An imaging system for capturing an image is described herein. The imaging system may be part of a camera, in particular a mobile phone camera. The imaging system may be arranged to capture photos or videos.

The imaging system comprises an objective which is arranged to create an image of an object plane in an image plane. The objective is arranged to gather light from an object being observed and focuses the light rays coming from the object to produce a real image at the image plane.

An optical path extends from the object plane to the image plane through multiple optical surfaces. In particular, the optical path may be folded by means of a mirror or a prism. Such mirror or prism may be arranged in front of a front optical surface. Thus, light entering the optical system is reflected by the mirror or prism before interacting with the multiple optical surfaces. An optical surface may be a refractive surface, which is arranged to interact with electromagnetic radiation, in particular light, in a predefined manner. An optical surface may be an interface between two optical media having different refractive indices, wherein both optical media are essentially transparent for light in the visible wavelength range.

The objective comprises a rigid lens and a tunable lens. The rigid lens may consist of glass or polymer. The rigid lens has predefined optical properties, which remain essentially constant under intended working conditions. In particular, the objective may comprise multiple rigid lenses. Each rigid lens provides two of the multiple optical surfaces. In particular, each rigid lens comprises two of the multiple optical surfaces, wherein both optical surfaces of the rigid are adjacent to a gaseous material, in particular air. In particular, the rigid lenses are not adjacent to a liquid material. In particular, each rigid lens has at least on curved surface, having a radius of curvature smaller than infinity. Thus, a flat transparent element with two planar surfaces is not considered to be a rigid lens in the context of the present disclosure. In particular, the rigid lens which is arranged closest to the image plane comprises two aspheric optical surfaces.

The tunable lens is arranged to control a distance of the object plane to the front optical surface by adjusting a refractive power of the tunable lens. In other words, the tunable lens is arranged to control the focus of the objective. In particular, the rigid lenses have a fixed distance to the image plane and the tunable lens is arranged to focus the objective to different distances of the object plane altering the focal power of the tunable lens.

The tunable lens comprises a front optical surface, wherein the front optical surface is one of the multiple optical surfaces which is closest to the object plane along the optical path. In particular, a planar cover element, which does not add to the refractive power of the objective is not considered to be part of the objective. For clarification, the imaging system may comprise a planar cover element which is not considered as being part of the objective.

The tunable lens comprises a main refractive surface, wherein the main refractive surface has the largest refractive power of all of the multiple optical surfaces of the imaging system. In particular, the main refractive surface has the smallest radius of curvature. In particular the main refractive surface has the largest refractive power of all of the multiple optical surfaces when the imaging system is focused to infinity. Here and in the following “infinity” describes a distance which is defined by the fact that the rays emanating from one point of the object enter the optics essentially parallel. In particular, “infinity” corresponds to a distance which is at least as large as the hyperfocal distance of the objective. The hyperfocal distance depends on the size of the objective. In particular “infinity” describes a distance of more than 10 meters, preferably more than 4 meters.

In particular, the optical system comprises an aperture stop, wherein the aperture stop is arranged at the tunable lens. In particular, the aperture stop is defined by the plane at which rays of different fields passing through the optical system intersect each other.

According to one embodiment, the imaging system for capturing an image, comprises an objective which is arranged to create an image of the object plane in the image plane. The optical path extends from the object plane to the image plane through the multiple optical surfaces. The objective comprises at least one rigid lens and the tunable lens, wherein the tunable lens comprises a front optical surface, wherein the front optical surface is one of the multiple optical surfaces which is closest to the object plane along the optical path. The tunable lens is arranged to control the distance of the object plane to the front optical surface by adjusting the refractive power of the tunable lens, and the tunable lens comprises a main refractive surface, wherein the main refractive surface has the largest refractive power of all of the multiple optical surfaces of the imaging system.

An imaging system described herein is based, inter alia, on the consideration, that common objectives with tunable lenses are typically designed by adding a tunable lens to a set of rigid lenses. The set of rigid lenses is typically optimized for imaging an object distance of infinity. Thus, the tunable lens is tuned zero refractive power, when the objective is focused to infinity, and the tunable lens only adds refractive power to the objective when the object distance is smaller than infinity. In this common configuration, the tunable lens increases the length of the objective and adds spherical aberrations when tuned to non-infinity distances of the object plane.

Amongst other things, the imaging system described herein makes use of the idea to integrate the tunable lens in the objective by replacing the foremost rigid lens with the tunable lens. Thus, the tunable lens has a non-zero refractive power when the objective is focused to infinity. Hence, the required change of the optical properties of the tunable lens when adjusting the focus of the objective is reduced, which reduces the optical aberrations, in particular spherical aberration. Advantageously, the integration of the tunable lens as first lens of the optical system reduces the total length of the objective, measured along the optical path.

According to one embodiment, the front optical surface is the main refractive surface.

According to one embodiment the front optical surface has the smallest asphericity of the multiple optical surfaces. In particular, the shapes of the optical surfaces may be described by means of the following formula:

Wherein an optical axis is presumed to lie in the direction z, and z(r) is the sag — the z-component of the displacement of the surface from the vertex, at distance r from the axis. The coefficients cti (alpha) describe the deviation of the surface from the axially symmetric quadric surface specified by a radius of R and K (kappa). If the coefficients cti are all zero, then R is the radius of curvature and K is the conic constant, as measured at the vertex (where r=0). In this case, the surface has the form of a conic section rotated about the optical axis, with a shape determined by K. In case, the coefficients ci are all zero, the smallest asphericity means that the front optical surface has the smallest deviation from K=1 of all optical surfaces.

According to one embodiment the tunable lens comprises a liquid in a liquid volume, a deformable optical surface and a rigid optical element, wherein the liquid volume is arranged adjacent to the deformable optical surface and the rigid optical element, and the liquid volume is arranged between the deformable optical surface and the rigid optical element along the optical path. A refractive power of the tunable lens is adjustable by adjusting a radius of curvature of the deformable optical surface, and the deformable optical surface is the front optical surface and/or the deformable optical surface is the main refractive surface. The tunable lens may be arranged to adjust the refractive power, by altering the pressure of the liquid in the liquid volume. For example, the tunable lens may comprise a flexible transparent membrane, which comprises the deformable optical surface. In particular, the membrane may be made from PDMS. The said pressure may be altered by pushing a shaping element against the flexible membrane.

In particular, the liquid volume is at least partially delimited by means of the deformable optical surface and the rigid optical element. A change in pressure of the liquid reduces or increases the bulging of the deformable optical surface.

According to one embodiment, the refractive power of the front optical surface exceeds the refractive power of the other optical surfaces of the multiple optical surfaces respectively by at least a factor of [X], when the tunable lens is adjusted to set a focus of the objective to infinity. The factor X is at least 1. 1 , in particular 1.2, preferably 1.5. In case the deformable optical surface is the front optical surface, the curvature of the deformable optical surface is adjusted so that the deformable optical surface has a larger refractive power than all other optical surfaces of the multiple optical surfaces. In case the deformable optical surface is not the front optical surface, the deformable optical surface is adjusted to have a smaller refractive power than the front optical surface, when the objective’s focus is set to infinity.

According to one embodiment, the deformable optical surface faces the object plane and the rigid optical element faces the image plane. In particular, the deformable optical surface is the front optical surface.

According to one embodiment the deformable optical surface faces the image plane, the rigid optical element faces the object plane and comprises the front optical surface, wherein the front optical surface has a radius of curvature of less than 3mm, preferably less than 1.5 mm. In other words, the front optical surface has a focal length of at most 5 mm, preferably at most 3.5 mm.

According to one embodiment, the rigid optical element comprises a first surface and a second surface, wherein the first surface faces away from the liquid and the second surface faces towards the liquid, wherein the first surface has a non-planar aspheric shape and/or the second surface has a non- planar aspheric shape. In particular, the second surface is directly adjacent to the liquid. For example, the second surface delimits the liquid volume in one direction along the optical path.

According to another embodiment, the rigid optical element comprises a first surface and a second surface, wherein the first surface faces away from the liquid and the second surface faces towards the liquid, wherein the first surface has planar or spheric shape.

According to one embodiment the imaging system according to one of the preceding claims comprises an image sensor, wherein the image sensor is arranged at the image plane and a module length, measured along the optical path from the front optical surface to the image sensor is at most 6.5 mm preferably at most 5 mm.

According to one embodiment, the tunable lens is arranged such that a change in curvature of the deformable surface due to thermal expansion of the liquid compensates a change of the refractive index of the liquid over temperature.

In particular, the liquid volume, a coefficient of thermal expansion of the liquid and a geometry of the liquid volume are selected such that the following relation is met: wherein f is the focal length, R is the radius of curvature of the tunable surface, T is the temperature and n is the refractive index of the liquid. According to one embodiment, the tunable lens comprises a biasing element and an actuator, wherein in a first state the actuator controls the refractive power of the tunable lens, and in a second state the biasing element is arranged to maintain a dedicated refractive power of the tunable lens when the actuator does not control the refractive power. In particular, the biasing element is arranged to define a tuning state when the actuator is switched off. For example, the tuning state is selected such that the objective is focused to infinity.

According to one embodiment the liquid has a first refractive index, and the rigid optical element has a second refractive index, wherein the first refractive index is at least 0.1 smaller than the second refractive index. For example, the material of the liquid has a refractive index of at most 1,3 and the material of the rigid optical element has a refractive index of at least 1,4.

Further advantages and advantageous embodiments and further embodiments of the result from the following embodiment examples shown in connection with the figures.

It shows:

Figures 1-4 show an exemplary embodiment of an imaging system with a front optical surface being a deformable optical surface, in a schematic sectional view with exemplary rays traced through the imaging system;

Figures 5-8 show an exemplary embodiment of an imaging system with rigid optical element comprising the front optical surface, in a schematic sectional view with exemplary rays traced through the imaging system;

Figures 9-12 show an exemplary embodiment of an imaging system with a front optical surface being deformable optical surface, in a schematic sectional view with exemplary rays traced through the imaging system;

Figures 13-16 show an exemplary embodiment of an imaging system with rigid optical element comprising the front optical surface, in a schematic sectional view with exemplary rays traced through the imaging system; and

Figures 17 and 18 show exemplary embodiments of an imaging system in a schematic sectional view.

Elements which are identical, similar or have the same effect are given the same reference signs in the figures. The figures and the proportions of the elements shown in the figures to one another are not to be regarded as to scale. Rather, individual elements may be shown exaggeratedly large for better representability and/or for better comprehensibility.

Figures 1-4 show an exemplary embodiment of an imaging system with a front optical surface being a deformable optical surface of a tunable lens, in a schematic sectional view with exemplary rays traced through the imaging system. The imaging system 1 is arranged to capture images and comprises an objective 10 which is arranged to create an image of an object plane 2 (not shown) in an image plane 3. An optical path 100 extends from the object plane 2 to the image plane through multiple optical surfaces. The objective 10 comprises multiple rigid lenses 120, wherein each of the rigid lenses comprises two of the multiple optical surfaces. The optical surfaces of the rigid lenses are adjacent to a gaseous medium, in particular air.

The objective 10 comprises a tunable lens 110 with a front optical surface 6, wherein the front optical surface 6 is one of the multiple optical surfaces which is closest to the object plane 2 along the optical path 100. In particular, the front optical surface 6 has the largest distance to the image plane 3 along the optical path 100.

The tunable lens 110 is arranged to control a distance of the object plane 2 to the front optical surface 6 by adjusting a refractive power of the tunable lens 110. Moreover, the tunable lens comprises a main refractive surface 7, wherein the main refractive surface 7 has the largest refractive power of all of the multiple optical surfaces of the imaging system. In the embodiment shown in figures 1-4, the front optical surface 6 is the main refractive surface 7.

The imaging system shown in figure 1 shows a schematic illustration of the imaging system being focused on infinity distance, wherein the imaging system has a field of view of 20°. In the tuning state of the tunable lens 110, the refractive power of the front optical surface 6 exceeds the refractive power of the other optical surfaces of the multiple optical surfaces respectively by at least a factor of 1.2, preferably by a factor of 1.5, when the tunable lens 110 is adjusted to set a focus of the objective 10 to infinity.

The optical system 1 comprises an aperture stop 8, wherein the aperture stop 8 is arranged at the tunable lens 110. Furthermore, a protective glass 9 is arranged between the rigid lenses 120 and the image plane 3.

Figure 2 shows the same exemplary embodiment as figure 1, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 100 mm. Figure 3 shows the same exemplary embodiment as figure 1, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 50 mm. Figure 4 shows the same exemplary embodiment as figure 1, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 25 mm.

The tunable lens 110 comprises a liquid in a liquid volume 113, a deformable optical surface 111 and a rigid optical element 112. The liquid volume 113 is arranged adjacent to the deformable optical surface 111 and the rigid optical element 112. The liquid volume 113 is arranged between the deformable optical surface 111 and the rigid optical element 112 along the optical path 100.

A refractive power of the tunable lens is adjustable by adjusting a radius of curvature of the deformable optical surface 111, and the deformable optical surface 111 is the front optical surface 6 and the deformable optical surface 111 is the main refractive surface 7. The deformable optical surface 111 faces the object plane 2 and the rigid optical element 112 faces the image plane 3. The rigid optical element 112 differs from the rigid lens in the feature that at least one optical surface of the rigid optical element is adjacent to a liquid material, which is particularly arranged in the liquid volume, while the rigid lens comprises two optical surface which are adjacent to a gaseous material.

Figures 5-8 show an exemplary embodiment of an imaging system 1, wherein the rigid optical element 112 comprises the front optical surface 6, in a schematic sectional view with exemplary rays traced through the imaging system 1. The embodiment shown in the figures 5 to 8 differs from the embodiment shown in the figures 1 to 4 in the feature, according to which the deformable optical surface 111 faces the image plane 3. The rigid optical element 112 faces the object plane 2 and comprises the front optical surface 6. The front optical surface 6 has a radius of curvature of less than 3 mm.

The imaging system shown in figure 5 shows a schematic illustration of the imaging system 1 being focused on infinity distance, wherein the imaging system 1 has a field of view of 30°. Figure 6 shows the same exemplary embodiment as figure 5, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 100 mm. Figure 7 shows the same exemplary embodiment as figure 5, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 50 mm. Figure 8 shows the same exemplary embodiment as figure 1, wherein the tunable lens 110 adjusts the focus of the imaging system 1 to a working distance of 25 mm.

Figures 9-12 show an exemplary embodiment of an imaging system 1 with a front optical surface 6 being the deformable optical surface 111 of the tunable lens 110, in a schematic sectional view with exemplary rays traced through the imaging system 1. The embodiment shown in figures 9-12 differs from the embodiment shown in figures 1-4 in the field of view angle. The embodiment of figures 1-4 has a field of view of 20° when focused to infinity. The embodiment shown in figures 9-12 has a field of view of 30° when focused to infinity.

Figures 13-16 show an exemplary embodiment of an imaging system with rigid optical element comprising the front optical surface, in a schematic sectional view with exemplary rays traced through the imaging system. The embodiment shown in figures 13-16 differs from the embodiment shown in figures 5-8 in the field of view angle. The embodiment of figures 5-8 has a field of view of 20° when focused to infinity. The embodiment shown in figures 13-16 has a field of view of 30° when focused to infinity.

Figures 17 shows an exemplary embodiment of an imaging system 1 in a schematic sectional view. The imaging system 1 is arranged to for capture an image. The imaging system 1 comprises an objective 10 which is arranged to create an image of the object plane 2 in the image plane 3. The optical path 100 extends from the object plane 2 to the image plane 3 through multiple optical surfaces. The objective comprises the rigid lens 120 and the tunable lens 110. The tunable lens 110 comprises the front optical surface 6, wherein the front optical surface 6 is one of the multiple optical surfaces which is closest to the object plane 2 along the optical path 100. The tunable lens 110 is arranged to control a distance 4 of the object plane 2 to the front optical surface 6 by adjusting a refractive power of the tunable lens 110. The tunable lens comprises the main refractive surface 7, wherein the main refractive surface 7 has the largest refractive power of all of the multiple optical surfaces of the imaging system 1. 1 the embodiment shown in figure 17, the front optical surface 6 is the main refractive surface 7. The front optical surface 6 has the smallest asphericity of the multiple optical surfaces.

The tunable lens comprises a liquid in a liquid volume 113, the deformable optical surface 11 and a rigid optical element 112, wherein the liquid volume 113 is arranged adjacent to the deformable optical surface 111 and the rigid optical element 112. The liquid volume 113 is arranged between the deformable optical surface 111 and the rigid optical element 112 along the optical path 100. A refractive power of the tunable lens 110 is adjustable by adjusting a radius of curvature of the deformable optical surface 111, and the deformable optical surface 111 is the front optical surface 6 and the deformable optical surface 111 is the main refractive surface 7.

The deformable optical surface 111 faces the object plane 2 and the rigid optical element 112 faces the image plane 3. The rigid optical element 112 comprises a first surface 1121 and a second surface 1122, wherein the first surface 1121 faces away from the liquid volume 113 and the second surface 1122 faces towards the liquid volume 113, wherein the first surface 1121 has a non-planar aspheric shape and/or the second surface 1122 has a non-planar aspheric shape. The liquid fills the liquid volume completely. In particular the liquid has a first refractive index and the rigid optical element has a second refractive index, wherein the first refractive index is at least 0.1 smaller than the second refractive index.

The imaging system 1 comprises an image sensor 20, wherein the image sensor 20 is arranged at the image plane 3 and a module length 5, measured along the optical path 100 from the front optical surface 6 to the image sensor 20 is at most 6.5 mm.

Figures 18 shows exemplary embodiment of an imaging system 1 in a schematic sectional view. The embodiment shown in figure 18 differs from the embodiment shown in figure 17 in the feature, according to which the deformable optical surface 111 faces the image plane 3. The rigid optical element 120 faces the object plane 2 and comprises the front optical surface 6. In particular, the first surface 1121 is the front optical surface 6.

A change in curvature of the deformable surface 111 due to thermal expansion of the liquid in the liquid volume 113 compensates a change of the first refractive index of the liquid over temperature. The tunable lens comprises a biasing element 115 and an actuator 116. The biasing element 115 is ar- ranged to provide a biasing force, which pulls the shaper 114 towards the container 115 along the optical path 100. The actuator 116 is arranged to exert an actuation force, which pushes the shaper 114 towards the container 115 along the optical path 100. In a first state the actuator 116 controls the refractive power of the tunable lens 110, by providing an adjustable actuation force. In a second state the biasing element 115 is arranged to maintain a dedicated refractive power of the tunable lens 110 when the actuator 116 does not control the refractive power. In particular, the actuator is switched off in the second state.

The invention is not limited to the embodiments by the description based thereon. Rather, the invention includes any new feature as well as any combination of features, which in particular includes any combination of features in the claims, even if that feature or combination itself is not explicitly stated in the claims or embodiments.

List of reference signs

1 Imaging system

10 Objective

100 Optical path

110 Tunable lens

120 Rigid lens

111 Deformable optical surface

112 Rigid optical element

113 Liquid volume

114 Shaper

115 Container

116 Biasing element

117 Actuator

2 Object plane

3 Image plane

20 Sensor

4 Distance

5 Module length

1121 First surface

1122 Second surface

6 Front optical surface

7 Main refractive surface

8 Aperture stop

9 Protective glass