CN105203102B - Sky polarization mode detection method and system based on s- wave plates - Google Patents

Abstract

The invention provides a sky polarization mode detection system and detection method. The technical solution one is: the detection system includes an s-wave plate, a polarizer, a hand-held light field camera, and a lens; the s-wave plate and the polarizer are located on the plane where the aperture of the lens is located, facing the lens and observing from the direction directly in front of the lens, The s-wave plate is located in front of the polarizer. Polarizers have a high extinction ratio and transmittance within the operating wavelength range of the s-wave plate. The second technical solution is: the detection method includes the following steps: first, a sky image is taken by the sky polarization mode detection system, and the gray value of the sky image is converted into the light intensity value of the target radiation. The second step is to obtain the sky polarization degree and polarization direction angle. The invention solves the problems of poor real-time performance and complicated system structure in the detection of the sky polarization mode in the existing method.

Description

Method and system for detection of sky polarization mode based on s-wave plate

technical field

The application of the invention belongs to the technical field of celestial navigation, and relates to a method and a system for detecting sky polarization patterns by using an s-wave plate and a hand-held light field camera.

Background technique

The polarization distribution information of sky light can be used for navigation, and it has the characteristics of strong anti-interference ability and low cost. In order to realize accurate sky polarized light navigation, it is necessary to accurately detect the polarization degree and polarization direction angle information of the sky in a large field of view. The degree of polarization and the angle of polarization direction information together are usually referred to as polarization mode. At present, there are two main methods for detecting polarization modes in the sky with large field of view, which are photodiode-based methods and camera imaging methods. The method based on photodiodes requires many photodiodes to point to different sky areas for measurement, so the system structure is complex and the implementation is also relatively difficult. The camera imaging method can directly image the sky area with a large field of view with the help of a wide-angle lens, but in order to measure the polarization mode, it is necessary to obtain multiple sky images in different polarization directions, which often requires multiple shots by one camera or simultaneous shots by multiple cameras There are two ways to realize it, the real-time performance of the former way is not strong, and the latter way will lead to complex system structure.

In conclusion, the existing methods either have poor real-time performance or complex system structures when realizing the detection of polarization patterns in the sky with a large field of view. The emergence of hand-held light-field cameras has made it possible to measure polarization patterns in the sky with a large field of view after a single imaging. The handheld light field camera is a new type of imaging device with a microlens array installed in front of the light-sensitive element of an ordinary camera. It was invented by Chinese scientist Ren Ng in 2005. Due to the existence of the microlens array, when the handheld light field camera shoots the sky, each microlens images the sky area under a certain viewpoint as a diffuse light spot. The diffuse spot is divided and utilized according to different regions to realize the acquisition of images under different polarization directions at the same view point of the sky, and the purpose of completing the detection of polarization modes of the sky with a large field of view can be achieved in one imaging.

Contents of the invention

The technical problem to be solved by the invention is: the invention provides a sky polarization mode detection method based on a sky polarization mode detection system. The present invention realizes the detection of the sky polarization mode by one-time imaging of the sky with a large field of view through a single hand-held light field camera, and is used to solve the poor real-time performance of the system due to the need for multiple measurements in the detection of the sky polarization mode in the existing method , or the problem of complex system structure due to the need for multiple sets of measurement units.

Technical scheme one of the present invention is:

A sky polarization pattern detection system, including s-wave plate, polarizer, hand-held light field camera, lens. It is characterized in that the s-wave plate and the polarizing plate are located on the plane where the aperture of the lens is located, facing the lens and viewed from the direction directly in front of the lens, and the s-wave plate is located in front of the polarizing plate. The polarizer has a higher extinction ratio and transmittance in the working wavelength range of the s-wave plate.

Technical scheme two of the present invention is:

A method for detecting a sky polarization mode, using the sky polarization mode detection system provided by the technical solution 1, specifically comprising the following steps:

The first step is to use the sky polarization mode detection system to take a sky image, and convert the gray value of the sky image into the light intensity value of the target radiation.

The second step is to obtain the sky polarization degree and polarization direction angle.

It is assumed that the microlens array of the hand-held light field camera in the sky polarization mode detection system includes M rows and N columns of microlenses; the coordinates of the pixel in the center of the imaging circular spot corresponding to any microlens Len _{m, n} in the image coordinate system are (X _m,n , Y _m,n ), the radius of the imaging circle is D pixels, where D≥5, 1≤m≤M, 1≤n≤N. The first coordinate element in the pixel coordinates of the center of the imaging circle represents the row number of the pixel in the image, and the second coordinate element represents the column number of the pixel in the image. The image coordinate system is agreed as follows: the upper left corner of the image is the origin of the coordinate system, the vertical downward direction in the image is the positive direction of the first coordinate element, and the horizontal right direction in the image is the positive direction of the second coordinate element. All the microlenses in the present invention have the same focal length, the same size, and the same radius of the imaging circular spot.

Obtain all image areas on the sky image plane whose distance from the pixel point (X _m,n , Y _m,n ) is less than or equal to D pixels, and the image area is circular, let it be G _m,n . Carry out Radon transformation on the circular image area G _{m, n} , select the projection angle range of Radon transformation from 0 to 180 degrees, and make the projection angle step of Radon transformation be S (the selection of the value of S should meet the requirement that S is divisible by 90 requirements), then a total of projection angles, record the sequence of projection angles formed by them as C, C={0,S,2S,...k×S,...,180}, For any projection angle k×S in the projection angle sequence C, there is a Radon transformation result of the projection angle, which is a sequence composed of 2D+1 values, denoted as R _k , and the Radon transformation of all projection angles The resulting sequence is denoted as Ra, Find any sequence in Ra The maximum value in is denoted as Rmax _k . Then for all the maximum value sequences obtained for all projection angles Find the maximum value in MaxR, the result is recorded as MaxMaxR, and the element number corresponding to the maximum value in MaxR is recorded as NmaxR, Then the element with the serial number NmaxR is obtained from the Radon transformation result of the projection angle NmaxR×S, and finally there is a polarization direction angle θ _m,n =NmaxR×S corresponding to the microlens Len _m ,n.

Select the element R _{K with the serial number K} in the Radon transformation result sequence Ra, The value of K should be determined by judging whether the value of the projection angle θ _{m, n} is greater than 90 degrees. If it is greater than 90 degrees, then If less than 90 degrees then A subsequence in R _K Find the minimum value in , which is recorded as MinMinR, where [] means rounding to an integer. The degree of polarization P m _{,n corresponding to the microlens Len m ,n} can be calculated as follows:

Calculate all microlenses Len _1,1 ,...Len _m,n ...,Len _M,N according to the above method, so as to obtain the sky polarization matrix P(P _1,1 ,...P _m,n ,...P _M,N ) and the polarization orientation angle matrix θ(θ _1,1 ,...θ _m,n ,...θ _M,N ). The degree of polarization matrix P and the polarization direction angle matrix θ are the required sky polarization mode measurement results.

The beneficial effects of the present invention are: by using a hand-held light field camera and a wide-angle lens with an s-wave plate and a polarizer added at the aperture position, a single shot of the sky with a large field of view can be obtained, which contains multiple polarizations at any viewing angle. The sky image in the direction of the sky, and then through the Radon transformation of the acquired image, the polarization degree and polarization direction angle information of any viewing angle in the sky can be obtained. Therefore, the present invention realizes the detection of the sky polarization mode through one imaging of the sky with a large field of view by a single camera, and solves the problems of poor real-time performance and complex system structure in the detection of the sky polarization mode in the existing method.

Description of drawings

Figure 1 is a schematic diagram of the principle of the sky polarization mode detection system provided by the present invention;

Figure 2 is the specific implementation flow chart;

Figure 3 is the physical picture of the s-wave plate;

Figure 4 is a physical picture of a fisheye lens with an s-wave plate and a polarizer inserted at the lens aperture;

Figure 5 is the physical map of the sky polarization mode detection system;

Figure 6 is a scene diagram of the indoor experiment;

Figure 7 is an image obtained by the sky polarization mode detection system when shooting a liquid crystal display;

Figure 8 is the partially enlarged result of Figure 7;

Figure 9 is the Radon transformation result of a microlens corresponding to the imaging circular spot

Fig. 10 is the measurement result of the polarization orientation angle distribution of the liquid crystal display;

FIG. 11 is the measurement result of the polarization degree distribution of the liquid crystal display.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Figure 1 is a schematic diagram of the principle of the sky polarization mode detection system (hereinafter referred to as the system) provided by the present invention. As shown in the figure, the system includes a lens 1 , an s-wave plate 3 , a polarizer 4 , and a handheld light field camera 7 . In order to obtain a larger imaging range of the sky as much as possible, the lens 1 is recommended to use a fisheye lens with a field of view of 180 degrees in this implementation example. The s-wave plate 3 and the polarizer 4 are located at the plane where the aperture 2 of the lens 1 is located. In this embodiment, the s-wave plate 3 is circular, and viewed from the direction facing the front of the lens, the s-wave plate is located in front of the polarizer. The polarizer has a higher extinction ratio and transmittance in the working wavelength range of the s-wave plate. The s-wave plate 3 was invented by Peter G. Kazansky, a professor at the University of Southampton. There are two types, namely the radial type and the angular type, both of which can be used in the present invention with no difference in effect. For a detailed introduction of the s-wave plate 3, please refer to the literature Polarization sensitive elements fabricated by femtosecond lasernanostructuring of glass, OPTICAL MATERIALS EXPRESS, 2011, 1(4):783-795. The microlens array 5 is located between the lens 1 and the photosensitive surface 6 of the camera. The microlens array 5 is composed of many microlenses with the same focal length and the same size. See the enlarged view of the microlens array in FIG. 8 . Enlarged details of the microlens array Figure 8 shows the result of the microlenses being arranged in a square shape. In practical applications, it can also be designed to be arranged in a hexagonal honeycomb shape to more efficiently utilize the space of the photosensitive surface 6 of the camera. The combination of the microlens array 5 and the photosensitive surface 6 of the camera constitutes a hand-held light field camera 7 . For a detailed introduction to the structure of the handheld light field camera, see Ren Ng's doctoral dissertation "Digital light field photography". In order to use the system conveniently, the sky image obtained by the system is processed subsequently, and the computer 9 can be connected to the system again. Computer 9 is responsible for camera control, image acquisition and storage, image processing and data calculation. The computer 9 can send a control signal 10 to the hand-held light field camera 7, and the arrow of the control signal 10 indicates the signal flow direction, and the hand-held light field camera 7 can also pack and send an image data packet 11 to the computer 9, and the arrow of the image data packet 11 indicates the data flow direction.

Figure 2 shows the workflow of the method of the present invention, wherein the process of converting the gray value of the sky image into the radiation intensity value in the first step is also called camera response curve calibration, using the method proposed by Debevec and Malik (ACM SIGGRAPH97 conference paper in 1997, Recovering high dynamic range radiance maps from photographs), the light intensity response curve that satisfies the relationship between the gray value of the image captured by the sky polarization mode detection system and the radiant light intensity of the shooting target can be obtained. After the sky polarization mode detection system is built, it is only necessary to calibrate the camera response curve once when it is used for the first time, and this step can be directly omitted for future use without repeated calibration. In this step, when the hand-held light field camera 7 shoots a pair of sky images, attention should be paid to controlling the exposure of the camera so as to reduce the size of the overexposed or underexposed area in the image as much as possible, because a poorly exposed image will reduce the image obtained by the present invention. Accuracy of sky polarization mode. The second step is to obtain sky polarization degree and polarization orientation angle. The definition of image coordinate system in this step is the same as the definition in "Digital Image Processing" (Author: Gonzalez, published by Electronic Industry Press in 2005, No. 8 pages). In this step, the Radon transform is also called "Radon transform" in Chinese. It is an image projection method invented by J.Radon. The literature has a detailed introduction to it: J.Radon, P.C.Parks.On the determination of functions from their integralvalues Along certain manifolds, IEEE Transactions on Medical Imaging, Vol. 5, No. 4, 1986, pp. 170–176.

Figure 3 is a physical picture of the combination of s-wave plate 3 and polarizer 4 that we customized and processed by ourselves. The s-wave plate 3 and the polarizer 4 are tightly fixed together with double-sided tape, and the white background in the figure is a liquid crystal display. The s-wave plate 3 is closer to the liquid crystal display than the polarizer 4 when viewed. Since the liquid crystal display can be considered as perfectly polarized light, the transmitted light of the combination of s-wave plate 3 and polarizer 4 placed in front of the display shows a change in brightness.

Figure 4 shows the real object of the lens 1 with the s-wave plate 3 and the polarizer 4 inserted in the plane where the aperture 2 is located, and the white background in the figure is still a liquid crystal display. Double-sided tape is used to bond and fix the edges of the s-wave plate 3 and the polarizer 4 to the edge of the aperture 2 in the lens 1, so as to avoid the relative position movement of the s-wave plate 3 in the aperture 2 in actual use.

Figure 5 is a physical map of the sky polarization mode detection system.

We carried out an indoor confirmatory experiment using the method and system described above. In the experiment, we use the liquid crystal display as the measurement object of the sky polarization mode detection system. Due to the inherent characteristics of liquid crystals, the light emitted by liquid crystal displays can be considered as perfectly polarized light. Figure 6 is an indoor experiment scene diagram, in which the liquid crystal display is placed close to the lens, the purpose is to fill the display as much as possible with the imaging field of view of the camera.

Figure 7 shows the image acquired by the sky polarization mode detection system when it takes one shot of the liquid crystal display. Figure 8 is the partially enlarged result of Figure 7. It can be seen in Figure 8 that there are multiple different brightness regions in the image formed by each microlens, which correspond to different polarization directions of multi-quadrant polarizers, and each brightness is different. The combination of the regions forms a pattern of regular and obvious changes in brightness and darkness.

Figure 9 shows the Radon transformation result of a certain microlens corresponding to the imaging circular spot. The abscissa in the figure represents the projection angle value (in degrees), and the ordinate corresponds to the serial number of the Radon transformation result curve under any projection angle. In the figure Points A and B are the minimum and maximum points of the entire Radon transformation result search respectively. X and Y in the figure respectively give the projection angle value corresponding to the point and the serial number of the Radon transformation result, and Z represents the point’s The projection result of the pixel light intensity value, that is, the Z coordinates of point A and point B respectively correspond to MinMinR and MaxMaxR in formula 1.

Figure 10 shows the measurement results of the polarization direction angle distribution of the liquid crystal display by the sky polarization mode detection system. The gray value in the image represents the value of the polarization direction angle, and the unit is degree. It can be seen that the measurement result of the polarization direction angle of the liquid crystal display is relatively uniform, which is consistent with the real situation. The value is about 45 degrees, which is consistent with the actual measurement. The value of the real polarization direction angle of the liquid crystal display is consistent. The measurement value of the polarization direction angle of the liquid crystal display is based on the vertical upward direction as the reference direction of 0 degree. The arrow in the figure indicates the position corresponding to the value of the polarization direction angle measurement result of the liquid crystal display on the gray scale indicator bar on the right. The value of the polarization direction angle of a measurement point of the liquid crystal display is displayed in the image. X and Y in the figure respectively give the coordinates of the point corresponding to the image coordinate system, and Z represents the measurement result of the polarization direction angle of the point. .

Figure 11 shows the measurement results of the polarization distribution of the liquid crystal display by the sky polarization mode detection system. The gray value in the image represents the value of the degree of polarization (the degree of polarization has no unit). It can be seen that the measurement result of the degree of polarization of the liquid crystal display is also relatively uniform, and its value is about 0.8. The arrow in the figure indicates the polarization of the liquid crystal display The value of the direction angle measurement result corresponds to the position on the gray scale indicator bar on the right. The polarization value of a measurement point of the liquid crystal display is displayed in the image. X and Y in the figure respectively give the coordinates of the point corresponding to the image coordinate system, and Z represents the measurement result of the polarization degree of the point. The light emitted by the liquid crystal display can be regarded as completely polarized light, so there is a certain deviation between the measured result of the degree of polarization and the real value, which is mainly caused by the lens not focusing accurately. Due to the insertion of an s-wave plate and a polarizer in the lens, it is difficult for the lens to achieve accurate focus on close-range objects.

Claims (2)

1. A sky polarization pattern detection system, comprising s-wave plate, polarizer, hand-held light field camera, lens, is characterized in that, s-wave plate and polarizer are positioned at the plane of the aperture place of lens, face lens from lens Observed in the direction directly in front of the s-wave plate, the s-wave plate is located in front of the polarizer; the polarizer has a higher extinction ratio and transmittance in the working wavelength range of the s-wave plate. 2. A sky polarization mode detection method utilizes the sky polarization mode detection system provided in claim 1, specifically comprising the steps of: The first step is to use the sky polarization mode detection system to take a sky image, and convert the gray value of the sky image into the light intensity value of the target radiation; The second step is to obtain the sky polarization degree and polarization direction angle: Assume that the microlens array of the hand-held light field camera in the sky polarization mode detection system includes _M rows and N columns of microlenses, all of which are the same; The coordinates below are (X _m,n , Y _m,n ), the radius of the imaging circle spot is D pixels, where D≥5, 1≤m≤M, 1≤n≤N; the pixel coordinates of the center of the imaging circle spot One coordinate element represents the row number of the pixel in the image, and the second coordinate element represents the column number of the pixel in the image; the image coordinate system is defined as follows: the upper left corner of the image is the origin of the coordinate system, and the vertical downward direction in the image is the positive direction of the first coordinate element, and the horizontal right direction in the image is the positive direction of the second coordinate element; Obtain all image areas on the sky image plane whose distance from the pixel point (X _m,n , Y _m,n ) is less than or equal to D pixels, the image area is a circle, let it be G _m,n ; for a circular image The region G _m,n is subjected to Radon transformation, and the projection angle range of Radon transformation is selected to be 0° to 180°, and the projection angle step size of Radon transformation is set to S, and the selection of the value of S should meet the requirement that S can be divisible by 90, then there are projection angles, record the projection angle sequence they form as C, C={0, S, 2S,...k×S,..., 180}, for any projection angle in the projection angle sequence C k×S, there is a Radon transformation result of the projection angle, which is a sequence composed of 2D+1 values, denoted as R _k , and the Radon transformation result sequence of all projection angles is denoted as Ra, find any sequence in Ra The maximum value in R _k is denoted as Rmax _k , then all the maximum value sequences obtained for all projection angles are Find the maximum value in MaxR, the result is recorded as MaxMaxR, and the element number corresponding to the maximum value in MaxR is recorded as, then the element with the sequence number NmaxR is obtained from the Radon transformation result with a projection angle of NmaxR×S, and finally There is a polarization direction angle θ m _{,n corresponding to the microlens Len m ,n} =NmaxR×S; Select the element with the sequence number K in the Radon transformation result sequence Ra The value of K should be determined by judging whether the value of the projection angle θ _{m, n} is greater than 90 degrees. If it is greater than 90 degrees, then If less than 90 degrees then A subsequence in R _K Find the minimum value in , which is recorded as MinMinR, where [] means rounding to an integer; the degree of polarization P _{m,n corresponding to the microlens Len m ,n} can be calculated as follows: (Formula 1) Calculate all microlenses Len _1,1 ,...Len _m,n ...,Len _M,N according to the above method, so as to obtain the sky polarization matrix P=(P _{1, 1} ,...P _m,n ,...P _M,N ) and polarization orientation angle matrix θ=(θ _1,1 ,...θ _m,n ,...θ _M,N ); The degree matrix P and the polarization direction angle matrix θ are the required sky polarization mode measurement results.