EP4689736A1

EP4689736A1 - Hybrid lens system

Info

Publication number: EP4689736A1
Application number: EP24711555.3A
Authority: EP
Inventors: Christin Weichelt; Markus Kamm; Alexander GATTO
Original assignee: Sony Europe BV; Sony Group Corp
Current assignee: Sony Europe BV; Sony Group Corp
Priority date: 2023-03-27
Filing date: 2024-03-15
Publication date: 2026-02-11
Also published as: CN120958351A; WO2024200053A1

Abstract

A hybrid lens system is provided, comprising one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; and a birefringent device arranged to intercept light passing through the metasurface; wherein the birefringent device is configured to extend a wavelength range of the hybrid lens system. Furthermore, a hybrid lens system is provided comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; wherein a longitudinal chromatic aberration and a lateral chromatic aberration of the hybrid lens system are configured to a predetermined amount to perform wavelength averaging to extend a wavelength range of the hybrid lens system.

Description

HYBRID LENS SYSTEM

BACKGROUND

Field of Disclosure:

This disclosure relates to a hybrid lens system.

Description of Related Art:

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure.

Optical lens systems are used in a wide range of devices. For example, optical lens systems are used in image capture devices, image magnification devices (such as telescopes, binoculars or microscopes), image projection devices and the like.

The optical performance of the lens system often limits the performance of a device in which that lens system is used. As such, there is a need for lens systems with improved optical performance.

Furthermore, as optical lens systems are used in a wide range of devices, there is a need to provide optical lens systems with a compact form factor. This makes it easier for the lens systems to be incorporated in a wider range of devices.

The need for a compact form factor of the optical lens system is exacerbated for certain types of devices, where the overall form factor of the device is limited. This includes certain types of devices such as portable electronic devices (e.g. a smartphone device) or medical imaging devices (e.g. an endoscopic imaging device).

As such, there is a need for optical lens systems with compact form factor and improved optical performance.

It is an aim of the present disclosure to address these issues.

SUMMARY

Aspects of the present disclosure are defined by the appended claims.

Further respective aspects and features of the present disclosure are defined in the appended claims.

In accordance with embodiments of the disclosure, an optical lens system with compact form factor and improved optical performance can be provided.

The present disclosure is not particularly limited to this advantageous technical effect. Other advantageous technical effects will become apparent to the skilled person when reading the disclosure. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Figure 1 illustrates an apparatus in accordance with embodiments of the disclosure;

Figure 2 illustrates a hybrid lens system in accordance with embodiments of the disclosure;

Figure 3 illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 4 illustrates a hybrid lens system in accordance with embodiments of the disclosure;

Figure 5 illustrates a hybrid lens system in accordance with embodiments of the disclosure;

Figure 6 illustrates a birefringent device in accordance with embodiments of the disclosure;

Figure 7A illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 7B illustrates a birefringent device in accordance with embodiments of the disclosure;

Figure 8 illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 9 illustrates example images simulated for a camera system in accordance with embodiments of the disclosure;

Figure 10 illustrates an example multi-lens array in accordance with embodiments of the disclosure;

Figure 11 illustrates a hybrid lens system in accordance with embodiments of the disclosure;

Figure 12A illustrates an example definition of a defocus value in accordance with embodiments of the disclosure;

Figure 12B illustrates an example back focus for a hybrid lens system in accordance with embodiments of the disclosure;

Figure 13 illustrates Module Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure; Figure 14A illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 14B illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 15A illustrates an example of a hybrid lens system in accordance with embodiments of the disclosure;

Figure 15B illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure; and

Figure 16 illustrates example images simulated for a camera system in accordance with embodiments of the disclosure.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

Referring to Figure 1 , an apparatus 1000 according to embodiments of the disclosure is shown. Typically, an apparatus 1000 according to embodiments of the disclosure is a computer device such as a personal computer or a terminal connected to a server. Indeed, in embodiments, the apparatus may also be a server. The apparatus 1000 is controlled using a microprocessor or other processing circuitry 1002. In some examples, the apparatus 1000 may be a portable computing device such as a mobile phone, laptop computer or tabletcomputing device. In other examples, the apparatus 1000 may be an image capture device (such as a digital camera) or the like.

The processing circuitry 1002 may be a microprocessor carrying out computer instructions or may be an Application Specific Integrated Circuit. The computer instructions are stored on storage medium 1004 which maybe a magnetically readable medium, optically readable medium or solid-state type circuitry. The storage medium 1004 may be integrated into the apparatus 1000 or may be separate to the apparatus 1000 and connected thereto using either a wired or a wireless connection. The computer instructions may be embodied as computer software that contains computer readable code which, when loaded onto the processor circuitry 1002, configures the processor circuitry 1002 to perform a method according to embodiments of the disclosure.

Additionally, an optional user input device 1006 is shown connected to the processing circuitry 1002. The user input device 1006 may be a touch screen or may be a mouse or stylist type input device. The user input device 1006 may also be a keyboard or any combination of these devices. In some examples, the user input device 1006 may be a device configured to receive audio input instructions from a user.

A network connection 1008 may optionally be coupled to the processor circuitry 1002. The network connection 1008 may be a connection to a Local Area Network or a Wide Area Network such as the Internet or a Virtual Private Network or the like. The network connection 1008 may be connected to a server allowing the processor circuitry 1002 to communicate with another apparatus in order to obtain or provide relevant data. The network connection 1002 may be behind a firewall or some other form of network security. The network connection can comprise a wired or wireless connection.

Additionally, shown coupled to the processing circuitry 1002, is a display device 1010. The display device 1010, although shown integrated into the apparatus 1000, may additionally be separate to the apparatus 1000 and may be a monitor or some kind of device allowing the user to visualise the operation of the system. In addition, the display device 1010 may be a printer, projector or some other device allowing relevant information generated by the apparatus 1000 to be viewed by the user or by a third party. Alternatively, the display device may include a head-mountable display, which can be used for viewing a virtual or augmented reality environment.

The apparatus 1000 of the present disclosure may also include a lens system (not shown). The lens system may, for example, be included as part of a camera or other image capture device. The camera system may include an image sensor. Alternatively, the image sensor may be included within the apparatus 1000.

As explained, in examples, the apparatus 1000 may be a portable electronic device such as a mobile phone (e.g. a “smart phone”) or the like. The form factor of a portable electronic device is often restricted, in order that the device retains its portability. Therefore, a lens system included in a portable electronic device (such as a lens system included as part of a camera in a mobile phone) must also have a small form factor. This makes it easier for the lens system to be incorporated into the portable electronic device.

A compact form factor may also be required for lens systems included in other types of devices (i.e. devices other than a portable electronic device). For example, lens systems included as part of robotic devices may be required to have a small or compact form factor. Likewise, the form factor of lens systems included as part of medical imaging devices (such as endoscopes) may also be restricted. In these cases, the compact form factor of the lens system can make it easier for the device to perform a desired function or operation.

It will be appreciated that the optical performance of a lens system used in a device may limit the performance of the device. For example, in an image capture device it is required that the lens system has good optical performance in order that high quality images (e.g. images free from optical aberrations or images with a wide Field of View) can be obtained.

However, it can be difficult to provide good optical performance (or, more generally, a desired level of optical performance) in a lens system with compact size. Accordingly, as explained also in the background, there is a need for optical lens systems with a compact form factor and improved optical performance. <Hybrid Lens System>

Metasurfaces consist of structures with feature sizes in the range of the wavelength of light or even lower. Accordingly, metasurfaces modify the phase of electromagnetic waves and can be designed to exhibit a certain lateral phase profile. For example, metasurfaces can convert planar waves into spherical waves and thereby act like a focusing lens. Flat and compact lenses can be realized by metasurface technology. This makes metasurfaces useful for constructing lens systems with compact form factor.

However, the phase profile induced by metasurfaces has a strong dependency on both wavelength and angle of incidence. Therefore, it is difficult to realize achromatic lenses with wide field of view (FoV).

Thus, the inventors have realized that a metasurface can be combined with one or more refractive lens elements in order to realize a hybrid lens system with compact form factor and improved optical performance (e.g. wide FoV).

As such, in accordance with embodiments of the disclosure, a hybrid lens system is provided.

Consider, now, Figure 2 of the present disclosure. Figure 2 illustrates a hybrid lens system in accordance with embodiments of the disclosure. In embodiments of the disclosure, a hybrid lens system (such as the hybrid lens system shown in Figure 2 of the present disclosure) is a lens system comprising one or more refractive lenses and at least one metasurface.

The hybrid lens system 2000 illustrated in Figure 2 of the present disclosure comprises a number of optical elements.

Specifically, the hybrid lens system 2000 comprises a metasurface 2012 and a plurality of refractive lenses 2002, 2004, 2006, and 2008. The metasurface 2012 and the plurality of refractive lenses 2002, 2004, 2006 and 2008 are arranged along an optical axis 2010 of the hybrid lens system 2000.

The number and type of refractive lenses present in the hybrid system is not particularly limited.

That is, while four refractive lenses 2002, 2004, 2006 and 2008 are shown in the example of Figure 2, the present disclosure is not particularly limited in this regard. The number of refractive lenses provided in the hybrid lens system may be much greater than the four refractive lenses shown in this example. Alternatively, only a single refractive lens may be provided in the hybrid lens system. The number of refractive lenses present in the hybrid lens system will depend upon the specific situation to which the hybrid lens system is applied. For example, the number of refractive lenses present in the hybrid lens system may vary depending on a type of image capture device in which the hybrid lens system is used. Furthermore, the type of refractive lens used in the hybrid lens may vary. As an example, a number of lenses used in the hybrid lens system may be aspheric lenses. However, in some examples, a number of the lenses used in the hybrid lens system may be spherical lenses. A mixture of both spherical and aspherical lenses may be provided in examples of the disclosure. Thus, the types of lenses used in the hybrid lens system depends upon the situation to which the embodiments of the disclosure are applied.

In general, the number of classical refractive lens elements required for a lens system can be reduced by including metasurfaces to the lens system and the overall system length can therefore be reduced. This is because flat and compact lenses can be realized by metasurface technology.

In the example of Figure 2 of the present disclosure, the metasurface is provided as a metalens 2012. That is, the metasurface 2012 is provided as a separate lens element within the hybrid lens system.

Consider, now, an example situation where an image capture device (such as a camera) is included in a portable electronic device (such as a mobile telephone). A typical lens of a mobile phone camera may have 5 lens elements. By applying a metasurface, the number of lens elements may be reduced to 4 lens elements and/or the performance could be improved. Furthermore, in some examples, the metasurface may be applied to one or more of the surfaces of the refractive lens elements, thus further reducing the overall system length. This would reduce the number of lens elements to 3. This makes the hybrid lens system even more compact.

As such, the hybrid lens system (such as hybrid lens system 2000) is more compact than a lens system with only refractive lens elements (i.e. a lens system without a metasurface).

Furthermore, the hybrid lens system may have improved optical performance compared to a lens system without a metasurface (such as a lens system with 5 refractive lenses) or a lens system with only metasurfaces (and no classical lens elements).

Accordingly, embodiments of the present disclosure provide a hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system.

Consider, now, Figure 3 of the present disclosure. Figure 3 of the present disclosure illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure.

Modulation Transfer Functions provide a measurement of the optical performance, or optical performance potential, of a lens system. There are many different factors that can affect the optical performance of a lens system. For example, diffraction and optical aberrations can affect the optical performance of a lens system. However, Modulation Transfer Functions provide a measure of lens performance which thus describes the optical performance of the lens system.

Specifically, Modulation Transfer Functions describe how a lens reproduces contrast as a function of spatial frequency (i.e. resolution). The y-axis of the Modulation Transfer Function curve (or chart) plots the contrast transfer ability through the lens. Thus, the maximum value on the y-axis is “1” (which corresponds to full transfer - i.e. transfer without loss of contrast - through the lens system). The x-axis of the Modulation Transfer Function curve corresponds to different spatial frequencies. In the example of Figure 3, the spatial frequency (on the x-axis) has units of cycles/mm.

In the example of Figure 3, the Modulation Transfer Functions for different field point positions for a hybrid lens system for 550nm with 4 refractive lenses and a metalens are shown. This is for a FoV of 84 degrees, an F-number of 1.7, with a focal length of 2.51mm, and an overall system length of 4.33mm. The hybrid lens system of Figure 3 is an example of a hybrid lens system as described with reference to Figure 2 of the present disclosure. The Modulation Transfer Function curves shown with solid lines are measured in tangential directions (T) across the field points.

Furthermore, Modulation Transfer Function curves of the same lens system measured in radial direction (R) are shown (dashed lines).

While the example of Figure 3 has been provided for a specific configuration of a hybrid lens system, it will be appreciated that the present disclosure is not particularly limited in this regard. In general, a hybrid lens system (with one or more refractive lens elements and a metasurface) will have improved optical performance compared to a lens system with only refractive lens elements (i.e. a lens system without a metasurface).

Thus, the hybrid lens system (such as hybrid lens system 2000) is both more compact and has improved optical performance compared to a lens system with only refractive elements (i.e. a lens system without a metasurface).

As explained with reference to Figures 2 and 3 of the present disclosure, a hybrid lens system (including one or more refractive lenses and a metasurface) has a more compact form factor and improved optical performance compared to a system with only refractive lens elements (i.e. a lens system without a metasurface). However, metasurfaces have a strong dependency on wavelength and angle of incidence. As such, a hybrid lens system which provides a combination of refractive lens elements with metasurfaces (such as that described with reference to Figures 2 and 3 of the present disclosure) still has significant chromatic aberration. Significant chromatic aberration may make the hybrid lens system less suitable for certain applications (e.g. for applications in image capture devices (such as cameras for mobile telephones)).

As such, a hybrid lens system with reduced chromatic aberration is desired. <Birefringent Device>

Accordingly, in embodiments of the disclosure, a hybrid lens system which displays reduced chromatic aberration is provided.

Consider, now, Figure 4 of the present disclosure. Figure 4 illustrates a hybrid lens system in accordance with embodiments of the disclosure.

The hybrid lens system is a lens system comprising one or more refractive lenses and at least one metasurface. That is, similar to the hybrid lens system described with reference to Figure 2 of the present disclosure, the hybrid lens system 3000 of Figure 4 of the present disclosure comprises at least one refractive lens 3002 and a metasurface 3012 arranged on an optical axis 3010 of the hybrid lens system 3000.

The metasurface 3012 may be provided as a metalens (i.e. an independent lens element of the hybrid lens system 3000). However, in other examples, the metasurface may be applied to a surface of the one or more refractive lenses 3002. This can further reduce the number of lens elements provided in the system and thus makes it easier to provide a lens system with compact form factor.

Furthermore, while only a single refractive lens 3002 is shown in this example, it will be appreciated that the present disclosure is not particularly limited in this regard. That is, at least one refractive lens 3002 is provided in the hybrid lens system 3000. There may be many more than one refractive lens provided in accordance with embodiments of the disclosure, depending on the situation to which embodiments of the disclosure are applied.

In addition, a birefringent device (such as a birefringent mask) is provided along the optical axis in order to intercept light passing through the metasurface 3012. In some examples, the birefringent device is arranged in proximity to (e.g. at) the aperture stop of the lens system. More generally, the birefringent device is arranged in proximity to the aperture stop or to its apparent images, the entrance or exit pupil of the hybrid lens system. In some examples, the birefringent mask may be included as a separate element in the hybrid lens system. However, in other examples, the birefringent mask may be incorporated into the structures of the metasurface as metaatoms. This is because meta structures of certain shape can be designed to exhibit polarization dependent light manipulation. Incorporating the birefringent mask into the metasurface may further reduce the form factor of the lens system.

In this regard, Figure 5 of the present disclosure illustrates a hybrid lens system in accordance with embodiments of the disclosure.

The hybrid lens system illustrated in Figure 5 provides an example of a hybrid lens system 3000 in accordance with embodiments of the disclosure. In the example configuration of Figure 5, the hybrid lens system 3000 comprises four refractive lens elements 3002, 3004, 3006 and 3008. In this specific example, the refractive lens elements are aspherical lenses. However, in other examples, these lenses may be spherical lenses. Alternatively, a mixture of both spherical and aspherical lenses may be provided in accordance with embodiments of the disclosure.

Furthermore, in the example configuration of Figure 5, the hybrid lens system 3000 further comprises a metasurface 3012. In this specific example, the metasurface is provided as a separate lens element. However, in other examples, the metasurface may be applied to the surface of one or more of the refractive lenses 3002, 3004, 3006 and 3008.

The refractive lenses 3002, 3004, 3006 and 3008 and the metasurface 3012 of the hybrid lens system 3000 are arranged along the optical axis 3010 of the hybrid lens system 3000.

In addition, a birefringent device (such as a birefringent mask) is provided along the optical axis in order to intercept light passing through the metasurface 3012. In this example, the metasurface lens is provided as a separate lens element within the hybrid lens system. However, the present disclosure is not particularly limited in this regard. In other examples, the birefringent mask may be incorporated into the structure of the metasurface (not shown in this example). In other words, meta structures of certain shape can be designed to exhibit polarization dependent light manipulation, in order that the metasurface exhibits the functionality the birefringent mask. Thus, in examples, the birefringent device is provided by the metasurface.

Thus, the example hybrid lens system of Figure 5 of the present disclosure provides an example of a hybrid lens system comprising 4 aspheric lenses, a metasurface (such as a metalens) and a birefringent device. Such a hybrid lens system may be used to realize an imaging lens with wide FoV (for example, lenses to be used in a portable electronic device (such as a mobile phone)).

In comparison to the hybrid lens system 2000 described with reference to Figure 2 of the present disclosure, the hybrid lens system 3000 of the present disclosure further comprises a birefringent device 3014. The birefringent device 3014 is also arranged on the optical axis of the hybrid lens system 3000. In fact, as shown in the example of Figure 4, the birefringent device 3014 is arranged to intercept light passing through the metasurface 3012.

A birefringent device is a device which affects different polarization states differently and position dependently. In some examples, the birefringent device may be a birefringent mask. In such examples, a birefringent device such as that described in US10684402B2 may be used in accordance with embodiments of the disclosure.

However, the birefringent device of US10684402B2 is used for the purpose of extending the depth of field of an imaging system. In contrast, the inventors have realised that providing a hybrid lens system as such as that is illustrated with reference to Figure 4 of the present disclosure, where a birefringent device is arranged to intercept light passing through the metasurface, reduces the chromatic aberration of the hybrid lens system and thus improves the optical performance of the hybrid lens system. Thus, in the present disclosure, the birefringent device has a function to extend the wavelength range of the hybrid lens system; this will be described in more detail below.

Accordingly, embodiments of the disclosure provide a hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; and a birefringent device arranged to intercept light passing through the metasurface; wherein the birefringent device is configured to extend the wavelength range of the hybrid lens system.

A more detailed discussion of the structure of the birefringent device will be provided.

Consider, now Figure 6 of the present disclosure. Figure 6 illustrates a birefringent device 3014 in accordance with embodiments of the disclosure. In this example, the birefringent device is a birefringent mask.

The birefringent mask 3014 illustrated in Figure 6 of the present disclosure comprises a ring-shaped structure of a birefringent layer. The orientation of the extraordinary (fast) axis of the birefringent layer alternates by 90 degrees between adjacent rings. The number and size of the rings, as well as the birefringence (d * An) of the birefringent mask can be optimized with the target to have similar Modulation Transfer Functions for all wavelengths in an extended wavelength range. Here, d is the thickness of the birefringent layer and An is the difference between the extraordinary and ordinary refractive index of the birefringent material of the birefringent layer.

Thus, by constraining the design of the birefringent mask (such as the number and sizes of the rings) - or by constraining the design of the structure of the metasurface - the hybrid lens system can be designed in order that all wavelengths in a target wavelength range have a similar Modulation Transfer Function (and thus similar optical performance). This reduces chromatic aberration of the hybrid lens system.

It will be appreciated that the present disclosure is not particularly limited to the specific example of the birefringent mask illustrated in Figure 6 of the present disclosure. That is, the birefringent mask 3014 illustrated in Figure 6 of the present disclosure is one example of a birefringent device which could be used in accordance with embodiments of the disclosure.

A hybrid lens system 3000 with one or more refractive lenses, a metasurface and a birefringent mask arranged on an optical axis of the hybrid lens system provides a lens system with compact form factor and improved optical performance (in particular, reduced chromatic aberration).

Consider, now, Figure 7A of the present disclosure. Figure 7A illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure. In particular, Figure 7A illustrates Modulation Transfer Function curves of a hybrid lens system such as hybrid lens system 3000 as described with reference to Figures 4 and 5 of the present disclosure.

As previously explained, Modulation Transfer Functions describe how a lens (or lens system) reproduces contrast as a function of spatial frequency (i.e. resolution). The y-axis of the Modulation Transfer Function curve (or chart) plots the contrast transfer ability through the lens. Thus, the maximum value on the y-axis is “1” (which corresponds to full transfer - i.e. transfer without loss of contrast - through the lens system). The x-axis of the Modulation Transfer Function curve corresponds to different spatial frequencies. In the example of Figure 7A, the spatial frequency (on the x-axis) has units of Ip/mm.

The Modulation Transfer Functions illustrated in Figure 7A of the present disclosure are provided for different wavelengths within the targeted wavelength range (here, 605nm to 635nm in 5 nm steps). These are provided for a hybrid lens system including a birefringent device (such as hybrid lens system 3000) and for a hybrid lens system without a birefringent mask. The modulation transfer functions for a hybrid lens system with a birefringent device are shown with solid lines in the example of Figure 7A. The modulation transfer functions for a hybrid lens system without a birefringent mask are shown with dashed lines in the example of Figure 7A.

Consider, now, Figure 7B of the present disclosure. Figure 7B illustrates a birefringent device in accordance with embodiments of the disclosure. Specifically, Figure 7B illustrates an example of a birefringent device which can be used in the hybrid lens system described with reference to Figure 7A of the present disclosure. The birefringent device of Figure 7B is a birefringent mask. The birefringent mask is a 3-ring mask with normalized radii of the rings of 0.682, 0.938 and 1.00 and a retardation (phase shift) of 2.271 rad = 0.361A.

However, while Figure 7B of the present disclosure illustrates one example configuration of a birefringent device, the present disclosure is not particularly limited in this regard. In some examples, a birefringent mask other than that described with reference to Figure 7B may be used in accordance with embodiments of the disclosure. In other examples, the structure of the metasurface may be constrained in order that the birefringent device is provided as part of the metasurface of the hybrid lens system.

If the Modulation Transfer Function curve is sufficiently larger than zero, the spatial information is not lost and can be reconstructed. Furthermore, it is important that the shapes of the Modulation Transfer Function curves are similar (i.e. within a predetermined range) for all wavelength within the specified wavelength range.

Consider, now, Figure 8 of the present disclosure. Figure 8 illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure.

Specifically, Figure 8 illustrates polychromatic modulation transfer functions for different field point positions for a 4 lens + Metalens system with birefringent mask (an example implementation of a hybrid lens system 3000 of the present disclosure), with the design wavelength of 620nm for wavelengths from 595 to 645nm in 2.5 nm steps (equal weight for each wavelength, wavelength range of 50nm).

The example Modulation Transfer Functions of Figures 7 and 8 of the present disclosure demonstrate that an extended wavelength range of 50-60nm can be achieved using a hybrid lens system including a birefringent device (such as hybrid lens system 3000) compared to approximately 10nm for a hybrid lens system without a birefringent mask.

It will be appreciated that the Modulation Transfer Functions illustrated in Figures 7A and 8 of the present disclosure are provided for a specific example implementation of a hybrid lens system. However, more generally, it will be appreciated that a hybrid lens system including a birefringent mask reduces the chromatic aberration of the hybrid lens system and thus improves the optical performance of the hybrid lens system. This means that the hybrid lens system can be used over an extended wavelength range. Thus, a lens system such as hybrid lens system 3000 is both more compact and has improved optical performance compared to a lens system with only refractive elements.

A hybrid lens system with a birefringent device (such as hybrid lens system 3000) may be used in a number of different example situations. For example, the hybrid lens systems with a birefringent device may be used in an image capture device. An example of an image capture device is a camera such as may be used in a portable electronic device (including, for example a mobile phone (e.g. a “smart phone”)).

An image capture device includes an image sensor configured to acquire image data. An example image sensor includes a CCD sensor or a CMOS sensor. An image resolution of an image sensor is not particularly limited. As an example, the image sensor may have 4K or 8K resolution. Furthermore, the image sensor may acquire image data comprising still image data (e.g. pictures) or moving image data (e.g. videos).

In examples, when a hybrid lens system is used with an image sensor in an image capture device it should be ensured (by the optimization target) that the modulation transfer curves do not hit the zero-line up to the Nyquist frequency of the camera system or to the maximum spatial frequency of the range of interest. The Nyquist frequency is one-half of the spatial sampling frequency of the image sensor. As an example, if the pixel size of the image sensor is 1.4 pm, then the Nyquist frequency is equal to 1 / (2 * 1.4 pm) = 357 mm-1.

If the MTF curve is sufficiently larger than zero, the spatial information is not lost and can be reconstructed by an inverse filter (though noise may be enhanced by the inverse filter). It is important that the shapes of the MTF curves are similar for all wavelength within the specified wavelength range to be able to apply a common wavelength invariant filter to the entire image data.

Consider, now, the example of Figure 9 of the present disclosure. Figure 9 illustrates example images simulated for a camera system in accordance with embodiments of the disclosure.

That is, the images of Figure 9 of the present disclosure are simulated for a camera system comprising an image sensor and a hybrid lens system comprising one or more refractive lenses, a metasurface and a birefringent mask (e.g. a hybrid lens system such as hybrid lens system 3000) - such as the lens design illustrated in Figure 5 of the present disclosure.

As explained, the use of the birefringent mask in the hybrid lens system enables a reduction of chromatic aberration - which thus enables the wavelength range of the hybrid lens system to be extended. However, use of a birefringent mask does cause a drop of the polychromatic modulation transfer curves at the higher spatial frequency. This can be seen in the example modulation transfer curves of Figures 7A and 8 of the present disclosure.

As such, in some examples, an inverse filter may be applied in order to reconstruct the image data for higher spatial frequencies (provided that the modulation transfer curve is sufficiently larger than zero).

An inverse filter may be applied by image processing circuitry included in the image capture device, for example. Alternatively, an inverse filter may be applied by an image processing device external to the image capture device. In examples, the image processing device external to the image capture device may be a server, which applies post-processing to image data captured by the image capture device.

Returning to Figure 9, the first panel 9000 of Figure 9 shows (simulated) images captured by an image capture system including a hybrid lens system 3000 (a hybrid lens system with a birefringent device) where an inverse filter has not been applied. As there is a drop of the modulation transfer curve at higher spatial frequency, it can be seen that some higher resolution information has been lost in the image data. On the other hand, the second panel 9002 of Figure 9 shows (simulated) images captured by the same image capture system, where an inverse filter (of filter size 11x11 in this specific example) has been applied. The use of the inverse filter enables image data for higher spatial frequencies to be reconstructed. Thus, the images in panel 9002 of Figure 9 are sharper than the images in panel 9000 of Figure 9 (where the inverse filter has not been applied).

As explained, it is important that the shapes of the Modulation Transfer Function curves are similar for all wavelength within the specified wavelength range to be able to apply a common wavelength invariant filter to the entire image data. <Multi-lens Array>

The use of a hybrid lens system with a birefringent device (such as the hybrid lens system 3000) enables a lens system with compact form factor and improved optical performance (in particular, reduced chromatic aberration) to be provided.

However, in certain situations, a further extension of the wavelength range of a lens system may be required. As an example, certain broadband imaging applications may require an extended wavelength range to be provided.

As such, in examples (such as for certain broadband imaging applications) the inventors have realized that a multi-lens array structure can be provided, with each lens of the multi-lens array being optimized for an assigned wavelength sub-range. This may further extend the wavelength range of the hybrid lens system.

Consider, now, Figure 10 of the present disclosure. Figure 10 illustrates an example multi-lens array in accordance with embodiments of the disclosure.

In this example, four hybrid lens systems are provided in a 2x2 array 1100.

The configuration of the 2x2 multi-lens array is shown in panel 1100A of Figure 10. Here, a first hybrid lens system is provided in the top-left corner of the multi-lens array. This first hybrid lens system is optimized for Red light (e.g. a wavelength of 590-640nm). A second hybrid lens system is provided in the top-right corner of the multi-lens array. This second hybrid lens system is optimized for Green light (e.g. a wavelength of 510-560nm). A third hybrid lens system is provided in the bottom-left corner of the multi-lens array. This third hybrid lens system is optimized for Blue light (e.g. 430-480nm). Finally, a fourth hybrid lens system is provided in the bottom-right corner of the multi-lens array. This fourth hybrid lens system is optimized for IR light (e.g. 910nm- 960nm).

A side profile of the multi-lens array is shown in panel 1100B of Figure 10. Here, the top-right and bottom-right lens systems of the multi-lens array can be seen (i.e. the hybrid lens systems optimized for Green and IR light respectively (located in the top-right and bottom-right positions of the 2x2 array)).

The multi-lens array structure of Figure 10 is therefore an example of a multi-lens array structure in accordance with embodiments of the disclosure, which is optimized for VIS-NIR imaging.

The multi-lens array of Figure 10 may then be used with an image capture device (e.g. an image sensor in a camera) in order to capture a broadband image. That is, when used with an image capture device, the captured image is divided into an array of sub-images, where each sub-images represents an image captured in the respective wavelength sub-range (i.e. Red, Greed, Blue and IR in this example). The captured sub-images will have a lateral disparity to each other, caused by the baseline between the lenses. The disparity depends on the distance between the captured objects. The images captured using the multi-lens array structure may then be corrected by digital post-processing (using any suitable image processing techniques) before overlaying all subimages into one broadband image.

Accordingly, the multi-lens array structure of Figure 10 may be used in order to enable broadband images to be obtained using a lens system with compact form factor and improved optical performance.

It will be appreciated that while the example multi-lens array of Figure 10 is shown for specific wavelength bands of Red, Green, Blue and IR light, the present disclosure is not particularly limited in this regard. More generally, each lens system of the multi-lens array may be optimized for any specific wavelength sub-range as required (e.g. depending on the situation (such as the type of object to be imaged)).

Furthermore, while the multi-lens array of Figure 10 is a 2x2 array structure, the present disclosure is not particularly limited in this regard. That is, the 2x2 array structure is provided as one example of a multi-lens array in accordance with embodiments of the disclosure. More generally, a multi-lens array structure may have any suitable size depending on the number of wavelength sub-ranges required. In examples, the multi-lens array may be arranged in an NxN array structure (where N is a positive integer). However, in other examples, the multi-lens array may be arranged in an NxM array structure (with both N and M are positive integers).

Furthermore, in examples, a single image sensor may be used in order to capture images from the multi-lens array. Alternatively, a plurality of image sensors may be provided, with each image sensor being configured to capture an image from one or more of the hybrid lenses of the multi-lens array. In some examples, a dedicated image sensor may be provided for each hybrid lens system of the multi-lens array (i.e. a dedicated image sensor for each of the Red, Green, Blue and IR optimized hybrid lenses of the example of Figure 10).

As such, a hybrid lens system with reduced chromatic aberration, and thus improved wavelength range, is desired.

In some examples of the disclosure, a birefringent device can be included in the hybrid lens system (either as a separate element of the hybrid lens system or as part of the metasurface). This has been explained with reference to Figures 4 to 10 of the present disclosure.

However, in other examples, the wavelength range of a hybrid lens system (comprising one or more refractive lenses and a metasurface) can be extended by utilizing an extended depth of field effect. That is, the inventors have realized that by utilizing an extended depth of field effect (and thus averaging over wavelength), the wavelength range of the hybrid lens system can be extended.

In an optical system which exhibits certain chromatic aberrations (such as longitudinal chromatic aberrations) different colour components (i.e. different wavelengths of light) are brought to focus in different planes in the image space. Taking Red, Green and Blue colours as an example, each of the Red, Green and Blue colour components of an image may be brought to focus at different planes in the image space. This means that there will be a variation in the sharpness of each colour component for a given object distance. Put another way, the in-focus position will vary across the different colour components. This variation in the sharpness across different colour components can be exploited in order to provide an extended depth of field for an image (e.g. by sharpness transfer between colour components of an image).

As explained, the inventors have realized that this extended depth of field effect can be utilized in order to provide an extension of the wavelength range of a hybrid lens system.

As such, in embodiments of the disclosure, a hybrid lens system which utilizes the extended depth of field effect in order to provide an extension of the wavelength range of a hybrid lens system is provided.

Consider, now, Figure 11 of the present disclosure. Figure 11 of the present disclosure illustrates a hybrid lens system in accordance with embodiments of the disclosure.

The hybrid lens system 4000 is a lens system comprising one or more refractive lenses 4002 and at least one metasurface 4012. That is, similar to the hybrid lens system 2000 described with reference to Figure 2 of the present disclosure, the hybrid lens system 4000 of Figure 11 of the present disclosure comprises at least one refractive lens 4002 and a metasurface 4012 arranged on an optical axis 4010 of the hybrid lens system 4000.

The metasurface 4012 may be provided as a metalens (i.e. an independent lens element of the hybrid lens system 4000). However, in other examples, the metasurface may be applied to a surface of the one or more refractive lenses 4002.

Furthermore, while only a single refractive lens 4002 is shown in this example, it will be appreciated that the present disclosure is not particularly limited in this regard. That is, at least one refractive lens 4002 is provided in the hybrid lens system 4000. There may be many more than one refractive lens provided in accordance with embodiments of the disclosure, depending on the situation to which embodiments of the disclosure are applied.

In some examples, the one or more refractive lenses may be spherical lenses. In other examples, at least one of the one or more refractive lenses may be aspheric lenses.

In order that the extended depth of field effect of the hybrid lens system can be exploited, the chromatic aberration of the hybrid lens system 4000 should be constrained. In particular, the longitudinal and lateral colour aberration of the hybrid lens system with the metasurface should be constrained during optimization of the lens system.

The chromatic back focal shift of an optical system can be used in order to characterize the variation of focal plane of the optical system with wavelength. Indeed, to constrain the longitudinal chromatic aberration in the optimization, the back focal shift is constrained in such a way that the focus shift over the targeted wavelength range is limited to a range which corresponds to the defocus range of +/- 1 .7-2A (the resulting absolute shift in pm is different depending on the lens system). Furthermore, in order that the lateral chromatic aberration is constrained, the Modulation Transfer Function curves in the sagittal and tangential direction of the field points should be made similar to each other (i.e. within a certain range of each other).

Consider, now, Figure 12A of the present disclosure. Figure 12A of the present disclosure provides an example definition of a defocus value in accordance with embodiments of the disclosure. In this equation (illustrated in Figure 12A) is the wavefront defocus in rad, A is the wavelength, NA is the Numerical Aperture of the lens system and Az is the difference between the infocus and defocus image position.

In examples, the defocus variation of the chromatic back focal shift should be within a limit derived from the extended depth of field and slightly extended (e.g. around +/- 1.7-2A) over the targeted wavelength range of the hybrid lens system 4000. That is, in examples, the defocus variation of the chromatic back focal shift is limited to a maximum of around 1.7-2A over the wavelength range of the hybrid lens system. Wavelength averaging is a property of the incoherent light. If there is a broader light spectrum in regard to wavelength, the Modulation Transfer Function will change compared to a situation where only a single wavelength of light is considered.

Consider, now, Figure 12B of the present disclosure. Figure 12B illustrates an example back focus shift for a hybrid lens system in accordance with embodiments of the disclosure.

The example of Figure 12B shows the longitudinal back focus shift of a hybrid lens system 4000 in accordance with embodiments of the disclosure. In this specific example, the lens system has 3 aspherical lenses and 1 metalens. The hybrid lens system has: F/1.7, 84deg FoV (field of view), 2.51 mm focal length and 4.51 mm system length. However, it will be appreciated that this example lens is merely one example configuration of a hybrid lens system 4000 which can be used in accordance with embodiments of the disclosure.

As can be seen in Figure 12B, the example hybrid lens system has a wavelength dependent focal shift at the image side of around +/- 25pm for a wavelength range of +/-30nm. This corresponds to a defocus range of +/-1 .75A.

Returning to Figure 11 , it will be appreciated that in some, optional, examples, the hybrid lens system 4000 may be provided as a camera system. In this situation, the camera system will include an image sensor 4016 and a controller 4018. As these are part of the camera system (and not the hybrid lens system) they are shown in dashed lines in the example of Figure 11.

The image sensor 4016 is configured to acquire image data. The controller 4018 is provided in order to control the image sensor. For example, the controller may receive image data from the image sensor 4016 and control the subsequent storage and/or display of this image data. In other examples, the controller 4018 may control the operation of the image sensor 4016.

The image sensor 4016 may be any suitable image sensor which can acquire image data. For example, the image sensor 4016 may be a CCD or CMOS image sensor or the like. However, the present disclosure is not particularly limited in this respect. The image sensor may also acquire image data of a resolution as required by the situation. For example, the image sensor may acquire 4K or 8K image data. In examples, the image data can be still image data (e.g. pictures). However, in other examples, the image data may include moving image data (e.g. videos). The present disclosure is not particularly limited in this regard.

In the example of Figure 11 , the image data acquired by the image sensor is then passed to the controller 4018.

In some examples, the controller 4018 may receive the image data from the image sensor 4016 directly. However, in other examples, the controller may indirectly receive the image data from the image sensor 4016 (e.g. via a storage or buffer). Once the image data is received from the image sensor, the controller 4018 is configured to perform processing on the image data (using any suitable image processing techniques).

In this manner, the hybrid lens system 4000 of Figure 11 provides an example of a hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; wherein a longitudinal chromatic aberration and a lateral chromatic aberration of the hybrid lens system are configured to a predetermined amount to perform wavelength averaging to extend a wavelength range of the hybrid lens system.

Consider, now, Figure 13 of the present disclosure. Figure 13 illustrates Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure.

In this example, polychromatic modulation transfer function curves for different field point positions for a hybrid lens system are shown. The hybrid lens system in this example comprises four refractive lenses and a metalens (metasurface as an independent lens element of the hybrid lens system). The hybrid lens system used in this example is a hybrid lens system such as hybrid lens system 4000, which utilizes wavelength averaging in order to provide an extended wavelength range. Specifically, the hybrid lens system of this example has a design wavelength of 620nm. The polychromatic modulation transfer function curves are provided for wavelengths from 595 to 645nm, in 2.5nm steps (a total range of 50nm). Equal weight has been applied for each wavelength. The extended wavelength range seen in this example arises from the averaging over wavelength (i.e. by utilizing the extended depth of field effect).

Thus, a hybrid lens system 4000 with wavelength averaging (such as that described with reference to Figure 11 of the present disclosure) is able to achieve an extended wavelength range of 50-60nm.

In some examples, a system stop may be positioned at (or in proximity to) the metasurface. This enables the lateral colour aberration to be constrained more efficiently during optimization of the hybrid lens system. More generally, the lens system stop or one of its apparent images is arranged at the metasurface. However, the present disclosure is not particularly limited in this regard.

As previously explained, a hybrid lens system including a birefringent mask reduces the chromatic aberration of the hybrid lens system (such as a hybrid lens system 2000) and thus improves the optical performance of the hybrid lens system. This means that the hybrid lens system 3000 can be used over an extended wavelength range. Thus, a lens system such as hybrid lens system 3000 is both more compact and has improved optical performance compared to a lens system with only refractive elements (or a hybrid lens system 2000). The hybrid lens system with wavelength averaging (such as the hybrid lens system 4000) provides an alternative to the hybrid lens system including the birefringent mask. That is, the hybrid lens system with wavelength averaging can also be used over an extended wavelength range. Thus, a lens system such as hybrid lens system 4000 is both more compact and has improved optical performance compared to a lens system with only refractive elements (or a hybrid lens system 2000).

A comparison between the hybrid lens system 3000 (with birefringent device) and the hybrid lens system 4000 (with wavelength averaging) is illustrated in Figures 14A and 14B of the present disclosure.

Specifically, Figures 14A and 14B of the present disclosure illustrate Modulation Transfer Function curves of a hybrid lens system in accordance with embodiments of the disclosure. The Modulation Transfer Function curves provided in Figure 14A of the present disclosure are provided for a hybrid lens system with a birefringent device (such as hybrid lens system 3000). The Modulation Transfer Function curves provided in Figure 14B of the present disclosure are provided for a hybrid lens system without a birefringent device (but with wavelength averaging (such as hybrid lens system 4000)).

Thus, Figures 14A and 14B provide a comparison of monochromatic Modulation Transfer Functions (635nm) for different field point positions near the edge of the designed wavelength range for a lens system with the design wavelength of 620nm between a hybrid lens system with a birefringent device and a hybrid lens system with wavelength averaging respectively.

An advantage of the birefringent device (such as a birefringent mask) is visible only when considering single wavelengths at the edges of the targeted wavelength range, where a good performance away from the design wavelength cannot be obtained for the lens system exploiting the wavelength averaging. Here the Modulation Transfer Function curves drop to zero and cannot be recovered for a hybrid lens system with wavelength averaging (see Figure 14B of the present disclosure). However as most natural objects reflect light over a broader range this will be only relevant in very limited applications. On the other hand, the hybrid lens system with wavelength averaging provides a higher Modulation Transfer Function at the lower spatial frequencies compared to the hybrid lens system with a birefringent device (as can be seen from a comparison of Figures 14A and 14B of the present disclosure). This can also be seen from a comparison of Figures 8 and 13 of the present disclosure.

Thus, the hybrid lens system with wavelength averaging (such as the hybrid lens system 4000) provides an alternative solution to the hybrid lens system including the birefringent mask.

As previously explained, a hybrid lens system 4000 is a hybrid lens system with one or more refractive lenses and a metasurface, which employs wavelength averaging. The number of refractive lenses included in the hybrid lens system is not particularly limited. That is, while certain examples of the hybrid lens system 4000 have been provided with reference to a lens system comprising 4 lenses, the present disclosure is not particularly limited in this regard. That is, the number of refractive lenses may be significantly more or less than this number.

Consider, now, the example of Figure 15A of the present disclosure. Figure 15A provides an example implementation of a hybrid lens system 4000 in accordance with embodiments of the disclosure.

Specifically, the hybrid lens system of Figure 15A is a hybrid lens system comprising 3 aspherical lenses and 1 metalens (a metasurface as an independent lens element). The lens system has F/1.7, 84 deg FoV (field of view), 2.51 mm focal length and 4.51 mm system length. Wavelength averaging is exploited for the hybrid lens system in order to provide the extended wavelength range.

Figure 15B of the present disclosure illustrates Modulation Transfer Function curves for a hybrid lens system in accordance with embodiments of the disclosure. Specifically, Figure 15B shows polychromatic Modulation Transfer Function Curves for a wavelength range of 60nm for different field points for the hybrid lens system of Figure 15A.

Thus, with an example implementation of a hybrid lens system such as that described with reference to Figure 15A of the present disclosure, an extended wavelength range of 60nm can be achieved for a hybrid lens system with compact form (e.g. a total of 4.51mm system length in this specific example).

In some examples, the hybrid lens system 4000 can be provided as a camera system comprising an image sensor. As such, in examples, it should be ensured (by the optimization target) that the Modulation Transfer Function curves do not hit the zero-line up to the Nyquist frequency of the image sensor or to the maximum spatial frequency of the range of interest. The Nyquist frequency is one-half of the spatial sampling frequency of the image sensor. As an example, if the pixel size of the image sensor is 1.4 pm, then the Nyquist frequency is equal to 1 / (2 * 1 .4 pm) = 357 mm-1.

If the Modulation Transfer Function curve is sufficiently larger than zero, the spatial information is not lost and can be reconstructed by an inverse filter (though noise may be enhanced by the inverse filter). It is important that the shapes of the Modulation Transfer Function curves are similar for all wavelength within the specified wavelength range to be able to apply a common wavelength invariant filter to the entire image data.

Consider, now, the example of Figure 16 of the present disclosure. Figure 16 illustrates an example set of images simulated for a hybrid lens system 4000 (with wavelength averaging) in accordance with embodiments of the disclosure. That is, the images of Figure 16 of the present disclosure have been simulated for a camera system comprising an image sensor and a hybrid lens system comprising one or more refractive lenses, a metasurface, where wavelength averaging is employed in order to provide an extended wavelength range - such as the lens design illustrated with reference to Figure 11 of the present disclosure (or the optional camera system which is also described with reference to this Figure).

As in the case with a birefringent mask, the wavelength averaging of embodiments of the disclosure may cause a decrease in the polychromatic Modulation Transfer Function curves at higher spatial frequencies. Accordingly, in some examples, an inverse filter may be applied in order to reconstruct the image data for higher spatial frequencies (provided that the modulation transfer curve is sufficiently larger than zero).

An inverse filter may be applied by image processing circuitry included in the image capture device (such as controller 4018). Alternatively, an inverse filter may be applied by an image processing device external to the image capture device. In examples, the image processing device external to the image capture device may be a server, which applies postprocessing to image data captured by the image capture device.

Returning to Figure 16, the first panel 16000 of Figure 16 shows (simulated) images captured by an image capture system including a hybrid lens system 4000 (a hybrid lens system with wavelength averaging) where an inverse filter has not been applied. As there is a drop of the modulation transfer curve at higher spatial frequency, it can be seen that some higher resolution information has been lost in the image data. On the other hand, the second panel 16002 of Figure 16 shows (simulated) images captured by the same image capture system, where an inverse filter (of filter size 11x11 in this specific example) has been applied. The use of the inverse filter enables image data for higher spatial frequencies to be reconstructed. Thus, the images in panel 16002 of Figure 16 are sharper than the images in panel 16000 of Figure 16 (where the inverse filter has not been applied).

As explained, it is important that the shapes of the Modulation Transfer Function curves are similar for all wavelength within the specified wavelength range to be able to apply a common wavelength invariant filter to the entire image data.

<Multi-lens Array>

The use of a hybrid lens system with wavelength averaging (such as the hybrid lens system 4000) enables a lens system with compact form factor and improved optical performance (in particular, reduced chromatic aberration) to be provided.

However, in certain situations, a further extension of the wavelength range of a lens system may be required. As an example, certain broadband imaging applications may require an extended wavelength range to be provided. As such, in examples (such as for certain broadband imaging applications) the inventors have realized that a multi-lens array structure can be provided, with each lens of the multi-lens array being optimized for an assigned wavelength sub-range. This may further extend the wavelength range of the hybrid lens system.

Consider, again, Figure 10 of the present disclosure. Figure 10 illustrates an example multi-lens array structure as can be used in accordance with embodiments of the disclosure. Hereinbefore, the example multi-lens array structure of Figure 10 of the present disclosure has been described with specific reference to the use of a hybrid lens system 3000 (a hybrid lens system with a birefringent device). However, the example multi-lens array structure of Figure 10 of the present disclosure (and other multi-lens array structures which have been described) may also be used with a hybrid lens system 4000 (a hybrid lens system with wavelength averaging) or a hybrid lens 2000. More generally, it will be appreciated that the hybrid lens system 4000 may be used in a multi-lens array structure within embodiments of the disclosure, in order to further extend the wavelength range of the hybrid lens system 4000.

Further discussion of the use of a multi-lens array structure comprising a hybrid lens system 4000 will not be provided for brevity of disclosure. <Clauses>

In addition, embodiments of the present disclosure can be defined in accordance with the following numbered clauses:

1) A hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; and a birefringent device arranged to intercept light passing through the metasurface; wherein the birefringent device is configured to extend a wavelength range of the hybrid lens system.

2) The hybrid lens system according to clause 1, wherein extending a wavelength range of the hybrid lens system comprises extending a wavelength range over which Modulation Transfer Functions of the hybrid lens system for all wavelengths are within a predetermined limit.

3) The hybrid lens system according to clause 1 or clause 2, wherein the birefringent device is arranged in proximity to the aperture stop or to its apparent images, the entrance or exit pupil of the hybrid lens system. 4) The hybrid lens system according to any preceding clause, wherein the birefringent device comprises a ring-shaped structure of a birefringent layer.

5) The hybrid lens system according to clause 4, wherein an extraordinary axis of the birefringent layer alternates by 90 degrees between adjacent rings of the birefringent device.

6) The hybrid lens system according to clause 4, wherein a number and a size of the rings of the birefringent device are configured to extend a wavelength range of the hybrid lens system.

7) The hybrid lens system according to clause 4, wherein a birefringence of the birefringent device is configured to extend a wavelength range of the hybrid lens system.

8) The hybrid lens system according to clause 7, wherein the birefringence of the birefringent device is defined as d*An, where d is the thickness of the birefringent layer and An is a difference between an extraordinary and ordinary refractive index of the birefringent material of the birefringent layer.

9) The hybrid lens system according to any preceding clause, wherein the metasurface is applied to a surface of one of the one or more refractive lenses.

10) The hybrid lens system according to any preceding clause, wherein the birefringent device is provided by the metasurface.

11) The hybrid lens system according to any preceding clause, wherein the birefringent device is provided by an element external to the metasurface.

12) A camera system comprising: an image sensor configured to acquire image data; and a hybrid lens system according to any preceding clause; wherein the hybrid lens system is optimized for the camera system in order that Modulation Transfer Functions of the hybrid lens system for all wavelengths within a wavelength range are greater than zero up to the Nyquist frequency of the image sensor or to the maximum spatial frequency of the range of interest. 13) The camera system according to clause 12, wherein an inverse filter is applied to the image data acquired by the image sensor.

14) The camera system according to clause 13, wherein the inverse filter is a common wavelength invariant filter.

15) A hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; wherein a longitudinal chromatic aberration and a lateral chromatic aberration of the hybrid lens system are configured to a predetermined amount to perform wavelength averaging to extend a wavelength range of the hybrid lens system.

16) The hybrid lens system according to clause 15, wherein extending a wavelength range of the hybrid lens system comprises extending a wavelength range over which Modulation Transfer Functions of the hybrid lens system for all wavelengths are within a predetermined limit.

17) The hybrid lens system according to clause 15 or clause 16, wherein a lens system stop or one of its apparent images is arranged at the metasurface.

18) The hybrid lens system according to any of clauses 15 to 17, wherein the hybrid lens system is configured such that the defocus variation of the chromatic back focal shift is limited to a maximum of around 1.7-2A over the wavelength range of the hybrid lens system.

19) A camera system comprising: an image sensor configured to acquire image data; and a hybrid lens system according to any of clauses 15 to 18; wherein the hybrid lens system is optimized for the camera system in order that Modulation Transfer Functions of the hybrid lens system for all wavelengths within a wavelength range are greater than zero up to the Nyquist frequency of the image sensor or to the maximum spatial frequency of the range of interest.

20) The camera system according to clause 19, wherein an inverse filter is applied to the image data acquired by the image sensor. 21) The camera system according to clause 20, wherein the inverse filter is a common wavelength invariant filter.

22) A multi-lens array comprising: a plurality of hybrid lens systems according to any of clauses 1 to 11 or a plurality of hybrid lens systems according to any of clauses 15 to 18, wherein the plurality of hybrid lens systems are arranged in an array, and wherein each hybrid lens of the plurality of hybrid lens systems is optimized for a wavelength sub-range of a specified wavelength range of the multi-lens array.

23) The multi-lens array according to clause 22, further comprising: an image sensor configured to capture a sub-image corresponding to each of the plurality of hybrid lens systems; and processing circuitry configured to: generate corrected sub-images by correcting a lateral disparity between captured sub-images based on a baseline between each of the corresponding hybrid lens systems; and generate broadband image data based on the corrected sub-images which have been generated.

In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Similarly, a data signal comprising coded data generated according to the methods discussed above (whether or not embodied on a non-transitory machine-readable medium) is also considered to represent an embodiment of the present disclosure.

It will be apparent that numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the technology may be practised otherwise than as specifically described herein.

It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and/or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and/or processors may be used without detracting from the embodiments.

Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of any embodiment may be physically, functionally, and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and/or processors.

Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique.

Claims

1. A hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; and a birefringent device arranged to intercept light passing through the metasurface; wherein the birefringent device is configured to extend a wavelength range of the hybrid lens system.

2. The hybrid lens system according to claim 1 , wherein extending a wavelength range of the hybrid lens system comprises extending a wavelength range over which Modulation Transfer Functions of the hybrid lens system for all wavelengths are within a predetermined limit.

3. The hybrid lens system according to claim 1 , wherein the birefringent device is arranged in proximity to the aperture stop or to its apparent images, the entrance or exit pupil of the hybrid lens system.

4. The hybrid lens system according to claim 1 , wherein the birefringent device comprises a ring-shaped structure of a birefringent layer.

5. The hybrid lens system according to claim 4, wherein an extraordinary axis of the birefringent layer alternates by 90 degrees between adjacent rings of the birefringent device.

6. The hybrid lens system according to claim 4, wherein a number and a size of the rings of the birefringent device are configured to extend a wavelength range of the hybrid lens system.

7. The hybrid lens system according to claim 4, wherein a birefringence of the birefringent device is configured to extend a wavelength range of the hybrid lens system.

8. The hybrid lens system according to claim 7, wherein the birefringence of the birefringent device is defined as d*An, where d is the thickness of the birefringent layer and An is a difference between an extraordinary and ordinary refractive index of the birefringent material of the birefringent layer.

9. The hybrid lens system according to claim 1, wherein the metasurface is applied to a surface of one of the one or more refractive lenses.

10. The hybrid lens system according to claim 1 , wherein the birefringent device is provided by the metasurface.

11. The hybrid lens system according to claim 1 , wherein the birefringent device is provided by an element external to the metasurface.

12. A camera system comprising: an image sensor configured to acquire image data; and a hybrid lens system according to claim 1 ; wherein the hybrid lens system is optimized for the camera system in order that Modulation Transfer Functions of the hybrid lens system for all wavelengths within a wavelength range are greater than zero up to the Nyquist frequency of the image sensor or to the maximum spatial frequency of the range of interest.

13. The camera system according to claim 12, wherein an inverse filter is applied to the image data acquired by the image sensor.

14. The camera system according to claim 13, wherein the inverse filter is a common wavelength invariant filter.

15. A hybrid lens system comprising: one or more refractive lenses and a metasurface arranged along an optical axis of the hybrid lens system; wherein a longitudinal chromatic aberration and a lateral chromatic aberration of the hybrid lens system are configured to a predetermined amount to perform wavelength averaging to extend a wavelength range of the hybrid lens system.

16. The hybrid lens system according to claim 15, wherein extending a wavelength range of the hybrid lens system comprises extending a wavelength range over which Modulation Transfer Functions of the hybrid lens system for all wavelengths are within a predetermined limit.

17. The hybrid lens system according to claim 15, wherein a lens system stop or one of its apparent images is arranged at the metasurface.

18. The hybrid lens system according to claim 15, wherein the hybrid lens system is configured such that the defocus variation of the chromatic back focal shift is limited to a maximum of around 1.7-2A over the wavelength range of the hybrid lens system.

19. A camera system comprising: an image sensor configured to acquire image data; and a hybrid lens system according to claim 15; wherein the hybrid lens system is optimized for the camera system in order that Modulation Transfer Functions of the hybrid lens system for all wavelengths within a wavelength range are greater than zero up to the Nyquist frequency of the image sensor or to the maximum spatial frequency of the range of interest.

20. The camera system according to claim 19, wherein an inverse filter is applied to the image data acquired by the image sensor.

21. The camera system according to claim 20, wherein the inverse filter is a common wavelength invariant filter.

22. A multi-lens array comprising: a plurality of hybrid lens systems according to claim 1 or a plurality of hybrid lens systems according to claim 15, wherein the plurality of hybrid lens systems are arranged in an array, and wherein each hybrid lens of the plurality of hybrid lens systems is optimized for a wavelength sub-range of a specified wavelength range of the multi-lens array.

23. The multi-lens array according to claim 22, further comprising: an image sensor configured to capture a sub-image corresponding to each of the plurality of hybrid lens systems; and processing circuitry configured to: generate corrected sub-images by correcting a lateral disparity between captured sub-images based on a baseline between each of the corresponding hybrid lens systems; and generate broadband image data based on the corrected sub-images which have been generated.