Fixed focus camera module with near-field image recognition

A camera apparatus includes an image sensor to output an image signal, a stop aperture, a lens assembly, and a controller. The lens assembly is disposed between the image sensor on an image side of the lens assembly and the stop aperture on an object side of the lens assembly. The lens assembly includes a plurality of lens elements that collectively induce axial chromatic aberration between red, green, and blue light. The controller is coupled to receive red, green, and blue channels of the image signal. The controller includes logic that causes the controller to use the blue channel without the red or green channels of the image signal to perform image recognition on objects captured in a near-field of the lens assembly and to use the blue, red, and green channels collectively when capturing images in a far-field of the lens assembly.

TECHNICAL FIELD

This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to fixed focus lens assemblies and camera modules.

BACKGROUND INFORMATION

Conventional digital imaging devices or cameras often include a lens assembly (which includes multiple lens elements) that focuses image light onto an image sensor that measures the image light and generates an image based on the measurements.FIG. 1illustrates a common configuration for a digital imaging device100, which includes an image sensor110and a lens assembly115including multiple lens elements120. Lens assembly115is disposed over image sensor110to focus image light125from an object onto the pixels of image sensor110. Lens assembly115may include convex and/or concave lens elements120that together give lens assembly115an effective focal length (“EFL”). The lens assembly115also provides a certain Depth of Field (“DOF”). DOF refers to the range of distances in the field of view of an image sensor that appear to be well focused in an image captured by image sensor110. The Field of View (“FOV”) of lens assembly115refers to the angular extent that is viewable through lens assembly115at a given moment.

Lens assembly115has a z-axis height H1, which is also referred to as the optical total track length (“TTL”). The optical TTL is typically influenced by the FOV and the size of image sensor110, as well as other design choices. A larger optical TTL may limit the applications or uses of digital imaging device100because of space constraints. Therefore, a lens assembly that allows a digital imaging device to capture high resolution images with a large FOV while reducing the optical TTL can be advantageous. A lens assembly that provides a large DOF where both far and near images are brought into focus is also desirable.

DETAILED DESCRIPTION

Head wearable displays are becoming portable computing devices that are used for everyday tasks such as capturing far-field (e.g., greater than 1 m object distance) full color pictures for personal use (e.g., memory savers, social media sharing, etc.) and capturing near-field (e.g., less than 1 m) images for image recognition tasks. Near-field image recognition tasks can include optical character recognition (“OCR”), bar code scanning, 2D code recognition (e.g., QR codes, data matrix codes, etc.), feature recognition, object recognition, etc. Accordingly, a camera module capable of acquiring high quality full color far-field images while also being capable of acquiring high quality near-field images acceptable for image recognition is desirable for head wearable displays.

Camera modules for use in head wearable displays should also have a FOV ranging between 80 to 110 degrees (and in particular between 88 and 100 degrees). In contrast, cell phone cameras typically have a FOV in the 64 to 75 degree range and don't have as tight form factor constraints. Other conventional lens assemblies, such as fish eye lenses, are designed to have very wide fields of view, typically above 120 degrees, but are not well suited for typical wearable computing tasks. Finally, such camera modules should also be compact and light weight. As such, fixed focus lenses provide the smallest and lightest form factors.

FIGS. 2 and 3are cross-sectional views illustrating a camera module200having a lens assembly that provides a large FOV with reduced optical TTL and improve near-field image recognition, in accordance with an embodiment of the disclosure.FIG. 2provides an unobstructed cross-sectional view of camera module200whileFIG. 3illustrates light ray bundles passing through the lens assembly. The illustrated embodiment of camera module200includes a stop aperture205, a lens assembly210, an infrared cut filter (“IRCF”)215, and an image sensor220. The illustrated embodiment of lens assembly210has six total lenses including: a lens L1 having surfaces S2 and S3, a lens L2 having surfaces S4 and S5, a lens L3 having surfaces S6 and S7, a lens L4 having surfaces S8 and S9, a lens L5 having surfaces S10 and S11, and a lens L6 having surfaces S12 and S13. The object225corresponds to surface S0, stop aperture205corresponds to surface S1, and IRCF215includes surfaces S14 and S15. The light incident side surface S16 of image sensor220is positioned at image plane230to capture object image235.

Lens assembly210provides a fixed focus camera module200with a large FOV in a compact form factor along the z-axis (the axis running parallel to the depth of field) while achieving good optical characteristics (e.g., acceptable optical distortion, well controlled field curvatures along tangential and sagittal directions, well controlled lateral color, etc.). Various embodiments of lens assembly210may range between 80 degrees and 110 degrees for the FOV. In one embodiment, camera module200has a diagonal FOV of 90 degrees with an optical TTL of 4.09 mm for image sensor220having a full image circle size of 5.6 mm, and stop aperture205providing an F-number of 2.4. In this embodiment, IRCF215is implemented as a blue glass IRCF having a thickness of 0.25 mm. Of course, camera module200may be implemented with its constituent components having other dimensions. For example, the F-number may typically vary between 2.0 to 2.4 for use in a head wearable display, though greater variances may be implemented.

Lens assembly210is purposefully designed to induce axial chromatic aberration (seeFIG. 4) such that light corresponding to one color channel has a substantially shorter focal distance than light corresponding to the remaining two color channels. In one embodiment, light corresponding to the dominate signal channel of an image sensor is selected to have a longer focal distance than at least one of the other signal channels. Since green light in a typically RGGB Bayer pattern color filter array provides the dominate signal channel (two green filters for each red and blue filter in a unit pixel) and blue light with a shorter wavelength naturally tends to have a shorter focal distance anyway, in one embodiment, lens assembly210is designed with axial chromatic aberration that intentionally focuses blue light at a substantially shorter focal distance than green and red light. By doing so, all three color channels of the image sensor can be used to acquire full color far-field pictures, while the monochrome blue channel can be analyzed when acquiring near-field pictures for image recognition. Since the blue contribution in the far-field is less significant than the combination of the green and red channels, far-field picture quality remains acceptable. Since the blue channel has a substantially shorter focal distance, the blue channel of near-field pictures is sharp. This enables a fixed focus camera to have good far-field characteristics for general image capture, while also having good near-field characteristics for image recognition. Image recognition need not be based upon full color images—particularly for OCR, bar code scanning, and 2D code recognition—which are typically based upon black and white objects.

In one embodiment, lens assembly210is designed to provide an object focal distance for green light that is greater than twice an object focal distance for blue light. For example, green light may have an object focal distance of approximately 1 m, while blue light has an object focal distance of approximately 0.4 m. Of course, lens assembly210may be designed with other object focal distances, but typically the object focal distance for green light will range between 0.7 m to 1.8 m, while the object focal distance for blue light will range between 0.2 m to 0.6 m to provide the above recited near-field and far-field characteristics in a fixed focus camera module. Thus, the blue channel with its shorter focal distance provides a sharper image in the near-field than the green channel with its longer focal distance.

Returning toFIG. 2, lens assembly210includes a total of six discrete lenses L1 to L6 with stop aperture205positioned in front of (i.e., on the object side of) the first lens L1 in the optical train. In other words, lens assembly201does not include any surfaces with optical power on the object side of stop aperture205.

In the illustrated embodiment, lens L1 has positive optical power, lens L2 has positive optical power, lens L3 has negative optical power, lens L4 has positive optical power, lens L5 has positive optical power, and lens L6 has negative optical power. Thus, lens assembly210includes six total lenses with four lenses having varying degrees of positive optical power and two lenses having varying degrees of negative optical power. Thus the total optical power of lens assembly210is distributed across the six lenses L1 to L6. The illustrated embodiments of lens L1 to L6 are discrete elements within intervening air gaps. These discrete lenses can be fabricated of a variety of materials (e.g., plastic, glass, etc.). In one embodiment, lens L1 to L6 are fabricated of injection molded plastics for high volume manufacturing.

Lens L1 is the first inline lens in the optical train and is positive, contributing to the total positive optical power. Lens L1 operates to reduce the large ray angle of the upper marginal ray for large off-axis field heights. This reduction of ray angle helps to reduce optical aberration for the upper marginal rays for the large off-axis field heights. The Abbe number is a measure of a transparent material's dispersion in relation to its index of refraction. In one embodiment, lens L1 is made of a material having the lowest Abbe number of the lenses with positive optical power, thereby contributing substantially to axial color aberration.

Lens L2 is the second inline lens in the optical train and is strongly positive, therefore contributing to the total positive optical power. Lens L2 is designed to have a high Abbe number thereby reducing its contribution to axial color aberration (dispersion).

Lens L3 is the third inline lens in the optical train and has negative optical power. The negative optical power of lens L3 reduces the field curvature of lens assembly210. Lens L3 may also be made of a material having low Abbe number and partially offsets axial color aberration induced by the positive power lenses.

Lens L4 is the fourth inline lens in the optical train and is weakly positive, but contributes to the overall positive optical power. In one embodiment, lens L4 has the weakest positive power of all the positive lenses.

Lens L5 is the fifth inline lens in the optical train and has positive optical power, contributing to the overall positive optical power. Lens L5 is strongly positive. In one embodiment, lens L5 has the strongest positive optical power (i.e., shortest focal length of the positive lenses). Lens L5 is made of a material having a high Abbe number thereby reducing its contribution to axial color aberration.

Lens L6 is the sixth inline lens in the optical train and has negative optical power. In one embodiment, lens L6 has the strongest negative optical power of the lenses having negative optical power. In the illustrated embodiment, lens L6 is the largest lens in lens assembly210. Lens L6 has an inflection point in the curvature of surface S13 and no inflection point in the curvature of surface S12. Lens L6 operates as a field corrector. The rays from different field heights fall on different regions of lens L6, which servers to correct field curvature, control optical distortion, and control the chief ray angle in the image space. For some implementations of image sensor220(e.g., CMOS image sensors), the chief ray angle in the image space should be kept below 32 degrees to maintain desirable quantum efficiency and low cross-talk. To achieve this, the chief ray angle for large field heights should be constrained. Lens L6 serves as a field corrector to maintain reasonable chief ray angles.

In the illustrated embodiment, lens L6 is the largest lens and larger than lens L5. The diameter of lens L6 is sufficiently large while the diameter of lens L5 sufficiently small, relative to lens L6, such that the convex shape of surface S11 extends into a recess formed by the concave surface S12 of lens L6. This design feature contributes to the overall compactness of the optical TTL.

IRCF215may be implemented using a variety of different types of filters to cut out the infrared spectrum. For example, IRCF215may be implemented as a pigmented or absorptive color filter (e.g., blue glass filter) or an interference filter. However, given that an interference filter operates by reflecting the IR wavelengths back into lens assembly210, these reflections may again bounce back towards image sensor220due to the refractive index interfaces at each lens surface. Accordingly, absorptive type IRCF may be more effective at removing infrared wavelengths. These wavelengths are removed, since they are not visible to the human eye, but may be picked up by image sensor220. In one embodiment, IRCF215is a blue glass infrared cut filter having a thickness of 0.25 mm.

Image sensor220is positioned such that its light sensitive surface S16 is coincident with the image plane230of lens assembly210. Image sensor220may be implemented using a variety of technologies including charged coupled devices (“CCD”) sensors or complementary metal-oxide-semiconductor (“CMOS”) sensors. In one embodiment, image sensor220is a 1/3.2″ 5 megapixel CMOS sensor.

FIGS. 12A & 12Bare tables that provide design parameters for implementing an example lens assembly according to the design guidelines set forth above. The table illustrated inFIG. 12Asummarizes the radius of curvature, thickness, material type, refractive index, Abbe number, and focal length of each surface S0-S16 (illustrated inFIG. 2) for one demonstrative implementation of camera module200. Similarly, the table illustrated inFIG. 12Bsets forth the conic constant (k) and aspheric coefficients describing the shape of each surface S2 through S13 illustrated inFIG. 2. The coefficients A4 through A16 represent the coefficients in the first seven series elements in the aspheric surface equation,

z⁡(r)⁢cr21+1-(1+k)⁢c2⁢r2+A4⁢r4+A6⁢r6+A8⁢r8+A10⁢r10+A12⁢r12+A14⁢r14+A16⁢r16,
where c represents the radius of curvature (1/radius), k represents the conic constant, and r represents the radius of the lens.

FIG. 4is a chart illustrating how the lens design described above, and detailed inFIGS. 12A-12B, purposefully induces axial chromatic aberration to improve near-field image recognition, in accordance with an embodiment of the disclosure. As illustrated, the illustrated embodiment of lens assembly210has an on-axis (at center of lens) axial chromatic separation A between blue light and red light of approximately 42 um. However, in other embodiments, the on-axis axial chromatic separation A may range between 30 um and 50 um. This is in contrast with conventional lens design which attempt to minimize axial chromatic separation. A conventional lens assembly may achieve an on-axis axial chromatic separation of about 10 um.

FIGS. 5A & 5Bare comparison charts illustrating a modulation transfer function (“MTF”) for green light at a 1.5 m object distance (far-field) for a conventional camera module versus camera module200, in accordance with an embodiment of the disclosure. MTF is the magnitude component of the optical transfer function, a well-known transfer function used to describe how an optical system projects light from an object to an image sensor. The closer the MTF value is to 1.0, the better the image quality of the optical system.

Conventional lenses have an on-axis axial color aberration of approximately 10 um. It is the goal of typical conventional lens designers to minimize axial color aberration, since it separates color images and reduces overall image quality.FIG. 5Aillustrates that a conventional lens design having about 10 um of on-axis axial color aberration has an on-axis MTF of about 0.87 for green light (555 nm wavelength). Correspondingly,FIG. 5Billustrates the on-axis MTF for camera module200and achieves a comparable on-axis MTF value of 0.87 at a 1.5 m object distance for green light (555 nm wavelength).

FIGS. 6A & 6Bare comparison charts illustrating the MTF for blue light at a 0.2 m object distance (near-field) for a conventional camera module versus camera module200, in accordance with an embodiment of the disclosure.FIG. 6Aillustrates that a conventional lens design having about 10 um of on-axis axial color aberration has an on-axis MTF of about 0.5 for blue light (470 nm wavelength). Correspondingly,FIG. 6Billustrates the MTF for camera module200and achieves an on-axis MTF value of 0.6 at a 0.2 m object distance for blue light (470 nm wavelength). Accordingly,FIGS. 5 and 6illustrate that camera module200has comparable image quality for green light in the far-field, but has 20 percent improvement in blue light image quality in the near-field over conventional camera modules with modest axial color aberration. It is noteworthy that the green channel of an image sensor having a typical Bayer color filter array is a more dominant contributor to the overall full color image than the red or blue channels.

FIGS. 7 and 8provide visual evidence of what the charts inFIGS. 5 and 6describe. In particular,FIG. 7Aillustrates a picture at a 1.5 m object distance (far-field) using all three red, green, and blue color channels of a conventional image sensor. Correspondingly,FIG. 7Billustrates a picture also at a 1.5 m object distance (far-field) using all three red, green, and blue color channels from camera module200. As illustrated the two pictures are very similar with little discernible image quality difference. However,FIG. 8Aillustrates a picture at a 0.2 m object distance (near-field) using all three red, green, and blue color channels of a conventional image sensor with low axial color aberration. It can be seen that the image quality if fairly low with significant blurriness. In contrast,FIG. 8Billustrates a picture at a 0.2 m object distance (near-field) using only the blue channel of camera module200. It can be seen that the image quality is significantly improved (sharper) overFIG. 8Aand suitable for OCR or other image recognition.

FIG. 9is a cross-sectional view illustrating a camera module900including a barrel style lens assembly, in accordance with an embodiment of the disclosure. The illustrated embodiment of camera module900includes a lens holder905, a lens assembly910, a stop aperture915, an IRCF920, an image sensor925, and a substrate930. Camera module900represents one possible form factor for implementing camera module200illustrated inFIGS. 2 and 3. Correspondingly, lens assembly910is an example implementation of lens assembly910, while stop aperture915, IRCF920, and image sensor925correspond to implementations of stop aperture205, IRCF215, and image sensor220, respectively.

Lens assembly910packages the discrete lens elements (L1 through L6) into a barrel style form factor, which threads into lens holder905. In one embodiment, lens assembly910includes male threads around its perimeter, which mate to female threads on the inside edge of lens holder905. This thread design facilitates offset adjustment to align the image plane230of lens assembly910with the light sensitive side S16 of image sensor925. Lens holder905also serves to seal image sensor925on top of substrate930and prevent dust or other contaminants for accumulating on image sensor925. It should be appreciated that lens assembly210may be implemented in other form factors than the barrel style illustrated inFIG. 9.

FIG. 10is a functional block diagram illustrating a camera system1000, in accordance with an embodiment of the disclosure. The illustrated embodiment of camera system1000includes camera module200, a controller1005, and memory1010. Camera system1000is well suited for integration within an head wearable display, but may also be used in various other products including smart phones, tablets, laptops, televisions, watches, etc. Camera module200provides a high resolution camera having a wide FOV, a compact z-axis form factor, and good near and far field characteristics in a fixed focus design. During operation, controller1005may perform a number of post imaging functions including cropping, compression, image analysis, etc. using instructions stored in memory1010. The acquired images may further be stored into memory1010, which may be implemented using volatile memory (DRAM, SRAM, etc.) or non-volatile memory (e.g., flash memory, etc.).

In one embodiment, controller1005includes hardware logic (or executed software logic stored in memory1010) to identify near-field objects. In one embodiment, identification of near-field object may be assumed when a user inputs a request to perform image recognition. Other image processing techniques, or otherwise, may be implemented to identify when an object is a near-field object upon which image recognition is to be performed. When it is determined that image recognition is to be performed upon a near-field object, controller1005ignores the red and green image signal channels output from camera module200and analyzes only the blue channel when performing image recognition. When capturing images of far-field objects, all three RGB color channels of the image signal are used.

FIG. 11is a perspective view illustrating a binocular head wearable display1100that may include a camera system1000, in accordance with an embodiment of the disclosure. The illustrated embodiment of binocular head wearable display1100includes two see-through displays1101that may present each eye of the user with a computer generated image (“CGI”) superimposed over there vision of the external world. Camera system1000may be used to capture images of the external world, which are then processed by controller1005to generate CGI that is context sensitive to the user's environment.

The see-through displays1101are mounted to a frame assembly, which includes a nose bridge1105, left ear arm1110, and right ear arm1115. Camera system1000may be disposed in any of portion of the frame assembly with a forward facing perspective. In other embodiments, camera module1000may be a rear facing camera positioned to capture eye images for eye tracking functionality, in addition to, or in place of, a forward facing camera. AlthoughFIG. 11illustrates a binocular embodiment, display1100may also be implemented as a monocular display.

The see-through displays1101are secured into an eye glass arrangement or head wearable display that can be worn on the head of a user. The left and right ear arms1110and1115rest over the user's ears while nose bridge1105rests over the user's nose. The frame assembly is shaped and sized to position each display in front of a corresponding eye of the user. Other frame assemblies having other shapes may be used (e.g., a visor with ear arms and a nose bridge support, a single contiguous headset member, a headband, goggles type eyewear, etc.).