HIGH RESOLUTION MINIATURE WIDE-ANGLE LENS

A miniature wide-angle lens including six optical elements and having a wide-angle total field of view between 110° and 140 also has a ratio of an optical lens total track length to an image footprint diameter between 0.85 and 0.95. The lens has a distortion profile creating a resolution curve having a maximum number of pixels/degree that is at least 1.75 times larger than the resolution value in a center of the field of view and at least 1.75 times larger than the resolution value at the edge of the field of view.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to the field of optical lenses and their design and more particularly of an optical construction designed for use on a high-resolution image sensor in a miniature mobile application.

Existing optical lenses fail to offer wide-angle fields of view between 110° and 140° for miniature consumer lenses having a ratio between the optical lens total track length and the image footprint diameter between 0.85 and 0.95 with a distortion profile creating a resolution curve having a maximum number of pixels/degree that is at least 1.75 times larger than the resolution value in the center of the field of view and at least 1.75 times larger than the resolution value at the edge of the field of view.

This unique combination of total field of view, ratio between the total track length and the image footprint diameter and resolution curve would allow for a miniature optical lens creating a high quality image on a larger sensor to be built, with a distortion profile offering the best tradeoff between keeping proportions and keeping straight lines for this kind of wide-angle lens. A new construction for a miniature wide-angle is required to achieve all of these requirements.

BRIEF SUMMARY OF THE INVENTION

To overcome all the previously mentioned issues, embodiments of the present invention present a novel optical lens construction having a wide-angle total field of view between 110° and 140° including at least six optical elements and having a ratio between the optical lens total track length and the image footprint diameter between 0.85 and 0.95. The lens has a distortion profile creating a resolution curve having a maximum number of pixels/degree that is at least 1.75 times larger than the resolution value in the center of the field of view and at least 1.75 times larger than the resolution value at the edge of the field of view, offering the best tradeoff between keeping proportions and keeping straight lines for this kind of wide-angle lens. In order to achieve the desired resolution curve and keep a good balance of image quality, the object-side surface of the first element is concave in a central region around the optical axis and convex in an outer region surrounding the central region, the image-side surface of the first element is convex in a central region around the optical axis and concave in an outer region surrounding the central region, the object-side surface of the last element is convex in a central region around the optical axis and concave in an outer region surrounding the central region and the image-side surface of the last element is concave in a central region around the optical axis and convex in an outer region surrounding the central region.

DETAILED DESCRIPTION OF THE INVENTION

The words “a” and “an”, as used in the claims and in the corresponding portions of the specification, mean “at least one.”

FIG. 1shows the layout100of the optical lens for a first embodiment according to the present invention. The lens includes six optical elements made of plastic material. In an alternate embodiment according to the present invention, at least one of the elements could also be made of glass or other optical material, including diffractive elements or meta-material element. From an object side to an image side, the lens includes a first element120, a second element122, an aperture stop124, a third element126, a fourth element128, a fifth element130, a sixth element132and a cover glass also acting as an IR filter134before an image plane136in which the lens forms an image. An optical axis115represents the central axis of symmetry of the optical lens and is perpendicular to the image plane136. An image sensor is placed at the image plane of the lens when it is forming a camera module. The first element120has a negative power in a paraxial region with a focal f1=−15.35 mm. The object-side surface of the first element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the first element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The second element122has a positive power in a paraxial region with a focal f2=16.41 mm. The object-side surface of the second element is convex. The image-side surface of the second element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The third element126has a positive power in a paraxial region with a focal f3=1.80 mm. The object-side surface of the third element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the third element is convex. The fourth element128has a negative power in a paraxial region with a focal f4=−3.36 mm. The object-side surface of the fourth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the fourth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The fifth element130has a positive power in a paraxial region with a focal f5=1.90 mm. The object-side surface of the fifth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the fifth element is convex. The sixth element132has a negative power in a paraxial region with a focal f6=−2.61 mm. The object-side surface of the sixth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the sixth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface.

Also on the figure, the rays150represent the rays coming from an object in the center of the field of view (object angle of 0°) while the rays160represent the rays coming from an object at the maximum field of view (object angle of 62.5°). Because of symmetry, the total field of view of this lens is twice this angle, for a total field of view of 125°. With this unique combination of 6 optical elements including 12 aspherical freeform surfaces, with 9 surfaces having at least one change of curvature from either concave to convex or convex to concave, this lens construction can achieve better distortion control as will be explained with respect toFIG. 4than existing prior art, while keeping a ratio between the optical lens total track length105and the image footprint diameter110of 0.905, which is within the target between 0.85 and 0.95, allowing a miniature optical lens that covers the full diagonal of large image sensors having generally resolutions of 5 to 25 MPx depending on the pixel size. The focal length of the full optical lens is 2.08 mm in this example embodiment, but in any wide-angle lens embodiment according to the present invention, the focal length is generally under 2.5 mm.

The table atFIG. 2shows the main parameters of the optical prescription for the optical lens for a first embodiment according to the present invention. In this table, surface 0 represents the object at an infinite distance from the lens, surfaces 1 to 4 and 6 to 13 represent the 6 aspherical optical elements, surface 5 represents the aperture stop, surfaces 14 and 15 represent the coverglass also acting as an IR filter and surface 16 is the image plane. For each surface, the radius, thickness, index of refraction and Abbe number are given. The materials used in this example have index and Abbe number values given in the table ofFIG. 2, but other values could be used in other embodiments of the current optical lens. In all embodiments, when V1 represents the Abbe number of the first lens element, V2 the Abbe number of the second lens element, V3 the Abbe number of the third lens element, V4 the Abbe number of the fourth lens element, V5 the Abbe number of the fifth lens element and V6 the Abbe number of the sixth lens element, the following conditions are respected: V1>40, V2>40, V3>40, V4<40, V5>40, V6<40.

The table ofFIG. 3shows the conic constant, the normalization radius and the aspherical coefficient for the 12 aspherical freeform surfaces in this optical lens for a first embodiment. For each surface, the sag Z at a given height r is given by the equation:

where c is the curvature (inverse of the radius of curvature from the table ofFIG. 2), k is the conic constant, αiare the aspherical coefficient from the table ofFIG. 3and p is the normalized radius coordinate obtained by dividing the coordinate r by the normalization radius from the table ofFIG. 3.

FIG. 4shows the resolution curve400resulting from the unique distortion profile of the optical lens for the first embodiment according to the present invention. The resolution curve is the mathematical derivative of the position curve, which is the image height in the image plane in μm as a function of the field of view angle in degree. The resolution curve is thus given in μm/degree as a function of the field of view angle in degree. The resolution curve for the optical lens according to the present invention has a maximum resolution of 66.4 μm/° at an object angle of 47.8° shown at420on the graph, a resolution value of 36.5 μm/° in the center where the object angle is 0° shown at410on the graph and a resolution value of 33.6 μm/° at the edge of the field of view where the object angle is 62.5° shown at430on the graph. The ratio between the maximum value and the central value is ≈1.82 and the ratio between the maximum value and the edge value is ≈1.98. Both of these ratios are higher than 1.75, allowing the ideal balance between keeping the straight lines in the object as straight as possible in the image as well as keeping the ideal proportions especially in the corners of the image without undesirable stretching.

FIG. 5shows the layout500of the optical lens for a second embodiment according to the present invention. The lens includes six optical elements made of plastic material. In an alternate embodiment according to the present invention, at least one of the elements could also be made of glass or other optical material, including diffractive elements or meta-material element. From an object side to an image side, the lens includes a first element520, a second element522, an aperture stop524, a third element526, a fourth element528, a fifth element530, a sixth element532and a cover glass also acting as an IR filter534before an image plane536in which the lens form an image. An optical axis515represents the central axis of symmetry of the optical lens and is perpendicular to the image plane536. An image sensor is placed at the image plane of the lens when it is forming a camera module. The first element520has a negative power in a paraxial region with a focal f1=−12.87 mm. The object-side surface of the first element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the first element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The second element522has a positive power in a paraxial region with a focal f2=14.19 mm. The object-side surface of the second element is convex. The image-side surface of the second element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The third element526has a positive power in a paraxial region with a focal f3=1.88 mm. The object-side surface of the third element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the third element is convex. The fourth element528has a negative power in a paraxial region with a focal f4=−3.48 mm. The object-side surface of the fourth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the fourth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The fifth element530has a positive power in a paraxial region with a focal f5=1.88 mm. The object-side surface of the fifth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the fifth element is convex. The sixth element532has a negative power in a paraxial region with a focal f6=−2.67 mm. The object-side surface of the sixth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the sixth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface.

Also on the figure, the rays550represent the rays coming from an object in the center of the field of view (object angle of 0°) while the rays560represent the rays coming from an object at the maximum field of view (object angle of 62.5°). Because of symmetry, the total field of view of this lens is twice this angle, for a total field of view of 125°. With this unique combination of 6 optical elements including 12 aspherical freeform surfaces, with 9 surfaces having at least one change of curvature from either concave to convex or convex to concave, this lens construction can achieve better distortion control as will be explained with respect toFIG. 8than existing prior art, while keeping a ratio between the optical lens total track length505and the image footprint diameter510of 0.904, which is within the target between 0.85 and 0.95, allowing a miniature optical lens that covers the full diagonal of large image sensors having generally resolutions of 5 to 25 MPx depending on the pixel size. The focal length of the full optical lens is 2.04 mm in this example embodiment, but in any wide-angle lens embodiment according to the present invention, the focal length is generally under 2.5 mm.

The table atFIG. 6shows the main parameters of the optical prescription for the optical lens for the second embodiment according to the present invention. In this table, surface 0 represents the object at an infinite distance from the lens, surfaces 1 to 4 and 6 to 13 represent the 6 aspherical optical elements, surface 5 represent the aperture stop, surfaces 14 and 15 represent the coverglass also acting as an IR filter and surface 16 is the image plane. For each surface, the radius, thickness, index of refraction and Abbe number are given. The materials used in this example have index and Abbe number values given in the table ofFIG. 6, but other values could be used in other embodiments of the current optical lens. In all embodiments, when V1 represents the Abbe number of the first lens element, V2 the Abbe number of the second lens element, V3 the Abbe number of the third lens element, V4 the Abbe number of the fourth lens element, V5 the Abbe number of the fifth lens element and V6 the Abbe number of the sixth lens element, the following conditions are respected: V1>40, V2>40, V3>40, V4<40, V5>40, V6<40.

The table ofFIG. 7shows the conic constant, the normalization radius and the aspherical coefficient for the 12 aspherical freeform surfaces in this optical lens for the second embodiment. For each surface, the sag Z at a given height r is given by the equation:

where c is the curvature (inverse of the radius of curvature from the table ofFIG. 6), k is the conic constant, αiare the aspherical coefficient from the table ofFIG. 7and p is the normalized radius coordinate obtained by dividing the coordinate r by the normalization radius from the table ofFIG. 7.

FIG. 8shows the resolution curve800resulting from the unique distortion profile of the optical lens for the second embodiment according to the present invention. The resolution curve is the mathematical derivative of the position curve, which is the image height in the image plane in μm as a function of the field of view angle in degree. The resolution curve is thus given in inn/degree as a function of the field of view angle in degree. The resolution curve for the optical lens according to the present invention has a maximum resolution of 65.8 μm/° at an object angle of 47.5° shown at820on the graph, a resolution value of 35.9 μm/° in the center where the object angle is 0° shown at810on the graph and a resolution value of 33.3 μm/° at the edge of the field of view where the object angle is 62.5° shown at830on the graph. The ratio between the maximum value and the central value is ≈1.83 and the ratio between the maximum value and the edge value is ≈1.98. Both of these ratios are higher than 1.75, allowing the ideal balance between keeping the straight lines in the object as straight as possible in the image as well as keeping the ideal proportions especially in the corners of the image without undesirable stretching.

FIG. 9shows the layout900of the optical lens for a third embodiment according to the present invention. The lens includes six optical elements made of plastic material. In an alternate embodiment according to the present invention, at least one of the elements could also be made of glass or other optical material, including diffractive elements or meta-material element. From an object side to an image side, the lens includes a first element920, a second element922, an aperture stop924, a third element926, a fourth element928, a fifth element930, a sixth element932and a cover glass also acting as an IR filter934before an image plane936in which the lens form an image. An optical axis915represents the central axis of symmetry of the optical lens and is perpendicular to the image plane936. An image sensor is placed at the image plane of the lens when it is forming a camera module. The first element920has a negative power in a paraxial region with a focal f1=−10.11 mm. The object-side surface of the first element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the first element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The second element922has a positive power in a paraxial region with a focal f2=14.43 mm. The object-side surface of the second element is convex. The image-side surface of the second element is convex. The third element926has a positive power in a paraxial region with a focal f3=1.90 mm. The object-side surface of the third element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the third element is convex. The fourth element928has a negative power in a paraxial region with a focal f4=−3.62 mm. The object-side surface of the fourth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the fourth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The fifth element930has a positive power in a paraxial region with a focal f5=2.01 mm. The object-side surface of the fifth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface. The image-side surface of the fifth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The sixth element932has a negative power in a paraxial region with a focal f6=−2.92 mm. The object-side surface of the sixth element is convex in a region around the optical axis. This central convex region is surrounded by a concave area in an outer region of its surface. The image-side surface of the sixth element is concave in a region around the optical axis. This central concave region is surrounded by a convex area in an outer region of its surface.

Also on the figure, the rays950represent the rays coming from an object in the center of the field of view (object angle of 0°) while the rays960represent the rays coming from an object at the maximum field of view (object angle of 62.5°). Because of symmetry, the total field of view of this lens is twice this angle, for a total field of view of 125°. With this unique combination of 6 optical elements including 12 aspherical freeform surfaces, with 9 surfaces having at least one change of curvature from either concave to convex or convex to concave, this lens construction can achieve better distortion control as will be explained with respect toFIG. 12than existing prior art, while keeping a ratio between the optical lens total track length905and the image footprint diameter910of 0.926, which is within the target between 0.85 and 0.95, allowing a miniature optical lens that covers the full diagonal of large image sensors having generally resolutions of 5 to 25 MPx depending on the pixel size. The focal length of the full optical lens is 2.01 mm in this example embodiment, but in any wide-angle lens embodiment according to the present invention, the focal length is generally under 2.5 mm.

The table atFIG. 10shows the main parameters of the optical prescription for the optical lens for the third embodiment according to the present invention. In this table, surface 0 represents the object at an infinite distance from the lens, surfaces 1 to 4 and 6 to 13 represent the 6 aspherical optical elements, surface 5 represents the aperture stop, surfaces 14 and 15 represent the coverglass also acting as an IR filter and surface 16 is the image plane. For each surface, the radius, thickness, index of refraction and Abbe number are given. The materials used in this example have index and Abbe number values given in the table ofFIG. 10, but other values could be used in other embodiments of the current optical lens. In all embodiments, when V1 represents the Abbe number of the first lens element, V2 the Abbe number of the second lens element, V3 the Abbe number of the third lens element, V4 the Abbe number of the fourth lens element, V5 the Abbe number of the fifth lens element and V6 the Abbe number of the sixth lens element, the following conditions are respected: V1>40, V2>40, V3>40, V4<40, V5>40, V6<40.

The table ofFIG. 11shows the conic constant, the normalization radius and the aspherical coefficient for the 12 aspherical freeform surfaces in this optical lens for the third embodiment. For each surface, the sag Z at a given height r is given by the equation:

where c is the curvature (inverse of the radius of curvature from table ofFIG. 10), k is the conic constant, αiare the aspherical coefficient from the table ofFIG. 11and p is the normalized radius coordinate obtained by dividing the coordinate r by the normalization radius from the table ofFIG. 11.

FIG. 12shows the resolution curve1200resulting from the unique distortion profile of the optical lens for the third embodiment according to the present invention. The resolution curve is the mathematical derivative of the position curve, which is the image height in the image plane in μm as a function of the field of view angle in degree. The resolution curve is thus given in inn/degree as a function of the field of view angle in degree. The resolution curve for the optical lens according to the present invention has a maximum resolution of 67.6 μm/° at an object angle of 47.6° shown at1220on the graph, a resolution value of 35.2 μm/° in the center where the object angle is 0° shown at1210on the graph and a resolution value of 33.4 μm/° at the edge of the field of view where the object angle is 62.5° shown at1230on the graph. The ratio between the maximum value and the central value is ≈1.92 and the ratio between the maximum value and the edge value is ≈2.02. Both of these ratios are higher than 1.75, allowing the ideal balance between keeping the straight lines in the object as straight as possible in the image as well as keeping the ideal proportions especially in the corners of the image without undesirable stretching.

All embodiments presented were using aspherical shapes with rotational symmetry, but any freeform surface with or without rotational symmetry could also be used according to the present invention. In some embodiments, at least one asymmetric freeform surface could be used to create an anamorphic image plane in which the focal length in a first direction is larger than the focal length in a second perpendicular direction. This optional stretching of the image in a direction is useful especially when the image sensor is of rectangular shape and the lens is optimal when having different magnifications in both main directions of the image sensor. In these cases, the field of view in a first direction could be different or not from the field of view in a second direction perpendicular to the first direction.

All of the above figures and example show embodiments of the miniature optical lens having a total field of view between 110° and 140°, but other similar embodiments could be possible with small departures from the present lens prescriptions. In most embodiments, the optical lenses have a ratio between the optical lens total track length and the image footprint diameter between 0.85 and 0.95. In most embodiments, the lenses have a distortion profile creating a resolution curve having a maximum number of pixels/degree that is at least 1.75 times larger than the resolution value in the center of the field of view and at least 1.75 times larger than the resolution value at the edge of the field of view. In most embodiments, in order to achieve the desired resolution curve and keeping a good balance of image quality, the object-side surface of the first element has a concave curvature in a central region around the optical axis and a convex curvature in an outer region surrounding the central region, the image-side surface of the first element has a convex curvature in a central region around the optical axis and a concave curvature in an outer region surrounding the central region, the object-side surface of the last element has a convex curvature in a central region around the optical axis and a concave curvature in an outer region surrounding the central region and the image-side surface of the last element has a concave curvature in a central region around the optical axis and a convex curvature in an outer region surrounding the central region. In addition to these four surfaces having a change of curvature from either concave to convex or convex to concave from the center to the edge of the surface, in most embodiments there are at least eight total surfaces having these changes of curvature.