WO2009006747A1

WO2009006747A1 - Optical element, illumination system and method of designing an optical element

Info

Publication number: WO2009006747A1
Application number: PCT/CH2008/000305
Authority: WO
Inventors: Daniel Dias; Ville Kettunen; Martin Salt; Markus Rossi
Original assignee: Heptagon Oy
Priority date: 2007-07-12
Filing date: 2008-07-08
Publication date: 2009-01-15
Also published as: TW200923449A

Abstract

The invention relates to an optical element (10) that comprises an at least partially transparent layer having a front surface and a back surface, wherein at least one of said front surface and back surface comprises optical structures, e.g. classical lens elements or optical microstructures (20), said optical structures representing a predetermined optical function of the optical element, e.g. a generally a beam shaping function. At least one of said front surface and back surface comprises further optical structures, preferably microstructures (30), wherein said further optical structures generate at least one informative and/or decorative element (32) that is visible on or through the front surface at least( under predetermined illumination conditions. As these further structures can be manufactured in the same step as the optical structures ensuring the basic optical function, it is possible to add an extra value to an optical element without complicating the manufacturing process or increasing the costs.

Description

OPTICAL ELEMENT, ILLUMINATION SYSTEM AND METHOD OF DESIGNING AN OPTICAL ELEMENT

FIELD OF THE INVENTION

The invention relates to an optical element whose surface is structured by means of optical structures, e.g. diffraction and/or refraction type optical microstructures or classical optical element, that represent a predetermined optical function. The invention further relates to an illumination system comprising at least one light source such as an electroluminescent element and at least one optical element whose surface is structured by means of such optical structures, in particular microstructures. The invention further relates to a method of designing an optical element.

BACKGROUND OF THE INVENTION

Optical elements that provide a predetermined optical function by means of optical structures, such as classical optical structures (e.g. convex/concave lenses) or optical microstructures, on at least one of its surfaces are known. The optical element comprises a layer of an at least partially transparent material whose thickness is generally much smaller than its extent in the other two dimensions. Optical structures are arranged on the front surface or the back surface or both surfaces. The layer may be a glass substrate with a thin layer of epoxy attached thereto, the epoxy having a surface profile representing said structures, in particular but not necessarily microstructures.

Optical microstructures comprise fine structures that are such that the wave nature of the light has to be taken into account and participates in the effect the optical element having such microstructures on one of its surfaces has upon radiation, in particular by refraction and/or diffraction. The optical microstructures have characteristic profile depths and profile widths of the order of a few wavelengths up to a few tens of wavelengths for refractive optics and of about one wavelength up to a few wavelengths for diffractive optics. For example, the characteristic depths/heights and often also widths are of typically a few micrometers, for example of 0.5 μm - 200 μm, typically around 20 μm. Such optical microstructures have, for example, the shape of a series of micro-ridges or micro-grooves arranged in an otherwise unstructured planar surface. As a rule of thumb, corresponding micro-optical elements have structures such that the phase relation of radiation present at different neighboring places on the structure is well-defined. This is opposed to classical, purely refractive optical elements, where the behavior of the radiation at different elements of the structure can be described in a geometrical optics model, and the wave nature is disregarded.

Optical elements with optical microstructures have the potential to shape the initially emitted light in almost any manner. This may be achieved by choosing for different areas of the optical element different types of microstructures and/or different characteristic depths/widths/distances of the microstructures. The overall profile can be calculated such that the overall optical element has a predetermined optical function, which is represented by the optical microstructures. Such an optical element and a corresponding design method are described in WO 2004/044995, for example, which is incorporated herein by reference. Optical elements with optical microstructures are especially beneficial in combination with one or more LED(s) as a light source, as the LED is an extended light source and cannot be regarded as a point source. This makes it especially difficult to achieve a good collimation with conventional optics.

A further advantage of using optical microstructures is that the optical elements can be essentially flat, as the microstructures do not significantly add to its thickness. A typical thickness of the optical element is 0.5 mm, a preferred range is 300 μm to 1.1 mm. Such optical elements can be manufactured in a mass process, e.g. by imprinting the desired profile on a transparent layer, e.g. of a plastic material or a resin, attached to a substrate, e.g. a glass wafer.

In the past years, digital cameras became increasingly popular, nowadays also as part of mobile phones. Especially cameras of a mobile phone have to be produced at low cost, and thus require a flash light that can be produced at low cost as well. It is known to use an optical element utilizing optical microstructures to achieve a collimating function for the flash light of such a digital camera. As an alternative, also classical optical structures can be used.

It is often desirable to add to a product comprising an optical element, e.g. such a mobile phone with a flash light, decorative and/or informative elements, e.g. a company's logo, a security label that ensures authenticity of the product, certain color effects, and the like. Such elements generally have to be applied to the product in a separate manufacturing step. - A -

It is an object of the present invention to provide an optical element having additional decorative and/or informative elements without making its manufacturing more complex or costly.

It is a further object of the present invention to simplify manufacture of products comprising one or more optical elements and additionally comprising decorative and/or informative elements.

SUMMARY OF THE INVENTION

These and other objects are achieved by an optical element as claimed in claim 1, by an illumination system as claimed in claim 15, and by a method as claimed in claim 18. Preferred embodiments are described in the dependent claims and the description, and are shown in the figures.

As described above, the optical element comprises an at least partially transparent layer having a front surface and a back surface, wherein at least one of said front surface and back surface comprises optical structures, such as optical microstructures or one or more classical optical elements, said optical structures representing a predetermined optical function of the optical element. This optical function is generally a beam shaping function: A light beam emitted by a light source is shaped to achieve a predetermined output distribution, e.g. with predetermined divergence angle(s). These optical structures required to provide the basic optical function of the optical element for generating a required output beam shape will also be designated as optically effective structures. If these structures are optical microstructures, they will be designated as optically effective microstructures. According to the invention, at least one of said front surface and back surface comprises further optical structures, in particular microstructures, wherein said further optical structures generate at least one informative and/or decorative element that is visible on or through the front surface at least under predetermined illumination conditions.

hi other words, the invention generates an informative and/or decorative effect on an optical element visible on or through its front surface by introducing a further variation in the surface profile of the front and/or back surface in addition to the surface profile required for generating the desired beam shaping effect, i.e. for fulfilling the main function of the optical element.

The inventors have discovered that by adding further optical structures, in particular microstructures, to the surface of an optical element, added value in different areas can be provided while not directly or not significantly contributing to the basic optical function of the optical element and without increasing the manufacturing costs.

By adding the further optical structures or changing the optically effective structure by superimposing the further optical structures, the following different functions can be achieved besides the basic beam shaping function of the optically effective structure, for example: The informative and/or decorative element may be a security feature, e.g. to assure that the product carrying the optical element or the optical element itself is genuine. Generally, structures that are only visible under special lighting conditions are preferred. The informative and/or decorative element may also carry branding information, e.g. be a company's logo visible on the optical element in order to identify the product as coming from that company. This element is preferably visible under all lightning conditions. In addition or alternatively, the informative and/or decorative element may have a decorative effect, e.g. may be iridescent or may lead to a visible pattern of lines or shapes visible on the optical element. The informative and/or decorative element may include design aspects from the rest of the device into which the optical element is installed, in order to carry through the overall design theme and/or to have more aesthetic impact, or to achieve a new visual effect for the device. Any further optical microstructure may fulfill any or all of the above functions at the same time.

The further optical structures can be spatially separated from the optically effective structures, or can be arranged in a spatially overlapping relationship, e.g. by superimposing a small modulation on the surface profile of the optically effective structures.

The optically effective structures are generally designed to shape a light beam emitted by a light source arranged at a predetermined location in the direct vicinity of the optical element; light source and optical element form an illumination system. The further structures are generally designed not to affect light emitted from this light source but to affect light emitted from a further light source, which will mostly be located outside the illumination system as such. This further light source may be diffuse ambient light or a light source brought in a certain spatial relationship with respect to the illumination system.

The optically active structures are preferably diffraction and/or refraction type microstructures. It is preferred that they have characteristic features as discussed in the introductory portion of this application, i.e. characteristic depths/heights and often also widths of typically a few or a few ten micrometers, for example of 0.5 μm - 200 μm, preferably around 20 μm. In a preferred embodiment, the optically active microstructures are designed to collimate light emitted by a light source. In case of microstructures, the further optical (micro-)structures are generally in the same size range as the optically effective (micro-)structures. Only if overlaid directly on the optically active structures they are shallower, from 50 ran up to 10 μm such that the optical function of these structures is not adversely affected. In this case the widths are preferably similar to those of the optically active structures.

The further optical structure may comprise one or more microgrooves, a diffusive texture, or a grating structure, for example. Microgrooves enable to form an outline of a company's logo, a security feature, a serial number, the outline of a decorative shape, a decorative pattern of lines, and the like. A diffusive texture, i.e. a random type microstructuring of the surface leads to a decoration that is visible from almost any angle under ambient light. Periodic microstructures (grating structures) create variable visibility with viewing angle and iridescent effects.

According to a further preferred embodiment of the invention, the further optical structure has none or a low visibility under illumination with a first range of incidence angles, and has an increased visibility under illumination with a second range of incidence angles. This can be achieved by one or more microgrooves arranged in the front or back surface of the layer, for example. They have generally a low visibility but light up when illuminated under a given incidence angle, e.g. by a laser.

A further preferred embodiment of the invention comprises a further optical structure that has none or a low visibility under illumination with light incident on the front surface or the back surface of the partially transparent layer, and has an increased visibility under illumination with light entering through a side face of the at least partially transparent layer. Such "outcoupling" arrangements are especially preferred in connection with security features. The corresponding illumination system may comprise an additional light source for directing light into the side face of the optical element.

The following general types of further optical structures are preferred:

1. Structures written directly over the active optical area of the optical element, i.e. the part of the front or back surface of the optical element that bears the optically effective structure. Generally, these further optical structures are microstructures that are intended to be weakly visible under ambient light and more strongly visible under very bright illumination, or illumination by laser. These further microstructures are created by a depth modulation of the profile of the optically effective structures. The original optically effective structures are designed to achieve a specified beam shaping effect, such as camera flash illumination or some other special beam shape. The typical depth of such a microstructure is in the tens of microns range. To add in the decorative/security features, a second microstructure modulation of a much lower depth is overlaid onto the first, i.e. the optically effective structure.

One key aspect of such an approach is that decorative or informative features can be added to the optically active part of the optical element without significantly influencing the output beam shape of the optical element.

It is also possible that the optically effective structures are arranged in the back surface and the further structures are arranged on the front surface of the optical element, but overlap one another as seen in a direction perpendicular to the optical element. 2. Structures written outside the active optical area of the optical element with varying degrees of visibility and visual effects. These further structures are generally intended to be clearly visible. These further microstructures can be of different types:

- Outline. Only the outline of the required shape is defined. This can most easily be done by a single microgroove tracing out the edge of the shape, but plenty of other structuring methods are possible. Such an outline is generally of low visibility under most lighting conditions.

- Diffusive texture. A random-type microstructuring of the surface leads to decoration that is visible from almost any angle under ambient light. The depth of the microstructure dictates visibility. Special effects such as variable depth across the decoration allows for varying visibility for different parts of the decoration.

Grating structures. Periodic microstructuring for creating variable visibility with viewing angle and iridescent (color or "rainbow") effects. Microgrooves are spaced with a defined period and diffractive effects create decorations that vary with viewing angle and/or position on the decoration/security feature.

3. Structures used for so-called outcoupling effects: Light from a LED or other light source is sent into the side of the transparent layer. This light is guided in the substrate and is effectively thrown out towards the viewer only in the regions carrying further optical structures. The result is an element that lights up only in the microstructured areas. The types of structures used can be the same as those discussed in section 2 above. Generally, microgrooves are highly effective: the structure is almost invisible until the illumination is turned on, after which it becomes highly visible. The inventive method of designing an optical element comprises the following steps:

Determining an optical function of the optical element. TMs is, for example done by taking into account the emission characteristics of a light source that is to be used in conjunction with the optical element, and the desired output beam shape. - Determining an additional function of the optical element, i.e. appearance of the informative/decorative element, e.g. a logo, decoration or a security feature, under a certain angle of incidence or certain range of incidence angles.

- Determining optical structures that represent said optical function, for example as discussed in WO2004/044995. - Determining further optical structures that represent said additional function, for example as discussed above.

Determining an overall surface profile of the optical element comprising said optical structures and said further optical structures, for example by superimposing the optical structures and the further optical structures or by arranging them side by side.

The optical element according to the invention is preferably manufactured in a wafer-scale replication process, which is known per se and comprises the following steps:

Mastering by laser beam writing. The optically effective and the further microstructures are written into a photoresist layer on a substrate using an intensity modulated laser beam. The photoresist layer is then developed to yield a surface relief structure. The laser intensity can be modulated with 65000 different values, for example, allowing highly precise, continuous surface relief to be created. All the required decoration/security features are included at this stage, along with the optical microstructures representing the beam shaping function of the optical element. Tooling: An inverted copy (tool) of the master is made by pouring a deformable material that can be hardened, e.g. a silicone material, e.g. liquid PDMS (Poly Dimethyl Siloxane) over the master, allowing it to solidify and then separating the hardened tool from the photoresist master. - Wafer-scale replication: The tool is then used as a mold in a wafer scale replication process, e.g. in a process as described in Preferred manufacturing methods are described in the co-pending US patent applications No. 10/541,008 (published as US 2006/0113701 and WO 2004/068198), 11/384,562, 11/384,563, 11/384,537, and 11/384,558 which are incorporated herein by reference. A thin layer of replication material in a deformable state, e.g. UV curable liquid epoxy, is placed onto a glass wafer. The tool is pressed into the replication material and exposed to UV light. The liquid solidifies under the UV, and the tool is separated, leaving an exact copy of the surface relief in the epoxy layer - a replicated wafer. Thereby a plurality of optical elements are produced simultaneously. The final parts can then be diced out of the wafer.

As an alternative to the wafer-scale replication step above, optical elements can also be made by injection molding. In this case, the replicated wafer from the process as discussed above is electroplated with Nickel. The Nickel is grown on top of the microstructured surface to a particular thickness, and is then separated from the wafer. The result is typically a Nickel plate or foil with an inverse copy of the microstructures on one side. This metallic copy is then inserted into an injection molding tool to provide the microstructure for the final molded part. The inserted Nickel part is often called the 'insert' or 'shim'.

Alternative methods for providing a master tend be more limited in their range of capabilities compared to laser writing, but it could be possible to create some of the above features and effects using the following: Electron beam lithography, with the structure either in photoresist or etched into a substrate (e.g. glass, silicon); photolithography, with the structure either in photoresist or etched into a substrate (e.g. glass, silicon), diamond turning/ruling.

For injection molding, certain structures and features could also be added directly to the Nickel shim by mechanical machining (e.g. precision drilling/grinding - rather for simple, large features), diamond ruling, sink erosion or photolithography (followed by chemical etch).

In any case it is possible to produce the optically active microstructures as well as the further microstructures in one single step, and preferably to produce a plurality of optical elements simultaneously on wafer scale.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic drawing of an optical element especially for collimating light emitted by one or more LEDs without decorative/informative design features; Figure 2 shows the optical element as shown in Fig. 1 with decorative/informative design features for branding purposes according to the invention;

Figure 3 shows an optical element according to the invention with low- visibility security features on the active area of the optical element; Figure 4a+b show schematically an example of adding a new decorative effect using optical microstructures with different visibilities under different lightning conditions Figure 5a shows an original surface structure of an optical element, for providing a specified beam shape;

Figure 5b shows a theoretical profile of a further optical microstructure to be superimposed on the surface structure of Fig. 5a; Figure 5c shows the surface structure as shown in Fig. 5a with addition of the further optical microstructure as shown in Fig. 5b for the decoration/security feature; the dotted lines show profile of original structure before addition of the modulation;

Figure 6 shows a sectional view of an optical element showing the profile of a microgroove type further microstructures for outline type decoration;

Figure 7 shows a sectional view of an optical element showing the profile of random further microstructures for diffusive decoration;

Figure 8 shows a sectional view of an optical element showing the profile of periodic further microstructures for a decoration with color and visibility effects that change with viewing angle;

Figure 9a-c show examples of the light outcoupling effect by the further microstructures with illumination from the side, from below, and from above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a plan view of an optical element 10 comprising a planar at least partially transparent layer 10' with a front surface 12 normally directed to a viewer, a back surface 14 normally directed to a light source, and side surfaces 16. The layer 10' is generally rectangular with length and width of some millimeters. The front surface 12 and/or back surface 14 is structured by means of optical microstructures 20 having the shape of circular microgrooves concentrically arranged around two symmetry centers A, B. In an alternative embodiment, there could be more or less symmetry centers, or even no symmetry. The microgrooves can also be elliptical or have even more complex shapes. The optical microstructures 20 constitute a Fresnel- type lens, for example. They are designed to shape a beam path of a light source according to predetermined requirements. For example, the microstructures 20 collimate light emitted by an extended light source, such as one or more LEDs, arranged behind the optical element 10, e.g. next to the back surface 14. In particular, the optical element 10 may be a collimating lens as part of a flash light of a digital camera, e.g. one of a mobile phone.

The optically effective microstructures 20 cover only a part of the front surface 12 of the optical element 10. This part is also designated as optically active area 18 of the optical element 10. The surface of the optical element 10 in its periphery 19, i.e. outside the active area 18, is unstructured. This optical element 10 does not comprise additional decorative and/or informative design features.

Fig. 2 shows the optical element 10 of Fig. 1 with additional informative/decorative elements 32 visible in the periphery 19 of the optical element 10. The informative/decorative elements 32, here the word "logo", are produced by further microstructures 30 on the optical element 10, e.g. microstructures of the types shown in Fig. 6, 7 or 8. These microstructures 30 may be arranged directly in the front surface 12, or they may be arranged in the back surface 14 such that they are visible on the front surface 12. For example, they might be visible as such through the layer 10', or an outcoupling arrangement as shown in Fig. 9a-c with visibility only under certain lightning conditions may be used. The informative/decorative elements 32 serve for branding purposes, for example. As they can be manufactured in the same process as the optically active microstructures 20, branding information can be added without additional manufacturing steps.

Fig. 3 shows an optical element 10 similar to that of Fig. 1 with optically effective microstructures 20 on its front surface 12 and with further optical microstructures 30 superimposed on the optically effective microstructures 20 leading to low-visibility security features 32, here the word "logo", on the active area 18 of the optical element 10.

The further microstructures 30 may be represented by a modulation of the optically effective microstructures 20, e.g. as shown in Fig. 5a-c. Alternatively, they may be arranged on the back surface 14 of the optical element 10, e.g. in an outcoupling arrangement as shown in Fig. 9a-c.

It is preferred that informative/decorative elements 32 acting as security features generally have a low visibility. Under certain illumination conditions, for example illumination under a predetermined angle of incidence, the visibility is preferably increased.

Such a variable visibility of the informative/decorative element 32 is schematically shown in Fig. 4a+b.

Fig. 5a shows schematically a sectional view of an optical element 10 with a front surface 12 comprising optically effective microstructures 20 and an unstructured back surface 14. The microstructures 20 are schematically shown as saw-tooth like profile, but could have any other shape suited for achieving the required optical function. The typical profile depths d are in the range of some ten micrometers, preferably around 20 μm. The typical profile lengths 1, e.g. the distances between two microgrooves, are in the range of 10 to 200 μm. The typical widths w (full width at half maximum) of the microstructures are in the range of .5 to 100 μm.

Fig. 5b shows a profile of further optical microstructures 30 that generate decorative or security design elements 32. This profile is superimposed on the profile of the microstructures 20 as shown in Fig. 5a and leads to an overall profile as shown in Fig. 5c. The further optical microstructures 30 cause, in a microscopic view, a modulation of the surface structure of the microstructures 20 as shown in Fig. 5c, and generate macroscopically visible decorative or security design elements 32 in the optically active area 18 of the optical element 10.

The superimposed microstructures 30 comprise rectangular grooves with a depth d' and a profile length 1' (length of one groove plus length of one protrusion between two grooves). In order not to alter the optical function represented by the optically active microstructures 20, the profile depth d' is smaller than the profile depth d of the original profile, e.g. d' in the range of 50 nm to 10 μm. Fig. 6 shows an example for producing a informative/decorative element 32 of a line type, here the outline of the letter "R". A microgroove 31 with here rectangular cross-section (depth d of 1 to 20 μm, width w' of 1 to 30 μm) is a highly effective microstructure. The microgroove 31 could be arranged in the front surface 12 (as shown here) or in the back surface 14 (not shown) and in or outside the optically active area of the optical element 10. Such an outline is generally of low visibility under most lightning conditions. Fig. 7 shows an example for producing a informative/decorative element 32 of a two- dimensional type, here the full area of the letter "R". The profile of the corresponding microstructures 30 comprises a diffusive texture, e.g. a plurality of grooves 31 ' or depressions arranged randomly in the front surface 12 of the optical element 10. This kind of profile generates a diffusive decoration that is visible under ambient light from all perspectives. The depth d of the microstructure 30 determines visibility. Special effects such as variable depth across the decoration allows for varying visibility for different parts of the decoration. The microstructure 30 could be arranged in the front surface 12 (as shown here) or in the back surface 14 (not shown) and in or outside the optically active area of the optical element 10.

Fig. 8 shows an example for producing a informative/decorative element 32 of a two- dimensional type, here the full area of the letter "R", with an additional iridescent effect, i.e. color and visibility effects that change with viewing angle (indicated by two different fillings of the letter R appearing under different viewing angles). The profile of the microstructure 30 is periodic and acts thus as a grating. For example, it comprises a plurality of grooves 31 arranged with constant spacing. The microstructure 30 could be arranged in the front surface 12 (as shown here) or in the back surface 14 (not shown) and in or outside the optically active area of the optical element 10.

Fig. 9a-c show different examples for an outcoupling arrangement. The optical element 10 is shown in a sectional view with a light source 40 arranged next to its back surface 14 and optically effective microstructures 20 on its front surface 12. Light is emitted by the light source 40 in the general direction of an optical axis 50 running generally perpendicular to the front and back surface 12, 14. The light beam 52 emitted by the light source and shaped by the optical element is shown in dashed lines. The back surface 14 (Fig. 9a+b) or the front surface 12 (Fig. 9c) comprises further microstructures 30 that do or not significantly not affect light emitted from the light source 40. These further microstructures 30 are designed to direct light emitted by a further light source 42 in the general direction of the optical axis 50, i.e. towards a viewer.

In Fig. 9a, the further light source 42 is arranged such that light is coupled into the optical element 10 via a side face 16. The light is guided through the optical element 10 by total internal reflection at the front and back surface 12, 14. The further microstructures at the back surface 14 change the angle of the reflected/scattered light such that at least part of it leaves the optical element 10 via the front surface 12 under a small angle with respect to the optical axis 50. The further light source 42 may be a part of the illumination system or may be an external light source.

In Fig. 9b, the same effect is achieved by arranging the further light source 42 such that light emitted thereby falls onto the back surface 14 under a small angle with respect to the plane of the back surface 14. The further microstructures 30 at the back surface 14 are designed such that light is directed into the layer under a small angle with respect to the optical axis 50 and at least part of it leaves the optical element 10 via the front surface 12 under a small angle with respect to the optical axis 50.

In Fig. 9c, the further microstructures 30 are arranged on the front surface 12. The further light source 42 is arranged such that light emitted thereby falls onto the front surface 12 under a small angle with respect to the surface. The further microstructures 30 at the front surface 12 are designed such that light is directed towards the viewer under a small angle with respect to the optical axis 50 without entering the optical element 10. The further light source 42 can preferably be switched on and off in order to selectively "activate" (i.e. make visible) the informative/design element generated by the further microstructures 30.

Claims

WHAT IS CLAIMED IS:

Optical element, comprising an at least partially transparent layer having a front surface and a back surface, wherein at least one of said front surface and back surface comprises optical structures, said optical structures representing a predetermined optical function of the optical element, and wherein at least one of said front surface and back surface comprises further optical structures, said further optical structures generating at least one informative and/or decorative element visible at least under predetermined illumination conditions.

2. Optical element as claimed in claim 1, wherein the optical structures are optical microstructures.

3. Optical element as claimed in claim 1 or 2, wherein the further optical structures are optical microstructures.

4. Optical element as claimed in claim 1 , wherein the optical structures and/or the further optical structures comprise at least one of diffraction type and refraction type optical microstructures.

5. Optical element as claimed in one of the preceding claims, wherein the further optical structures do not significantly change the predetermined optical function of the optical element.

6. Optical element as claimed in one of the preceding claims, wherein the optical structures are arranged in a first area of at least one of said front surface and back surface and the further optical structures are arranged in a second area of at least one of said front surface and back surface, and wherein the first are and the second area are distinct from one another.

7. Optical element as claimed in one of the preceding claims, wherein the optical structures are arranged in a central region of the front surface of the optical element, and wherein the further optical structures are arranged in a periphery of the front surface of the optical element.

8. Optical element as claimed in one of claims 1 to 5, wherein the optical structures and the further optical structures are arranged in the same surface of the optical element in an overlapping relationship.

9. Optical element as claimed in claim 8, wherein the further optical structures are represented by depth modulations superimposed on the optical structures.

10. Optical element as claimed in one of the preceding claims, wherein the further optical structures comprise at least one of a microgroove, a diffusive texture, and a grating structure.

11. Optical element as claimed in one of the preceding claims, wherein the further optical structure has none or a low visibility under illumination with a first range of incidence angles, and has an increased visibility under illumination with a second range of incidence angles.

12. Optical element as claimed in one of the preceding claims, wherein the further optical structure has none or a low visibility under illumination of the front surface or the back surface, and has an increased visibility under illumination with light entering through a side face of the at least partially transparent layer.

13. Optical element as claimed in one of the preceding claims, wherein characteristic profile depths of the further optical structures are smaller than characteristic profile depths of the optical microstructures.

14. Optical element as claimed in claim 11, wherein the characteristic profile depths of the optical structures are in the range of some ten micrometers, preferably 20 μm, and the characteristic profile depths of the further optical structures are in the range of 50 run to 10 μm.

15. Illumination system, comprising at least one light source and an optical element as claimed in one of the preceding claims.

16. Illumination system as claimed in claim 15, wherein the further optical structure has an increased visibility under illumination of a side face of the optical element or under illumination of a front surface or a back surface with a predetermined range of incidence angles, and has none or low visibility under other illumination conditions, further comprising at least one further light source arranged such that light emitted by the further light source enters the optical element through the side face or falls onto the front surface or the back surface under said predetermined range of incidence angles.

17. Illumination system as claimed in claim 15 or 16, especially for a flash light for a camera or for a mobile communication device, wherein the optical structures are configured such that light emitted by the light source is collimated by the optical element, and wherein the further optical structures are configured such that they exhibit a predetermined visible pattern, e.g. a company's logo or a security feature, under predetermined illumination conditions.

18. Method of designing an optical element, especially as claimed in one of claims 1-14, comprising the following steps: determining an optical function of the optical element; - determining an additional function of the optical element; determining optical structures that represent said optical function;

- determining further optical structures that represent said additional function; determining an overall surface profile of the optical element comprising said optical structures and said further optical structures.