Patent Publication Number: US-2021191077-A1

Title: High resolution, wide fov static lens assembly

Description:
BACKGROUND 
     Lens systems with multiple lens elements are well known for cameras. For a static lens, obtaining a wide field of view (FOV) can be challenging because of the resulting distortion. Most fisheye lenses have poor corner modulation transfer functions (MTF), causing distortion around the edges. Modern consumer electronic devices also have small form factors, making it desirable to have cameras which do not take up much room on the device. It is desirable to have a static lens system with a wide FOV and minimal distortion and aberrations, which can be used for a video conferencing system. 
     SUMMARY 
     This disclosure describes various embodiments that relate to compact static lens assemblies. 
     A static lens system is disclosed and includes the following in one embodiment: a lens assembly housing defining a front opening; an imaging sensor; a lens assembly disposed within the lens assembly housing and comprising: first, second, third, fourth, fifth and sixth lens elements, the sixth lens element being adjacent to the imaging sensor and the first lens element being adjacent to the front opening; wherein the first through third and sixth lens elements are concave lenses have a negative focal length and the fourth and fifth lens elements are concave lens and have a positive focal length. 
     In one embodiment, an aperture stop is provided between the third and fourth lens elements, and a field stop is provided between the fourth and fifth lens elements. An IR filter is provided between the sixth lens element and the imaging sensor. The first and second lens elements have curvatures that increase the FOV, while the fifth and sixth lens elements have curvatures that reduce color aberrations. All the lens elements are spherical lenses. 
     Preferably, the FOV is at least 75 degrees and the F-Theta distortion is less than 3.5%. In one embodiment, the lens design provides a FOV of 79.5 degrees and an F-Theta distortion of less than 3%. Lens elements 1 and 2 are used to increase the FOV. Lens elements 5 and 6 are used to reduce color aberration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  shows a perspective view of an exemplary camera module suitable for use with the described embodiments; 
         FIG. 2  shows a cross-sectional view of lens elements making up a lens assembly according to an embodiment; 
         FIG. 3  shows a cross-section view of a lens assembly that illustrates how the lens elements are positioned within a lens housing according to an embodiment; 
         FIG. 4  shows modulation transfer function (MTF) charts for the lens assembly of  FIG. 2  according to an embodiment; 
         FIG. 5  shows sagittal and tangential field curvature lines representing change in field curvature across a field of view of the lens assembly depicted in  FIG. 2 ; and 
         FIG. 6  is a simplified block diagram of a system configured to operate a videoconferencing system, according to certain embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure relate generally to optics for imaging devices, and in particular to optics suitable for use with videoconferencing devices, according to certain embodiments. 
     In the following description, various embodiments of a small form-factor imaging device will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that certain embodiments may be practiced or implemented without every detail disclosed. Furthermore, well-known features may be omitted or simplified in order to prevent any obfuscation of the novel features described herein. 
     Compact, high-resolution lens assemblies can help bring high quality imagery to a relatively small form factor device. Simply increasing the resolution of an imaging sensor without commensurately improving the quality of the optics supporting that imaging sensor may only result in higher bandwidth streams of imagery. An overall quality of the lens should be designed so that each pixel of the imaging sensor can be resolved. In today&#39;s market where  2   k  and  4   k  video are becoming more common, keeping a small form factor and overall cost of the lens assembly competitive while also resolving the additional detail made possible by higher density image sensors is challenging. 
     These and other embodiments are discussed below with reference to  FIGS. 1-6 ; however, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
       FIG. 1  shows a perspective view of an exemplary camera module  100  suitable for use with the described embodiments. Camera module  100  includes a protective cover  102  over a lens housing  108  enclosing a lens assembly and mounted on a base  110 . A cylindrical ring  104  is connected to ring  106 . Ring  106  with knobs can be rotated for assembly during manufacturing to achieve the proper distance. The components of module  100  can be formed from metal or polymeric materials. Protective cover  102  can have a negligible amount of magnification and be optically clear to allow the lens assembly within lens housing  108  to operate without substantial degradation. In some embodiments, protective cover  106  can be sealed to cylindrical ring  102  to prevent moisture from intruding into an interior volume defined by lens housing  108 . 
       FIG. 2  shows a cross-sectional view of lens elements making up a lens assembly  200 .  FIG. 2  shows the lens elements positioned to minimize an overall focal length of lens assembly  200 . Lens assembly  200  is made up of from front to back lens elements 1-6. The lens elements form a (−) (−) (−) (+) (+) (−) arrangement of negative and positive focal length elements. The lens elements in one embodiment are all made of glass, to improve performance and reduce manufacturing complexity. In some embodiments, the lens elements are designed to provide minimal F-Theta distortion without resorting to expensive molding glass. 
     Lens elements 5 and 6 are used to reduce color aberration. Color or chromatic aberration from earlier lens elements cause different wavelengths of light (different colors) to have differing focal lengths. Lens elements 5 and 6 are an achromatic doublet that brings different colors (in particular red and blue) to the same focus. Alternately, other lens shapes could be used to correct the chromatic aberrations. 
     At the back of the lens group is an IR filter  202  and a sensor  204 . The IR filter is an infrared light blocking element or coating preventing most infrared light from reaching digital sensor  204 . In some embodiments, the infrared coating can block 98-99% of light having a wavelength between 700 nm and 1000 nm. This IR filter can prevent degraded image capture performance caused by IR light being incorrectly captured and presumed to be visible light. It should be noted that in some embodiments, the IR filter could instead be a coating added to one surface of lens element 6, removing the need for a separate IR filter  202 . An IR filter coating could also take other forms such as a film layer adhered to one side of another lens element. 
     Table (1) depicted below shows various other exemplary technical features of lens assembly  200  depicted in  FIG. 2 . It should be noted that other designs, materials and other technical features may vary and the below technical specifications should not be construed as limiting. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Distance to 
               
               
                 Element 
                 R1 
                 R2 
                 Material 
                 Index 
                 Abbe number 
                 Focal Length 
                 Distance 
                 Image sensor 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                   
                 Glass 
                 1.835 
                 40.7 
                 −7.152 
                 1.7 
                   
               
               
                   
                   
                 (Air Gap) 
                   
                   
                   
                 1.813 
               
               
                 2 
                   
                 Glass 
                 1.697 
                 63.4 
                 −6.009 
                 0.743 
                 16.268 
               
               
                   
                   
                 (Air Gap) 
                   
                   
                   
                 1.921 
               
               
                 3 
                   
                 Glass 
                 1.62 
                 32.2 
                 −33.933 
                 2.119 
                 12.228 
               
               
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0.35 
               
            
           
           
               
            
               
                 Aperture Stop 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0.162 
                   
               
               
                 4 
                   
                   
                 Glass 
                 1.871 
                 42.7 
                 3.719 
                 3.958 
                 7.758 
               
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 −0.122 
               
            
           
           
               
            
               
                 Field Stop 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0.225 
                   
               
               
                 5 
                   
                   
                 Glass 
                 1.618 
                 55.5 
                 3.885 
                 2.786 
                 4.869 
               
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0 
               
               
                 6 
                   
                   
                 Glass 
                 1.946 
                 19.3 
                 −4.491 
                 0.748 
                 4.121 
               
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 2.721 
               
               
                 7 
                   
                   
                 Glass (IR Cut) 
                 1.5168 
                   
                 100000 
                 0.3 
                 1.1 
               
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0.2 
               
               
                   
                   
                   
                 Glass (Sensor Top Cover) 
                   
                   
                   
                 0.4 
               
               
                   
                   
                   
                 (Air Gap) 
                   
                   
                   
                 0.5 
               
               
                   
                   
                   
                 Sensor 
                   
                   
                   
                   
               
               
                   
                   
                   
                 Total Track 
                   
                   
                   
                 20.524 
               
            
           
           
               
               
               
               
               
            
               
                 Focal Length 2.4 
                   
                   
                   
                   
               
               
                 Image sensor Size: SONY IMX317 (Image height = 3.56 mm - radius) 
               
               
                 Lens Image circle: 6.5 mm (Diameter) 
               
               
                 Image sensor Size: SONY IMX335 (Image height = 3.24 mm - radius) 
               
               
                 Lens Image circle: 6.5 mm (Diameter) 
               
               
                   
               
            
           
         
       
     
     Table (1) shows preferred characteristics of each of lens elements 1-6 and IR filter  202  (element 7). The lens material, glass, is shown for each element, along with the intermediate air gaps. An index of refraction of the lens material is then shown, along with the locations of the aperture stop and field stop. Next, the Abbe number of each lens element is shown. The Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material&#39;s dispersion (variation of refractive index versus wavelength). High values of V indicate low dispersion. Higher Abbe values indicate less chromatic aberration. Next, the focal length of each lens element (in millimeters) is shown. Next, the distance from the previous element or width of a lens element is shown. If the material is “air gap”, then the distance from the previous lens element is shown. If the material shows “Glass”, then it is the width of the lens element. The last column shows the distance of each element to the image sensor, in mm. In embodiments, the total focal length is less than 3 mm, with a total distance, or “length” of the lens being less than 30 mm. In the embodiment shown in Table 1, the numbers provided account for a total focal length of 2.4 mm, with a total distance, or depth of the lens, being 20.524 mm. 
     In the embodiment shown in Table 1, the focal length of each of the first and second lens elements is less than a quarter of the focal length of the third lens element. The focal length of the fourth and fifth lens elements are each less than two thirds of the focal lengths of the first and second lens elements, and are opposite in sign. The fifth and sixth lens elements are coupled together, with a convex surface of the fifth lens element mating with a concave surface of the sixth lens element. 
     In alternate embodiments, the focal lengths and distances to the image sensor may be varied slightly within a range and still provide good results. In particular, a change in the value of one element within the acceptable range can be compensated for by changes in the value of one or more other elements, within their ranges. Such acceptable ranges are set forth in the equations below in Table (2). 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 No. 
                 Acceptable range 
                 Description 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 −3.3 &lt; f1/f &lt; −2.6 
                 f1 is the focal length of 
               
               
                   
                   
                   
                 element 1; f is the overall 
               
               
                   
                   
                   
                 focal length of the lens 
               
               
                   
                   
                   
                 (elements 106) 
               
               
                   
                 2 
                 −3.0 &lt; f2/f &lt; −2.4 
                 f2 is the focal length of 
               
               
                   
                   
                   
                 element 2 
               
               
                   
                 3 
                 0.6 &lt; |f/f3| + |f/f4| &lt; 1   
                 f2 is the focal length of 
               
               
                   
                   
                   
                 element 2, f3 is the focal 
               
               
                   
                   
                   
                 length of element 3 
               
               
                   
                 4 
                 1.1 &lt; |f/f5| + |f/f6| &lt; 1.5 
                 F5 is the focal length of 
               
               
                   
                   
                   
                 element 5, f6 is the focal 
               
               
                   
                   
                   
                 length of element 6 
               
               
                   
                 5 
                 256 &lt; Vd2 − Vd3 &lt; 40 
                 Vd2 is the Abbe number of 
               
               
                   
                   
                   
                 element 2, Vd3 is the Abbe 
               
               
                   
                   
                   
                 number of element 3 
               
               
                   
                 6 
                  30 &lt; Vd5 − Vd6 &lt; 45 
                 Vd5 is the Abbe number of 
               
               
                   
                   
                   
                 element 5, Vd6 is the Abbe 
               
               
                   
                   
                   
                 number of element 6 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 3  shows a cross-sectional view of lens assembly  300  positioned within lens assembly housing  300 . As depicted, lens assembly housing  300  can be a multi-part housing that includes ring/cylinder housing  104  and lens housing  108 . Housing  104  contains lens elements 1-6, aperture stop  208  and field stop  210 . Housing  104  can be screwed into lens housing  108  using threaded surfaces  302  for assembly during manufacturing. The IR filter  202  and imaging sensor  204  are mounted in lens housing  108 . By turning housing  104  using knobbed ring  106 , the distance between lens elements 1-6 and the imaging sensor can be varied. 
       FIG. 4  shows modulation transfer functions (MTF) charts for the focal lengths of lens assembly  200 . MTF is the spatial frequency response of an imaging system or element. It is the contrast at a given spatial frequency relative to low frequencies. MTF is a useful measure of true or effective resolution, since it accounts for the amount of blur and contrast over a range of spatial frequencies.  FIG. 4  shows the modulus of the Optical Transfer Function (OTF) versus spacial frequency. These curves illustrate the high resolution capabilities of lens assembly  200 . The curves shown range from curve  402  (0.6540 mm—Tangential) to curve  404  (differential limit—tangential). 
       FIG. 5  shows sagittal and tangential field curvature lines representing change in field curvature across a field of view of lens assembly  200  for lens assembly  200 .  FIG. 5A  plots the field curvature for different colors, as shown on a plot of degrees versus mm.  FIG. 5A  shows curves  502  (0.4600—tangential),  504  (0.5500—tangential),  506  (0.6200—tangential),  508  (0.4600—sagittal),  510  (0.6200—sagittal) and  512  (0.5500—sagittal). For a maximum field of 79.5 degrees, the maximum field curvature sagittal is 0.0614 mm, and the maximum field curvature tangential is 0.0356 mm. 
       FIG. 5B  shows degrees versus percentage of F-Theta distortion.  FIG. 5B  shows that the field of view (FOV) is 79.5 degrees, and the maximum distortion is 2.7815%. 
     Examples of Systems for Operating Videoconferencing Devices 
       FIG. 6  is a simplified block diagram of a system  600  configured to control camera module  100 , according to certain embodiments. In this embodiment, camera module  100  is used in a video conferencing system. System  600  includes processor(s)  610 , operational logic  620 , movement tracking system  630 , input detection system  650 , and power management system  660 . Each of system blocks  620 - 660  can be in electrical communication with the processor(s)  610 . System  600  may further include additional systems that are not shown or discussed to prevent obfuscation of the novel features described herein. 
     In certain embodiments, processor(s)  610  can include one or more microprocessors (μCs) and can be configured to control the operation of system  600 . Alternatively, processor(s)  610  may include one or more microcontrollers (MCUs), digital signal processors (DSPs), or the like, with supporting hardware and/or firmware (e.g., memory, programmable I/Os, etc.), as would be appreciated by one of ordinary skill in the art. In some embodiments, multiple processors may provide an increased performance in system  600  speed and bandwidth. It should be noted that although multiple processors may improve system  600  performance, they are not required for standard operation of the embodiments described herein. 
     Operational logic  620  can include any combination of software, firmware, or hardware that can perform the various steps, operations, and functions associated with videoconferencing system  100 , as described above with respect to  FIGS. 1-5B . For instance, operational logic  620  can control settings and operating parameters such as recording resolution, focus, magnification and lens assembly azimuth and inclination. Operational logic  620  can be stored in any suitable non-transitory computer-readable storage media that can store program code and/or data that provide the functionality of embodiments of the present disclosure. That is, operational logic  620  can store one or more software programs to be executed by processors (e.g., in processor(s)  610 ). It should be understood that “software” can refer to sequences of instructions that, when executed by processing unit(s) (e.g., processors, processing devices, etc.), cause system  600  to perform certain operations of software programs. The instructions can be stored as firmware residing in read only memory (ROM) and/or applications stored in media storage that can be read into memory for processing by processing devices. Software can be implemented as a single program or a collection of separate programs and can be stored in non-volatile storage and copied in whole or in-part to volatile working memory during program execution. From a storage subsystem, processing devices can retrieve program instructions to execute in order to execute various operations described herein. In some embodiments, the memory associated with operational logic  620  can include RAM, ROM, solid-state memory, magnetic or optically-based memory systems, removable media (e.g., “thumb drives,” SD cards, flash-based devices), or other types of storage media known in the art. One of ordinary skill in the art would understand the many variations, modifications, and alternative embodiments thereof. 
     Movement tracking system  630  can be configured to track a movement of participants in a videoconferencing session. In certain embodiments, one or more optical or auditory sensors can be used for movement and active speaker determination. Optical sensors can take the form of infrared sensors for tracking movement toward and away from the videoconferencing device and auditory sensors can take the form of one or more directional microphones for identifying an active speaker. For example, movement tracking system  630  can provide movement data to a host computer to control magnification and orientation of an imaging device. Movement tracking system  630  can report movement information to processor(s)  610 . 
     Communications system  640  can be configured to provide wireless communication between videoconferencing system  100  and a host computing device, according to certain embodiments. Communications system  640  can employ any suitable wireless communication protocol including, but not limited to Bluetooth®-based communication protocols (e.g., BLE), IR, ZigBee®, ZWire®, Wi-Fi (IEEE 802.11), Thread, Logi® protocols, or other suitable communication technology to facilitate wireless bidirectional communication between videoconferencing system  100  and a host computing device. System  600  may optionally comprise a hardwired connection to a host computing device. For example, videoconferencing device  100  can be configured to receive a Universal Serial Bus (e.g., USB-C) cable to enable bidirectional electronic communication between videoconferencing device  100  and a host computing device. Some embodiments may utilize different types of cables or connection protocol standards to establish hardwired communication with other entities. 
     Input detection system  650  can be configured to detect a touch or touch gesture on one or more buttons, touch sensitive surfaces, or the like, on videoconferencing system  100 . Input detection system  650  can include one or more touch sensitive surfaces, touch sensors, buttons, controls, or other user interface, as would be understood by one of ordinary skill in the art. Touch sensors generally comprise sensing elements suitable to detect a signal such as direct contact, electromagnetic or electrostatic fields, or a beam of electromagnetic radiation. Touch sensors can be configured to detect at least one of changes in the received signal, the presence of a signal, or the absence of a signal. 
     Power management system  660  can be configured to manage power distribution, recharging, power efficiency, and the like, for videoconferencing  100 . In some embodiments, power management system  660  can include a battery (not shown), a USB based recharging system for the battery (not shown), power management devices, and a power grid within system  600  to provide power to each subsystem (e.g., accelerometers, gyroscopes, etc.). In certain embodiments, the functions provided by power management system  660  may be incorporated into processor(s)  610 . The power source can be a replaceable battery, a rechargeable energy storage device (e.g., super capacitor, Lithium Polymer Battery, NiMH, NiCd), or a corded power supply (e.g., via USB-C port—see  FIG. 1 ). One of ordinary skill in the art would understand the many variations, modifications, and alternative embodiments thereof. 
     It should be appreciated that system  600  is illustrative and that variations and modifications are possible. System  600  can have other capabilities not specifically described here (e.g., mobile phone, global positioning system (GPS), power management, one or more cameras, various connection ports for connecting external devices or accessories, etc.). Further, while system  600  is described with reference to particular blocks, it is to be understood that these blocks are defined for convenience of description and are not intended to imply a particular physical arrangement of component parts. Further, the blocks need not correspond to physically distinct components. Blocks can be configured to perform various operations, e.g., by programming a processor or providing appropriate control circuitry, and various blocks might or might not be reconfigurable depending on how the initial configuration is obtained. Embodiments of the present invention can be realized in a variety of apparatuses including electronic devices implemented using any combination of circuitry and software. Furthermore, aspects and/or portions of system  600  may be combined with or operated by other sub-systems as required by design. For example, operational logic  620  may operate within processor(s)  610  instead of functioning as a separate entity. The foregoing embodiments are not intended to be limiting and those of ordinary skill in the art with the benefit of this disclosure would appreciate the myriad applications and possibilities. 
     Other variations are within the spirit of the present disclosure. Thus, while the disclosed techniques are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the disclosure, as defined in the appended claims. For example, the videoconferencing term should be construed broadly and could also refer to a webcam or action camera. Use of the described lens assembly with other imaging system types such as DSLRs, mirrorless and cinema cameras should also be deemed to be within the scope of contemplated use. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening. The phrase “based on” should be understood to be open-ended, and not limiting in any way, and is intended to be interpreted or otherwise read as “based at least in part on,” where appropriate. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the disclosure and does not pose a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure.