Compact imaging lens system

A compact imaging lens system includes, in order from the object side to the image side, a first lens (1) of negative refractive power, a free-form surface prism (2) of positive refractive power and a second lens (3) of positive refractive power. The free-form surface prism has an incidence surface (S3), a reflection surface (S4) and an exit surface (S5). The first lens is disposed on the side of the incidence surface, and the second lens is disposed on the side of the exit surface. The free-form surface prism functions equivalent to a right-angle prism with aspheric surfaces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging lens system, and particularly relates to a compact imaging lens system suitable for use in an image pick-up device.

2. Description of Prior Art

Integration of photoelectric technology into an image pick-up device is now a trend in current technology development. To fulfill the portability requirement, the image pick-up device has to be light in weight and small in size. In the selection of imaging lens shapes and materials for use in the image pick-up device, the conventional spherical ground glass lenses have been widely adopted because there is a wide range of materials available for making such lenses and color aberration can be more easily corrected. However, the spherical ground glass lenses have the disadvantages of uneasy correction of spherical aberration and astigmatic aberration in the case of a small F-number and a large wide angle of field. To improve the above-mentioned drawbacks existed in the conventional spherical ground glass lenses, aspheric glass or plastic lenses have been introduced to help reduce these aberrations. The inclusion of aspheric lens elements in a lens system significantly increases image quality and reduces barrel distortion associated with wide-angle lenses. Further, since one aspheric lens element can replace several spherical lens elements in the lens system to perform the same aberration correction function, reduction of the amount of lens elements in the system is also enabled, which makes the lens system smaller and lighter. However, for a conventional glass lens system, to effectively correct off-axis aberrations and color aberrations associated with a wide angle of view, a relatively larger amount of lens elements must be employed. This makes such a glass lens system long in length, large in volume and high in cost, which deviates from the current compact trend. In addition, the machining of an aspheric glass lens element is very difficult, which further limits the application of such a glass lens system in a compact digital product. Comparatively, an aspheric plastic lens element is easy to machine and low in cost. Therefore, aspheric plastic lens elements are widely used in compact imaging lens systems to reduce the length of the entire system.

For a wide-angle lens system, important design considerations include correction of off-axis aberrations and color aberrations associated with a wide angle of view, serious distortions and so on. Accordingly, a wide-angle lens system design is more difficult, and various problems may arise such as unmachinability of a lens element shape and too large chief ray angles. Various wide-angle lens system designs have been disclosed in, for example, U.S. Pat. Nos. 4,493,537, 5,251,073, and 4,525,038. However, a design, which ensures effective correction of various aberrations at a very short total length while giving attention to actual machinability, is rare. Accordingly, a prism type design and a free-form surface prism type design have been proposed to reduce the total length of the lens system. The term “free-form surface” means a curved surface that is neither a plane nor part of a sphere, and the term “free-form surface prism” means a prism having a free-form surface on at least one surface. A free-form surface prism is described in, for example, U.S. Pat. No. 6,323,892. The above-mentioned prism type design folds the optical path by 90 degrees by means of a 45° reflecting mirror that has no aberration correction function.

The application of a free-form surface prism has the following advantages: (1) The total length of a lens system can be reduced by several light reflections in the free-form surface prism to obtain the light path folding effect; (2) As no chromatic aberration exists on a reflection surface of the free-form surface prism, the amount of constituting elements in the lens system can be reduced since no additional lens elements are needed to compensate a large amount of chromatic aberrations which occur when employing conventional refraction elements; (3) Assembly of the lens system is facilitated since the positional relationship between optical surfaces on the free-form surface prism are fixed. However, conventional free-form surface prisms are generally complicate in construction, which makes its design, machining and fixing difficult. Therefore, there still remains room for improvement.

SUMMARY OF THE INVENTION

Accordingly, the main object of the present invention is to provide a compact imaging lens system that employs a free-form surface prism to reduce the total length of the lens system by light reflection.

Another object of the present invention is to provide a compact imaging lens system that is simple in structure, easy in machining, low in cost, and has a wide angle of view and good image quality.

To achieve the above objects of the present invention, a compact imaging lens system in accordance with the present invention, which is suitable for use in an image pick-up device, includes, in order from an object side to an image side, a first lens of negative refractive power, a prism of positive refractive power and a second lens of positive refractive power. The prism has an incidence surface, a reflection surface and an exit surface. The first lens is disposed on the side of the incidence surface of the prism, and the second lens is disposed on the side of the exit surface of the prism. The first lens is a biconcave lens having a first concave surface facing the object to be imaged and a second concave surface on the image side. The prism is in the form of a free-form surface prism that functions equivalent to a right-angle prism with aspheric surfaces. The incidence surface of the prism faces the second surface of the first lens, the reflection surface of the prism is inclinedly disposed on an optical axis of the lens system, and the exit surface of the prism faces toward an image plane. Both the incidence surface and the exit surface of the prism are convex surfaces, and the reflection surface of the prism is a planar surface or a curved surface. The second lens is disposed between the free-form surface prism and the image plane, and is in the form of a biconvex lens, a concave-convex or a convex-concave lens. An aperture stop is further disposed between the second lens and the free-form surface prism.

At least one of the first and second surfaces of the first lens is made aspheric. Both the incidence surface and the exit surface of the prism are aspheric surfaces. The second lens may also be an aspheric lens.

The first lens, the prism and the second lens are all made of plastics. Alternatively, the second lens also may be made of glass.

The first lens, the prism and the second lens of the present compact imaging lens system satisfy the following conditional expressions:
1.3<|f1/f|<2.3;
2.5<f2/f<5.0;
2.5<f3/f<4.0
where f represents effective focal length of the entire lens system, and f1, f2 and f3 represent focal lengths of the first lens, the prism and the second lens, respectively.

Compared with the prior art, the present compact imaging lens system is only composed of two lenses and a prism, and thus has the advantages of simple structure, easy assembly, wide angle of view, small size and light weight. By application of a free-form surface prism, the optical axis of the present lens system is rotated by 90 degrees to fold the optical path, whereby the total length and the volume of the present lens system can be reduced. At the same time, various aberrations also can be compensated to obtain high image resolution. Further, by replacing conventional glass lens elements with injection molded plastic lens elements having aspheric surfaces, the production cost of the present lens system is significantly reduced, the optical length of the present lens system is shortened and the production yield is also increased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compact imaging lens system in accordance with the present invention is suitable for use in an image pick-up device, so that an object to be imaged can be imaged onto an image sensor such as a CCD (Charge-Coupled Device) sensor or CMOS (Complimentary Metal-Oxide Semiconductor) sensor.

Referring toFIG. 1, a compact imaging lens system in accordance with the concept of the present invention includes, in order from the object side to the image side, a first lens1of negative refractive power, a prism2of positive refractive power and a second lens3of positive refractive power. The prism2and the first lens1are arranged in a juxtaposed manner. The second lens3is disposed below the prism2and adjacent to the image side. The incident light from the object to be imaged is transmitted sequentially through the first lens1, the prism2and the second lens3, and then is focused onto an image sensor (image plane)4such as a CCD sensor or CMOS sensor to obtain a clear image.

The first lens1is a biconcave lens for collecting light from the object to be imaged. The first lens1has a first concave surface S1facing the object and a second concave surface S2facing the prism2. At least one of the first and second concave surfaces S1and S2is made aspheric for compensating distortion and lateral color aberrations.

The prism2is in the form of a free-form surface prism that functions equivalent to a right-angle prism with aspheric surfaces. It should be understood that, while the intersect angle arrangement of the prism2is described as a right angle and the right angle is preferred for improving machining stability and tolerance allowance, in some embodiments other intersect angles may also be employed. The prism2simultaneously implements several optical functions that include providing three surfaces for aberration correction and rotating the optical axis of the lens system by 90 degrees. The prism2has an incidence surface S3facing the second concave surface S2of the first lens1, a reflection surface S4inclinedly disposed on the optical axis, and an exit surface S5toward the image plane4. Both the incidence surface S3and the exit surface S5are convex surfaces and are made aspheric, and the reflection surface S4is a planar surface or a curved surface. The first lens1is disposed on the side of the incidence surface S3of the prism2, and the second lens3is disposed on the side of the exit surface S5of the prism2.

The second lens3having two opposite surfaces S7, S8is disposed between the free-form surface prism2and the image plane4, and is in the form of a biconvex lens, a concave-convex or a convex-concave lens. The second lens3serves to compensate spherical aberrations and on-axis color aberrations. The second lens3may be a spherical convex lens or an aspheric convex lens having at least one aspheric surface, the latter of which is preferred for aberration collection purposes.

An aperture stop5is further disposed between the second lens3and the free-form surface prism2for controlling the amount of light that enters. The present lens system may further include a glass cover6between the aperture stop5and the image plane4. Optical functional films, such as an anti-reflection film and an IR (infrared) reflecting film, may be coated on the glass cover6to obtain a better image quality.

As described above, the first lens1, the free-form surface prism2and the second lens3of the present imaging lens system may all be made aspheric. Thus, while ensuring efficient correction of color aberrations and off-axis aberrations, the number of constituting elements of the present lens system can be significantly reduced, the total length of the present lens system thus can be shortened and the system weight also can be significantly reduced. To reduce production costs, the first lens1, the prism2and the second lens3are preferably all made of plastics. Alternatively, the second lens3may be made of glass.

In the present compact imaging lens system, the light from the object to be imaged is first transmitted through the first lens1, incident into the prism2via the incidence surface S3of the prism2, then reflected by the reflection surface S4to the exit surface S5of the prism2, and finally incident into the second lens3through the exit surface S5of the prism2and the aperture stop5. The interior light reflection within the prism2by the reflection surface S4rotates the optical axis of the present lens system by 90 degrees to achieve the optical path folding effect. Accordingly, in comparison with a conventional refraction optical system, the size of the present optical system can be significantly reduced. Further, the three surfaces of the free-form surface prism2, i.e., the incidence refracting surface S3, the reflection surface S4and the exit refracting surface S5, perform refracting, reflecting and aberration collecting functions equivalent to three individual lenses. Therefore, in comparison with a reflecting member only having a reflecting function for optical path folding, the free-form surface prism2has multiple functions one of which is aberration correction. In addition, compared with a conventional design having corresponding lenses or reflectors disposed in the air, since the prism2body is constructed with a transparent material having a higher refractive index than the air, the optical path length of the present lens system employing the prism2can be elongated while shortening the total length thereof.

To assure good optical performance, the first lens1, the prism2and the second lens3of the present compact imaging lens system satisfy the following conditional expressions:
1.3<|f1/f|<2.3  (1)
where f represents the effective focal length of the entire lens system and f1 represents the effective focal length of the first lens1and when |f1/f| exceeds the upper limit, astigmatism correction becomes insufficient, and when it is below the lower limit, axial color aberrations increase;
2.5<f2/f<5.0  (2)
where f2 represents the effective focal length of the prism2and when f2/f exceeds the upper limit lateral color aberrations increase, and when it is below the lower limit, spherical aberrations and coma aberrations are difficult to correct;
2.5<f3/f<4.0  (3)
where f3 represents the effective focal length of the second lens3and when f3/f exceeds the upper limit, that is, when the focal length of the second lens3is too long, the total length of the present lens system becomes increasingly long, and when it is below the lower limit, coma aberrations and astigmatism aberrations are difficult to correct.

The present invention will be more fully understood by describing four numerical embodiments as follows in combination with corresponding figures and graphs.

As shown inFIG. 2, a compact imaging lens system in accordance with Numerical Embodiment 1 of the present invention includes a first lens11of negative refractive power, a free-form surface prism21of positive refractive power, an aperture stop51, a second lens31of positive refractive power, a glass cover61and an image plane41. The first lens11is a biconcave lens. The free-form surface prism21has a convex incidence surface S31, a planar reflecting surface S41and a convex exit surface S51. The second lens31is a biconvex lens.

The numerical data of Numerical Embodiment 1 will be shown below. In each numerical embodiment, “i” indicates the order of the surface from the object side (including lens surfaces, the aperture stop, the glass cover and the image plane), Ri indicates the curvature radius of the ith surface, D/T indicates the ith member thickness or the distance between the ith surface and the (i+1)th surface, and Nd and Vd indicate the refractive index and Abbe number of the ith member, respectively, for d-line.

Surface (i)Ri (mm)D/T (mm)NdVdS0∞S11−3.7401.11.513657.4S211.901.70S314.0482.7261.513657.4S41∞−2.7261.513657.4S512.692−1.5S61∞−0.1S71−2.491−0.71.53657.4S8118.681−1.77S91∞−0.81.5163364.1S101∞
In the above table, as the coordinate rotates 90 degrees at the reflecting surface S41of the prism21, negative distances are shown.

In this numerical embodiment, the first lens11, the prism21and the second lens31are all made aspheric. The aspheric surfaces thereof are expressed by the following equation:

z=c⁢⁢h21+[1-(k+1)⁢c2⁢h2]12+A⁢⁢h4+B⁢⁢h6+C⁢⁢h8+D⁢⁢h10
where z is sag value along the optical axis; c is the base curvature (1/radius) of the surface; h is the semi-diameter height; k is the conic coefficient; and A, B, C and D are the 4th-order, 6th-order, 8th-order and 10th-order aspheric coefficients, respectively.

Specifically, the two surfaces S11, S21of the first lens11, the incidence surface S31and the exit surface S51of the prism21, and the surface S71of the second lens31are all made aspheric. Aspheric coefficients for these aspheric surfaces are illustrated below:

In this numerical embodiment, the respective values of the above conditions (1), (2) and (3), and the effective focal length, the field of view, the total length and the F-number of the present lens system are listed in the table as below.

Effective Focal Length of Lens System (f)1.3 mmField of View (F.O.V)120 degreesTotal Length of Lens System13.13 mmF-number2.82|f1/f|1.77f2/f3.34f3/f3.33

As illustrated in the above table, the respective values of |f1/f|, f2/f and f3/f are 1.77, 3.34 and 3.33, which are all within a corresponding range specified by condition (1), (2) or (3).

FIGS. 3-6respectively show graphs of longitudinal spherical aberration, field sags, distortion and lateral color aberration of Numerical Embodiment 1 of the present compact imaging lens system. From these graphs, it can be seen that the present compact imaging lens system of Numerical Embodiment 1 provides a high level of optical performance.

As shown inFIG. 7, a compact imaging lens system in accordance with Numerical Embodiment 2 of the present invention includes a first lens12of negative refractive power, a free-form surface prism22of positive refractive power, an aperture stop52, a second lens32of positive refractive power, a glass cover62and an image plane42. The first lens12is a biconcave lens. The free-form surface prism22has a convex incidence surface S32, a planar reflecting surface S42and a convex exit surface S52. The second lens32is a concave-convex lens. The numerical data of Numerical Embodiment 2 will be shown below.

Surface (i)Ri (mm)D/T (mm)NdVdS0∞S12−6.01.051.513657.4S222.041.80S329.1842.8431.513657.4S42∞−3.01.513657.4S522.216−1.5S62∞−0.206S725.204−0.71.53657.4S821.732−2.12S92∞−0.81.5163364.1S102∞
In the above table, as the coordinate rotates 90 degrees at the reflecting surface S42of the prism22, negative distances are shown.

In this numerical embodiment, the first lens12, the prism22and the second lens32are all made aspheric. The aspheric surfaces thereof are expressed by the following equation:

z=c⁢⁢h21+[1-(k+1)⁢c2⁢h2]12+A⁢⁢h4+B⁢⁢h6+C⁢⁢h8+D⁢⁢h10
And the definition of the variables, z, c, h, k, and A, B, C, and D, are given in the previous Numerical Embodiment 1.

Specifically, the two surfaces S12, S22of the first lens12, the incidence surface S32and the exit surface S52of the prism22, and the two surfaces S72, S82of the second lens32are all made aspheric. Aspheric coefficients for these aspheric surfaces are illustrated below:

In this numerical embodiment, the respective values of the above conditions (1), (2) and (3), and the effective focal length, the field of view, the total length and the F-number of the present lens system are listed in the table as below.

Effective Focal Length of Lens System (f)1.33 mmField of View (F.O.V)132 degreesTotal Length of Lens System14.0 mmF-number2.89|f1/f|2.14f2/f3.17f3/f3.56

As illustrated in the above table, the respective values of |f1/f|, f2/f and f3/f are 2.14, 3.17 and 3.56, all of which are within a corresponding range specified by condition (1), (2) or (3).

FIGS. 8-11respectively show graphs of longitudinal spherical aberration, field sags, distortion and lateral color aberration of Numerical Embodiment 2 of the present compact imaging lens system. From these graphs, it can be seen that the present compact imaging lens system of Numerical Embodiment 2 provides a high level of optical performance.

As shown inFIG. 12, a compact imaging lens system in accordance with Numerical Embodiment 3 of the present invention includes a first lens13of negative refractive power, a free-form surface prism23of positive refractive power, an aperture stop53, a second lens33of positive refractive power, a glass cover63and an image plane43. The first lens13is a biconcave lens. The free-form surface prism23has a convex incidence surface S33, a planar reflecting surface S43and a convex exit surface S53. The second lens33is a convex-concave lens. The numerical data of Numerical Embodiment 3 will be shown below.

Surface (i)Ri (mm)D/T (mm)NdVdS0∞S13−4.401.11.513657.4S231.901.70S333.7142.4631.513657.4S43∞−2.6431.513657.4S532.698−1.448S63∞−0.1S73−1.244−0.71.53657.4S83−2.508−1.289S93∞−0.81.5163364.1S103∞
In the above table, as the coordinate rotates 90 degrees at the reflecting surface S43of the prism23, negative distances are shown.

In this numerical embodiment, the first lens13, the prism23and the second lens33are all made aspheric. The aspheric surfaces thereof are expressed by the following equation:

z=c⁢⁢h21+[1-(k+1)⁢c2⁢h2]12+A⁢⁢h4+B⁢⁢h6+C⁢⁢h8+D⁢⁢h10
And the variables of the equation, including z, c, h, k, and A, B, C and D, are defined previously.

Specifically, the two surfaces S13, S23of the first lens13, the incidence surface S33and the exit surface S53of the prism23, and the two surfaces S73, S83of the second lens33are all made aspheric. Aspheric coefficients for these aspheric surfaces are illustrated below:

In this numerical embodiment, the respective values of the above conditions (1), (2) and (3), and the effective focal length, the field of view, the total length and the F-number of the present lens system are listed in the table as below.

Effective Focal Length of Lens System (f)1.3 mmField of View (F.O.V)120 degreesTotal Length of Lens System12.06 mmF-number2.83|f1/f|1.88f2/f3.16f3/f3.12

As illustrated in the above table, the respective values of |f1/f|, f2/f and f3/f are 1.88, 3.16 and 3.12, all of which are within a corresponding range specified by condition (1), (2) or (3).

FIGS. 13-16respectively show graphs of longitudinal spherical aberration, field sags, distortion and lateral color aberration of Numerical Embodiment 3 of the present compact imaging lens system. From these graphs, it can be seen that the present compact imaging lens system of Numerical Embodiment 3 provides a high level of optical performance.

As shown inFIG. 17, a compact imaging lens system in accordance with Numerical Embodiment 4 of the present invention includes a first lens14of negative refractive power, a free-form surface prism24of positive refractive power, an aperture stop54, a second lens34of positive refractive power, a glass cover64and an image plane44. The first lens14is a biconcave lens. The free-form surface prism24has a convex incidence surface S34, a curved (convex) reflecting surface S44and a convex exit surface S54. The second lens34is a biconvex lens. The numerical data of Numerical Embodiment 4 will be shown below.

Surface (i)Ri (mm)D/T (mm)NdVdS0∞S14−3.8321.11.513657.4S241.901.70S343.8842.5311.513657.4S44−1434.006−2.5311.513657.4S542.794−1.5S64∞−0.1S74−2.403−0.71.5163364.1S8412.981−1.684S94∞−0.81.5163364.1S104∞
In the above table, as the coordinate rotates 90 degrees at the reflecting surface S44of the prism24, negative distances are shown.

In this numerical embodiment, both the first lens14and the prism24are made aspheric. The aspheric surfaces thereof are expressed by the following equation:

Specifically, the two surfaces S14, S24of the first lens14, and the incidence surface S34and the exit surface S54of the prism24are all made aspheric. Aspheric coefficients for these aspheric surfaces are illustrated below:

In this numerical embodiment, the respective values of the above conditions (1), (2) and (3), and the effective focal length, the field of view, the total length and the F-number of the present lens system are listed in the table as below.

Effective Focal Length of Lens System (f)1.3 mmField of View (F.O.V)120 degreesTotal Length of Lens System12.64 mF-number2.82|f1/f|1.7f2/f3.2f3/f3.07

As illustrated in the above table, the respective values of |f1/f|, f2/f and f3/f are 1.7, 3.2 and 3.07, all of which are within a corresponding range specified by condition (1), (2) or (3).

FIGS. 18-21respectively show graphs of longitudinal spherical aberration, field sags, distortion and lateral color aberration of Numerical Embodiment 4 of the present compact imaging lens system. From these graphs, it can be seen that the present compact imaging lens system of Numerical Embodiment 4 provides a high level of optical performance.

As described above, the present compact imaging lens system is only composed of two lenses and a prism, and thus has the advantages of simple structure, easy assembly, wide angle of view, small size and light weight. By application of a free-form surface prism, the optical axis of the present lens system is rotated by 90 degrees to fold the optical path, whereby the total length and the volume of the present lens system can be reduced. At the same time, various aberrations also can be compensated to obtain high image resolution. Further, by replacing conventional glass lens elements with injection molded plastic lens elements having aspheric surfaces, the production cost of the present lens system is significantly reduced, the optical length of the present lens system is shortened and the production yield is also increased.

It should be noted that the present compact imaging lens system is a wide-angle lens system having an angle of view larger than 120 degrees. Therefore, the present lens system is more suitable for use in vehicles, monitoring systems and network video systems, although it also may be used as a common wide-angle image capture lens. The overall optical length of the present lens system is only about 7.5 mm, which facilitates the application of the present lens system into vehicles, anti-thief systems, computer peripheral systems and even mobile phones. The construction of the present lens system not only shortens the total length of the present lens system, saves production cost, reduces tolerance, but assures a high level of optical performance as well.