Dispersion polarization spectral imaging system and method based on Savart polariscope channel

Abstract

The invention belongs to the technical field of optical metrology testing, and particularly relates to a dispersion polarization spectral imaging system and method based on a Savart polariscope channel. The system comprises a demodulation module, and a front lens group, a polarization modulation module and a grating spectrometer which are sequentially arranged along the direction of an optical axis, the front lens group receives parallel light reflected by a target and enables the parallel light to enter the polarization modulation module, the polarization modulation module separates the parallel light to obtain interferograms with different wavelengths, and the grating spectrometer is used for receiving the interferograms. And the grating spectrometer performs dispersion on the interferograms with different wavelengths, and the demodulation module demodulates the interferograms with different wavelengths at different positions to obtain a polarization image of the target. According to the invention, target multispectral polarization information detection can be realized, the target multispectral polarization information can be obtained, and the detection efficiency is greatly improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical metrology testing, and particularly relates to a Savart polarizer channel dispersion polarization spectrum imaging system and method. BACKGROUND

[0002] With the development of the times, people's demand for remote sensing data has gone beyond the qualitative or semi-quantitative description of the basic characteristics of ground objects, which puts forward new requirements for the quantitative interpretation of geophysical parameters for space optical remote sensors, and promotes the iteration of high precision and high stability.

[0003] In the field of polarization detection, the spatial modulation polarization imaging technology is a kind of polarization information detection technology, which specifically comprises: connecting four wedge-shaped prisms with different optical axis directions with a camera, forming an interference pattern on a CCD area detector, and then demodulating the interference pattern to obtain a polarization image. However, due to the existence of the wedge angle of the wedge-shaped prism, the system has the phenomena of beam deflection and beam separation, so that only part of the light is imaged on the CCD after passing through the wedge-shaped prism, thereby affecting the imaging quality. Therefore, someone proposes to replace the wedge-shaped prism with two Savart polarizers with different optical axes and a half-wave plate. However, due to the half-wave plate inside the system, there will be an installation angle error during installation and adjustment, which introduces an additional phase delay into the system, resulting in a decrease in polarization information demodulation accuracy. At the same time, this system can only realize single-band polarization image detection, and frequent replacement of filters is required to realize multi-band detection, which greatly reduces the detection efficiency. Therefore, it is an urgent technical problem for researchers in the field to optimize the structure of the spatial modulation polarization spectrum imaging system, avoid the influence of the installation angle error, and improve the detection efficiency.

[0004] At present, due to the half-wave plate inside the spatial modulation polarization imaging system based on the Savart polarizer, there will be an installation angle error during installation and adjustment, which introduces an additional phase delay into the system, resulting in a decrease in polarization information demodulation accuracy. In addition, this spatial modulation polarization imaging system can only realize single-band polarization image detection, and frequent replacement of filters is required to realize multi-band detection, which greatly reduces the detection efficiency. SUMMARY

[0005] Therefore, the present application aims to provide a Savart polarizer channel dispersion polarization spectrum imaging system and method to solve the problem that the existing spatial modulation polarization imaging system based on the Savart polarizer has an installation angle error during installation and adjustment of the half-wave plate, which introduces an additional phase delay into the system, resulting in a decrease in polarization information demodulation accuracy. The present application can realize multi-spectral polarization information detection of the target, and can obtain multi-spectral polarization information of the target, greatly improving the detection efficiency.

[0006] To achieve the above object, the technical solution created by the present invention is implemented as follows: A channel dispersion polarization spectral imaging system based on a Savart polarizer includes a demodulation module and a filter, a front mirror group, a polarization modulation module, and a grating spectrometer sequentially arranged along an optical axis. The filter spectrally filters light reflected by a target, allowing incident light within a specific wavelength range to pass through. The front mirror group receives the incident light passed by the filter and allows the incident light to be incident on the polarization modulation module. The polarization modulation module separates parallel light to obtain interference patterns of different wavelengths. The grating spectrometer disperses the interference patterns of different wavelengths. The demodulation module demodulates the interference patterns of different wavelengths at different positions to obtain Stokes vector polarization images of the target at different wavelengths. The polarization modulation module includes a first Savart polarizer, a second Savart polarizer and a linear polarizer coaxially arranged in sequence along the optical axis direction; a right-handed Cartesian space rectangular coordinate system is established with the horizontal plane direction as the X-axis, the vertical horizontal plane direction as the Y-axis, the optical axis direction as the Z-axis, and the light propagation direction as the positive direction of the Z-axis. The first Savart polarizer has two mutually orthogonal optical axes, wherein the first optical axis is 45° to the positive direction of the Z-axis in the YOZ plane, and the second optical axis is 45° to the positive direction of the Z-axis in the XOZ plane. The second Savart polarizer has two mutually orthogonal optical axes, and the two optical axes of the second Savart polarizer are projected on any one of the XOZ plane, the YOZ plane and the XOY plane. The angles between the two optical axes obtained by the projection and any coordinate axis of the corresponding plane are 45° and 135° respectively.

[0007] Furthermore, the channel dispersion polarization spectral imaging system also includes a detector connected to the demodulation module, and the detector is used to display Stokes vector polarization images of the target at different wavelengths.

[0008] Furthermore, the polarization modulation module emits a first beam of linear polarized light, a second beam of linear polarized light, a third beam of linear polarized light, and a fourth beam of linear polarized light.

[0009] A channel dispersion polarization spectral imaging method based on a Savart polarizer is implemented by a channel dispersion polarization spectral imaging system based on a Savart polarizer, specifically comprising the following steps: S1: The light reflected by the target passes through the filter and the front mirror group and is incident on the first Savart polarizer for shearing, obtaining the shearing amount of two beams of linear polarized light. : ; in, is the refractive index of o-light of the first Savart polarizer, ne is the refractive index of e light for the first Savart polariscope, t is the thickness of a single plate in the first Savart polariscope; S2: After the two linearly polarized lights are sheared by the second Savart polariscope, four linearly polarized lights are obtained, and the optical path difference between the four linearly polarized lights and the optical axis is calculated by the following formula: ; ; ; ; ; ; ; wherein, is the focal length of the channel dispersion type polarized light spectrum imaging system, , and are intermediate parameters for expressing the optical path difference between the four linearly polarized lights and the optical axis, and have no actual physical meaning, , is the pixel position on the detector, is the optical path difference between the first linearly polarized light and the optical axis, is the optical path difference between the second linearly polarized light and the optical axis, is the optical path difference between the third linearly polarized light and the optical axis, is the optical path difference between the fourth linearly polarized light and the optical axis; S3: Based on the calculation result of step S2, the complex amplitude of the four linearly polarized lights is expressed as: ; ; ; ; ; wherein, is the wavelength of the incident light transmitted by the filter, and are the amplitudes perpendicular and parallel to the horizontal plane, is the phase of the first linearly polarized light and the optical axis, is the phase of the second linearly polarized light and the optical axis, is the phase of the third linearly polarized light and the optical axis, is the phase of the fourth linearly polarized light and the optical axis; S4: Based on the complex amplitude of the four linearly polarized lights,​ , the interference light intensity I obtained by superimposing four linearly polarized lights is calculated: ; ; ; ; ; ; ; ; wherein, , , and are the Stokes vectors of the target, , b, , c, , a, e and g are all intermediate variables and have no physical meaning; S5: The grating spectrometer disperses the interference patterns of different wavelengths, and the demodulation module converts the interference patterns of different wavelengths at different positions into frequency spectra through Fourier transform: ; ; ; wherein, D is the pixel size of the detector, N is the number of pixels in a row or a column of the detector, , , , respectively correspond to the frequency spectrum information of , , , , is a spatial frequency coordinate value, and are both width parameters associated with spatial frequency characteristics; S6: Based on the interference light intensity I obtained by superimposing four linearly polarized lights, a two-dimensional low-pass filter with a width parameter and a width parameter is selected to filter the frequency spectrum, and a two-dimensional inverse Fourier transform is performed on the filtered frequency spectrum to obtain the different wavelength Stokes vector polarization images of the target: ; ; ; ; wherein real represents taking the real part, imag represents taking the imaginary part, is the S0 component of the Stokes vector corresponding to the wavelength of the incident light, is the S1 component of the Stokes vector corresponding to the wavelength of the incident light, is the S2 component of the Stokes vector corresponding to the wavelength of the incident light, is the S3 component of the Stokes vector corresponding to the wavelength of the incident light.

[0010] Compared with the prior art, the present application can achieve the following beneficial effects: The Savart polarizer channel dispersion polarization spectrum imaging system and method based on the Savart polarizer channel dispersion polarization spectrum imaging system and method can avoid the problem of installation angle error in the process of installing the half-wave plate in the traditional structure, can realize target multi-spectral polarization information detection, improve the data demodulation precision of the instrument, and can obtain target multi-spectral polarization information. The present application will play an important role in the structure design of the spatial modulation polarization imaging system, and has wide engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0011] The accompanying drawings, which form a part of the present application, are used to provide a further understanding of the present application, and the illustrative embodiments of the present application and their description serve the purpose of explaining the present application. The accompanying drawings should not be used to limit the present application unduly. Figure 1 Part of the structure of the Savart polarizer channel dispersion polarization spectrum imaging system based on the Savart polarizer channel dispersion polarization spectrum imaging system according to the present application embodiment is shown in the figure; Figure 2 The structure of the polarization modulation module according to the present application embodiment is shown in the figure; Figure 3 The flowchart of the Savart polarizer channel dispersion polarization spectrum imaging method based on the Savart polarizer channel dispersion polarization spectrum imaging method according to the present application embodiment is shown in the figure.

[0012] Explanation of reference signs: 0, filter; 1, front lens group; 2, polarization modulation module; 3, grating spectrometer; 4, first Savart polarizer; 5, second Savart polarizer; 6, linear polarizer. DETAILED DESCRIPTION

[0013] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and do not constitute a limitation on the present application.

[0014] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other in the case of no conflict.

[0015] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application. In addition, the terms "first", "second" and the like are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined with "first", "second" and the like can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0016] In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood through specific circumstances.

[0017] The present application will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0018] As Figures 1-2 shown, the present application proposes a channel dispersion type polarization spectrum imaging system based on Savart polariscope, which comprises a demodulation module and a filter 0, a pre-mirror group, a polarization modulation module and a grating spectrometer arranged in sequence along the optical axis direction, wherein the filter 0 performs spectral filtering on the light reflected by the target, so that the incident light of a specific wavelength range is transmitted, the pre-mirror group receives the incident light transmitted by the filter 0, and makes the incident light incident to the polarization modulation module, the polarization modulation module separates the parallel light to obtain interference patterns of different wavelengths, the grating spectrometer disperses the interference patterns of different wavelengths, the demodulation module demodulates the interference patterns of different wavelengths at different positions to obtain different wavelength Stokes vector polarization images of the target; The polarization modulation module comprises a first Savart polarizer, a second Savart polarizer and a linear polarizer which are coaxially arranged in sequence along the optical axis direction; a right-hand Cartesian coordinate system is established with the parallel horizontal plane direction as the X axis, the vertical horizontal plane direction as the Y axis, the optical axis direction as the Z axis, and the light propagation direction as the positive direction of the z axis (i.e. the right-hand Cartesian coordinate system is established with the right hand holding the Z axis, the four fingers turning from the positive direction of the X axis to the positive direction of the Y axis, and the thumb pointing to the positive direction of the Z axis); the first Savart polarizer has two mutually orthogonal optical axes, wherein the first optical axis is 45° to the positive direction of the Z axis in the YOZ plane, and the second optical axis is 45° to the positive direction of the Z axis in the XOZ plane; the second Savart polarizer has two mutually orthogonal optical axes, and the two optical axes of the second Savart polarizer are projected in any one of the XOZ plane, the YOZ plane and the XOY plane, and the included angles between the two optical axes obtained by the projection and any coordinate axis of the corresponding plane are 45° and 135° respectively.

[0019] It should be noted that the present application proposes a channel dispersion type polarization spectrum imaging system based on a Savart polarizer, which is combined with a grating spectrometer. The structure mainly comprises a pre-lens group, a polarization modulation module and a grating spectrometer. The pre-lens group is used to obtain parallel light and make it incident on the polarization modulation module. The polarization modulation module comprises two Savart polarizers with different optical axis directions (hereinafter referred to as the first Savart polarizer as SP1 and the second Savart polarizer as SP2) and a linear polarizer, which are used to produce beam splitting and obtain a constant optical path difference directly related to the amount of beam splitting. The optical axis direction of SP1 is perpendicular and parallel to the horizontal plane, and the optical axis direction of SP2 is ±45° to the horizontal plane. The grating spectrometer is used to disperse different wavelengths of light and obtain interference patterns of different wavelengths.

[0020] The principle of the channel-dispersive polarization spectral imaging system based on Savart polarizers is as follows: First, a beam of parallel light reflected from a target remains parallel after passing through a Galilean front-end lens. This parallel light beam is converted to two linearly polarized beams by SP1. Because the two optical axes of SP1 are perpendicular and parallel to the horizontal plane, respectively (let's define the horizontal plane as the x-axis and the direction perpendicular to the horizontal plane as the y-axis), the two beams are laterally displaced, with the vibration directions of the two linearly polarized beams being perpendicular and parallel to the x-axis, respectively. The two linearly polarized beams then pass through SP2. Because the two optical axes of SP2 are oriented at ±45° to the horizontal plane, the two linearly polarized beams are converted to four linearly polarized beams. The vibration directions of the four linearly polarized beams are each at ±45° to the x-axis in the xoy plane. The four linearly polarized beams are separated from each other with a constant optical path difference. Next, the four beams are normalized by a linear polarizer, with their vibration directions perpendicular to the x-axis. Dispersion by a grating spectrometer produces interference patterns at different wavelengths. Finally, the interferogram is demodulated to obtain the polarization image of the target.

[0021] In some embodiments, the channel dispersion polarization spectral imaging system further includes a detector connected to the demodulation module, and the detector is used to display Stokes vector polarization images of the target at different wavelengths.

[0022] In some embodiments, the polarization modulation module emits a first beam of linear polarized light, a second beam of linear polarized light, a third beam of linear polarized light, and a fourth beam of linear polarized light.

[0023] like Figure 3 As shown, the present invention provides a channel dispersion polarization spectral imaging method based on Savart polarizer, which is implemented by a channel dispersion polarization spectral imaging system based on Savart polarizer, and specifically includes the following steps: S1: The light reflected by the target passes through filter 0 and front lens group 1 and is incident on the first Savart polarizer for shearing, obtaining the shearing amount of two beams of linear polarized light. : ; in, is the refractive index of o-light of the first Savart polarizer, is the refractive index of the e-light of the first Savart polarizer, and t is the thickness of the single plate in the first Savart polarizer; S2: After the two linearly polarized beams are sheared by the second Savart polarizer, four linearly polarized beams are obtained. The optical path difference between the four linearly polarized beams and the optical axis is calculated by the following formula: ; ; ; ; ; ; ; wherein, is the focal length of the channel dispersion type polarization spectral imaging system, , and are intermediate parameters expressing the optical path difference of the four linearly polarized lights and the optical axis, without actual physical meaning, , are the pixel positions on the detector, is the optical path difference of the first linearly polarized light and the optical axis, is the optical path difference of the second linearly polarized light and the optical axis, is the optical path difference of the third linearly polarized light and the optical axis, is the optical path difference of the fourth linearly polarized light and the optical axis; S3: based on the calculation result of step S2, the complex amplitude of the superposition of the four linearly polarized lights is expressed as: ; ; ; ; ; wherein, is the wavelength of the incident light transmitted by the filter 0, and are the amplitudes of the vertical and horizontal planes, respectively, is the phase of the first linearly polarized light and the optical axis, is the phase of the second linearly polarized light and the optical axis, is the phase of the third linearly polarized light and the optical axis, is the phase of the fourth linearly polarized light and the optical axis; S4: based on the complex amplitude of the superposition of the four linearly polarized lights , the interference light intensity I obtained by the superposition of the four linearly polarized lights is calculated: ; ; ; ; ; ; ; ; wherein, , , and are the target Stokes vectors, , b, , c, , a, e and g are intermediate variables without physical meaning. It should be noted that the interference light intensity I is simplified from the following formula: ; wherein, is the conjugate of , is the conjugate of , is the conjugate of .

[0024] S5: The grating spectrometer disperses the interference patterns of different wavelengths, so that the interference patterns of different wavelengths are at different positions of the detector, and the demodulation module converts the interference patterns of different wavelengths at different positions into frequency spectra through Fourier transform: ; ; ; wherein, D is the size of the pixel of the detector, N is the number of pixels of a row or a column of the detector, , , , respectively correspond to the frequency spectrum information of , , , , is a spatial frequency coordinate value, and are both width parameters associated with spatial frequency characteristics; S6: Based on the interference light intensity I obtained by superimposing the four beams of linearly polarized light, a two-dimensional low-pass filter with a width parameter and a width parameter is selected to filter the frequency spectrum, and a two-dimensional inverse Fourier transform is performed on the filtered frequency spectrum to obtain the target different wavelength Stokes vector polarization image: ; ; ; ; wherein real denotes taking a real part, and imag denotes taking an imaginary part, is a S0 component of the Stokes vector corresponding to the wavelength of the incident light, is a S1 component of the Stokes vector corresponding to the wavelength of the incident light, is a S2 component of the Stokes vector corresponding to the wavelength of the incident light, is a S3 component of the Stokes vector corresponding to the wavelength of the incident light.

[0025] The application realizes target polarization spectrum image restoration, can avoid the angle error in the installation and adjustment of the half-wave plate in the traditional structure, and makes the system introduce additional phase delay, thereby avoiding the problem of the decline of the polarization information demodulation precision, and can obtain the multi-spectrum polarization information of the target, and greatly improves the detection efficiency.

[0026] It should be understood that the various forms of flow shown above can be used to reorder, add or delete steps. For example, each step described in the present disclosure can be executed in parallel, sequentially or in a different order, as long as the desired results of the technical solutions of the present disclosure can be achieved, which is not limited herein.

[0027] The above detailed description does not constitute a limitation on the protection scope of the present application. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent replacement and improvement within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A channel-dispersed polarization spectroscopic imaging system based on a Savart polariscope, characterized by: The channel dispersion type polarization spectrum imaging system comprises a demodulation module and a filter, a front lens group, a polarization modulation module and a grating spectrometer arranged in sequence along an optical axis direction, wherein the filter performs spectral filtering on target reflected light, and transmits incident light in a specific wavelength range; the front lens group receives the incident light transmitted by the filter, and causes the incident light to be incident on the polarization modulation module; the polarization modulation module separates parallel light, and obtains interference patterns of different wavelengths; the grating spectrometer disperses the interference patterns of different wavelengths; and the demodulation module demodulates the interference patterns of different wavelengths at different positions, and obtains different wavelength Stokes vector polarization images of the target; The polarization modulation module comprises a first Savart polarizer, a second Savart polarizer and a linear polarizer coaxially arranged in sequence along the optical axis direction; a right-hand Cartesian space rectangular coordinate system is established with a horizontal plane direction as an X axis, a vertical horizontal plane direction as a Y axis, an optical axis direction as a Z axis, and a light propagation direction as a positive direction of the Z axis; the first Savart polarizer has two mutually orthogonal optical axes, wherein a first optical axis is 45° to the positive direction of the Z axis in a YOZ plane, and a second optical axis is 45° to the positive direction of the Z axis in an XOZ plane; the second Savart polarizer has two mutually orthogonal optical axes, and the two optical axes of the second Savart polarizer are projected in any one of an XOZ plane, a YOZ plane and an XOY plane, and the angles between the two optical axes obtained by the projection and any coordinate axis of the corresponding plane are 45° and 135° respectively.

2. The Savart polarizing mirror based channel-dispersed polarization spectroscopic imaging system according to claim 1, wherein: The channel dispersion type polarization spectrum imaging system further comprises a detector connected to the demodulation module, and the detector is used to display the different wavelength Stokes vector polarization images of the target.

3. The Savart polarizing mirror based channel-dispersed polarization spectroscopic imaging system according to claim 1, wherein: The polarization modulation module emits first, second, third and fourth linear polarized light beams.

4. A Savart polariscope-based channel-dispersed polarization spectroscopic imaging method, implemented based on the Savart polariscope-based channel-dispersed polarization spectroscopic imaging system of claim 3, characterized in that: Specifically comprising the following steps: S1: the light of the target reflection is incident to the first Savart polarizer in sequence through the filter and the front lens group to obtain the shearing amount of two linearly polarized lights : ； wherein the refractive index of o light for the first Savart polarizer, the refractive index of e light for the first Savart polarizer, t is the thickness of a single plate in the first Savart polarizer; S2: After the two linear polarized light beams are sheared by the second Savart polarizer, four linear polarized light beams are obtained, and the optical path difference between the four linear polarized light beams and the optical axis is calculated by the following formula: ； ； ； ； ； ； ； wherein, is the focal length of the channel dispersion type polarization spectroscopic imaging system, , and are intermediate parameters expressing the optical path difference of the four linearly polarized lights and the optical axis, without actual physical meaning, , is the pixel position on the detector, is the optical path difference of the first linearly polarized light and the optical axis, is the optical path difference of the second linearly polarized light and the optical axis, is the optical path difference of the third linearly polarized light and the optical axis, is the optical path difference of the fourth linearly polarized light and the optical axis; S3: Based on the calculation result of step S2, the complex amplitudes of the four linearly polarized lights are combined to obtain a linearly polarized light with a polarization direction of 45° and a linearly polarized light with a polarization direction of 135°. is represented as ； ； ； ； ； wherein is the wavelength of the incident light transmitted through the filter, and are the amplitudes of the perpendicular and parallel horizontal components, respectively, is the phase of the first beamline polarized light with respect to the optical axis, is the phase of the second beamline polarized light with respect to the optical axis, is the phase of the third beamline polarized light with respect to the optical axis, is the phase of the fourth beamline polarized light with respect to the optical axis; S4: Complex amplitude based on four beams of linearly polarized light , calculate the interference light intensity I obtained by superimposing four beams of linearly polarized light: ； ； ； ； ； ； ； ； wherein , , and Stokes vector targeted, , b, , c, , a, e and g are intermediate variables without physical meaning; S5: The grating spectrometer disperses the interference patterns of different wavelengths, and the demodulation module converts the interference patterns of different wavelengths at different positions into frequency spectra through Fourier transform: ； ； ； wherein D is the size of a pixel of the detector, N is the number of pixels in a row or a column of the detector, , , , respectively represent the spectral information of , , , , is a spatial frequency coordinate value, and are both width parameters associated with the spatial frequency characteristics. S6: interference light intensity obtained based on superposition of four beam linearly polarized lights , a two-dimensional low-pass filter with a width parameter of and a width parameter of is selected to filter the frequency spectrum, and inverse two-dimensional Fourier transform is performed on the filtered frequency spectrum to obtain different wavelength Stokes vector polarization images of the target: ； ； ； ； wherein real denotes taking the real part, and imag denotes taking the imaginary part, S0 is a component of the Stokes vector corresponding to the incident light wavelength, S1 is a component of the Stokes vector corresponding to the incident light wavelength, S2 is a component of the Stokes vector corresponding to the incident light wavelength, S3 is a component of the Stokes vector corresponding to the incident light wavelength.

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