TECHNICAL FIELD

The present invention relates to optical imaging apparatuses in general. More specifically, the present invention relates to an optical imaging apparatus with a wide-angle imaging lens system and a large image sensor, for instance, for a camera of a mobile device, such as a smartphone, tablet computer and the like.

BACKGROUND

Many smartphones comprise a wide-angle imaging system (“the camera”) that has a field of view (FOV) of about 60 to 90 degrees and a ratio of total track length (TTL) to effective focal length (EFL) that is larger than 1. Trying to increase the resolution by increasing the size of the image sensor leads to an increased EFL and, thus, to an increased total length of the imaging system. Thus, there is a need for an optical imaging system that allows using large image sensors (for instance, image sensors having a diameter of 1 inch and larger), while keeping the overall total track length (TTL) of the camera short, i.e. having a TTL/EFL ratio of less than 1.

SUMMARY

It is an object of the invention to provide an optical imaging apparatus with a wide-angle imaging lens system and a large image sensor, for instance, for a camera of a mobile device, such as a smartphone, tablet computer and the like.

The foregoing and other objects are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

In brief, embodiments of the invention provide an imaging lens system for a camera in which the TTL/EFL ratio can be reduced to values less than 1 when the camera is in an inactive state. Thus, embodiments of the invention provide a compact camera module, when the camera is in the inactive state. In order to use the camera, a first group of optical elements of the imaging system is moved along an optical axis away from the image sensor into an active position, resulting in a TTL/EFL ratio greater than 1 to enable high performance optical imaging. Superior image quality can thus be achieved. When the camera is in the inactive state, the lens system is collapsed, reducing the overall size of the imaging system. In addition, embodiments of the invention make use of an imprinted polymer layer or coating on one or both surfaces of at least one of the lenses of the imaging system, facilitating lens centration and spacing as well as axial and tip or tilt alignment. The imprinted polymer layer can improve the optical image quality and reduce the TTL due to an achromatic functionality of the combination of lens and imprinted polymer layer.

More specifically, according to a first aspect an imaging apparatus for capturing an image is provided. The imaging apparatus comprises a first group of optical elements on the optical axis of the imaging apparatus, wherein the first group of optical elements comprises a plurality of lenses and defines an aperture stop of the imaging apparatus. Moreover, the imaging apparatus comprises an image sensor and a second group of optical elements located between the image sensor and the first group of optical elements on the optical axis. The second group of optical elements comprises a field flattening lens having a concave aspheric front surface. The plurality of lenses of the first group of optical elements comprise a first set of lenses defining an achromat arranged in front of the aperture stop and a second set of lenses arranged behind the aperture stop. Thus, an imaging apparatus is provided with a wide-angle imaging lens system allowing for a large image sensor.

In this application, the words “fronf’and “rear”, when referring to an optical element that transmits light, refer to an entrance side and an exit side of the optical element, respectively. Light enters the optical element on the front side and leaves the optical element on the rear side. A lens has a front surface and a rear surface. Light (e.g. from a an object) enters the lens via the front surface and leaves the lens via the rear surface (e.g. toward an image sensor).

In a further possible implementation form, the first group of optical elements comprises a diaphragm defining the aperture stop. Alternatively, the aperture stop may be defined "virtually" by the plurality of lenses of the first group of optical elements.

In a further possible implementation form, a front surface shape and a rear surface shape of the field flattening lens are configured to correct a field curvature aberration. Advantageously, this allows ensuring a proper field curvature aberration, even if a tilt error of the first group of optical elements is present.

In a further possible implementation form, a front and a rear surface shape of the field flattening lens are configured to provide an incident chief ray angle between -30° and +30°, i.e. smaller than |30|° relative to a surface normal of the front and rear surface of the field flattening lens. Advantageously, this allows ensuring a proper field curvature aberration, even if a tilt error of the first group of optical elements is present.

In a further possible implementation form, the imaging apparatus comprises a group actuator configured to move the first group of optical elements along the optical axis between an active position and an inactive position for adjusting a distance d between the first group of optical elements and the second group of optical elements. Advantageously, this allows to have a compact imaging system in the inactive position, while providing an improved image quality in the active position.

In a further possible implementation form, the first group of optical elements has a distance d from the second group of optical elements along the optical axis, wherein the ratio of the distance d to an image height lies in the range of 0.4 to 0.8 in the active position of the first group of optical elements.

In a further possible implementation form, the imaging apparatus comprising an Near- infrared cut-off filter (NIR-filter) and/or a cover glass arranged between the second group of optical elements and the image sensor, i.e. in front of the image sensor. Advantageously, this allows protecting the image sensor by the cover glass and providing a more realistic color reproduction of images by blocking light outside of the visible spectral range by means of the NIR filter.

In a further possible implementation form, the f-number, i.e. F/# of the imaging apparatus is between 2.4 and 1.6. Thus, advantageously, the imaging apparatus allows providing a good imaging performance even in low light conditions as well as larger possible Modulation Transfer Function (MTF) due to an improved diffraction limit.

In a further possible implementation form, the plurality of lenses of the first group of optical elements have a clear aperture diameter smaller than 10 mm. Thus, advantageously, the imaging apparatus can be provided with a compact form factor.

In a further possible implementation form, the distance between the second group of optical elements and the image sensor is smaller than 0.6 mm.

In a further possible implementation form, wherein the first set of lenses defining the achromat comprises a glass lens with an imprinted polymer layer on a front surface or a rear surface of the glass lens, wherein the imprinted polymer layer has an aspheric surface shape and wherein the aspheric surface shape of the imprinted polymer layer differs from a shape of the front surface or the rear surface of the glass lens. Because of the different shapes the imprinted polymer layer may act as an additional lens with different refractive power compared to the glass lens.

In a further possible implementation form, the glass lens comprises a first material having a first refractive power Pi and a first Abbe number V _± and the imprinted polymer layer comprises a second material having a second optical refractive power P ₂ and a second Abbe number V ₂ , wherein V ₂ - P ₁ + V ₁ - P ₂ is approximately equal to 0.

In a further possible implementation form, the glass lens has a concave aspheric surface on a side facing the imprinted polymer layer. Additionally or alternatively, the imprinted polymer layer has a concave aspheric surface on a side opposite the side facing the glass lens.

In a further possible implementation form, the imprinted polymer layer comprises a mechanical interlock for aligning the glass lens and/or the first set of lenses. Advantageously, this allows aligning the glass lens and/or the first set of lenses with respect to one of the further lenses following along the optical path and/or the image sensor.

In a further possible implementation form, the focal length of the glass lens and the focal length of the imprinted polymer layer have different signs, i.e. one focal length is larger than zero and the other is smaller than zero.

In a further possible implementation form, the glass lens has a refractive index larger than 1.58 and the imprinted polymer layer has a refractive index smaller than 1.58. Here and in the following the refractive indices and the Abbe numbers are defined for an exemplary reference wavelength of 587,56 nm.

In a further possible implementation form, the glass lens has an Abbe number smaller than 45 and the imprinted polymer layer has an Abbe number larger than 45.

In a further possible implementation form, the absolute value of the ratio between a focal length of the glass lens and a focal length of the imprinted polymer layer is in the range between 1.5 and 2.5. In a further possible implementation form, the first group of optical elements comprises a third set of lenses arranged between the first set of lenses and the aperture stop, wherein the third set of lenses has a positive refractive power.

In a further possible implementation form including the third set of lenses, the glass lens has a refractive index smaller than 1.58 and the imprinted polymer layer has a refractive index larger than 1.58.

In a further possible implementation form including the third set of lenses, the glass lens has an Abbe number larger than 45 and the imprinted polymer layer has an Abbe number smaller than 45.

In a further possible implementation form including the third set of lenses, the absolute value of the ratio between a focal length of the glass lens and a focal length of the imprinted polymer layer is smaller than 1.

In a further possible implementation form, the first set of lenses defining the achromat comprises a first lens and a second lens, wherein the first lens of the first set of lenses has a positive refractive power and the second lens of the first set of lenses has a negative refractive power or wherein the first lens of the first set of lenses has a negative refractive power and the second lens of the first set of lenses has a positive refractive power.

In a further possible implementation form, the first lens of the first set of lenses has a refractive index larger than 1.58 and the second lens of the first set of lenses has a refractive index smaller than 1.58.

In a further possible implementation form, the first lens of the first set of lenses has an Abbe number smaller than 45 and the second lens of the first set of lenses has an Abbe number larger than 45.

In a further possible implementation form, the absolute value of the ratio between the focal length of the first lens of the first set of lenses and the focal length of the second lens of the first set of lenses is in the range between 1.5 and 2.5.

In a further possible implementation form including the first lens and the second lens of the first set of lenses, the first group of optical elements comprises a third set of lenses arranged between the first set of lenses and the aperture stop, wherein the third set of lenses has a positive refractive power.

In a further possible implementation form including the first lens and the second lens of the first set of lenses and the third set of lenses, the first lens of the first set of lenses has a refractive index smaller than 1.58 and the second lens of the first set of lenses has a refractive index larger than 1.58.

In a further possible implementation form including the first lens and the second lens of the first set of lenses and the third set of lenses, the first lens of the first set of lenses has an Abbe number larger than 45 and the second lens of the first set of lenses has an Abbe number smaller than 45.

In a further possible implementation form including the first lens and the second lens of the first set of lenses and the third set of lenses, the absolute value of the ratio between the focal length of the first lens of the first set of lenses and the focal length of the second lens of the first set of lenses is smaller than 1.

In a further possible implementation form not including the third set of lenses, the second set of lenses comprises a first lens, a second lens and a third lens, wherein the third lens of the second set of lenses is arranged closer to the second lens group than the first lens of the second set of lenses and the second lens of the second set of lenses is arranged between the first lens and the third lens of the second set of lenses, wherein the first lens of the second set of lenses has a positive refractive power, the second lens of the second set of lenses has a negative refractive power and the third lens of the second set of lenses has a positive refractive power.

In a further possible implementation form not including the third set of lenses, the third lens of the second set of lenses has a convex aspheric surface on a side facing the second lens group.

In a further possible implementation form not including the third set of lenses, the first lens of the second set of lenses has a refractive index smaller than 1.58, the second lens of the second set of lenses has a refractive index larger than 1.58 and the third lens of the second set of lenses has a refractive index smaller than 1.58. In a further possible implementation form not including the third set of lenses, the first lens of the second set of lenses has an Abbe number larger than 45, the second lens of the second set of lenses has an Abbe number smaller than 45 and the third lens of the second set of lenses has an Abbe number larger than 45.

In a further possible implementation form not including the third set of lenses, the absolute value of the ratio between the focal length of the first lens of the second set of lenses and the focal length of the second lens of the second set of lenses is in the range between 0.5 and 1 and/or the absolute value of the ratio between the focal length of the second lens of the second set of lenses and the focal length of the third lens of the second set of lenses is in the range between 1 and 2.

In a further possible implementation form including the third set of lenses, the second set of lenses comprises a first lens, a second lens and a third lens, wherein the third lens of the second set of lenses is arranged closer to the second lens group than the first lens of the second set of lenses and the second lens of the second set of lenses is arranged between the first lens and the third lens of the second set of lenses, wherein the first lens of the second set of lenses has a negative refractive power, the second lens of the second set of lenses has a negative refractive power and the third lens of the second set of lenses has a positive refractive power.

In a further possible implementation form including the third set of lenses, the third lens of the second set of lenses has a convex aspheric surface on a side facing the second lens group.

In a further possible implementation form including the third set of lenses, the first lens of the second set of lenses has a refractive index larger than 1.58, the second lens of the second set of lenses has a refractive index larger than 1.58 and the third lens of the second set of lenses has a refractive index smaller than 1.58.

In a further possible implementation form including the third set of lenses, the first lens of the second set of lenses has an Abbe number smaller than 45, the second lens of the second set of lenses has an Abbe number smaller than 45 and the third lens of the second set of lenses has an Abbe number larger than 45.

In a further possible implementation form including the third set of lenses, the absolute value of the ratio between the focal length of the first lens of the second set of lenses the and focal length of the second lens of the second set of lenses is in the range between 0.5 and 1 and/or wherein the absolute value of the ratio between the focal length of the second lens of the second set of lenses and the focal length of the third lens of the second set of lenses is in the range between 1 and 5.

According to a further aspect an electronic device, for instance, a smartphone, tablet computer or the like is provided comprising an imaging apparatus according to the first aspect.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 is diagram illustrating an imaging apparatus according to an embodiment;

Fig. 2a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus of figure 1 for different FOVs;

Fig. 2b is a diagram illustrating the lateral color aberration of the imaging apparatus of figure 1 for different wavelengths;

Fig. 2c is a diagram illustrating the distortion of the imaging apparatus of figure 1;

Fig. 2d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus of figure 1 for different FOVs;

Fig. 2e is a table listing exemplary lens parameters of the imaging apparatus of figure 1;

Fig. 2f is a table listing the asphere coefficients of the imaging apparatus of figure 1;

Fig. 3 is diagram illustrating an imaging apparatus according to a further embodiment;

Fig. 4a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus of figure 3 for different FOVs; Fig. 4b is a diagram illustrating the lateral color aberration of the imaging apparatus of figure 3 for different wavelengths;

Fig. 4c is a diagram illustrating the distortion of the imaging apparatus of figure 3;

Fig. 4d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus of figure 3 for different FOVs;

Fig. 4e is a table listing exemplary lens parameters of the imaging apparatus of figure 3;

Fig. 4f is a table listing the asphere coefficients of the imaging apparatus of figure 3;

Fig. 5 is diagram illustrating an imaging apparatus according to a further embodiment;

Fig. 6a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus of figure 5 for different FOVs;

Fig. 6b is a diagram illustrating the lateral color aberration of the imaging apparatus of figure 5 for different wavelengths;

Fig. 6c is a diagram illustrating the distortion of the imaging apparatus of figure 5;

Fig. 6d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus of figure 5 for different FOVs;

Fig. 6e is a table listing exemplary lens parameters of the imaging apparatus of figure 5;

Fig. 6f is a table listing the asphere coefficients of the imaging apparatus of figure 5; and

Figs. 7a and 7b are schematic cross-sectional views of different embodiments of an imprinted achromatic lens of an imaging apparatus according to an embodiment.

In the following identical reference signs refer to identical or at least functionally equivalent features. DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is defined by the appended claims.

For instance, it is to be understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

Figure 1 is diagram illustrating an imaging apparatus 100 according to an embodiment. In an embodiment, the imaging apparatus is provided in the form of a camera for an electronic device, such as a smartphone, a tablet computer or the like.

The imaging apparatus 100 comprises a first group of optical elements 105 on the optical axis. The first group of optical elements 105 includes a plurality of lenses, namely in the embodiment shown in figure 1 a first set or group of lenses 110 and a second set or group of lenses 130, and defines an aperture stop 170 of the imaging apparatus 100. The first set of lenses 110 defines an achromat and is arranged in front of the aperture stop 170, while the second set of lenses 130 is arranged behind the aperture stop 170. As will be appreciated, the spatial relationships between the different optical elements of the imagining apparatus 100, e.g. the first and second set of lenses 110 and 130 being in front of and behind the aperture stop 170, are defined herein relative to the path of the optical light along the optical axis through the imaging apparatus 100 (which in figure 1 travels from left to right through the imaging apparatus 100). In the embodiment shown in figure 1, the first set of lenses 110 defining an achromat comprises a first lens 111a and a second lens 113a. In an embodiment, the first lens 111a has a positive refractive power, while the second lens 113a has a negative refractive power or vice versa. In an embodiment, the first lens 111a and/or the second lens 113a may comprise a plastic material. In an embodiment, the aperture stop 170 may be provided by a diaphragm. In a further embodiment, the aperture stop 170 may be defined "virtually" by the plurality of lenses of the first group of optical elements 105, for instance, the first set of lenses 110 and the second set of lenses 130 shown in figure 1.

Moreover, the imaging apparatus 100 shown in figure 1 comprises an image sensor 160 configured to convert the incident light into one or more electrical signals and arranged on the optical axis such that the second group of optical elements 140 is located between the image sensor 160 and the first group of optical elements 105, i.e. the first set of lenses 110 and the second set of lenses 130. As will be described in more detail further below, the first group of optical elements 105 is movable along the optical axis relative to the second group of optical elements 140 and the imaging sensor 160 for varying a distance d between the first group of optical elements 105 and the second group of optical elements 140. The second group of optical elements 140 comprises a field flattening lens 141 having a concave aspheric front surface on a side opposite to the image sensor 160. In an embodiment, the front surface shape and a rear surface shape of the field flattening lens 141 are configured to correct a field curvature aberration. Advantageously, this allows ensuring a proper field curvature aberration, even if a tilt error of the first group of optical elements 105 is present. In an embodiment, a front and a rear surface shape of the field flattening lens 141 are configured to provide an incident chief ray angle (CRA) in the range of -30° to +30°, i.e. smaller than |30|° relative to a surface normal of the front and rear surface of the field flattening lens 141.

In the embodiment shown in figure 1, the second set of lenses 130 comprises a first lens 131 , a second lens 133 and a third lens 135. In the embodiment shown in figure 1, the first lens 131 of the second set of lenses 130 has a positive refractive power, the second lens 133 of the second set of lenses 130 has a negative refractive power and the third lens 135 of the second set of lenses 130 has a positive refractive power. As can be taken from figure 1, the third lens 135 of the second set of lenses 130 may have a convex aspheric surface on the exit side, i.e. the side facing the second set of lens group 140 and the image sensor 160. In an embodiment, the imaging apparatus 100 comprises a group actuator configured to move the first group of optical elements 105, i.e. the first set of lenses 110 and the second set of lenses 130 together with the aperture stop 170, along the optical axis between an active position and an inactive position for adjusting the distance d between the first group of optical elements 105, including in the embodiment shown in figure 1 the first set of lenses 110 and the second set of lenses 130, and the second group of optical elements 140 and, thus, the image sensor 160, which is located at a fixed distance relative to the second group of optical elements 140. Advantageously, this allows to have a compact imaging system in the inactive position, while providing an improved image quality in the active position. In an embodiment, the distance d between the movable first group of optical elements 105 and the fixed second group of optical elements 140 along the optical axis is such that the ratio of the distance d to an image height lies in the range of 0.4 to 0.8 in the active position of the first group of optical elements 105, i.e. in the use position of the imaging apparatus 100. In an embodiment, the fixed distance between the second group of optical elements 140, including the field flattening lens 141, and the image sensor 160 is smaller than 0.6 mm.

In the embodiment shown in figure 1 , the imaging apparatus 100 further comprises a near- infrared cut-off filter and/or a cover glass 150 arranged between the second group of optical elements 140 and the image sensor 160, i.e. in front of the image sensor 160.

Figures 2a to 2d show diagrams illustrating the optical performance of the imaging apparatus 100 shown in figure 1. More specifically, figure 2a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus 100 of figure 1 for different FOVs, figure 2b is a diagram illustrating the lateral color aberration of the imaging apparatus 100 of figure 1 for different wavelengths, figure 2c is a diagram illustrating the distortion caused by the imaging apparatus 100 of figure 1 and figure 2d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus 100 of figure 1 for different FOVs.

Figure 2e shows a table listing exemplary lens parameters of the imaging apparatus 100 of figure 1 , including the surface type, the radius, the thickness, the refractive index, the Abbe number and the conic constant of each lens, while figure 2f shows a table listing the asphere coefficients of the different lens surfaces of the imaging apparatus 100 of figure 1. Here and in the following the refractive indices and the Abbe numbers are defined for an exemplary reference wavelength of 587,56 nm. As will be appreciated by the person skilled in the art, the surface numbers used in figures 2e and 2f correspond to the order of the lenses of the imaging apparatus of figure 1 from left to right, i.e. along the optical path. For instance, the surfaces 2 and 3 identified in tables 2e and 2f are the front or entry surface and the rear or exit surface of the first lens 111a of the first set of lenses 110, respectively.

The sag height z(h) profile of the asphere surfaces given in tables 2e and 4e can be calculated by the following equation: z(h)=ch ² /{1+[1-(k+1)c ² h ² ] ^1/2 }+Ah ⁴ +Bh ⁶ +Ch ⁸ +Dh ¹⁰ +Eh ¹² +Fh ¹⁴ +Gh ¹⁶ where c is the radius of curvature, h is the transversal distance w.r.t the optical axis, k is the conic constant and A, B, C, D, E, F and G are aspheric coefficients, which are given in table 2f

TTLr _est is the total track length of the optical system 100 in its resting position. It is given in table 2e. It is roughly given by TTL _rest “ TTL - d

In the embodiment shown in figure 1, the first lens 111a of the first set of lenses 110 has a refractive index larger than 1.58 and the second lens 113a of the first set of lenses 110 has a refractive index smaller than 1.58. In an embodiment, the first lens 111a of the first set of lenses 110 has an Abbe number smaller than 45 and the second lens of the first set of lenses has an Abbe number larger than 45. In an embodiment, the absolute value of the ratio between the focal length of the first lens 111a of the first set of lenses 110 and the focal length of the second lens 113a of the first set of lenses 110 is in the range between 1.5 and 2.5.

In the embodiment shown in figure 1, the first lens 131 of the second set of lenses 130 has a refractive index smaller than 1.58, the second lens 133 of the second set of lenses 130 has a refractive index larger than 1.58 and the third lens 135 of the second set of lenses 130 has a refractive index smaller than 1.58. Moreover, the first lens 131 of the second set of lenses 130 has an Abbe number larger than 45, the second lens 133 of the second set of lenses 130 has an Abbe number smaller than 45 and the third lens 135 of the second set of lenses 130 has an Abbe number larger than 45. In an embodiment, the absolute value of the ratio between the focal length of the first lens 131 of the second set of lenses 130 and the focal length of the second lens 133 of the second set of lenses 130 is in the range between 0.5 and 1 and the absolute value of the ratio between the focal length of the second lens 133 of the second set of lenses 130 and the focal length of the third lens 135 of the second set of lenses 130 is in the range between 1 and 2. In an embodiment, the f-number, i.e. F /# of the imaging apparatus 100 is between 2.4 and 1.6. In an embodiment, the plurality of lenses of the first group of optical elements 105 have a clear aperture diameter smaller than 10 mm.

A variant of the imaging apparatus 100 of figure 1 is shown in figure 3. The imaging apparatus 100 shown in figure 3 primarily differs from the imaging apparatus 100 shown in figure 1 in that the first set of lenses 110 defining the achromat comprises a glass lens 111b with an imprinted polymer layer 113b on a front surface thereof. In a further embodiment, the imprinted polymer layer 113b may be coated on the rear surface or on both surfaces of the glass lens 111 b as well. In order to provide the achromatic functionality the imprinted polymer layer 113b has an aspheric surface shape which differs from the shape of the front surface of the glass lens 111b. Because of the different surface shapes the imprinted polymer layer 113b may act as an additional lens with different refractive power compared to the glass lens 111b.

In an embodiment, the glass lens 111b comprises a first material having a first refractive power P _± and a first Abbe number V _± and the imprinted polymer layer 113b comprises a second material having a second optical refractive power P ₂ and a second Abbe number V ₂ , wherein V ₂ - P- _i + V- _i · P ₂ is approximately equal to 0. In an embodiment, the glass lens 111b has a concave aspheric surface on a side facing the imprinted polymer layer 113b. Additionally or alternatively, the imprinted polymer layer 113b has a concave aspheric surface on a side opposite the side facing the glass lens 111b. In an embodiment, the focal length of the glass lens 111b and the focal length of the imprinted polymer layer 113a have different signs, i.e. one focal length is larger than zero and the other is smaller than zero.

Figures 4a to 4d show diagrams illustrating the optical performance of the imaging apparatus 100 shown in figure 3. More specifically, figure 4a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus 100 of figure 3 for different FOVs, figure 4b is a diagram illustrating the lateral color aberration of the imaging apparatus 100 of figure 3 for different wavelengths, figure 4c is a diagram illustrating the distortion caused by the imaging apparatus 100 of figure 3 and figure 4d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus 100 of figure 3 for different FOVs.

Figure 4e shows a table listing exemplary lens parameters of the imaging apparatus 100 of figure 3, including the surface type, the radius, the thickness, the refractive index, the Abbe number and the conic constant of each lens, while figure 4f shows a table listing the asphere coefficients of the different lens surfaces of the imaging apparatus 100 of figure 3.

In the embodiment shown in figure 3, the glass lens 111b may have a refractive index larger than 1.58 and the imprinted polymer layer 113b may have a refractive index smaller than 1.58. Moreover, the glass lens 111b may have an Abbe number smaller than 45, while the imprinted polymer layer 113b may have an Abbe number larger than 45. In an embodiment, the absolute value of the ratio between the focal length of the glass lens 111b and the focal length of the imprinted polymer layer 113b may be in the range between 1.5 and 2.5.

A variant of the imaging apparatus 100 of figure 3 is shown in figure 5. The imaging apparatus 100 shown in figure 5 primarily differs from the imaging apparatus 100 shown in figure 3 and the imaging apparatus 100 shown in figure 1 in that in the imaging apparatus 100 shown in figure 5 the first group of optical elements 105 further comprises a third set of lenses 120, including a lens 121 , wherein the third set of lenses 120 is arranged between the first set of lenses 110 and the aperture stop 170 and has a positive refractive power.

Figures 6a to 6d show diagrams illustrating the optical performance of the imaging apparatus 100 shown in figure 5. More specifically, figure 6a is a diagram illustrating the polychromatic Modulation Transfer Function (MTF) performance of the imaging apparatus 100 of figure 5 for different FOVs, figure 6b is a diagram illustrating the lateral color aberration of the imaging apparatus 100 of figure 5 for different wavelengths, figure 6c is a diagram illustrating the distortion caused by the imaging apparatus 100 of figure 5 and figure 6d is a diagram illustrating the polychromatic Through-Focus-MTF performance of the imaging apparatus 100 of figure 5 for different FOVs.

Figure 6e shows a table listing exemplary lens parameters of the imaging apparatus 100 of figure 5, including the surface type, the radius, the thickness, the refractive index, the Abbe number and the conic constant of each lens, while figure 6f shows a table listing the Qbfs asphere coefficients of the different lens surfaces of the imaging apparatus 100 of figure 5.

The sag height profile of the Q-type asphere surfaces given in table 6e is based on the Qbfs surface description given in in G. W. Forbes, “Manufacturability estimates for optical surfaces”, Opt. Express, Vol. 19, 9923-9941 (2011). The corresponding Qbfs coefficients are given in table 6f. In the embodiment shown in figure 5, the glass lens 111b may have a refractive index smaller than 1.58, while the imprinted polymer layer 113b may have a refractive index larger than 1.58. Moreover, the glass lens 111b may have an Abbe number larger than 45, while the imprinted polymer layer 113b may have an Abbe number smaller than 45. In an embodiment, the absolute value of the ratio between the focal length of the glass lens 111b and the focal length of the imprinted polymer layer 113b may be smaller than 1.

Like the imaging apparatus 100 of figures 1 and 3 the imaging apparatus 100 shown in figure 5 comprises the second set of lenses 130 with the first lens 131, the second lens 133 and the third lens 135. In the embodiment shown in figure 5 the first lens 131 of the second set of lenses 130 has a negative refractive power, the second lens 133 of the second set of lenses 130 has a negative refractive power and the third lens 135 of the second set of lenses 130 has a positive refractive power. In the embodiment shown in figure 5, the third lens 135 of the second set of lenses 130 has a convex aspheric surface on a side facing the second lens group 140.

In the embodiment of figure 5, the first lens 131 of the second set of lenses 130 has a refractive index larger than 1.58, the second lens 133 of the second set of lenses 130 has a refractive index larger than 1.58 and the third lens 135 of the second set of lenses 130 has a refractive index smaller than 1.58. Moreover, the first lens 131 of the second set of lenses 130 has an Abbe number smaller than 45, the second lens 133 of the second set of lenses 130 has an Abbe number smaller than 45 and the third lens 135 of the second set of lenses 130 has an Abbe number larger than 45. In an embodiment, the absolute value of the ratio between the focal length of the first lens 131 of the second set of lenses 130 and the focal length of the second lens 133 of the second set of lenses 130 is in the range between 0.5 and 1 and the absolute value of the ratio between the focal length of the second lens 133 of the second set of lenses 130 and the focal length of the third lens 135 of the second set of lenses 130 is in the range between 1 and 5.

According to a further variant not shown in the figures the imaging apparatus 100 of figure 1 , where the achromat is defined be the first lens 111a and the second lens 113a of the first set of lenses 110, may also comprise the third set of lenses 120 shown in figure 5 having a positive refracting power. In such an embodiment, the first lens 111a of the first set of lenses 110 has a refractive index smaller than 1.58 and the second lens 113a of the first set of lenses 110 has a refractive index larger than 1.58. Moreover, the first lens 111a of the first set of lenses 110 has an Abbe number larger than 45 and the second lens 113a of the first set of lenses 110 has an Abbe number smaller than 45. In an embodiment, the absolute value of the ratio between the focal length of the first lens 111a of the first set of lenses 110 and the focal length of the second lens 113a of the first set of lenses 110 is smaller than 1.

Figures 7a and 7b show two embodiments for the first set of lenses 110 with the glass lens 111b and the imprinted polymer layer 113b for defining an achromat. In the embodiment shown in figure 7b the imprinted polymer layer 113b comprises a mechanical interlock 701 for aligning the glass lens 111b and/or the first set of lenses 110. Advantageously, this allows aligning the glass lens 111b and/or the first set of lenses 110 with respect to one of the further lenses following along the optical path, such as the lens 131, and/or the image sensor 160. The polymer imprinted layer 113b may be provided by means of a polymer imprinting process that allows to form additional structures, such as the interlock 701, with very high precision (given by the master) and in precise alignment with the imprinted lens surface.

The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments of the invention (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments (unit = step).

In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described apparatus embodiment is merely exemplary. For example, the unit division is merely logical function division and may be other division in actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments. In addition, functional units in the embodiments of the invention may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit.