TITLE

Method for producing an optical structure and optical structure

BACKGROUND

The present invention relates to a method for producing an optical structure by means of three-dimensional printing, wherein in a first step, a base structure is provided, wherein in a second step, a first substructure is printed on the base structure by means of targeted placement of one or more droplets of printing ink, wherein in a third step, the first substructure is at least partially cured.

Such optical structures are known and are used e.g. as lenses. In recent years, with the rise of rapid prototyping and in particular additive manufacturing, it has been shown that such optical structures can easily be produced by these manufacturing techniques, in particular by means of three-dimensional printing. The optical structures may thus be produced in a highly customizable, quick and cost-efficient way.

It is desirable, e.g. for ophthalmic lenses, microscopy and/or semiconductor production, to create optical structures with very small dimensions, e.g. in the region of microns or even nanometers, or to create larger optical structures comprising in turn optical microstructures of the afore-mentioned dimensions.

Yet, this poses problems for the current three-dimensional printing technology as the structures that may be printed by modern three-dimensional printing systems are larger than those required for such microstructures (the term microstructure is generally used to denote smaller structures, such as nanostructures, as well).

Therefore, in order to produce the desired microstructures, lots of efforts go into the task of producing smaller droplet sizes - until now to little effect. SUMMARY

Hence, it is a purpose of the present invention to provide a method for producing an optical structure, which does not show the described disadvantages of the prior art, but allows for an easy, quick and cost-efficient production of an optical structure with very small structural sizes.

According to the present invention, this object is achieved by a method for producing an optical structure by means of three-dimensional printing,

- wherein in a first step, a base structure is provided,

- wherein in a second step, a first substructure is printed on the base structure by means of targeted placement of one or more droplets of printing ink,

- wherein in a third step, the first substructure is at least partially cured, wherein process parameters of the second step and/or the third step are chosen such that the first substructure comprises at least two regions, wherein the first substructure comprises a gradient, preferably a non-uniform spatial distribution, of at least one optical property between said at least two regions.

The method according to the invention advantageously allows for a conventionally known three-dimensional printing process to be used to create smaller structures by purposefully providing a first substructure with a non-homogeneous optical property. In particular, by controlling the gradient and/or spatial distribution of the optical property, microstructures within the optical structure may advantageously be realized. Within the context of this application, the term microstructures is not intended to be limited to structures with sizes in the range of several micrometers, but also smaller structures with sizes in the range of several nanometers, i.e. it includes nanostructures as well. Furthermore, the term microstructure is used within the scope of this invention synonymously with the term optical microstructure, i.e. a microstructure used for optical purposes.

The embodiments described in conjunction with this subject matter of the present invention also apply to the further subject matters of the present invention and vice versa.

The two regions do not necessarily represent sharply delimited areas. In particular, the optical property may continuously change from one region to the other region. Preferably, in each of the two regions, the optical property is non-constant, but changes according to the gradient. Alternatively or additionally, at least one of the two regions comprises an at least substantially constant value for the optical property. Most preferably, the gradient is directed perpendicularly to the boundary area between the two regions at a particular point.

It is known that optical properties may change in the course of printing and/or curing. It is believed that especially at the surface boundaries, oxygen inhibition of the material causes the material composition to be altered, but the exact physical causes have not yet been fully understood. In order to mitigate these effects, it has been known from the state of the art to control the printing and/or curing process in such a way to maintain a substantially constant optical property, in particular a uniform distribution of the at least one optical property, within the first substructure. This may for example be achieved by maximizing the time between printing and curing and/or by minimizing the curing time. The applicant has surprisingly found that this previously unwanted effect can in fact be used and controlled to obtain a gradient of the at least one optical property.

The person skilled in the art acknowledges that a gradient is a vector and therefore only describes the direction of the maximum rate of increase of the optical property at a given point. The spatial distribution on the other hand, at least as understand within the context of the present invention, means the distribution of (scalar) values of the optical property for all points in a given volume. The non-uniformity means that the optical property is not constant at all points in the volume. In particular, the optical property changes, preferably constantly and/or linearly and/or exponentially, in at least one direction. Furthermore, the person skilled in the art acknowledges that the distribution may comprise certain symmetries. For example, preferably, the first substructure is at least partially rotationally symmetric and the optical property may comprise a radial gradient, i.e. the optical property is maximal at the outer edges and lowest at a center. In other cases, the gradient may be generally vertically aligned, i.e. the optical property is maximal at the top and lowest at a bottom.

The optical structure is a three-dimensional structure preferably intended to at least partially transmit light and hence be used for optical purposes. The optical structure is more preferably intended for use with the visible spectrum. Preferably, in order to serve an optical purpose, the optical structure is at least partially optically transparent and/or translucent, in particular at least for a predetermined range of wavelengths, such as the visible spectrum.

Preferably, the printing ink comprises a translucent or transparent component. More preferably, the printing ink comprises at least one photo-polymerizable component. The at least one photo-polymerizable component is most preferably a monomer that polymerizes upon exposure to radiation, e.g. ultra-violet (UV) light. The deposited droplets are preferably pin cured, i.e. partially cured, after deposition. More preferably, the viscosity of at least one component of the printing ink is increased. Pin curing is most preferably carried out after deposition of the respective droplet or after deposition of an entire or only part of a layer. Alternatively, pin curing is carried out at certain intervals, e.g. after printing of every second layer (or sublayer). Preferably, curing may comprise actively and/or passively curing, wherein in particular passively curing includes letting the droplets dry or cure over time, whereas actively curing includes acting upon the deposited droplets, e.g. submitting the droplets to additional energy such as electromagnetic radiation, in particular UV light.

The present invention hence advantageously only requires a single printing ink. In particular, the two regions are not produced by means of different printing inks. Instead, the process parameters of the second step and/or the third step are chosen such that the first substructure comprises at least two regions.

According to a preferred embodiment of the present invention, in a fourth step at least a second substructure is printed at least partially on top of the first substructure for completing the optical structure, wherein the fourth step preferably includes a curing substep, in which the second substructure is at least partially cured. The optical structures therefore preferably comprises at least two (partial) layers of printing ink, in particular a plurality of layers.

Furthermore, by combining at least two layers, it is advantageously possible to take advantage of the gradient of the at least one optical property to create one or more microstructures.

In the context of the present invention, three-dimensional printing of a structure comprises building up the structure from layers of printing ink. These are obtained through a targeted placement of droplets of printing ink at least partially side by side. The droplets of printing ink are preferably ejected from nozzles of a print head, typically in a substantially vertical direction towards a base structure, though ejecting at an angle is possible as well. Droplets of layers constituting the following layer are at least partly ejected towards the previously deposited layer, such that the three-dimensional structure is built up layer by layer.

Preferably, the three-dimensional printing is a multi-jet printing. More preferably, a three- dimensional printer, in particular a multi-jet printer, is used for the three-dimensional printing.

Preferably, the base structure is a substrate, a lens blank and/or a three-dimensionally printed structure in itself. In particular, the base structure may comprise a base layer printed upon a substrate. This is particularly advantageous as the resulting base structure may be produced such as to comprise a very smooth surface, which is of particular importance when creating optical microstructures. Furthermore, the material of the base layer may be chosen such as to allow for an optimal bonding to both the substrate on the one side and the first substructure on the other side. Preferably, the base structure comprises glass and/or a polymer, in particular cellulose triacetate (TAC), cyclic olefin copolymer (COC), polyethylene terephthalate (PET), polycarbonate (PC) and/or Polymethyl methacrylate (PMMA), which is also known as acrylic glass or plexiglass. Those materials are well-known and tested materials for optical purposes and therefore particularly suited for the production of the optical structure according to the present invention. Preferably, within the context of the present invention, it is assumed and preferred that the base structure is generally flat and therefore comprises a substantially flat surface area. Of course, the base structure may as well comprise a curved surface. All features and explanations apply equivalently in this case.

According to a preferred embodiment of the present invention, the second step and/or the fourth step are at least partially carried out in a multi-pass printing mode, wherein preferably a layer printed in a multi-pass printing mode comprises multiple, preferably complementary, sublayers which are printed in subsequent sublayer printing steps, wherein at least one sublayer printing step is followed by an at least partial curing step. This allows in a very advantageous manner for a highly flexible and customizable production of optical structures, in particular comprising microstructures. A multi-pass printing mode preferably comprises the printing head making several passes, in particular back and/or forth, wherein during each pass, a sublayer of the layer is printed. Each sublayer may be subjected to a separate curing step, only the completed layer may be subjected to a curing step or a curing step is performed in regular or irregular intervals, e.g. as soon as a predetermined amount of sublayers are printed and/or after a predetermined time after printing has passed.

According to a preferred embodiment of the present invention, the first substructure is a droplet, a sublayer and/or at least one layer, wherein preferably the second substructure is a droplet, a sublayer and/or at least one layer. In particular, the second substructure most preferably comprises multiple layers. It is thus advantageously possible to embed a microstructure such as a microlens within the optical structure. Three-dimensional printing usually comprises deriving from a predetermined shape of the optical structure to be printed a print model. This print model may be layer based and/or voxel based. A voxel is a value in a regular three-dimensional grid and therefore corresponds substantially to a pixel in a two- dimensional grid. The print model may therefore preferably comprise a plurality of voxels, wherein the print head is controlled such as to fill predetermined voxels with one or more droplets of printing ink. Thus, in the context of the present invention, the first substructure may comprise one or more voxels, which correspond to one or more droplets of printing ink per voxel. In this sense, the first substructure may consist of one or more droplets at the location of a voxel. For example, a simple model of a first substructure is a drop-like and/or dome-like shape on the base structure. Such a first substructure is in particular produced by depositing one or more droplets of printing ink on substantially the same location of the base structure. In particular, the optical structure may comprise only the base structure and the first substructure or it may comprise additional elements or structures. Yet, due to the gradient of the at least one optical property, the optical behavior or effect of the optical structure is preferably not the same as for an optical structure of the same shape and material, but with a uniform distribution of the optical property, such as pursued by the three- dimensional printing technologies known from the prior art. Alternatively, the first substructure is a sublayer or layer comprising a plurality of, i.e. at least two, droplets of printing ink deposited at least partially side by side. In this case, the layer preferably extends over a two-dimensional area of the base structure. In particular, the shape of the first substructure preferably takes into consideration the predetermined shape and/or optical behavior of a desired microstructure. It is hence particularly advantageously possible to create an optical structure with a predetermined shape but with finely tunable optical behavior compared to an optical structure with the same shape but with a (spatially) constant optical property.

According to a preferred embodiment of the present invention, the first region is arranged at an outermost surface of the first substructure, wherein the second region is arranged at an inner and/or lower part of the first substructure compared to the first region, wherein in particular the optical property is maximal in the first region and/or minimal in the second region or the optical property is minimal in the first region and/or maximal in the second region. Hence, it is herewith advantageously possible to create a first substructure which has at its outer surface, which is preferably at least partially a boundary to a subsequent substructure, a different optical property than at its core and/or in regions below the outer surface. This may advantageously be used to create optical desired optical effects. In particular, a microlens or another optical microstructure, such as a textured anti-reflective coating, may thus be produced without changing the droplet size and/or the size of the first substructure itself.

According to a preferred embodiment of the present invention, during the second and/or fourth step, print lines are created between at least two adjacent layers and/or sublayers, in particular between the first substructure and the second substructure, wherein the print lines are arranged such that they create an optical metastructure, in particular a diffraction grating. Such print lines often occur when printing a layered structure and are usually considered an unwanted artefact. By taking advantage of the print lines, it is advantageously possible to make use of such structures to create predetermined and desired optical effects. For example, such print lines may comprise a diameter of about 100-300 micrometer, in particular around 200 micrometer, and a height of about 5-20 nanometer, in particular around 10 nanometer.

According to a preferred embodiment of the present invention, a discontinuous transition of the at least one optical property is created between the first region of the first substructure and an adjacent region of the second substructure. It is hence advantageously possible to provide the optical structure with optical effects which are caused by the difference of the optical property at the boundary between the first substructure and the second substructure.

According to a preferred embodiment of the present invention, the parameters comprise at least one of a printing velocity, printing angle, droplet ejection velocity, air exposition time, printing ink temperature, ambient temperature, ambient gas composition, curing time, irradiation type, irradiation intensity, curing time, printing ink composition, printing ink viscosity, printing ink surface tension, oxygen level. At least one of those parameters is chosen such as to create the gradient of the at least one optical property of the first substructure. In particular, the oxygen level may refer to the oxygen level of the ambient gas and/or to the oxygen level of the printing ink.

According to a preferred embodiment of the present invention, the optical property is one of a refractive index, a polarization, an opacity, a birefringence, a filter wavelength, a transmittance, a reflectance, an absorptance. In a particularly preferred embodiment, the optical property is at least a refractive index. By imbuing the first substructure with a gradient of its refractive index, and in particular by creating a predetermined spatial distribution of the refractive index, additional optical effects can be achieved. For example, in a particularly advantageous way, a microlens effect or an anti-reflective effect may be obtained in this way, in particular by means of a textured coating and/or a gradient-index film.

A further subject matter of the present invention is an optical structure, in particular produced by a method according to the present invention, wherein the optical structure comprises a base structure and at least one first substructure, characterized in that the first substructure comprises at least two regions, wherein the first substructure comprises a gradient, preferably a non-uniform spatial distribution, of at least one optical property between said at least two regions. The embodiments and advantages described in conjunction with this subject matter of the present invention also apply to the further subject matters of the present invention and vice versa.

According to a preferred embodiment of this subject matter of the present invention, the at least one optical property is a refractive index, a polarization, an opacity, a birefringence, a filter wavelength, a transmittance, a reflectance, an absorptance. In a particularly preferred embodiment, the optical property is at least a refractive index. By providing an optical structure, in particular a first substructure of an optical structure, with a gradient of its refractive index, optical effects can be achieved, which are not limited by the structural size of the first substructure and/or the optical structure. For example, in a particularly advantageous way, a microlens or an anti-reflective coating, in particular by means of a textured coating and/or a gradient-index film may be provided.

According to a preferred embodiment of this subject matter of the present invention, the first region is arranged at an outermost surface of the first substructure, wherein the second region is arranged at an inner and/or lower part of the first substructure than the first region, wherein in particular the optical property is maximal in the first region and/or minimal in the second region or the optical property is minimal in the first region and/or maximal in the second region. By creating a first substructure whose refractive index is maximal at its outer surface, a microlens may be easily created. In the same way, other optical microstructures may be realized as well.

According to a preferred embodiment of this subject matter of the present invention, the first substructure is a sublayer, in particular a sublayer of a multi-pass layer, wherein the sublayer comprises at least one vacancy in outermost surface, wherein a subsequent sublayer is printed such as to fill the vacancy, such that the resulting layer, in particular the resulting multi-pass layer, comprises a boundary between the sublayers with a predefined optical property distribution, wherein the resulting layer preferably comprises a flat and/or smooth outer surface. Hence, in a particularly advantageous embodiment, the optical structure may comprise the shape of a generally flat and smooth layer, while the optical structure comprises microlenses realized at the boundaries between layers at the position of the vacancies due to the gradient of preferably a refractive index. This advantageously allows for the production of a flat lens.

According to an advantageous embodiment of this subject matter of the present invention, the optical structure further comprises a coating layer, wherein the coating layer is preferably produced by means of three-dimensional printing. Preferably, the coating layer provides shielding against ultraviolet radiation, color correction and/or anti-reflective properties. It is herewith advantageously possible to protect the optical structure from external damaging and/or deteriorating influences.

Still a further subject matter of the present invention is a printing system for three- dimensional printing, comprising a print head comprising at least one nozzle for ejecting droplets of printing ink towards a base structure, and a control unit for controlling the print head, wherein the printing system is configured to produce an optical structure, in particular an optical structure according to the present invention, with a method according to the present invention.

The embodiments and advantages described in conjunction with this subject matter of the present invention also apply to the further subject matters of the present invention and vice versa.

Preferably, the printing system comprises means for curing as well. Those may be active and/or passive curing means. More preferably, the means for curing are adapted such as to allow for a pin curing.

Preferably, the control unit is configured such as to derive a layer- and/or voxel-based model of the predetermined three-dimensional shape of the optical structure to be printed. More preferably, the control unit is configured to control the print head to produce the optical structure according to the model.

According to the present invention, the optical structure and in particular the first substructure is produced by three-dimensional printing, in particular ink-jet printing, e.g. multi-jet printing, which is a known additive manufacturing technique. Additive manufacturing is well known and a particularly versatile and reliable production technique. In the sense of the present invention, printing, in particular three-dimensional printing, of a structure comprises building up the structure from layers of printing ink, preferably through a targeted placement of droplets of printing ink at least partially side by side and in vertically stacked layers. The droplets of printing ink are ejected from one or more nozzles of a print head, typically towards a base structure such as a substrate. Droplets of layers constituting a second and subsequent layers are at least partly ejected towards a previously deposited layer, such that the three-dimensional structure is built up layer by layer. Preferably, the printing system is configured for multi-pass mode printing as detailed before, thus subdividing each layer into a plurality of sublayers.

According to an advantageous embodiment of the present invention, the base structure comprises glass and/or a polymer, in particular cellulose triacetate (TAC), cyclic olefin copolymer (COC), polyethylene terephthalate (PET), polycarbonate (PC) and/or Polymethyl methacrylate (PMMA), which is also known as acrylic glass or plexiglass. Those materials are well-known and tested materials for optical purposes and therefore advantageously allow for flexible optical structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1a/b show a first advantageous embodiment of a method according to the present invention;

Figures 2a/b show a second advantageous embodiment of a method according to the present invention;

Figures 3a-d show a third advantageous embodiment of a method according to the present invention.

DETAILED DESCRIPTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and for illustrative purposes may not be drawn to scale.

Where an indefinite or definite article is used when referring to a singular noun, e.g. “a”, “an”, “the”, this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, further and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order, except for the method steps. It is to be understood that the io terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

In Figures 1a and 1b, a first advantageous embodiment of a method according to the present invention is shown. The present invention relates to a method for producing an optical structure 1 by means of three-dimensional printing.

One or more droplets of printing ink are ejected from one or more nozzles of a print head in a targeted manner, directed at a base structure 2. The print head is preferably controlled by a control unit, which controls the targeted placement of the droplets according to a predetermined model in order to build up the desired three-dimensional shape of the optical structure 1. The droplets are preferably deposited at least partially side by side such as to yield a layer (or sublayer) and even more preferably at least partially on top of each other such as to stack layers.

In Figures 1a and 1b, an exemplary optical structure 1 is shown on a base structure 2. The base structure is preferably flat and even more preferably it comprises a smooth surface. Most preferably, the base structure 2 as well as the optical structure 1 are at least partially translucent and/or transparent, at least for a predetermined range of wavelengths, such as e.g. the visible spectrum. Light may thus travel through the base structure 2 and the optical structure 1 , in which it is subjected to certain optical effects, such as in particular refraction, diffraction, birefringence, reflection, etc. Here, the optical structure 1 comprises a drop-like or dome-like shape. It is hence rotationally symmetric around a center point and may therefore be described by means of a radial vector pointing in a radial outward direction r. The exemplary embodiment described herein serves to illustrate basic principles and is therefore chosen to comprise a relatively simple shape. The same principles also apply to more complex shapes.

The method comprises a first step, in which the base structure 2 is provided. The base structure 2 may be a common substrate, it may be a lens blank and/or it may be a structure produced in turn by three-dimensional printing. For example, the base structure may comprise a smooth and/or flat base layer printed upon a substrate.

In a second step, a first substructure 3 is printed on the base structure 2. Here, the first substructure 3 corresponds substantially to the optical structure 1, i.e. there are no further substructures. The first substructure 3 may consist in one or more droplets of printing ink. Preferably, the first substructure 3 is a layer.

It is particularly preferred that the three-dimensional printing is carried out in a multi-pass printing mode. This means in particular that each layer is divided into sublayers and the print head prints a sublayer or at least parts of a sublayer during each pass (back and forth) over the base structure 2. Together, the sublayers yield a layer. Potential subsequent layers are stacked on top of the layer in the same manner.

After printing the first substructure 3, it is at least partially cured in a third step. The curing may be an active or a passive curing. Preferably, it is an active pin curing by means of irradiation, in particular using LIV radiation. The printing ink preferably comprises a photo- polymerizable monomer which polymerizes upon exposure to (UV) radiation.

If a multi-pass printing mode is used, a curing substep may be performed after at least one sublayer printing step.

Figure 1a shows the situation immediately after printing. The printing ink comprises a material comprising at least one optical property. In the following, although other optical properties are possible as well, for reasons of simplicity, only a refractive index will be discussed. During or immediately after printing, the refractive index is substantially constant throughout the first substructure 3, with other words, the first substructure 3 comprises a uniform distribution of the refractive index.

This situation corresponds as well to the optical structures produced by methods known from the prior art. Those are produced in such a way as to obtain such a well-defined uniform distribution of the refractive index.

The applicant has now surprisingly found that if certain process parameters during printing, i.e. in particular the second step, and/or during curing, i.e. the third step, are tuned accordingly, the resulting first substructure 3 will exhibit a non-uniform distribution of the refractive index, i.e. the first substructure 3 comprises a gradient of the refractive index. This is schematically shown in Figure 1b. At least two regions may be roughly distinguished. A first region 10 at the outermost, i.e. the top, surface of the first substructure 3, and a second region 11 within the volume of the first substructure 3, i.e. below the surface and/or near its center. In the first region 10, for reasons not yet fully understood, the refractive index will be higher than in the second region 11. lt should be noted that those regions are not clearly defined or distinguished. The refractive index preferably changes gradually. Nevertheless, in the first region 10, the refractive index is in average higher than in the second region 11.

The resulting gradient of the refractive index is thus here directed parallelly to the radial direction r.

This effect is preferably achieved by letting the printing ink settle a bit before curing, i.e. having a longer waiting time between printing and curing, i.e. the second step and the third step.

The resulting refraction of light passing through the optical structure 1 is different for the first substructures 3 shown in Figures 1a and 1b. In particular, additional optical effects may be achieved and e.g. a microlens may be obtained with conventional, i.e. larger, structural sizes of the optical structure 1.

In Figures 2a and 2b, a second advantageous embodiment of a method according to the present invention is schematically depicted. This second embodiment substantially corresponds to the first embodiment discussed with reference to Figures 1a and 1b. It is therefore generally referred to the corresponding explanations above.

Here, the first substructure 3 is not more clearly shaped as a flat layer. Again, Figure 2a shows the situation immediately after printing and/or before curing. The first substructure 3 comprises a substantially uniform distribution of the refractive index throughout the entire substructure 3.

In Figure 2b, the resulting optical structure 1 with the base structure 2 and the first substructure 3 s shown. As in Figure 1 , the first substructure 3 comprises a first region 10 with a higher refractive index at the outer surface of the first substructure 3 and a second region 11 with a lower refractive index within its volume, i.e. at a distance from the outer surface.

In Figures 3a to 3d, a third advantageous embodiment of a method according to the present invention is schematically depicted. This third embodiment substantially corresponds to the first embodiment discussed with reference to Figures 1a and 1b and Figures 2a and 2b. It is therefore generally referred to the corresponding explanations above. Here, the first substructure 3 is printed as a layer, but it comprises a vacancy 4 at a predetermined location, i.e. at a predetermined position of the layer, one or more droplets are not deposited on purpose. This is shown in Figure 3a.

After an at least partial curing step, the first substructure 3 comprises a gradient of the refractive index as discussed before and shown in Figure 3b. The gradient is arranged such that the first substructure 3 comprises a non-uniform distribution of the refractive index.

Now, in a fourth step, a second substructure 4 is printed on the first substructure 3. Here, the second substructure 4 is configured such that it only comprises material at the location of the vacancy 5. This is shown in Figure 3c. Of course, the second substructure could be designed as a layer covering the entire first substructure 3 (or at least some areas of it) as well. In this case, more droplets are deposited at the location of the vacancy 5 compared to other locations.

Again, preferably a curing step is carried out which might either be configured such that the second substructure 4 comprises a substantially uniform distribution of the refractive index (or the printing step for printing the second substructure is carried out accordingly), such as shown in Figure 3c, or it is designed such that it comprises a gradient of the refractive index as well. This case is depicted in Figure 3d.

The resulting optical structure 1 according to the third embodiment will therefore comprise a flat and smooth layer and is hence basically indistinguishable from e.g. the optical structure 1 as depicted in Figure 2b. Yet, internally, it comprises a non-uniform, but well-defined distribution of the refractive index. At the boundaries between the first substructure 3 and the second substructure 4, there is a notable difference of refraction index, which will impact its optical behavior. The optical structure will e.g. comprise a microlens embedded in its otherwise flat structure.

It should be noted that the third embodiment, although having been described as separate layer printing steps, could also be realized in a multi-pass printing mode. In this case, the first substructure 3 corresponds to a first sublayer and the second substructure 4 corresponds to a second sublayer. Otherwise, the same explanations apply. REFERENCE SIGN LIST

1 Optical structure

2 Base structure 3 First substructure

4 Second substructure

5 Vacancy

10 first region

11 second region r radial direction