CN114061803A - Circular polarization photoelastic stress measuring system and measuring method - Google Patents
Circular polarization photoelastic stress measuring system and measuring method Download PDFInfo
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Abstract
The invention discloses a circular polarization photoelastic stress measuring system and a measuring method, and belongs to the technical field of photoelastic measurement. The circular polarization photoelastic stress measuring system comprises a dual-wavelength light source, a polarizer, an achromatic quarter wave plate, a polarization analyzer, an imaging lens and a photoelectric detector. The circular polarization photoelastic stress measuring instrument disclosed by the invention is simple in structure, convenient to use and high in measuring speed; the instrument adopts mature structural processes, has low processing cost, can realize large-size processing, has no limit on the size of a sample, and has wider measurement range; the circular polarization photoelastic stress measuring method disclosed by the invention adopts dual-wavelength light source illumination, and avoids the problems that isochromatic lines and isochromatic lines are difficult to distinguish and the level of the stripes is difficult to determine in the process of inverse trigonometric function calculation by recording the distribution and the tiny difference of stress stripes under different light sources, so that the accurate stress magnitude can be obtained finally.
Description
Technical Field
The invention relates to a circular polarization photoelastic stress measuring system and a measuring method, and belongs to the technical field of photoelastic measurement.
Background
Photoelastic measurement is an important technology for researching birefringence characteristics or stress distribution in optical materials and optical devices, and has the advantages of rapidness, intuition and high precision. The principle of photoelastic measurement is that the refractive index distribution of an optical material or an optical element changes with the internal principal stress difference or structural change, and the magnitude of the principal stress difference and the principal stress direction in the material or the internal structure of the element can be estimated by measuring the refractive index distribution. The existing photoelastic measuring device is mainly divided into a circular polarization stress measuring instrument and a parallel plane polarization stress measuring instrument.
As shown in fig. 3, the circular polarization stress measuring apparatus includes a beam expanding system composed of a laser source 10, a polarizer 2, a focusing lens 11 and a collimating lens 9, a sample 4 to be measured, an imaging lens 6, a quarter-wave plate 12 adjustable by a mechanical rotating device, an analyzer 5 and a photodetector 7. By controlling the relative angle between the quarter-wave plate 12 and the sample 4 to be detected, the linearly polarized light passing through the polarizer 2 is still linearly polarized after passing through the quarter-wave plate 12, the angle of the change of the polarization axis direction of the polarizer 2 is in direct proportion to the birefringence phase difference of the sample 4 to be detected, and the angle can be detected by rotating the analyzer 5. The specific operation process is as follows, firstly, the light transmission axis of the polarizer 2 and the fast axis of the quarter-wave plate 12 are adjusted to the set x-axis direction, the sample 4 to be measured is placed, two main stress directions of the sample 4 to be measured form 45 degrees with the x-axis and the y-axis respectively, the sample 4 to be measured and the analyzer 5 are controlled to rotate, and the extinction position is searched. After the fast axis of the quarter-wave plate 12 and the sample 4 to be measured are respectively adjusted to the x-axis direction (consistent with the transmission axis of the polarizer 2), the analyzer 5 is controlled to rotate for a circle, the extinction position is searched again, the rotated angle phi between the two extinction positions is recorded, and the birefringence phase difference sigma of the sample 4 to be measured can be obtained to be 2 phi according to the read phi value. According to the measuring method, optical elements such as the quarter-wave plate 12 and the like which can rotate are added into a light path, the mechanical movement of the elements is included for a plurality of times in the measuring process, the light path structure and the measuring process are both complicated objectively, and the measuring instrument is very heavy when the caliber is large, so that the application range is greatly limited.
As shown in fig. 4, the parallel plane polarization stress measuring instrument includes a monochromatic point light source 13, a polarizer 2, a collimating lens 9, a sample 4 to be measured, an imaging lens 6, an analyzer 5 and a photodetector 7. The stress fringe distribution is generally determined by the principal stress direction θ and the principal stress difference σ. The value theta cannot be accurately measured on the isochromatic line stripes by the parallel plane polarization stress measuring instrument, the monochromatic light source is changed into white light illumination by a five-step color phase shift method, the refractive indexes of light rays with different wavelengths are different, so that the main stress difference sigma of the light rays with different wavelengths passing through the same position of the sample 4 to be measured is different, and the centers of the isochromatic line black stripes with different wavelengths in the white light spectrum are at different points, so that the isochromatic line under the white light illumination always has available isochromatic line information, and the problem of measuring the stress direction theta is solved. However, the method cannot calculate the stress magnitude, and if the stress magnitude needs to be calculated, the experimental device needs to be modified, so that the measurement is inconvenient, and the measurement can only be performed in a laboratory due to the complex experimental system and high requirement on precision, so that the application range of the photoelastic measurement is limited.
Disclosure of Invention
The invention provides a circular polarization photoelastic stress measuring system and a measuring method, aiming at solving the problems that the existing photoelastic measuring device is complex in structure, the size of an element to be measured is limited, the system cost is high, the application range is limited, the stress cannot be calculated and the like.
The first purpose of the invention is to provide a circular polarized light photoelastic stress measuring method, which is realized based on a double-wavelength circular polarized light photoelastic stress measuring instrument;
the double-wavelength polarized light photoelastic stress measuring instrument comprises: the device comprises a dual-wavelength light source, a polarizer (2), an achromatic quarter wave plate (3), a polarization analyzer (5), an imaging lens (6) and a photoelectric detector (7); the polarizer (2), the achromatic quarter-wave plate (3), the analyzer (5), the imaging lens (6) and the photoelectric detector (7) are sequentially positioned on a transmission path of emergent light of the dual-wavelength light source;
the method comprises the steps of obtaining main stress difference distribution with phase wrapping by adopting dual-wavelength light source illumination solution, and splicing continuous main stress difference distribution after performing unwrapping processing on the main stress difference sigma (x, y) obtained by solving an inverse trigonometric function by adopting an unwrapping algorithm.
Optionally, the method includes:
the method comprises the following steps: turning on the dual-wavelength light source, adjusting the switch, and placing the light source at wavelength λ1In the light emitting mode, the included angle between the polarization axis angle of the polarizer (2) and the fast axis of the achromatic quarter-wave plate (3) is adjusted to be 45 degrees, the polarization axis angle of the analyzer (6) is arbitrary, so that light passing through the achromatic quarter-wave plate (3) is circularly polarized light, and the collected image is marked as I01(x, y) adjusting the light source to a wavelength λ2Lighting mode, collecting image as I02(x,y)；
Step two: a sample (4) to be measured is put between the polarizer (2) and the analyzer (5), and a light source is placed at the wavelength lambda1Lighting mode, collecting image as I11(x, y) adjusting the light source to a wavelength λ2Lighting mode, collecting image as I12(x,y)；
Step three: and calculating the main stress difference sigma (x, y) of the sample (4) to be measured according to the images collected in the first step and the second step.
Optionally, the step three of calculating the magnitude σ (x, y) of the principal stress difference of the sample to be measured (4) includes:
the main stress difference size sigma (x, y) of the sample (4) to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
a second object of the present invention is to provide a circular polarization photoelastic measuring system, wherein the measuring instrument includes: the device comprises an image acquisition module and a main stress difference calculation module; the image acquisition module is connected with the main stress difference calculation module;
the image acquisition module includes: the device comprises a dual-wavelength light source, a polarizer (2), an achromatic quarter wave plate (3), a polarization analyzer (5), an imaging lens (6) and a photoelectric detector (7); the polarizer (2), the achromatic quarter-wave plate (3), the analyzer (5), the imaging lens (6) and the photoelectric detector (7) are sequentially positioned on a transmission path of emergent light of the dual-wavelength light source;
the main stress difference calculation module is used for calculating the size of the main stress difference according to the picture information acquired by the image acquisition module, and the calculation process comprises the following steps:
the main stress difference size sigma (x, y) of the sample (4) to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
optionally, the dual-wavelength light source is a dual-wavelength area light source (1).
Optionally, the dual-wavelength light source includes: the dual-wavelength point light source comprises a dual-wavelength point light source (8) and a collimating lens (9), and emergent rays of the dual-wavelength point light source (8) are emitted out after passing through the collimating lens (9).
Optionally, the dual-wavelength light source is controlled by a switch, and is configured to control light with different wavelengths and frequencies to be emitted.
Optionally, the achromatic quarter wave plate (3) adjusts linearly polarized light into circularly polarized light.
Optionally, the analyzer (5) adjusts the angle by rotating around its own deflection axis.
Optionally, in the measurement system, a sample to be measured is placed between the achromatic quarter wave plate (3) and the analyzer (5).
The invention has the beneficial effects that:
(1) the invention adopts a dual-wavelength light source and a circular optical system, abandons a photoelastic modulator and a heavy mechanical rotating device in the prior art, and achieves the advantages of simple structure, convenience and low cost;
(2) according to the invention, by means of the dual-wavelength light source and recording the distribution and the small difference of the stress stripes under different light sources, the problems that isochromatic lines and isochromatic lines are difficult to distinguish and the stripe level is difficult to determine in the process of calculating the inverse trigonometric function are solved, so that the accurate stress magnitude can be obtained.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a schematic structural diagram according to a first embodiment of the present invention;
FIG. 2 is a schematic structural diagram of a third embodiment of the present invention;
FIG. 3 is a schematic structural diagram of a circular polarization stress measuring apparatus in the prior art of photoelasticity measurement;
FIG. 4 is a schematic structural diagram of a parallel plane polarization stress measuring apparatus in the prior art photoelasticity measurement;
in the figure, 1, a dual wavelength surface light source; 2. a polarizer; 3. an achromatic quarter wave plate; 4. a sample to be tested; 5. an analyzer; 6. an imaging lens; 7. a photodetector; 8. a dual wavelength surface light source; 9. a collimating lens; 10. a laser light source; 11. a focusing lens; 12. a quarter wave plate; 13. single wavelength point light source
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The first embodiment is as follows:
the embodiment provides a circular polarization photoelasticity measurement system, and the system includes: the device comprises an image acquisition module and a main stress difference calculation module; the image acquisition module is connected with the main stress difference calculation module;
referring to fig. 1, the image acquisition module of the present embodiment includes: the double-wavelength area light source 1 is characterized in that a polarizer 2, an achromatic quarter-wave plate 3, an analyzer 5, an imaging lens 6 and a photoelectric detector 7 are sequentially arranged on a transmission path of emergent light of the double-wavelength area light source 1. The dual-wavelength area light source 1 can be controlled by a switch and can sequentially emit light with the wavelength of lambda1And a wavelength of λ2Two lights with similar frequencies.
The main stress difference calculation module is used for calculating the size of the main stress difference according to the picture information acquired by the image acquisition module, and the calculation process comprises the following steps:
the main stress difference size sigma (x, y) of the sample (4) to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
wavelength λ in the present embodiment1Selecting 510nm wavelength lambda2520nm was chosen. The polarizer 2 and the analyzer 5 can rotate around the deflection axis thereof, and a sample 4 to be tested is placed between the polarizer 2 and the analyzer 5. The photoelectric detector 7 converts the light information transmitted from the imaging lens 6 into an electric signal and collects a fringe pattern generated by the irradiation of the dual-wavelength area light source 1 on the sample 4 to be measured.
Example two
The embodiment discloses a circular polarization photoelastic stress measuring method, which applies the circular polarization photoelastic stress measuring instrument described in the first embodiment to measure. The method comprises the following steps of acquiring fringe patterns generated by irradiating a sample to be measured 4 by two light sources with different frequencies, and calculating and processing the fringe images containing birefringence information to obtain the main stress direction and the main stress difference of the sample to be measured 4, thereby completing quantitative photoelasticity measurement:
the method comprises the following steps: turning on the light source, placing the light source at a wavelength λ1In the light emitting mode, the angle of the polarization axis of the polarizer 2 is adjusted to be 0 degrees, the angle of the fast axis of the achromatic quarter-wave plate 3 is 45 degrees, and the angle of the polarization axis of the analyzer 5 is arbitrary, namely, the light passing through the quarter-wave plate 3 is circularly polarized light. The collected image is marked as I01Adjusting the light source to a wavelength λ2Light emission pattern, collection chartLike a notation I02；
Step two: placing a sample 4 to be measured between the achromatic quarter-wave plate 3 and the analyzer 5, and placing a light source at a wavelength lambda1Lighting mode, collecting image as I11Adjusting the light source to a wavelength λ2Lighting mode, collecting image as I12；
Step three: calculating the main stress difference sigma (x, y) of the sample 4 to be measured according to the images acquired in the first step and the second step;
the main stress difference sigma (x, y) of the sample to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
by adopting the dual-wavelength illumination provided by the embodiment, the main stress difference distribution with phase wrapping can be obtained by unwrapping, and the continuous main stress difference distribution can be spliced after unwrapping the main stress difference sigma (x, y) obtained by unwrapping the inverse trigonometric function by adopting the conventional unwrapping algorithm.
EXAMPLE III
The embodiment discloses a circular polarization photoelasticity measurement system, includes: the device comprises an image acquisition module and a main stress difference calculation module; the image acquisition module is connected with the main stress difference calculation module;
the image acquisition module of this embodiment is shown in fig. 2, and includes a dual-wavelength point light source 8, and a collimating lens 9, a polarizer 2, an achromatic quarter-wave plate 3, a sample 4 to be measured, an imaging lens 6, an analyzer 5, and a photodetector 7 are sequentially disposed on a propagation path of light emitted from the dual-wavelength point light source 8. The dual-wavelength point light source 8 can be controlled by a switch and can sequentially emit light with the wavelength of lambda1And a wavelength of λ2Two lights with similar frequencies.
The circular polarization photoelastic stress measuring system of the present embodiment can also apply the circular polarization photoelastic stress measuring method disclosed in the second embodiment to measure.
Example four
In order to verify the measurement accuracy of the circular polarization photoelastic stress measurement method disclosed in the second embodiment, the present embodiment derives according to the measurement method provided in the second embodiment, specifically, according to the measurement method provided in the second embodiment
Is determined by the sign of
Is that
Is also in the falling interval.
Suppose that
And
both in the ascending or descending interval, and therefore at their inflection points, i.e. at the points of inflection
And
there will likely be one near n piIn the ascending interval and one belonging to the descending interval, the stress optical coefficient c and the sample thickness t of the material are constant values, so that the stress optical coefficient c and the sample thickness t are increased along with the increase of sigma (x, y)
At the beginning, error is generated to
When the error disappears, the error is eliminated,
the time error is maximum, and the error value is
And actual value
By comparison, the relative error is
Wavelength λ in the present embodiment1And wavelength lambda2625nm and 650nm, respectively, and the relative measurement error is 3.85%.
In the general photoelastic measurement method, the required range is 10 fringe orders, so the error analysis measuring range of the embodiment
Under this condition, the error of the present embodiment
The wavelength is about 0.385, and the error range of the photoelastic measurement technology is within the error requirement range, so that the photoelastic measurement device can achieve the purposes of simplifying the structure of the photoelastic measurement device and simultaneously meeting the measurement accuracy of circular polarization photoelastic stress.
Some steps in the embodiments of the present invention may be implemented by software, and the corresponding software program may be stored in a readable storage medium, such as an optical disc or a hard disk.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (10)
1. A circular polarization photoelastic stress measuring method is characterized in that the method is realized based on a double-wavelength circular polarization photoelastic stress measuring instrument;
the double-wavelength polarized light photoelastic stress measuring instrument comprises: the device comprises a dual-wavelength light source, a polarizer (2), an achromatic quarter wave plate (3), a polarization analyzer (5), an imaging lens (6) and a photoelectric detector (7); the polarizer (2), the achromatic quarter-wave plate (3), the analyzer (5), the imaging lens (6) and the photoelectric detector (7) are sequentially positioned on a transmission path of emergent light of the dual-wavelength light source;
the method comprises the steps of obtaining main stress difference distribution with phase wrapping by adopting dual-wavelength light source illumination solution, and splicing continuous main stress difference distribution after performing unwrapping processing on the main stress difference sigma (x, y) obtained by solving an inverse trigonometric function by adopting an unwrapping algorithm.
2. The method of claim 1, wherein the calculating of the principal stress difference σ (x, y) comprises:
the method comprises the following steps: turning on the dual-wavelength light source, adjusting the switch, and placing the dual-wavelength light source at wavelength λ1Adjusting the included angle between the polarization axis angle of the polarizer (2) and the fast axis of the achromatic quarter-wave plate (3) to be 45 degrees in a light emitting mode, wherein the polarization axis angle of the analyzer (6) is arbitrary, so that light passing through the achromatic quarter-wave plate (3) is circularly polarized light, and an acquired image is recorded as I01(x, y) adjusting the dual wavelength light source to a wavelength λ2Lighting mode, collecting image as I02(x,y)；
Step two: a sample (4) to be measured is placed between the polarizer (2) and the analyzer (5), and the dual-wavelength light source is placed at the wavelength lambda1Luminescence pattern, collectionThe image is marked as I11(x, y) adjusting the dual wavelength light source to a wavelength λ2Lighting mode, collecting image as I12(x,y)；
Step three: and calculating the main stress difference sigma (x, y) of the sample (4) to be measured according to the images collected in the first step and the second step.
3. The method according to claim 2, wherein the step three of calculating the magnitude of the principal stress difference σ (x, y) of the sample (4) comprises:
the main stress difference size sigma (x, y) of the sample (4) to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
4. a circularly polarized photoelastic measurement system, comprising: the device comprises an image acquisition module and a main stress difference calculation module; the image acquisition module is connected with the main stress difference calculation module;
the image acquisition module includes: the device comprises a dual-wavelength light source, a polarizer (2), an achromatic quarter wave plate (3), a polarization analyzer (5), an imaging lens (6) and a photoelectric detector (7); the polarizer (2), the achromatic quarter-wave plate (3), the analyzer (5), the imaging lens (6) and the photoelectric detector (7) are sequentially positioned on a transmission path of emergent light of the dual-wavelength light source;
the main stress difference calculation module is used for calculating the size of the main stress difference according to the picture information acquired by the image acquisition module, and the calculation process comprises the following steps:
the main stress difference size sigma (x, y) of the sample (4) to be tested satisfies the following relational expression:
wherein, I1(x, y) represents at a wavelength of λ1Normalized fringe intensity distribution after elimination of background light under illumination of light source, I2(x, y) represents at a wavelength of λ2The normalized stripe intensity distribution after the influence of background light is eliminated under the illumination of the light source, c is the stress optical coefficient of the sample to be measured, and t is the thickness of the sample to be measured;
the formula is further written as:
if λ1<λ2Then is at
When the temperature of the water is higher than the set temperature,
then
Is [0, π/2]Within a range of decreasing functions, i.e.
If λ1>λ2Then, then
Is [ pi/2, pi]Increasing functions within the range, i.e.
The turning parts of the function are distinguished by judging the increase and decrease of the function, the part with increasing stripes is reserved, the part with decreasing stripes is reversed by taking the negative sign, a stripe graph I with jumping discontinuous points is obtained, and the stripe intensity variation trend in the graph is the same as the variation trend of the main stress difference, namely:
when I is1(x，y)-I2(x,y)>When 0, let I equal to I1(x, y); when I is1(x，y)-I2When (x, y) is less than or equal to 0, making I ═ I1(x，y)；
The main stress difference size sigma (x, y) of the sample (4) to be detected is as follows:
5. system according to claim 4, characterized in that said dual wavelength light source is a dual wavelength area light source (1).
6. The system of claim 4, wherein said dual wavelength light source comprises: double wavelength point light source (8) and collimating lens (9), the emergent ray of double wavelength point light source (8) passes through jet-out behind collimating lens (9).
7. The system of claim 4 wherein said dual wavelength light source is controlled by a switch for controlling the emission of light of different wavelengths and frequencies.
8. A system according to claim 4, characterized in that the achromatic quarter waveplate (3) adjusts linearly polarized light to circularly polarized light.
9. System according to claim 4, characterized in that the analyzer (5) is angularly adjusted by rotation about its own deflection axis.
10. System according to claim 4, characterized in that in the system a sample to be measured is placed between the achromatic quarter waveplate (3) and the analyzer (5).
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