WO2021170221A1

WO2021170221A1 - Imaging system for an electronic device

Info

Publication number: WO2021170221A1
Application number: PCT/EP2020/054928
Authority: WO
Inventors: Mikko Juhola; Antonie VERHOEVEN; Gaurav BOSE
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2020-02-25
Filing date: 2020-02-25
Publication date: 2021-09-02
Also published as: CN115151844A; EP4085280A1; CN115151844B

Abstract

An imaging system comprising a lens system (1) comprising a plurality of lenses (2), a diffractive optical element (3), and a sensor (4). The lens system (1), diffractive optical element (3), and sensor (4) share an optical axis (O), and the diffractive optical element (3) is arranged between the lens system (1) and the sensor (4). Incident rays of light enter the lens system (1) at a plurality of first angles (α1) relative the optical axis (O), and emergent rays of light reach the sensor (4) at a plurality of second angles (α2) relative the optical axis (O). The second angles (α2) are smaller than the first angles (α1). This solution facilitates a comparatively short imaging system, i.e. having a short total track length.

Description

IMAGING SYSTEM FOR AN ELECTRONIC DEVICE

TECHNICAL FIELD

The disclosure relates to an imaging system for an electronic device, the imaging system comprising a lens system and a sensor.

BACKGROUND

When designing imaging systems, e.g. cameras, for electronic devices, several difficulties have to be overcome. Electronic devices such as mobile phones preferably have as small outer dimensions as possible, while imaging systems inevitably require certain dimensions in order to provide sufficiently good image sharpness, spatial frequency, sensitivity etc.

Furthermore, the demand for higher performance imaging systems in electronic devices requires even larger sensors and optics, resulting in a product which is comparatively thick and bulky. Additionally, there is the constant challenge of providing mobile electronic devices with imaging systems covering wide angles yet having as short track length as possible, in order to be able to minimize the dimensions of the electronic device.

Diffractive optical elements are often combined with classical imaging optical systems to correct e.g. color aberrations. However, there are well-known challenges in making a highly efficient, wide spectral range diffractive element with a large range of incident angles. Hence, diffractive optical elements are mainly used in long total track length cameras, where the range of incident angles to diffractive element is small, and not in smaller electronic devices such as smartphones, where one challenge is to achieve as short track length as possible.

SUMMARY

It is an object to provide an improved imaging system for an electronic device. The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided an imaging system comprising a lens system comprising a plurality of lenses, a diffractive optical element, and a sensor, the lens system, diffractive optical element, and sensor sharing an optical axis, the diffractive optical element being arranged between the lens system and the sensor, incident rays of light entering the lens system at a plurality of first angles relative the optical axis, emergent rays of light reaching the sensor at a plurality of second angles relative the optical axis, the second angles being smaller than the first angles.

This solution facilitates a comparatively short imaging system, i.e. having a short total track length. Due to the efficient light direction changing properties of the diffractive optical element, the ray angles can be controlled at a steep angle ray path which is subsequently directed towards the sensor by means of the diffractive optical element. This allows a shorter than usual imaging system for special applications such as time of flight or ultrawide angle fish eye lenses.

In a possible implementation form of the first aspect, the second angles are less than ±20° relative the optical axis. This small angle facilitates an as short track length as possible for the imaging system.

In a further possible implementation form of the first aspect, the lens system redirects the rays of light such that the rays of light have third angles, the third angles being larger than the first angles and smaller than the second angles.

In a further possible implementation form of the first aspect, the diffractive optical element is arranged closer to the sensor than the plurality of lenses along the optical axis, allowing the rays of light to reach an as large area as possible before turning them towards the sensor.

In a further possible implementation form of the first aspect, the diffractive optical element comprises a grating, used to manipulate the phase, amplitude, and the propagation direction of incident rays of light.

In a further possible implementation form of the first aspect, a configuration of the grating is asymmetrical in at least one direction perpendicular to the optical axis, allowing the configuration of the diffractive optical element to vary, from its center at the optical axis, with increasing light direction changing power. In a further possible implementation form of the first aspect, the grating comprises a plurality of slanted grating elements, and the configuration comprises adjusting periods between adjacent grating elements. Asymmetric gratings may be used to optimize the efficiency for higher angular and spectral ranges.

In a further possible implementation form of the first aspect, angles of the grating element slants, in relation to the optical axis, increase when distances between the grating elements and the optical axis increase in a direction perpendicular to the optical axis.

In a further possible implementation form of the first aspect, the period decreases when a distance between the grating element and the optical axis increases in the direction perpendicular to the optical axis. The chief ray incidence towards the diffractive optical element requires a large bending angle to the sensor. The grating period is smaller in this region, while the period increases laterally towards the optical axis where the bending needs to be less.

In a further possible implementation form of the first aspect, no grating elements are located along the optical axis. As the period increases, it gives rise to parasitic diffraction orders lowering the diffraction efficiency in the desired direction. At the optical axis O, it is therefore preferred that the diffractive optical element does not have a grating configuration in order to avoid any stray light hitting the sensor.

In a further possible implementation form of the first aspect, the lens system comprises ultra wide angle lenses, facilitating time of flight or other near-infrared imaging systems having large fields-of-view.

According to a second aspect, there is provided an electronic device comprising the imaging system according to the above.

This and other aspects will be apparent from the embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 shows a schematic illustration of an imaging system in accordance with one embodiment of the present invention;

Fig. 2 shows a partial side view of a diffractive optical element of an imaging system in accordance with one embodiment of the present invention;

Fig. 3 shows a schematic illustration of an imaging system in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to an electronic device (not shown) comprising an imaging system.

As shown in Fig. 1, the imaging system comprises a lens system 1 comprising a plurality of lenses 2, a diffractive optical element 3, and a sensor 4. The lens system 1, diffractive optical element 3, and sensor 4 share an optical axis O, and the diffractive optical element 3 is arranged between the lens system 1 and the sensor 4.

The diffractive optical element 3 may be arranged closer to the sensor 4 than the plurality of lenses 2 along the optical axis O.

The lens system 1 may comprise any suitable kind of lenses 2, including ultra-wide angle lenses, such as fish eye lenses, as shown in Fig. 3.

Incident rays of light firstly pass through the lenses 2, secondly pass through the diffractive optical element 3, and thirdly reaches the sensor 4. The incident rays of light enter the lens system 1 at a plurality of first angles al relative the optical axis O, and emergent rays of light reach the sensor 4 at a plurality of second angles a2 relative the optical axis O. The second angles a2 are smaller than the first angles al, hence, the lenses 2 and the diffractive optical element 3 focuses the indecent light onto the sensor 4.

The second angles a2 may be less than ±20° relative the optical axis O, i.e. cover a total area of up to 40°.

The lens system 1 redirects the rays of light such that the rays of light have third angles a3, the third angles a3 being larger than the first angles al and smaller than the second angles a2.

This solution facilitates a comparatively short imaging system, i.e. having a short total track length. Due to the efficient light direction changing properties of the diffractive optical element 3, the incident rays of light can be controlled at a steep angle ray path which is subsequently directed towards the sensor 4 by means of the diffractive optical element 3. This allows a shorter than usual imaging system for special applications such as time of flight or ultra-wide angle fish eye lenses.

The total track length of the imaging system roughly depends on at how steep angles one can get the maximum field of view ray path to extend through the lenses 2 to the sensor 4. Nevertheless, it is also necessary to get the rays of light to the sensor pixels at small enough angles, typically the maximum chief ray angle to the sensor 4 is around 40°, as previously mentioned. Some sensors, like time of flight cameras cannot use such big angles, wherefore it is necessary the imaging system allows the ray angles to be converted to close to 0°. This allows production of time of flight cameras, or other near-infrared cameras, with large fields of view, as well as visible range ultrawide angle cameras with a less aggressive diffractive optical element 3, such that the system’s efficiency instead could be optimized for a wider spectral range.

The diffractive optical element 3 may comprise a grating 5, shown in more detail in Fig. 2. The diffractive optical element 3 is located on the object side, and configured on a planar substrate in order to reduce Fresnel losses. Gratings 5 may be used to manipulate the phase, amplitude, and the propagation direction of incident rays of light. Asymmetric gratings (ex: parallelograms) may be used to optimize the efficiency for higher angular and spectral ranges. The slant angle, height and the fill factor of the gratings are used for optimizing efficiency, and the periods are chosen to ensure the transmission of first diffraction order normal to the direction of the sensor 4. The chief ray incidence from the lenses 2 towards the diffractive optical element 3 requires a larger bending angle to the sensor 4. The grating period is smaller in this region, while the period increases laterally towards the optical axis O where the bending needs to be less. As the period increases, it gives rise to other parasitic diffraction orders lowering the diffraction efficiency in the desired direction. At the optical axis O, it is therefore preferred that the diffractive optical element 3 does not have a grating configuration in order to avoid any stray light hitting the sensor 4.

In one embodiment, a configuration of the grating 5 is asymmetrical in at least one direction perpendicular to the optical axis O.

The grating 5 may comprises a plurality of slanted grating elements 5a, and the configuration may comprise adjusting the periods between adjacent grating elements 5a.

The angles b of the grating element slants 5a, in relation to the optical axis O, may increase when the distances between the grating elements 5a and the optical axis O increase in a direction D1 perpendicular to the optical axis O.

The period may decrease when the distance between a grating element 5a and the optical axis O increases in the direction D1 perpendicular to the optical axis O. Preferably, there are no grating elements 5a located along the optical axis O, and possibly not in an area directly adjacent the optical axis O either. The size if this no-grating area depends on the acceptance angle of the sensor 4. As such, the size of no-grating area grows with the tangent of the acceptance angle.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The reference signs used in the claims shall not be construed as limiting the scope. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure. As used in the description, the terms

“horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate.

Claims

1. An imaging system comprising a lens system (1) comprising a plurality of lenses (2), a diffractive optical element (3), and a sensor (4), said lens system (1), diffractive optical element (3), and sensor (4) sharing an optical axis (O), said diffractive optical element (3) being arranged between said lens system (1) and said sensor

(4), incident rays of light entering said lens system (1) at a plurality of first angles (al) relative said optical axis (O), emergent rays of light reaching said sensor (4) at a plurality of second angles (a2) relative said optical axis (O), said second angles (a2) being smaller than said first angles (al).

2. The imaging system according to claim 1, wherein said second angles (a2) are less than ±20° relative said optical axis (O).

3. The imaging system according to claim 1 or 2, wherein said lens system (1) redirects said rays of light such that said rays of light have third angles (a3), said third angles (a3) being larger than said first angles (al) and smaller than said second angles (a2).

4. The imaging system according to any one of the previous claims, wherein said diffractive optical element (3) is arranged closer to said sensor (4) than said plurality of lenses (2) along said optical axis (O).

5. The imaging system according to any one of the previous claims, wherein said diffractive optical element (3) comprises a grating (5).

6. The imaging system according to claim 5, wherein a configuration of said grating (5) is asymmetrical in at least one direction perpendicular to said optical axis (O).

7. The imaging system according to claim 5 or 6, wherein said grating (5) comprises a plurality of slanted grating elements (5a), and wherein said configuration comprises adjusting periods (p) between adjacent grating elements (5a).

8. The imaging system according claim 7, wherein angles (b) of said grating element slants, in relation to said optical axis, increase when distances (d) between said grating elements (5a) and said optical axis (O) increase in a direction (Dl) perpendicular to said optical axis (O).

9. The imaging system according to claim 8, wherein said period decreases when a distance (d) between said grating element (5a) and said optical axis (O) increases in said direction (Dl) perpendicular to said optical axis (O).

10. The imaging system according to any one of claims 7 to 9, wherein there no grating elements (5a) are located along said optical axis (O).

11. The imaging system according to any one of the previous claims, wherein said lens system (1) comprises ultra-wide angle lenses (2).

12. An electronic device comprising the imaging system according to any one of claims 1 to 11