is arranged outside the focal plane of the microlenses.

The pamphlet EP 3 015 279 A1 discloses a method according to the preamble of claim 1.

If the metallic motif layer is subjected to demetallization with a finely focused laser beam from different angles and thus successively at different points in the focal plane, until the partial areas with the desired line width are demetallized, scanning the entire surface of the lenticular image is usually very complex and tedious. In order to shorten the duration of the process, it was therefore proposed to arrange the metallic motif layer outside the focal plane of the (micro)lenses, so that during laser demetallization an expanded image of the incident laser radiation results in the plane of the motif layer. In this case, the demetallization can be significantly faster be carried out, but the defocusing produces blurred tilting images with image changes that are no longer clearly defined. Proceeding from this, the invention is based on the object of specifying a method of the type mentioned at the outset which avoids the disadvantages of the prior art and which, particularly at high production speeds, enables the production of sharply delimited demetallized partial areas of adjustable line width in a lenticular screen image.

This object is solved by the features of the independent claim. Developments of the invention are the subject matter of the dependent claims.

According to the invention a method as claimed in claim 1 is defined.

In a preferred variant of the method, the lenticular grid image is designed to represent n≧2 target images, and a line width is selected for the demetallized partial areas to be produced, which is preferably between 0.6*d _ML /n and 1.4*d _ML /n between 0.8*d _ML /n and 1.2*d _ML /n, particularly preferably between 0.9*d _ML /n and 1.1*d _ML /n, where d _ML is the diameter of the microlenses. The number n of target images to be displayed is in particular 2, 3, 4 or 5.

Within the scope of this description, microlenses are lenses whose size is below the resolution limit of the naked eye in at least one lateral direction. The microlenses can in principle be spherical or aspherical, but the use of plano-convex cylindrical lenses is currently preferred, so that with the method mentioned, a lenticular grid image with a lens grid of a plurality of plano-convex micro-cylindrical lenses is advantageously provided. For micro cylinder lenses, the term "diameter" always refers to the dimension perpendicular to the cylinder axis. The micro-cylindrical lenses can be of any length; for example, when used in security threads, they can correspond to the overall width of the thread and be several millimeters.

According to the invention, the metallic motif layer of the lenticular image is arranged essentially in the focal plane of the microlenses, which means in particular that the distance between the metallic motif layer and the focal plane is less than 25%, preferably less than 10% and particularly preferably less than 5% of the focal length of the microlenses amounts to.

The resolving power D of the microlenses of the lenticular image is determined by the Airy relationship D(λ) = 2.44*λ*f/ d _ML , where f is the focal length of the microlenses, λ is the light wavelength and d _ML is the diameter of the microlenses. The marking laser source is then advantageously selected in such a way that the resolving power D(λ) deviates from the line width of the demetallized partial areas to be produced by less than 15%, preferably by less than 10%.

A readily available laser source is advantageously used as the marking laser source, such as an Nd:YAG laser, a frequency-doubled Nd:YAG laser, a frequency-tripled Nd:YAG laser or an Er:glass laser. In principle, of course, other laser sources with other wavelengths, such as the diode lasers available for numerous wavelengths, can also be used as long as they are only suitable for the demetallization of the metallic motif layer. If two or more different laser sources of different wavelengths are used, line widths of different sizes can easily be implemented in a security element.

In an advantageous development of the invention, it is provided that the laser power of the marking laser source is adjusted for fine tuning in order to adapt the line width of the demetallized partial areas produced to the selected line width.

Advantageously, a lens raster image is provided whose lens raster has microlenses with a lens diameter between 5 μm and 20 μm and whose lens period is between 100% and 125% of the lens diameter.

The lens raster can border on air, but in particular it can also be embedded in an embedding layer whose refractive index preferably differs from the refractive index of the microlenses by 0.2 or more.

Further exemplary embodiments and advantages of the invention are explained below with reference to the figures, which are not shown to scale and proportions in order to improve clarity.

Fig. 1

eine schematische Darstellung einer Banknote mit einem erfindungsgemäßen Sicherheitselement in Form eines Fenstersicherheitsfadens, der ein Kippbild mit drei unterschiedlichen Sollbildern enthält,

Fig. 2

schematisch den Aufbau des Fenstersicherheitsfadens der Fig. 1 im Querschnitt,

Fig. 3

eine Schemaskizze eines Linsenrasterbilds zur Erläuterung des erfindungsgemäß verwendeten Prinzips, und

Fig. 4

schematisch den Aufbau eines Fenstersicherheitsfadens nach einem anderen Ausführungsbeispiel der Erfindung im Querschnitt.

Show it:

1

a schematic representation of a banknote with a security element according to the invention in the form of a window security thread, which contains a tilting image with three different target images,

2

schematic of the structure of the window security thread 1 in cross section,

3

a schematic sketch of a lenticular image to explain the principle used according to the invention, and

4

Schematically the structure of a window security thread according to another embodiment of the invention in cross section.

The invention will now be explained using the example of security elements for banknotes and other documents of value. figure 1 shows a schematic representation of a banknote 10 which is provided with a security element according to the invention in the form of a window security thread 12. The window security thread 12 emerges in window areas 14 on the surface of the banknote 10 while being embedded in the interior of the banknote 10 in the land areas 16 lying between them.

In the window areas 14, the security thread 12 shows a tilting image which presents the viewer with a different target image 18A, 18B or 18C from three different viewing directions 30A, 30B, 30C. The target images 18A-18C each show a motif that is formed from visually recognizable and contrasting metallic motif parts 20 and demetallized motif parts 22A, 22B, 22C.

Specifically, the window security thread 12 of the exemplary embodiment shows a sequence of euro symbols 22A against a shiny metallic background 20 when viewed at an angle 30A from above, while a sequence of coat of arms motifs 22B against a shiny metallic background 20 when viewed perpendicularly 30B and a Sequence of number motifs 22C in the form of the denomination "10" against a shiny metallic background 20 is visible. When the bank note is tilted, the appearance of the window security thread 12 in the window areas 14 changes back and forth between the three target images 18A, 18B, 18C, depending on the viewing direction.

figure 2 12 schematically shows the structure of the window security thread 12 of FIG 1 in cross section. The window security thread 12 has a carrier 32 in the form of a transparent plastic film, for example a PET film. The upper side of the carrier 32 is provided with a lens grid in the form of a plurality of parallel plano-convex cylindrical lenses 34 which have a radius of curvature R=4 μm and a lens diameter d _ML =7 μm and are arranged in a lens grid with a lens period of L=8 μm. In the embodiment of 2 the lenticular grid borders on air, so that the cylindrical lenses with n _lens = 1.5 and n _air = 1 have a focal length of f = 3R = 12 µm.

A motif layer 40 made of aluminum is formed on the underside of the carrier 32 and has demetallized partial regions 42 spaced apart in the grid of the cylindrical lenses 34 . The carrier 32, the cylindrical lenses 34 and the motif layer 40 are matched to one another in such a way that the motif layer 40 is in the focal plane of the cylindrical lenses 34.

For illustration shows figure 2 a section of the lenticular grid image in which the motif layer 40 contains demetallized sub-areas 42 only in the areas 44B visible when viewed perpendicularly 30B. The areas 44A and 44C visible when viewed obliquely from above (viewing direction 30A) or obliquely from below (viewing direction 30C) do not have any demetallization in the section shown, so that the viewer looks at metal areas of the motif layer 40 from these directions. Although the individual demetallized partial areas 42 represent narrow strips arranged in the grid of the cylindrical lenses, they settle when viewed from the different viewing directions due to the focusing Action of the cylindrical lenses 34 to the desired sequence of motifs 18A - 18C together.

Because of the small dimensions of the cylindrical lenses 34, a large number of metallic or demetallized partial areas work together in the reconstruction of the motifs 18A-18C. For example, with a height of the demetallized motif parts 22A - 22C of 2 mm and a lens period of the cylindrical lenses of L=8 μm, the demetallized partial areas 42 that take part in the reconstruction of the motifs "Euro symbol", "Coat of Arms" and "Sequence of digits 10" are distributed over an area of the motif layer 40 covered by 2 mm/8 µm = 250 cylindrical lenses.

As in 2 also shown, the window security thread 12 typically contains further layers, such as a full-surface color layer 45, which allows coloring of the demetallized motif parts 22A - 22C, an opaque white layer 46 and a heat-sealing lacquer layer 48. However, these or other functional layers are not essential for the present invention and are therefore not described in detail.

When designing the motif image of a lenticular grid image for displaying three target images, it has proven to be particularly advantageous if the line width D _real of the demetallized partial areas 42 is essentially one third of the diameter d _ML of the microlenses 34 . Analogously, the advantageous line width of the demetallized partial areas in a lenticular grid image for the display of two target images is essentially half the microlens diameter, and generally with a number n of target images to be displayed essentially one nth of the diameter d _ML of the microlenses. In this way, on the one hand, the available area of the motif layer is optimally utilized and, on the other hand, Tilting the lenticular image achieves a clearly defined switching between the different target images.

In order to achieve this advantageous line width, the motif layer 40 is conventionally scanned with a finely focused laser beam at different angles, for example, until partial areas 42 of the desired width are demetallized, or the motif layer is arranged outside the focal plane of the microlenses 34 to increase the process speed, so that During laser demetallization, an expanded and thus broader image of the incident laser radiation results in the plane of the motif layer. However, both variants have disadvantages with regard to the process duration or the quality of the target images generated, as already explained above.

To remedy this, the solution according to the invention uses the wavelength-dependent resolution of the optical system formed by the microlenses in order to obtain a desired line width without defocusing through a targeted selection of the wavelength of the laser radiation used for the demetallization.

wobei λ die Lichtwellenlänge, d_ML den Durchmesser der Mikrolinsen und f die Fokuslänge der Mikrolinsen darstellt. Die Größe D wird auch als Auflösungsvermögen bezeichnet, da zwei Punkte von einem optischen System gerade noch trennbar sind, wenn ihre Beugungsscheibchen (bzw. Beugungslinien bei Zylinderlinsen) einander zur Hälfte überdecken. Das beugungsbegrenzte Auflösungsvermögen des optischen Systems der Mikrolinsen 34 führt somit selbst bei optimaler Fokussierung der einfallenden Laserstrahlung zu einer bestimmten, von der Laserwellenlänge abhängigen Ausdehnung des Fokusbereichs.For a more detailed explanation of the principle used, reference is made to 3 even a parallel light beam 50 from the microlenses 34 is not imaged onto a point or, in the case of cylindrical lenses, onto an infinitely narrow line due to diffraction effects, but produces a diffraction disk or an elongated diffraction line 52 with a diameter $$\mathrm{D}\left(\mathrm{\lambda }\right)=2.44*\mathrm{\lambda }*\mathrm{f}/{\mathrm{i.e}}_{\mathrm{ML}}$$

where λ is the light wavelength, d _ML is the diameter of the microlenses and f is the focal length of the microlenses. The quantity D is also referred to as the resolving power, since two points can just about be separated from an optical system if their diffraction disks (or diffraction lines in the case of cylindrical lenses) overlap by half. The diffraction-limited resolving power of the optical system of the microlenses 34 thus leads to a specific extension of the focus area, which is dependent on the laser wavelength, even when the incident laser radiation is optimally focused.

While the limited resolving power is usually viewed as a limitation and disadvantage, the present invention uses the wavelength-dependent size of the diffraction spot in a targeted manner in order to easily generate demetallizations of a desired line width in the focal plane and thus with maximum image sharpness.

gewählt. Die oben angegeben Beziehung (1) für den Durchmesser D des Beugungsflecks 52 kann nach der Wellenlänge aufgelöst und der gewünschte Wert der Linienbreite D_ziel für den Durchmesser des Beugungsflecks 52 eingesetzt werden, um so eine ideale Ziel-Laserwellenlänge zu erhalten: $${\mathrm{\lambda }}_{\mathrm{ziel}}=\mathrm{0,41}*{\mathrm{D}}_{\mathrm{ziel}}*{\mathrm{d}}_{\mathrm{ML}}/\mathrm{f}$$

Specifically, for example, in the embodiment of 2 the metallic motif layer 40 of the lenticular pattern image, which is initially still full-surface, can be provided with demetallized partial areas in order to produce the target images 18A-18C. Since there are to be three image areas 44A - 44C under each microlens 34, the target line width for the demetallized partial areas is 42 $${\mathrm{D}}_{\mathrm{goal}}={\mathrm{i.e}}_{\mathrm{ML}}/3=2.3\mathrm{\mu m}$$

chosen. The above relationship (1) for the diameter D of the diffraction spot 52 can be solved for wavelength and the desired value of the linewidth D _target can be substituted for the diameter of the diffraction spot 52 to obtain an ideal target laser wavelength: $${\mathrm{\lambda }}_{\mathrm{goal}}=0.41*{\mathrm{D}}_{\mathrm{goal}}*{\mathrm{i.e}}_{\mathrm{ML}}/\mathrm{f}$$

With a target line width of D _target = 2.3 µm, the lens diameter d _ML = 7 µm and the focal length of the microlenses f = 12 µm, equation (2) results in a target laser wavelength of λ _target = 550 nm.

A frequency-doubled Nd:YAG laser with a wavelength of λ = 532 nm is therefore chosen as a readily available marking laser source for demetallization. At this wavelength, the diameter of the diffraction disk is D=2.2 μm according to equation (1) and thus essentially corresponds to the desired target line width D _target =2.3 μm with a deviation of only about 4%.

In the case of demetallization, it can also be taken into account that the demetallized line width D _real in practice does not always result in exactly the value for D calculated according to equation (1), but that the line width actually achieved also depends slightly on the laser power used. Specifically, that area of the focused laser beam in which the laser intensity exceeds the threshold required for the demetallization of the metallic motif layer is decisive for the demetallization. Since the laser intensity drops very sharply at the edge of the diffraction spot, only a small variation of the actual line width D _real can be achieved by increasing or decreasing the laser intensity, but in practice it is suitable for fine tuning.

In addition to the adaptation of the line width achieved by the wavelength-dependent resolution, the wavelength dependence of the refractive index n of the lens material can also be used to achieve a further To achieve variation and in particular an increase in line width. With the refractive index n of the lens material, which generally varies as a function of the wavelength, the focal length f of the microlenses used also varies depending on the wavelength of the incident radiation.

In the present invention, the demetallization takes place in such a way that the metallic motif layer lies essentially in the focal plane of the microlenses when the security element is viewed as desired in the visible spectral range. For example, if the microlenses are exposed to an IR laser (e.g. an Nd:YAG laser with λ = 1064 nm), then, depending on the material used for the microlenses, an additional broadening of the lines can result from the fact that the focus length at 1064 nm is already deviates significantly from the focal length in the visible spectral range. When the metallic motif layer is exposed to laser radiation, the conditions are therefore similar to those in the known method described above, in which the motif layer is arranged in a targeted manner outside the focal plane of the microlenses. In contrast to this known method, however, in the case of the present invention there is an arrangement “outside the focal plane” only at the wavelength used for demetallization.

After the marking laser source has been selected and the laser intensity to be used for the demetallization has been determined (and, if necessary, the refractive index of the lens material), the metallic motif layer 40 is illuminated through the microlenses 34 from three irradiation directions 30A, 30B, 30C in the form of the motifs 18A-18C Laser radiation applied to produce the desired demetallized portions 42 in the metallic layer 40 motif.

If the lenticular image of 2 Demetallizations with other line widths can be produced in the metallic motif layer 40, for example an Nd:YAG laser with λ=1064 nm and a focus width of D=4.4 μm, a frequency-tripled Nd:YAG laser with λ = 355 nm and a focus width of D = 1.5 µm, or an Er:glass laser with λ = 1540 nm and a focus width of D = 6.4 µm can be used. By using two or more different laser sources of different wavelengths, line widths of different sizes can also be used in a security element in a simple manner.

In a second specific exemplary embodiment, the 4 lenticular image 60 shown are provided with two target images which become visible when viewed obliquely from above (viewing direction 30A) or obliquely from below (viewing direction 30C).

The upper side of the carrier 62 is provided with a lens grid in the form of a plurality of parallel plano-convex cylindrical lenses 64 which have a radius of curvature R=4 μm and a lens diameter d _ML =7 μm and are arranged with a lens period of L=8 μm. In the exemplary embodiment, the lens material of the cylindrical lenses 64 has a refractive index n _lens =1.6, the refractive index of the carrier film 62 is n _film =1.64. In addition, the cylindrical lenses 64 are embedded in an embedding layer 66 with a refractive index n embedding=1.33.

On the underside of the carrier are as in the embodiment of the 2 a metallic motif layer 40, a full-surface color layer 45, an opaque white layer 46 and a heat-sealing lacquer layer 48 are arranged.

gewählt. Für die Berechnung der Ziel-Laserwellenlänge mit Hilfe der oben angegebenen Beziehung (2) wird noch die Fokuslänge der Mikrolinsen 64 benötigt, die sich im vorliegenden, eingebetteten Fall zu $$\mathrm{f}={\mathrm{n}}_{\mathrm{Folie}}/\left({\mathrm{n}}_{\mathrm{Linse}}-{\mathrm{n}}_{\mathrm{Einbettung}}\right)*\mathrm{R}=\mathrm{24,3}\mathrm{\mu m}$$

ergibt. Mithilfe von Beziehung (2) ergibt sich aus diesen Angaben eine Ziel-Laserwellenlänge von λ_ziel = 410 nm.Since there should be space for two image areas under each microlens, the target line width for the demetallized partial areas to be produced is 42 in the present exemplary embodiment $${\mathrm{D}}_{\mathrm{goal}}={\mathrm{i.e}}_{\mathrm{ML}}/2=3.5\mathrm{\mu m}$$

chosen. The focal length of the microlenses 64 is also required for the calculation of the target laser wavelength using the relationship (2) given above $$\mathrm{f}={\mathrm{n}}_{\mathrm{foil}}/\left({\mathrm{n}}_{\mathrm{lens}}-{\mathrm{n}}_{\mathrm{embedding}}\right)*\mathrm{R}=24.3\mathrm{\mu m}$$

results. With the help of equation (2), a target laser wavelength of λ _target = 410 nm results from this information.

In this case, a frequency-tripled Nd:YAG laser with a wavelength of λ=355 nm is chosen as the readily available marking laser source for the demetallization. Since the diameter of the diffraction disc has a slightly smaller diameter (D = 3.1 µm) than the target line width (deviation 11%) at this wavelength according to equation (1), the marking laser source is operated with high laser intensity during demetallization, to slightly increase the demetallized line width D _real and to approach the target line width.

If the lenticular image of 4 Demetallizations with other line widths can be produced in the metallic motif layer, for example an Nd:YAG laser with λ=1064 nm and a focus width of D=9.0 μm, a frequency-doubled one, can also be used as a readily available laser source Nd:YAG laser with λ=532 nm and a focus width of D=4.7 μm or an Er:glass laser with λ=1540 nm and a focus width of D=13.0 μm can be used.

Reference List

10

bank note

12

window security thread

14

panes

16

web areas

18A, 18B, 18C

target images

20

metallic motif parts

22A, 22B, 22C

demetallized motif parts

30A, 30B, 30C

viewing directions

32

carrier

34

cylindrical lenses

40

motif layer

42

demetallized sections

44A, 44B, 44C

visible areas

45

full color layer

46

opaque white layer

48

heat seal lacquer layer

50

parallel beam of light

52

diffraction disc

60

lenticular image

62

carrier

64

cylindrical lenses

66

bedding layer

Claims (10)

1. A method for manufacturing a security element having a lenticular image for depicting one or more target images that are visible only from predetermined viewing directions and whose motifs are formed by visually perceptible, contrasting metallic and demetalized sub-regions of a motif layer, wherin in the method

   - a lenticular image having a lens grid composed of a plurality of microlenses (34) and a metallic motif layer (40) arranged spaced apart from the lens grid is provided,

   - the refractive effect of the microlenses (34) defining a focal plane and the metallic motif layer (40) being arranged substantially in said focal plane,

   - a line width is chosen for the demetalized sub-regions (42) to be produced in the metallic motif layer (40),
   characterized in that

   - a marking laser source having a laser wavelength λ is selected such that the resolving power D(λ) of the microlenses of the lenticular image at the selected laser wavelength A substantially corresponds to the line width of the demetalized sub-regions (42) to be produced, wherein the resolving power D(λ) of the microlenses of the lenticular image is determined by the formula D(λ) = 2.44* λ*f/d_ML, where f is the focal length of the microlenses and d_ML is the diameter of the microlenses, and

   - the metallic motif layer (40) is impinged on through the microlenses (34) with laser radiation of the selected marking laser source to produce demetalized sub-regions (42) in the metallic motif layer.
2. The method according to claim 1, characterized in that the lenticular image is adapted for depicting n ≥ 2 target images, and for the demetalized sub-regions to be produced, a line width is chosen that is between 0.6*d_ML/n and 1.4*d_ML/n, preferably between 0.8*d_ML/n and 1.2*d_ML/n, particularly preferably between 0.9*d_ML/n and 1.1*d_ML/n, where d_ML is the diameter of the microlenses.
3. The method according to claim 1 or 2, characterized in that a lenticular image having a lens grid composed of a plurality of micro-cylindrical lenses is provided.
4. The method according to at least one of claims 1 to 3, characterized in that a lenticular image is provided whose metallic motif layer is arranged at a distance from the focal plane that is less than 25%, preferably less than 10% of the focal length of the microlenses.
5. The method according to at least one of claims 1 to 4, characterized in that the marking laser source is adjusted in such a way that D differs from the line width of the demetalized sub-regions to be produced by less than 15%, preferably by less than 10%.
6. The method according to at least one of claims 1 to 5, characterized in that, as a marking laser source, a Nd:YAG laser, a frequency-doubled Nd:YAG laser, a frequency-tripled Nd:YAG laser or an Er:glass laser is used.
7. The method according to at least one of claims 1 to 6, characterized in that two or more different marking laser sources of different wavelengths are used.
8. The method according to at least one of claims 1 to 7, characterized in that, for fine control, the laser power of the marking laser source is adjusted to adapt the line width of the produced demetalized sub-regions to the chosen line width.
9. The method according to at least one of claims 1 to 8, characterized in that a lenticular image is provided whose lens grid comprises microlenses having a lens diameter between 5 µm and 20 µm and whose lens period is between 100% and 125% of the lens diameter.
10. The method according to at least one of claims 1 to 9, characterized in that a lenticular image is provided whose lens grid is embedded in an embedding layer whose refractive index preferably differs from the refractive index of the microlenses by 0.2 or more.

Images