CN110018564B

CN110018564B - Optical imaging system for large depth of field continuation of wavefront coding space

Info

Publication number: CN110018564B
Application number: CN201910183874.8A
Authority: CN
Inventors: 张淑兰
Original assignee: Beijing Weijing Optical Technology Co ltd
Current assignee: Beijing Weijing Optical Technology Co ltd
Priority date: 2019-03-12
Filing date: 2019-03-12
Publication date: 2020-06-02
Anticipated expiration: 2039-03-12
Also published as: CN110018564A

Abstract

An optical lens imaging combination unit and an optical imaging system with a wavefront coding space and large depth of field continuation are disclosed, the optical lens imaging combination unit comprises an odd-order phase coding plate (1) and an objective lens group (2), the objective lens group consists of seven single lenses and is sequentially arranged along an optical axis towards a detector protective glass window (3); the odd-order phase encoding plate is positioned on the outermost side of the optical lens imaging combination unit, one surface facing outwards is a plane, the other surface is a free-form surface, and the surface type of the free-form surface is as follows: z (X, Y) ═ AX³+BXY³+CX²Y+DY³Wherein A is 9.8576 × 10^‑3，B＝2.1324×e^‑5，C＝2.1324×e^‑5，D＝9.8576×10^‑3. The depth of field of camera imaging can be greatly expanded, and the range is expanded to 5-110 km; the camera is characterized in that only one coding lens is added on the side of the optical lens of the camera, the electronic circuit is added with related decoding and application algorithms, the cost of volume, weight and power consumption is low, mechanical focusing is not needed in the distance change process, the time delay is small, the implementation of a guidance control algorithm is facilitated, and the camera has a strong application prospect in the fields of attack and defense confrontation and automatic driving in a high-mobility scene.

Description

Optical imaging system for large depth of field continuation of wavefront coding space

Technical Field

The invention relates to an optical imaging system and method for expanding the depth of field of a visible light optical system by using a method of combining optical design and computational optics, in particular to an optical imaging system and method for expanding the depth of field continuation of the visible light optical system by using a wavefront coding imaging technology.

Background

In recent years, with the continuous upgrading and upgrading of detector devices and the continuous improvement of the requirements of various applications on detection systems, higher requirements are also put on the performance and the optimized design of visible light optical systems. In general, the receiving area of the visible light detector is relatively small, and the depth of field of the visible light optical system is not too large. For example, the conventional techniques CN106324832A, CN106199956A, CN204719330U, etc. all have the defect that the corresponding depth of field is not large enough when the field of view is large. Therefore, it is imperative to find a suitable system and method for further extending the depth of field of the visible light optical system.

Particularly, the visible light imager on the satellite adopts a fixed-focus imager with high reliability and simple mechanism so as to avoid focusing response delay brought by a focusing mechanism. On a fixed-focus camera with high reliability, the intelligent recognition task is realized at a very low time cost, the requirement of time-delay-free clear imaging under a high maneuvering condition is met, and the traditional fixed-focus optical system is difficult to guarantee.

A computational optical scheme of depth of field extension is adopted on a fixed focus optical system. On the lens of the optical system, a phase mask of wave front modulation is added in an optical processing mode, so that in a traditional fixed focus camera with a simple mechanism, on the premise of not changing the design of the original optical system, the field depth expansion is realized by means of a computational optics method, the real-time rapid decoding processing on a satellite is realized, and the real-time clear imaging in a large field depth range is further ensured.

Disclosure of Invention

In order to solve the above technical problems in the background art, the present invention provides a method for realizing clear imaging with an ultra-large depth of field (5m to 110km, which is about 39 times of the depth of field of the original focus system) without changing the hardware conditions of the optical system and the image sensor of the fixed focus camera, and a design of an encoding optical system.

The invention provides an optical imaging system for wavefront coding space large depth of field continuation, which comprises an optical lens imaging combination unit, wherein the optical lens imaging combination unit comprises an odd-order phase coding plate (1) and an objective lens group (2), and is characterized in that: the objective lens group (2) is composed of a first single lens (L1), a second single lens (L2), a third single lens (L3), a fourth single lens (L4), a fifth single lens (L5), a sixth single lens (L6) and a seventh single lens (L7), wherein the odd phase encoding plate (1), the first single lens (L1), the second single lens (L2), the third single lens (L3), the fourth single lens (L4), the fifth single lens (L5), the sixth single lens (L6) and the seventh single lens (L7) are sequentially arranged along an optical axis towards a detector protection glass window (3); the odd-order phase coding plate (1) is positioned at the outermost side of the optical lens imaging combination unit, one outward surface of the odd-order phase coding plate is a plane, and the other outward surface of the odd-order phase coding plate is a free-form surface; one surface of the first single lens (L1) of the objective lens group (2) facing the odd-order phase encoding plate (1) is a convex surface, and the other surface is a concave surface; the second single lens (L2) is a biconvex lens; the third single lens (L3) is a meniscus lens, one surface of the third single lens facing the odd-order phase encoding plate (1) is a convex surface, and the other surface of the third single lens is a concave surface; the fourth single lens (L4) is a biconvex lens; the fifth einzel lens (L5) is a biconcave lens; the sixth single lens (L6) is a meniscus lens, one surface facing the odd-order phase encoding plate (1) is a concave surface, and the other surface is a convex surface; one surface of the seventh single lens (L7) facing the odd-order phase encoding plate (1) is a convex surface, and the other surface is a concave surface.

Preferably, the face shape of the additional free-form surface of the odd-order phase encoded sheet material (1) is: z (X, Y) ═ AX³+BXY²+CX²Y+DY³Wherein: a 9.8576 × 10^-3，B＝2.1324×e^-5，C＝2.1324×e^-5，D＝9.8576×10^-3。

Preferably, the Norm radius (Norm radius) of the odd phase encoded plate (1) is 20 mm.

Preferably, the surface test PV/Rms of the outward facing side of the odd phase encoded panel (1) is 0.5um/60 nm.

Preferably, the aperture stop of the objective lens group (2) is located on the convex surface of the first singlet lens (L1).

Preferably, one surface of the free curved surface of the odd-order phase coding plate (1) is close to the aperture diaphragm surface.

Preferably, the optical parameters of each lens of the objective lens group satisfy the following table:

preferably, the optical parameters of the optical lens imaging combination unit satisfy the following conditions:

wave band: 0.45-0.75 μm,

F/#：6、

Focal length: 127mm +/-2 mm,

Visual field: (6. + -. 0.1) × (6. + -. 0.1) ° C,

Transmittance: tau is more than or equal to 0.8,

Image quality (20 ℃): when omega is less than or equal to 0.7 omega max and 76lp/mm, the MTF is more than or equal to 0.5; when omega is more than 0.7 omega max and 76lp/mm, MTF is measured to be more than or equal to 0.45;

no thermalization: in the range of 0-40 ℃, the energy concentration ratio of a single pixel (6.5 mu m multiplied by 6.5 mu m) is not less than 50 percent; in the range of-40 ℃ to 60 ℃, the energy concentration ratio of a single pixel (6.5 mu m multiplied by 6.5 mu m) is not less than 45 percent;

temperature compensation mode: passive compensation;

extinction ratio: not more than 1X 10-5;

a detector: 2048 × 2048;

and (3) pixel size: 6.5 μm.

Preferably, the odd-order phase coding plate material is an SP1516 resin material;

preferably, the odd-order phase encoding plate has a thickness of 4 mm.

Preferably, the optical imaging system with the wavefront coding space extended with the large depth of field can clearly perform imaging detection in the range of 5 m-110 km, and decode the coded image to obtain a clear image close to the diffraction limit under the field of view.

The invention has the following advantages:

1. the introduction of the wavefront coding (WFC) technology can achieve the purpose of a larger depth of focus (depth of field) while ensuring the luminous flux and imaging resolution of the visible optical system, while also suppressing astigmatism, spherical aberration, chromatic aberration, and aberration due to defocus caused by mounting errors and temperature variations.

2. The method is simple to operate (only a pure phase mask plate is arranged at the position of the optical system diaphragm), and the purpose of imaging with ultra-large focal depth is achieved on the premise of not changing the size of the detector pixel.

The system for realizing ultra-large depth-of-field imaging is based on a wavefront coding imaging mechanism as a theoretical basis, not only can increase the depth of field of an image, but also can eliminate the damage of the traditional image amplification to local details, such as edges and other features, and is an in-depth excavation for the potential characteristics of the wavefront coding imaging technology.

3. The real-time imaging is realized, the decoding algorithm is simple, the electronic operation process time is short, and the purpose of real-time imaging with super-large depth of field (focal depth) can be realized.

Experimental verification of a wavefront coded imaging system by a cubic phase plate. The result shows that the processing method of the ultra-large depth-of-field imaging based on the wavefront coding mechanism has great advantages in the aspect of accurate description of images beyond a focus point.

Drawings

FIG. 1 shows the optical structure of the system after an odd-order phase encoding plate is added;

FIG. 2, imaging at different object distances wavefront coded system, where FIGS. 2(a) and 2(b) show the cases of object distances of 5m and 110km, respectively;

FIG. 3, encoded MTF for a wavefront coding system, where FIGS. 3(a) and 3(b) show encoded MTF cases at object distances of 5m and 110km, respectively;

FIG. 4, decoded MTF of a wavefront coding system;

FIG. 5, raw optical system MTF, where FIGS. 5(a) and 5(b) show the encoded MTF cases for object distances of 5m and 110km, respectively;

FIG. 6, a wavefront coded system, encodes a PSF, where FIGS. 6(a) and 6(b) show the encoded PSF at object distances of 5m and 110km, respectively;

FIG. 7, wavefront map at pupil of wavefront coding system, wherein FIGS. 7(a) and 7(b) show the wavefront map at object distances of 5m and 110km, respectively;

FIG. 8, a wavefront coding system encoded image, wherein FIGS. 8(a) and 8(b) correspond to wavefront coding system encoded images showing object distances of 5m and 110km, respectively;

FIG. 9, original image at system 5 m;

FIG. 10, a wavefront coding system decoded image, wherein FIGS. 10(a) and 10(b) correspond to wavefront coding system encoded images showing object distances of 5m and 110km, respectively;

FIG. 11, the encoded image is compared with the decoded image, wherein FIGS. 11(a) and 11(b) show the original effect at object distances of 5m and 110km, respectively; FIGS. 11(c) and 11(d) show the improved effect at object distances of 5m and 110km, respectively;

FIG. 12, the MTF of a wavefront coding system under different temperature conditions, where FIGS. 12(a) and 12(b) correspond to the case when the object distance is 5m and the temperature is-40 ℃ and 60 ℃ respectively; FIGS. 12(c) and 12(d) correspond to the case where the object distance is 110km and the temperature is-40 ℃ and 60 ℃ respectively.

Detailed Description

An optical imaging system for wavefront coding space large depth of field continuation comprises an optical lens imaging combination unit, wherein the optical lens imaging combination unit comprises an odd-order phase coding plate 1 and an objective lens group 2; the objective lens group 2 consists of a first single lens L1, a second single lens L2, a third single lens L3, a fourth single lens L4, a fifth single lens L5, a sixth single lens L6 and a seventh single lens L7; the odd phase encoding plate 1, the first single lens L1, the second single lens L2, the third single lens L3, the fourth single lens L4, the fifth single lens L5, the sixth single lens L6 and the seventh single lens L7 are sequentially arranged along the optical axis towards the detector protection glass window 3; the odd-order phase coding plate 1 is positioned on the outermost side, the outward surface is a plane, and the other surface is a free curved surface; one surface of the first single lens L1 of the objective lens group 2 facing the odd-order phase encoding plate 1 is a convex surface, and the other surface is a concave surface; the second single lens L2 is a biconvex lens; the third single lens L3 is a meniscus lens, and one surface facing the odd-order phase encoder plate 1 is a convex surface, and the other surface is a concave surface; the fourth single lens L4 is a biconvex lens; the fifth single lens L5 is a biconcave lens; the sixth single lens L6 is a meniscus lens, and one surface facing the odd-order phase encoder plate 1 is a concave surface, and the other surface is a convex surface; the seventh single lens L7 has a convex surface on one side facing the odd-order phase encoder plate 1 and a concave surface on the other side.

The aperture diaphragm of the objective lens group 2 is positioned on the convex surface of the first single lens L1, and one surface of the free curved surface of the odd-order phase coding plate 1 is close to the aperture diaphragm surface.

The optical parameters of the objective lens group are shown in table 1 below.

Table 1: optical parameters of each lens of the objective lens group

(1) Design and design parameters of odd-order phase encoding plate (i.e. phase plate)

SP1516 resin material is selected for use to odd phase mask plate material, and thickness is 4mm, and left side (front surface) is the plane, and right side (back surface) face type does: z (X, Y) ═ AX³+BXY²+CX²Y+DY³Wherein:

A＝9.8576×10^-3，B＝2.1324×e^-5，C＝2.1324×e^-5，D＝9.8576×10^-3。

norm radius (Norm radius) is 10 mm; the front surface test PV/Rms was 0.5um/60 nm.

The corresponding technical parameters met by the odd-order phase coding plate are respectively as follows:

depth of field for clear imaging: 5 m-110 km;

wave band: 0.45-0.75 μm;

F/#：6；

focal length: 127mm +/-2 mm;

visual field: (6 ± 0.1) °;

a detector: 2048 × 2048;

and (3) pixel size: 6.5 μm.

The optimized system structure with the addition of the odd-order phase encoding plate is shown in fig. 1, the encoding surface of the added odd-order phase encoding plate is positioned on the front surface of the first lens on the image side of the pupil of the optical imaging system, and the phase distribution is directly added into the original lens without adding optical elements.

(2) Depth of field scalability design and analysis

FIG. 2 shows the structure of the optical system with different object distances after the odd phase encoding plate is added. The first lens on the left in the figure is the odd phase encoding plate.

As can be seen from fig. 2, the light rays do not converge at the focal plane position after the odd-order phase encoding plate is added, because the odd-order phase encoding plate affects the modulation of the wavefront, and the divergence degree of the light rays within the depth of field is generally the same.

Modulation Transfer Function (MTF)

Fig. 3 and 4 are an encoded MTF and a decoded MTF, respectively, for a wavefront coding system. As shown in FIG. 3, the amplitude of the encoded MTF is relatively low at object distances of 5m and 110km, and the deviation from the diffraction limit is large, and at this time, after the odd-order phase encoding plate is added, the system introduces a certain optical aberration, so that the transfer capacity of the system is reduced. However, the MTF has no frequency cutoff and almost maintains a consistent amplitude distribution at two object distance positions, which means that the same filter can be used to decode the images at the two object distance positions to obtain a clear image. The decoding MTF of fig. 4 is obtained by decoding with a decoding filter constructed by using 5m central field MTF, and at two object distance positions, the MTF of each field is close to the diffraction limit, but there is a certain oscillation, and this time, the transfer capability is not affected due to the slight difference of MTF at different object distance positions.

It can also be seen from fig. 3 that the encoded MTFs at object distances of 5m and 110km are not very consistent in the low frequency part, because the depth of field required by the system is too large (out-of-focus is about 3mm at the focal plane position), which will have some effect on the decoded image. But the designed MTF has no cutoff in the pass band and still meets the requirement of the depth of field range.

For ease of comparison, FIG. 5 shows MTFs at different object distance positions for the original system without the use of wavefront coding techniques. As can be seen from the figure, when the system images at a position of a short distance of 5m, the imaging performance of the system is seriously degraded, and the MTF causes the image to be seriously blurred, so that the target cannot be resolved. Compared with MTF (modulation transfer function) of the applied wavefront coding technology, the performance and technical index of the extended depth of field of a system designed by the wavefront coding technology are verified.

② Point Spread Function (PSF)

Fig. 6 shows that the PSFs are encoded by the designed wavefront coding system, and it can be seen from the figure that under the condition of different object distances, the PSFs are all subjected to asymmetric dispersion, and the dispersion degrees can be judged to be almost the same through the coordinate size, so that the large depth of field performance of the designed system is further embodied. Furthermore, the PSF at 5m varies somewhat from the PSF position at 110km, which is more clearly visible by the phase change of the wavefront at the pupil, as the PSF is related to the wavefront change, as shown in fig. 7. As can be seen from fig. 7, at different object distance positions, the wavefront phase distribution is different, closer to the cubic phase profile at an object distance of 110km, and a certain deviation occurs at an object distance of 5m, which is the same as the result of the PSF. When image decoding is performed using a single-position PSF, artifacts will appear in the decoded image, and some analysis will be performed in the image simulation.

coded image simulation

And simulating the coded image and the decoded image, and verifying the large-depth-of-field imaging performance of the system. The coded image is obtained by optical design software Zemax, in the simulation process, the number of FFT grids is 512, the size of a detector pixel is 6.5 mu m, and the number of samples in two directions is 5 respectively. The PSF used in the simulation was the PSF for the location with object distance of 110km, as shown in fig. 6 (b). Fig. 8 is a coded image of positions of different object distances, and for comparison, fig. 9 shows a simulated image of an original system at an object distance of 5 m. As can be seen from fig. 8, the encoded image at object distances of 5m and 110km exhibited blur, and the diffusion direction was the same as the PSF diffusion direction.

In addition, compared with the simulated image at 5m of the original system in FIG. 9, the image degradation of the wavefront coding system is not serious, and the blurring of the image is approximately consistent at two object distance positions, so that the image decoding can be realized through the same filter, and a clear image can be obtained.

decoding image simulation

The encoded image is decoded using the 5m object distance position PSF of fig. 6, the decoding algorithm is the wiener filtering described above, and the decoded image is shown in fig. 10. As shown in fig. 10, the target is restored after filtering, the boundary thereof is clearly visible, but there is a certain artifact, because there is a certain difference between the PSFs used in actual simulation and decoding, and the PSFs also have a spatial variation characteristic at different phase positions, but this artifact can be eliminated in an actual system, and when decoding an image through the actual system PSF, the artifact in the image will be effectively suppressed.

Fig. 11 compares the imaging effect of the original optical system at 5m and 110km with the improved effect of the system, and it can be seen that the clear imaging capability of the optical system added with the odd-order phase plate is improved.

(3) Wavefront decoding timeliness

The number of covered pixels of the dynamic window is 512 multiplied by 512 when the detection is carried out, the filtering process is a convolution process, and when Wiener is used, the data calculation amount is shown in the following table. When the DSP chip ADSP-TS201 is used as a calculation core, the time for parallel processing of 1 frame of image by 4 DSPs is about 12.4 ms; when processed by the GPU platform, the processing time for a single frame of image is about 7 ms.

Table 2: 512 x 512 image Wiener filtering deconvolution data calculated quantity

	Two-dimensional FFT	Dot matrix divider	Frequency domain data addition	Total calculated amount
					Number of multiplications	18×218	6×218	218	25×218
Number of times of addition	36×218	6×218	218	43×218

In addition, to realize image decoding under the condition of a dynamic window, the PSF function at the window is required to be used, and as can be seen from a system MTF curve, the response of each field is almost consistent, and the system has spatial invariance, so that the image decoding can be carried out by using the PSF at the central field position, and the decoding algorithm is the same and does not influence the timeliness. However, the actual measurement PSF is needed to be used for image decoding during decoding, so that the image quality of the decoded image and the SNR of the system can be effectively ensured.

(4) Wavefront coding with thermal distortion elimination

The technical index requires that the working temperature range of the system is-40-60 ℃, and the thermal performance of the designed wavefront coding system is verified through system performance simulation under different temperature conditions. Because defocusing aberration is often introduced by the temperature change of the optical system, the wavefront coding system added with the odd-order phase coding plate also has a certain correction effect on the thermal defocusing of the system. The MTF of the system under different temperature conditions can embody the thermal performance of the system, and the MTF under different temperatures is analyzed.

Fig. 12 shows MTF of a wavefront coding system under different temperature conditions, and compared with the above simulated MTF, the MTF only shows a slight change under different temperature conditions, and the frequency of the MTF still has no cutoff in a pass band, so that coded images under different temperatures can still be restored by PSF under room temperature conditions. The technical specification is verified by the MTF results on the thermal performance requirements of the optical system.

(5) Conclusion

In conclusion of simulation results, the wave-front coding technology can solve the problem that a given system can clearly image and detect within 5-110 km, the timeliness and the performance of eliminating thermal distortion and the like meet the environmental use requirements, and then the digital image restoration technology is used for decoding the coded image, so that the clear image close to the diffraction limit under the view field can be obtained.

Claims

1. An optical lens imaging combination unit comprising an odd phase encoding plate (1) and an objective lens group (2), characterized in that: the objective lens group (2) is composed of a first single lens (L1), a second single lens (L2), a third single lens (L3), a fourth single lens (L4), a fifth single lens (L5), a sixth single lens (L6) and a seventh single lens (L7), wherein the odd phase encoding plate (1), the first single lens (L1), the second single lens (L2), the third single lens (L3), the fourth single lens (L4), the fifth single lens (L5), the sixth single lens (L6) and the seventh single lens (L7) are sequentially arranged along an optical axis towards a detector protection glass window (3);

the odd-order phase coding plate (1) is positioned at the outermost side of the optical lens imaging combination unit, one outward surface of the odd-order phase coding plate is a plane, and the other outward surface of the odd-order phase coding plate is a free-form surface;

the surface type of the free-form surface of the odd-order phase coding plate (1) is as follows:

wherein:

，

，

，

；

one surface of the first single lens (L1) of the objective lens group (2) facing the odd-order phase encoding plate (1) is a convex surface, and the other surface is a concave surface; the second single lens (L2) is a biconvex lens; the third single lens (L3) is a meniscus lens, one surface of the third single lens facing the odd-order phase encoding plate (1) is a convex surface, and the other surface of the third single lens is a concave surface; the fourth single lens (L4) is a biconvex lens; the fifth einzel lens (L5) is a biconcave lens; the sixth single lens (L6) is a meniscus lens, one surface facing the odd-order phase encoding plate (1) is a concave surface, and the other surface is a convex surface; one surface of the seventh single lens (L7) facing the odd-order phase encoding plate (1) is a convex surface, and the other surface is a concave surface.

2. An optical lens imaging combining unit as claimed in claim 1, wherein: the norm radius of the odd-order phase coding plate (1) is 20 mm.

3. An optical lens imaging combining unit as claimed in claim 2, wherein: the surface test PV/Rms of the outward surface of the odd-order phase coding plate (1) is 0.5um/60 nm.

4. An optical lens imaging combining unit as claimed in claim 1, wherein: the aperture diaphragm of the objective lens group (2) is positioned on the convex surface of the first single lens (L1).

5. An optical lens imaging combining unit as claimed in claim 4, wherein: one surface of the free curved surface of the odd-order phase coding plate (1) is close to the aperture diaphragm surface.

6. An optical lens imaging combining unit as claimed in claim 1, wherein: the odd-order phase encoding plate material is SP1516 resin material, and the thickness is 4 mm.

7. An optical imaging system with wavefront coding and large depth of field prolongation in space, which is characterized by comprising the optical lens imaging combination unit according to any one of claims 1-6.