CN104360463A - Three-distal coaxial illumination imaging optical system - Google Patents

Abstract

In the tri-telecentric coaxial illumination imaging optical system of the present invention, the front objective lens group and the rear objective lens group constitute a bi-telecentric imaging optical system, and the front objective lens group, the beam splitting prism and the illumination objective lens group constitute a bi-telecentric coaxial illumination optical system. System; the aperture stop AS ₁ is located on the image focal plane of the front objective lens group and the object focal plane of the rear objective lens group, and the aperture stop AS ₂ is located on the image square focal plane of the front objective lens group and the illumination objective lens group on the object focal plane. The invention couples two double-telecentric systems to form a triple-telecentric system, which has dual functions of coaxial illumination and imaging. The field of view of the imaging system reaches 180mm, and the imaging distortion is less than -0.1%; the resolution of the full field of view reaches 200lp/mm, and the imaging quality is uniform. Uneven lighting defects.

Description

Tri-telecentric coaxial illumination imaging optical system

technical field

The invention belongs to optical imaging technology, in particular to an optical system applied to machine vision detection with large field of view, high resolution and low distortion, in particular a three-telecentric coaxial illumination imaging optical system.

Background technique

Using machine vision instead of human eyes to carry out high-precision, high-speed photoelectric automatic online inspection is one of the most important development directions in the field of industrial online inspection in the past ten years. The imaging method with traditional fixed-focus or zoom lens is simple and low-cost, but has the following defects: 1. The image distortion is relatively large, especially for large field of view imaging, and the large distortion will seriously affect the measurement accuracy; 2. For a certain depth The imaging of the object will project the side of the object into the image, which is easy to be confused with the image of the detection surface and reduce the measurement accuracy; 3. The out-of-focus imaging of ordinary fixed-focus or zoom lenses will cause large measurement errors.

The telecentric optical path can overcome the above shortcomings, and is very suitable for the application in the field of automatic online inspection of machine vision. Therefore, the use of telecentric optical systems has increased in recent years.

The telecentric light path is divided into the object-space telecentric light path and the image-space telecentric light path. The principle is to place the aperture stop on the focal plane of the image space and the focal plane of the object space respectively, so that the chief rays of the object space and the image space are parallel to the optical path. axis. Combining these two telecentric optical paths constitutes a double-telecentric imaging optical path, that is, the position of the aperture stop in the middle is not only the image focal plane of the front objective lens group, but also the object focal plane of the rear objective lens group. In this way, the principal rays of the object and image are parallel to the optical axis, combining the advantages of the object and image telecentric optical paths, eliminating object and image distortions, and further improving the detection accuracy. There are two ways of telecentric light path illumination: external illumination and coaxial internal illumination. External illumination is relatively simple to implement, but it is prone to uneven illumination, especially for large field of view illumination. Because the coaxial internal lighting needs to use part of the optical path, the design is more complicated, the cost is high, and the brightness is slightly lower, but the uniformity is good, and the internal lighting is required in the detection occasions with high requirements.

Chinese patent 200710038508.0 proposes a symmetrical bi-telecentric projection optical system, which has the advantage of a resolution of 700lp/mm when the magnification is -1, and is suitable for applications in the field of high-precision lithography. The disadvantage is that the whole system uses 18 lenses, which is difficult to install and adjust, and uses more expensive special glass such as SFPL51Y, which is more expensive. The field of view is 31.446mm, and the working distance is 29mm, which is not suitable for large field of view detection occasions.

Chinese patent 201010242686.7 proposes a coaxial bi-telecentric imaging optical system, which has the advantage of using fewer lenses to achieve high-magnification bi-telecentric imaging, and the distortion is less than 0.1%. The uniformity is good; the field of view diaphragm of Koehler illumination is variable, and the size of the illumination field of view can be controlled. The disadvantage is that the field of view is only 19.2mm, which is not suitable for the inspection of parts with a large field of view; the structure of Koehler lighting is complicated and difficult to install and adjust.

In US Pat. No. 5,715,050, a bi-telecentric optical measurement system with coaxial illumination is proposed. The advantage of this system is that the position error of the object image plane has a low impact on the measurement accuracy, but the magnification of the system is low, and the size of the illuminated area cannot be controlled, so high measurement accuracy cannot be obtained and it also needs additional after the double telecentric imaging system The optical system is used to ensure a good contrast image, which increases the design cost and the difficulty of installation and calibration.

Invention content

The purpose of the present invention is to overcome the defects of the prior art, to provide a three-telecentric coaxial illumination and imaging optical system for machine vision detection with large field of view, low magnification, high resolution, and coaxial internal illumination, and to realize the imaging optical system with a diameter within 180mm. High-precision imaging and detection of the measured object.

The technical scheme of the present invention to achieve the above object is as follows: the three-telecentric coaxial illumination imaging optical system includes a double-telecentric imaging optical path formed by placing the aperture stop on the focal plane of the image side and the focal plane of the object side, and is characterized in that: Front objective lens group G ₁ , beam splitting prism BS, rear objective lens group G ₂ and illumination objective lens group G ₃ , front objective lens group G ₁ and rear objective lens group G ₂ form a bi-telecentric imaging optical system, front objective lens group G _1. The beam-splitting prism BS and the illumination objective lens group _G3 form a bi-telecentric coaxial illumination optical system;

Aperture stop AS ₁ is located on the image focal plane of the front objective lens group G ₁ and on the object focal plane of the rear objective lens group G ₂ , so that the front objective lens group G ₁ and the rear objective lens group G ₂ form a bi-telecentric imaging light path. Aperture stop AS ₂ is located on the image focal plane of the front objective lens group G ₁ and on the object focal plane of the illumination objective lens group G ₃ , so that the front objective lens group G ₁ and the illumination objective lens group G ₃ form a bi-telecentric illumination optical path .

In the front objective lens group G ₁ , from the object plane to before the dichroic prism BS, a positive biconvex lens L ₁ , a negative meniscus lens L ₂ , a positive meniscus lens L ₃ , and a negative meniscus lens L are arranged sequentially. ₄ and a negative meniscus lens L ₅ , wherein the positive meniscus lens L ₃ and the negative meniscus lens L ₄ form a positive meniscus doublet.

The beam-splitting prism BS is formed by double-gluing two right-angle prisms, _and the reflection-transmittance ratio is 1: ₁ . In this way, the uniform illumination of the large field of view on the object surface is realized; the light reflected from the measured object passes through the front objective lens group _G1 , and then enters the rear objective lens group _G2 through the dichroic prism to reach the CCD target surface for imaging.

The rear objective lens group G ₂ is composed of 6 lenses. From the dichroic prism BS to the CCD target surface, a variable imaging aperture stop AS ₁ , a negative meniscus lens L ₆ , and a negative meniscus lens L are arranged in sequence. _7. A positive meniscus lens L ₈ , a positive biconvex lens L ₉ , a negative meniscus lens L ₁₀ and a positive meniscus lens L ₁₁ , wherein the positive biconvex lens L ₉ and the negative meniscus lens L ₁₀ form a positive doublet Group.

The illumination objective lens group G ₃ is composed of 3 lenses, which are sequentially arranged along the optical axis from the aperture stop AS ₂ and include a positive biconvex lens L ₁₂ , a negative meniscus lens L ₁₃ and a positive plano-convex lens L ₁₄ , wherein the positive The double-convex lens L ₁₂ and the negative meniscus lens L ₁₃ form a positive doublet.

The bi-telecentric imaging system uses an area-array CCD camera with a target surface of 1" and a pixel size of 4.5 μm×4.5 μm.

The illumination source of the bi-telecentric illumination imaging system is an LED surface light source with a diameter of 7mm, and the illumination uniformity of the entire field of view is within 10%.

The bi-telecentric imaging optical system in the present invention has an object distance of 288.2mm, an object field of view of 175mm; an image distance of 23.58mm, an image field of 16mm, and a magnification of 0.0914×; The resolution reaches 200lp/mm, and the distortion is less than 0.1%.

The invention couples two double-telecentric systems to form a triple-telecentric system, which has dual functions of coaxial illumination and imaging. The bi-telecentric illumination system is coupled with the bi-telecentric imaging system through the beam splitting prism BS, and the front objective lens group _G1 is the common part of the bi-telecentric imaging optical path and the bi-telecentric coaxial illumination optical path, which plays a dual role of illumination and imaging.

From the point of view of the imaging optical path, the field of view of the system reaches 180mm, which is a large field of view in the bi-telecentric system; the imaging distortion is less than -0.1%; the resolution of the full field of view reaches 200lp/mm, and the imaging quality is uniform. From the point of view of the illumination light path, the use of bi-telecentric epi-axial coaxial illumination can achieve uniform illumination of the entire field of view, avoiding the defect of uneven illumination caused by coaxial ring light illumination.

Description of drawings

Figure 1: Structural diagram of the three-telecentric coaxial illumination and imaging optical system of the present invention;

Figure 2: Structural diagram of the front objective lens group;

Figure 3: The aberration curve of the front objective lens group;

Figure 4: Structural diagram of the rear objective lens group;

Figure 5: The aberration curve of the rear objective lens group;

Figure 6: Structural diagram of the bi-telecentric imaging system;

Figure 7: Aberration curve diagram of the bi-telecentric imaging system;

Figure 8: MTF curve diagram of the bi-telecentric imaging system;

Figure 9: Structural diagram of the illumination objective lens group;

Figure 10: Structural diagram of the bi-telecentric lighting system;

Figure 11: Illumination quality analysis diagram of the bi-telecentric lighting system.

Detailed ways

Firstly, according to the requirement that the object field of view D=175mm and the resolution requirement is 0.05mm, it is determined that the phase receiving device uses a 1"CCD camera, and the pixel size is 4.5μm×4.5μm, so the calculated magnification is β=16/175= 0.0914×. Then according to the magnification requirements, determine the focal length of the front objective lens group G ₁ as f' ₁ =45.5mm, and the focal length of the rear objective lens group G ₂ as f' ₂ =497mm. For the front objective lens group G ₁ and the rear objective lens group respectively The lens group G ₂ is optically designed separately, and the aperture stop AS ₁ is placed at the focal point of the image square of the front objective lens group G ₁ , which coincides with the object focal point of the rear objective lens group G ₂ , forming a double-telecentric optical path.

The front objective lens group G ₁ adopts a reverse optical path design, as shown in Figure 2. From left to right along the optical axis are aperture stop AS ₁ , negative meniscus lens L ₅ , negative meniscus lens L ₄ , positive meniscus lens L ₃ , negative meniscus lens L ₂ , and positive biconvex lens L ₁ , where The positive meniscus lens L ₃ and the negative meniscus lens L ₄ form a double cemented group. The angle of view of the incident parallel light is 2ω=20°, and the chief ray of the outgoing light is parallel to the optical axis. After aberration optimization design, it can be seen from Figure 3 that the spherical aberration, astigmatism, and field curvature of the front objective lens group G ₁ on the image plane are within 0.5mm, and the distortion is less than -0.2%.

The rear objective lens group G ₂ is designed with a forward optical path, and the structure diagram is shown in Figure 4. From left to right along the optical axis are aperture stop AS ₁ , negative meniscus lens L ₆ , negative meniscus lens L ₇ , positive meniscus lens L ₈ , positive biconvex lens L ₉ , negative meniscus lens L ₁₀ and positive meniscus lens Lunar lens L ₁₁ , wherein the positive double-convex lens L ₉ and the negative meniscus lens L ₁₀ form a positive double cemented group. The aperture stop AS ₁ is located at the focal plane of the object, the field angle of the incident parallel light is 2ω=20°, and the chief ray of the outgoing light is parallel to the optical axis. After aberration optimization design, it can be seen from Figure 5 that the spherical aberration, astigmatism, and field curvature on the image plane of the rear objective lens group G ₂ are within 0.1mm, and the distortion is less than -0.2%.

Flip the lens of the front objective lens group G ₁ and connect it with the rear objective lens group G ₂ at the aperture stop AS ₁ to form a bi-telecentric imaging system as shown in Figure 6 . The system is an afocal system, the chief ray of incident light is parallel to the optical axis, and the chief ray of outgoing light is also parallel to the optical axis, forming a double-telecentric structure. From the aberration analysis curve in Figure 7, it can be seen that spherical aberration, astigmatism, and curvature of field are all within 0.1mm; since the distortion numbers of the front objective lens group G ₁ and the rear objective lens group G ₂ are equal, the total distortion is less than -0.1%, less than Distortion of ordinary object-space telecentric and image-space telecentric structures. It can be seen from the MTF curve in Figure 8 that the resolution of the full field of view reaches 200lp/mm, and the resolution of the corresponding image receiving device is higher than 100lp/mm, that is, the pixel size is less than 10μm, it can be used with this system.

The design method of the illumination objective lens group G ₃ is similar to that of the rear objective lens group G ₂ , but because the illumination system does not require high aberrations and considering the cost factor, only three lenses are used. The structure is shown in Figure 9. From left to right along the optical axis are aperture stop AS ₂ , positive biconvex lens L ₁₂ , negative meniscus lens L ₁₃ and positive plano-convex lens L ₁₄ , wherein positive biconvex lens L ₁₂ and negative meniscus lens L ₁₃ form a positive doublet Group. The illumination objective lens group _G3 and the front objective lens group _G1 are connected at the aperture stop to form a double telecentric coaxial illumination system as shown in FIG. 10 . The chief ray of the light emitted from the light source surface of the system is parallel to the optical axis, and the chief ray of the emitted light is also parallel to the optical axis, forming a double-telecentric structure. Fig. 11 is a luminance analysis diagram of the illuminated object surface. It can be seen from the figure that the illumination brightness of the entire field of view is relatively uniform, and the illumination uniformity on the X-axis and Y-axis is basically within 10%, so it can provide uniform illumination for the entire object surface.

Flip the lens of the front objective lens group G ₁ shown in Figure 2, and connect it with the rear objective lens group G ₂ shown in Figure 2 at the aperture stop AS ₁ to form the bi-telecentric imaging system shown in Figure 6 . Flip the lens of the front objective lens group G ₁ shown in Figure 2, and connect the illumination objective lens group G ₃ shown in Figure 9 at the aperture stop AS ₂ to form a double telecentric coaxial lighting system shown in Figure 10. Finally, the double-telecentric imaging system shown in Figure 6 is combined with the double-telecentric coaxial illumination system shown in Figure 10 through the beam splitting prism BS to form the triple-telecentric coaxial illumination imaging of the present invention shown in Figure 1 optical system.

Claims (7)

1. A three-telecentric coaxial illumination imaging optical system, including a bi-telecentric imaging optical path formed by placing the aperture stop at the focal plane of the image and the focal plane of the object. It is characterized in that it includes a front objective lens group G ₁ , Prism BS, rear objective lens group G ₂ and illumination objective lens group G ₃ , front objective lens group G ₁ and rear objective lens group G ₂ form a bi-telecentric imaging optical system, front objective lens group G ₁ , dichroic prism BS and illumination objective lens Group G ₃ constitutes a double telecentric coaxial illumination optical system; Aperture stop AS ₁ is located on the image-space focal plane of the front objective group G ₁ and on the object-space focal plane of the rear objective group G ₂ , and the aperture stop AS ₂ is located on the image-space focal plane of the front objective group G ₁ And on the object focal plane of the illumination objective lens group _G3 . 2. The three-telecentric coaxial illumination and imaging optical system according to claim 1, characterized in that: the front objective lens group G ₁ is sequentially provided with a positive double-convex lens L ₁ from the object plane to before the beam splitting prism BS , a negative meniscus lens L ₂ , a positive meniscus lens L ₃ , a negative meniscus lens L ₄ and a negative meniscus lens L ₅ , wherein the positive meniscus lens L ₃ and the negative meniscus lens L ₄ form a positive meniscus Monthly double glue group. 3. The three-telecentric coaxial illumination and imaging optical system according to claim 1, characterized in that: the beam splitting prism BS is formed by double bonding of two right-angle prisms, and the reflection-transmittance ratio is 1:1, which is used to The light emitted from the illumination source and reflected by the illumination objective lens group _G3 enters the front objective lens group _G1 , thereby realizing uniform illumination of a large field of view on the object plane; the light reflected from the measured object passes through the front objective lens group _G1 After that, it enters the rear objective lens group _G2 through the beam splitter BS to reach the CCD target surface for imaging. 4. The three-telecentric coaxial illumination and imaging optical system according to claim 1, characterized in that: the rear objective lens group _G2 , from the beam splitting prism BS to the CCD target surface, sequentially set variable imaging aperture light Stop AS ₁ , a negative meniscus lens L ₆ , a negative meniscus lens L ₇ , a positive meniscus lens L ₈ , a positive biconvex lens L ₉ , a negative meniscus lens L ₁₀ and a positive meniscus lens L ₁₁ , wherein the positive double-convex lens L ₉ and the negative meniscus lens L ₁₀ form a positive doublet. 5. The three-telecentric coaxial illumination and imaging optical system according to claim 1, characterized in that: the illumination objective lens group G ₃ is arranged sequentially from the aperture stop AS ₂ along the optical axis and includes a positive biconvex lens L _12. A negative meniscus lens L ₁₃ and a positive plano-convex lens L ₁₄ , wherein the positive biconvex lens L ₁₂ and the negative meniscus lens L ₁₃ form a positive doublet. 6. The three-telecentric coaxial illumination and imaging optical system according to claim 1, characterized in that: the bi-telecentric imaging system is used in conjunction with a CCD with a target surface of 1" and a pixel of 4.5 μm×4.5 μm camera. 7. The triple-telecentric coaxial illumination imaging optical system according to claim 1, characterized in that: the illumination light source of the double-telecentric coaxial illumination system is an LED surface light source with a diameter of 7mm, and the illumination uniformity of the entire field of view is Within 10%.