CN109580182B - Method and device for measuring refractive index of curved optical element based on Brewster's law - Google Patents
Method and device for measuring refractive index of curved optical element based on Brewster's law Download PDFInfo
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Abstract
The invention discloses a method and a device for measuring the refractive index of a curved optical element based on Brewster's law, and belongs to the field of optical precision testing. The method adopts convergent laser beams to replace small-caliber parallel laser beams as incident light, focuses expanded beam laser on the surface vertex of a medium to be measured, eliminates the problem caused by beam divergence by using smaller spot area, obtains the intensity distribution of reflected light by using an area array detector, converts the straight-through quantity of the minimum value of detected light intensity into the darkest position of a detected laser dark spot, realizes the accurate quantification of the Brewster angle by using digital image processing, is insensitive to time-varying stray light and motion errors, solves the precision problem caused by motion, shortens the time of single measurement, and realizes the accurate measurement of the refractive index parameter of a curved surface optical element. Compared with the traditional Brewster angle method measuring device, the device has the advantages that the structure is simplified, the measuring precision is effectively improved, and the device cost and the measuring time are reduced.
Description
Technical Field
The invention relates to a method and a device for measuring the refractive index of a curved optical element, in particular to a method and a device for measuring the refractive index of the curved optical element based on Brewster's law, and belongs to the technical field of optical precision testing.
Background
The optical refractive index of a medium is a physical quantity that describes how slow the speed of light in the medium slows relative to the speed of light in a vacuum. For an optical system, the refractive index of the optical elements that make up it directly affects the performance of the optical system. With the development of modern science and technology, the performance requirements of optical systems are increasing, and therefore, the accurate measurement of the optical refractive index of an optical element is also important.
The method for measuring the optical refractive index of the medium by using the Brewster's angle method is a measuring method based on the Brewster's law. The contents of brewster's law are: for an isotropic optical medium, if there is a standard plane interface between the medium and another medium, when a light beam is incident on the interface from one of the media at a brewster angle, its reflected light is linearly polarized light with only an S component. By using the method, the refractive index of the medium to be measured can be calculated by finding the position of the Brewster angle under the condition of knowing the refractive index of the medium where the incident light is located.
The Brewster's angle method for measuring the refractive index of a medium is a non-contact measurement method and can be used for directly measuring the refractive index of a planar optical element. At present, the idea of measuring the refractive index of a medium based on the brewster angle method is to use a single-frequency laser with a small aperture as a light source, use an intensity detector such as a photodiode, and move the detector by a mechanical means to find the position of the brewster angle. For example, chinese patents CN200910234908.8CN201810424804.2 and CN200920282961.0 propose probe moving methods based on different mechanical structures.
However, the conventional brewster's law measurement has its inherent limitation that the medium to be measured must have a standard plane that serves as a medium interface. This is because if a parallel laser beam with a certain aperture is used to irradiate a curved surface, the laser beam may be greatly converged or diverged after being reflected by the curved surface according to the difference of the concave and convex of the curved surface, and is no longer a parallel beam, which may bring great difficulty to the detection of the reflected light and reduce the measurement accuracy. At present, most of the surface types of the shaped optical elements are curved surfaces and do not meet the test conditions. The concept of direct measurement of the refractive index of the molded optical element is violated if it is still desired to sample the test medium.
In addition, due to the adoption of the method for moving the detector for searching the Brewster angular position, the angle measurement precision completely depends on the angle control precision and the judgment precision of the position of the minimum value of the light intensity, and the method is easily influenced by the power fluctuation of a light source, the noise of the detector and the like.
Disclosure of Invention
The invention aims to solve the problem that the traditional Brewster's law measuring method can only detect the limitation of a plane medium to be measured, improve the universality and the practicability of measurement, and provide a measuring method and a measuring device for the refractive index of a curved surface optical element based on the Brewster's law.
The method adopts convergent laser beams to replace small-caliber parallel laser beams as incident light, focuses expanded beam laser on the surface vertex of a medium to be measured, eliminates the problem caused by beam divergence by using smaller spot area, obtains the intensity distribution of reflected light by using an area array detector, converts the straight-through quantity of the minimum value of detected light intensity into the darkest position of a detected laser dark spot, realizes the accurate quantification of the Brewster angle by using digital image processing, is insensitive to time-varying stray light and motion errors, solves the precision problem caused by motion, shortens the time of single measurement, and realizes the accurate measurement of the refractive index parameter of a curved surface optical element.
The method comprises the following steps:
step one, constructing a refractive index measuring light path for converging laser beams, putting a medium to be measured into the light path, and adjusting parameters, wherein the parameters comprise a space position and a pitching angle. And after the parameters are adjusted, fixing the medium to be measured, and continuously sampling the light field image information of the reflection light path for multiple times by the area array detector to obtain and store digital image information.
And step two, carrying out noise reduction processing and edge extraction on the acquired digital image signals.
And step three, positioning the central position of the light spot of the view field. And defining coordinates (x, y) of each pixel on the image plane by taking the coordinate as an origin, determining the corresponding relation between each position and the angle of the reflected light beam according to system element parameters, and establishing a function theta (x, y).
Step four, establishing a distribution relation I (x, y) between the coordinates of each pixel element and the signal intensity, searching the position with the lowest signal intensity through comparison, returning the corresponding coordinate value, calculating the corresponding angle theta, and enabling the theta to be equal to the thetaB。
Step five, calculating the refractive index of the medium to be measured:
n＝n1×tanθB(1)
wherein n is the refractive index of the medium to be measured, and n is1Is refractive index of air, thetaBIs the brewster angle. .
Based on the method, the invention provides a device for measuring the refractive index of the curved optical element based on the Brewster's law. The device comprises a laser, a beam expander, a collimating mirror, a first linear polarizer, a convergent lens, a second linear polarizer, an area array detector and a calculation processing system, wherein:
the laser emits small-caliber parallel laser beams, and the beams are expanded into wide-caliber parallel laser beams through the beam expander. The expanded light passes through a first linear polarizer, the polarization angle is adjusted to be 60 degrees, and the light is incident to the convergent lens perpendicular to the main plane of the convergent lens. The light beam converged by the converging lens is adjusted to be in the horizontal direction by the second linear polarizer, and finally converged to the focal position of the converging lens. The focus of the convergent lens coincides with the vertex of the reflecting surface of the medium to be measured, the included angle of the incident light central beam and the vertex position tangent plane of the reflecting surface in the horizontal direction is determined according to the type of the medium material to be measured, and the placing angle of the medium to be measured is regulated by taking the included angle as the reference. The light beam incident on the surface of the medium to be measured is reflected by the medium, the reflected light and the incident light have the same divergence degree and angle, are received by the area array detector and are converted into digital images, and finally are guided into a computing processing system for computing.
The laser is a visible light waveband single-wavelength laser, and the specific wavelength is determined according to the refractive index measurement requirement.
The working spectral range of the beam expander is adjusted along with the laser.
The aperture of the convergent lens is not smaller than the aperture of the expanded light beam, and the focal depth of the convergent lens is not larger than 1/20 of the focal length.
The medium to be measured is a non-coated medium and has a specular reflection surface, and the surface shape of the specular reflection surface is continuous and random;
the detector is an area array detector, and the spatial resolution of the detector is required to be not less than 1024 × 1024 in order to ensure the testing precision.
In the device, the component parameters and the position parameters between the components are required as follows:
defining the diameter of a beam after expansion as D, the focal length of a convergent lens as f, the vertical distance from the vertex of the surface of a medium to be measured to a photosensitive surface of a detector as L, and the side length of the photosensitive surface of the detector in the direction with smaller pixel number as b, calculating to obtain the range of an incident angle as theta 1-theta 2 according to the type of the medium material to be measured, wherein the angular width delta theta 1-theta 2 satisfies that Brewster angles corresponding to possible refractive indexes of the medium to be measured are all in the range of theta 1-theta 2, and the parameters need to satisfy the following inequality groups:
the stated meaning is that the reflected light of the medium to be tested near the Brewster angle is ensured to be within the angle range of the incident beam of the system, and simultaneously, the incident light of all the incident angles can be received by the detector after being reflected, so that the measuring range and the measuring precision are ensured.
Advantageous effects
Compared with the prior art, the method and the device of the invention have the following advantages and characteristics:
(1) the method for measuring the refractive index of the curved optical element based on the Brewster's law is provided for the first time, and is characterized in that a convergent light beam is used for replacing the incidence of a parallel light beam, so that the problem of limitation on the surface type when the refractive index is measured by the traditional Brewster's angle method can be solved, and the method is also suitable for a planar optical element, thereby expanding the application range of an instrument and being beneficial to realizing the generalization of the refractive index measurement of the optical element.
(2) According to the method, the area array detector is used for replacing a traditional intensity detector in the Brewster angle method for measuring the refractive index, the constant flow of the minimum value of the detected light intensity is converted into the darkest position of the detected laser dark spot, the dynamic errors caused by light intensity fluctuation of the light source and movement of the detector are solved, and the measurement precision is improved.
(3) Compared with the traditional Brewster angle method refractive index measuring device, the device has the advantages that the structure is simplified, the measuring precision is effectively improved, the device cost and the measuring time are reduced, and the device meets the design requirements of modern instruments.
Drawings
FIG. 1 is a block diagram of the apparatus according to an embodiment of the present invention;
FIG. 2 is a flow chart of the method of the present invention;
in the figure 1, 1-laser, 2-beam expander, 3-collimating mirror, 4-first linear polarizer, 5-convergent lens, 6-second linear polarizer, 7-medium to be measured, 8-area array detector and 9-computing processing system.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
Examples
A device for measuring the refractive index of a curved optical element based on Brewster's law is shown in figure 1 and comprises a laser 1, a beam expander 2, a collimating mirror 3, a first linear polarizer 4, a converging lens 5, a second linear polarizer 6, an area array detector 8 and a calculation processing system 9.
In the measurement process, a He-Ne laser with a center wavelength of 632.8nm is adopted as a laser 1, the diameter of a light spot is 2mm, the working wavelength of a beam expander 2 is 450-780nm, the beam expansion magnification is 6 times, the extinction ratio of a first linear polarizer 4 to a first linear polarizer 6 is 100:1, the transmittance is greater than 50%, the horizontal direction is taken as a reference, the polarization angle of the first linear polarizer 4 is 60 degrees, the polarization angle of a second linear polarizer 6 is 0 degree, a converging lens 5 is a double-cemented lens, the clear aperture d is 25.4mm, the focal length f is 75mm, the focal depth is about 3.4mm through calculation, an area array detector 8 is an area array CCD with a resolution of 2048 × 2048, the exposure time is set to be 40ms, the sampling frequency is 7Hz, and the collected signals are stored as a gray scale image with 8 depths.
According to the relevant requirements of the device, the position and the state of the detector are determined by taking the included angle of the incident light central beam and the vertex position tangent plane of the reflecting surface of the prism as the reference, wherein the included angle of the incident light central beam and the tangent plane in the horizontal direction is set to be 55 degrees, and the refractive index range of the common glass material is 1.4-1.7, and the corresponding Brewster angle range is 50-60 degrees.
As shown in fig. 2, using the method for measuring the refractive index of the converging beam optical element based on brewster's law, the process for the rectangular prism of K9 material to be measured in the proposed device is as follows:
according to the scheme provided by the device and the element parameters, corresponding devices and instruments are selected, necessary mechanical parts for fixing and the like are included, and an experimental light path is built.
Firstly, according to the technical scheme of the first step of the method, a complete plane of a right-angle prism is selected as a reflecting surface, the position of the prism is adjusted, the prism is focused on the reflecting surface through a converging lens 5, the prism is rotated in the horizontal direction by taking a focusing point of a light beam on the plane as a center, an angle is detected by using equipment such as a dial and the like until an included angle between the reflecting plane and an optical axis of an incident light path is 55 degrees, the position and the angle of the prism are fixed, and an area array detector 8 is arranged in an emergent light path, so that emergent light spots are normally incident on a photosensitive surface of a CCD detector. And after the light path adjustment is finished, turning on a CCD driving switch to continuously acquire a plurality of images.
Then, the collected picture information is transmitted to the calculation processing system 9, the noise is reduced by averaging a plurality of images, and then the edge extraction is performed on the images.
Then, according to the technical scheme of the third step of the method, the central position of the extracted image is calculated, an x-y coordinate system is established by taking the central position as an origin, the coordinates of each point in the laser dark spot are defined, and the corresponding relation theta (x, y) of the coordinates and the angle is established.
Then, according to the technical scheme of the fourth step of the method, the distribution relation I (x, y) between the coordinates of each pixel element and the signal intensity is established, the position with the lowest signal intensity is searched by comparison, the corresponding coordinate value is returned, the corresponding angle theta is calculated, and the theta is made to be thetaB。
And finally, calculating according to the technical scheme of the fifth step of the method, and measuring the optical refractive index of the prism.
The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
1. The method for measuring the refractive index of the curved optical element based on the Brewster's law is characterized by comprising the following steps of:
step one, constructing a refractive index measuring light path for converging a laser beam, putting a medium to be measured into the light path, and adjusting parameters, wherein the parameters comprise a spatial position and a pitching angle; after the parameters are adjusted, fixing the medium to be measured, and continuously sampling the light field image information of the reflection light path for multiple times by the area array detector to obtain and store digital image information;
the refractive index measurement light path is constructed in the following mode:
the laser emits a small-caliber parallel laser beam, and the small-caliber parallel laser beam is expanded into a wide-caliber parallel laser beam through a beam expander; the expanded light passes through a first linear polarizer, the polarization angle is adjusted to be 60 degrees, and the light is incident to the convergent lens perpendicular to the main plane of the convergent lens; the light beams converged by the converging lens are adjusted to be in the horizontal direction by the second linear polarizer, and finally converged to the focal position of the converging lens; the light beam incident on the surface of the medium to be measured is reflected by the medium, the reflected light and the incident light have the same divergence degree and angle, and are received by the area array detector and converted into digital images;
step two, carrying out noise reduction processing and edge extraction on the acquired digital image signals;
thirdly, positioning the central position of a light spot of a view field, defining coordinates (x, y) of each pixel on an image surface by taking the central position as an original point, determining the corresponding relation between each position and the angle of a reflected light beam according to system element parameters, and establishing a function theta (x, y);
step four, establishing a distribution relation I (x, y) between the coordinates of each pixel element and the signal intensity, searching the position with the lowest signal intensity through comparison, returning the corresponding coordinate value,and calculating the corresponding angle theta, and making theta equal to thetaB；
Step five, calculating the refractive index of the medium to be measured:
n＝n1×tanθB(1)
wherein n is the refractive index of the medium to be measured, and n is1Is refractive index of air, thetaBIs the brewster angle.
2. The measuring device for the refractive index of the curved optical element based on the Brewster's law is characterized by comprising a laser (1), a beam expander (2), a collimating mirror (3), a first linear polarizer (4), a converging lens (5), a second linear polarizer (6), an area array detector (8) and a computing and processing system (9);
wherein, the laser (1) emits a small-caliber parallel laser beam, and the beam is expanded into a wide-caliber parallel laser beam through the beam expander (2); the expanded light passes through a first linear polarizer (4), the polarization angle is adjusted to be 60 degrees, and the light is incident to a convergent lens (5) in a way of being vertical to the main plane of the convergent lens (5); the light beams converged by the converging lens (5) are adjusted to be in the horizontal direction by a second linear polarizer (6) and finally converged to the focal position of the converging lens (5); the focus of the convergent lens (5) is superposed with the vertex of the reflecting surface of the medium to be measured (7), the included angle of the incident light central beam and the vertex position tangent plane of the reflecting surface in the horizontal direction is determined according to the material type of the medium to be measured (7), and the placing angle of the medium to be measured (7) is regulated by taking the included angle as the reference; the light beam incident to the surface of the medium to be measured (7) is reflected by the medium, the reflected light and the incident light have the same divergence degree and angle, are received by the area array detector (8) and are converted into digital images, and finally are guided into the calculation processing system (9) for calculation;
in the measuring device, the element parameters and the position parameters between the elements are required as follows:
the diameter of a beam after expansion is defined as D, the focal length of a converging lens (5) is defined as f, the vertical distance from the surface vertex of a medium to be detected (7) to a photosensitive surface of a detector is defined as L, and the side length in the direction with smaller pixel number of the photosensitive surface of an area array detector (8) is defined as b;
the above parameters satisfy the above inequality group.
3. The device for measuring the refractive index of a curved optical element based on Brewster's law as claimed in claim 2, wherein the laser (1) is a single wavelength laser in the visible light band, and the specific wavelength is determined according to the refractive index measurement requirement.
4. -the device for measuring the refractive index of a curved optical element based on brewster's law according to claim 2, characterized in that the operating spectral range of the beam expander lens (3) is adjusted with the laser) (1).
5. The device for measuring the refractive index of a curved optical element based on Brewster's law as claimed in claim 2, wherein said converging lens (5) has an aperture not smaller than the aperture of the expanded beam and a focal depth not greater than 1/20 of the focal length.
6. The device for measuring the refractive index of a curved optical element based on Brewster's law as claimed in claim 2, wherein said medium to be measured (7) is an uncoated medium, and there is a specular reflection surface, which is continuous and arbitrary in surface shape.
7. The device for measuring the refractive index of a curved optical element based on Brewster's law as claimed in claim 2, wherein the spatial resolution of the area array detector (8) is not less than 1024 × 1024.
8. The device for measuring the refractive index of a curved optical element based on Brewster's law as claimed in claim 2, wherein the angle between the central beam of incident light and the tangent plane at the apex of the reflecting surface of the prism in the horizontal direction is 55 ° at its midpoint, and the position and state of the area array detector (8) are determined based on this angle.
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| TWI725827B (en) * | 2020-04-24 | 2021-04-21 | 力晶積成電子製造股份有限公司 | Image sensing module |
| CN117491976B (en) * | 2023-12-27 | 2024-04-02 | 武汉灵途传感科技有限公司 | Transmit-receive coaxial laser detection system, adjusting method thereof and laser radar |
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