Lens refractive index measuring method based on digital holographic microscopy

Abstract

The invention provides a method for measuring the refractive index of a lens based on digital holographic microscopy. The method comprises the following steps: S1, collecting a digital holographic micrograph of a to-be-measured lens; s2, processing the digital holographic microscopic image to obtain a phase image of the lens to be measured; s3, acquiring phase data of two points in the phase diagram of the lens to be measured; s4, acquiring thickness data of the two points of the lens to be measured; and S5, obtaining the refractive index of the lens to be measured according to the phase data and the thickness data of the two points.

Description

Technical Field

[0001] The present disclosure relates to the field of optical measurement technology, and in particular to a lens refractive index measurement method based on digital holographic microscopy. Background Art

[0002] Digital holographic microscopy is a combination of digital holography and optical microscopy, and is another major breakthrough after digital holography. The resolution of the camera, that is, the size of the pixel, is one of the main reasons that limit the resolution of digital holography. In order to improve the imaging resolution, a microscopic imaging system can be added to the object light path to amplify the object light wave, thereby reducing the field of view in exchange for higher resolution. The maturity of digital holographic microscopy technology has also made it widely used in biomedical imaging, surface morphology detection, optical information encryption and storage, and other fields.

[0003] Optical lenses, especially miniature and aspherical transparent plastic lenses, are widely used in various industries with the development of materials and processing technology. For example, injection molded lenses, which are widely used in imaging lenses, have replaced glass lenses in various fields due to their free geometry, low weight, high imaging quality and low production cost. However, due to the inherent molecular orientation of the induced plastic material during curing during the injection molding process, the optical properties of the lens are easily changed. Summary of the invention

[0004] In view of this, in order to at least partially solve at least one of the above-mentioned technical problems, the present disclosure provides a lens refractive index measurement method based on digital holographic microscopy.

[0005] In order to achieve the above purpose, the technical solution of the present invention is as follows:

[0006] According to an embodiment of one aspect of the present disclosure, a lens refractive index measurement method based on digital holographic microscopy is provided, including operations S1-S5: Operation S1: collecting a digital holographic micrograph of the lens to be measured; Operation S2: processing the digital holographic micrograph to obtain a phase diagram of the lens to be measured; Operation S3: obtaining phase data of two points in the phase diagram of the lens to be measured; Operation S4: obtaining thickness data at the above two points of the lens to be measured; and Operation S5: obtaining the refractive index of the lens to be measured according to the phase data and thickness data of the two points.

[0007] According to an embodiment of the present disclosure, in operation S1, in a digital holographic process for a lens to be tested, reference light and object light are interfered at a set angle to obtain a digital holographic micrograph.

[0008] According to an embodiment of the present disclosure, operation S1 includes: building a cage-structured digital holographic microscopy system; adjusting the position of the lens to be measured so that the interference fringe imaging on the detector plane is clearly focused; and recording a hologram when the interference fringe contrast is high and the fringes are dense.

[0009] According to the embodiment of the present disclosure, in the digital holographic process, the reference light and the object light are tilted at a set angle. Interference is performed to form a digital holographic micrograph distribution The mathematical expression is as follows:

[0010] ;

[0011] Wherein, S represents the object light distribution, R represents the reference light distribution, x represents the spatial coordinate in the x direction), y represents the spatial coordinate in the y direction, i represents a complex number, and k represents the wave number.

[0012] According to an embodiment of the present disclosure, in operation S2, the processing of the digital holographic micrograph includes Fourier transform, window function filtering, inverse Fourier transform, and phase unwrapping. Specifically, the phase of the object light is reconstructed by Fourier transform, inverse Fourier transform, and window function filtering; and the phase of the object light is calculated by phase unwrapping.

[0013] According to an embodiment of the present disclosure, operation S2 includes: intercepting the portion of the digital holographic micrograph where the lens is located; performing Fourier transform on the intercepted digital holographic micrograph to obtain a spectrum diagram; using a window function to select one of the ±1 levels of the spectrum diagram for filtering; performing an inverse Fourier transform on the filtering result and extracting the phase part to obtain a wrapped phase; and unwrapping the wrapped phase.

[0014] According to the embodiment of the present disclosure, in operation S3, the phase ψ and the lens thickness are obtained by the optical path calculation formula: and refractive index The relationship is:

[0015] ;

[0016] Where λ represents the wavelength of the test light.

[0017] According to an embodiment of the present disclosure, in operation S4, thickness data at two points on the lens to be tested is obtained by tomography.

[0018] According to an embodiment of the present disclosure, in operation S5, the refractive index n of the lens to be tested is obtained by restoring the numerical values ​​of two points on the phase of the lens to be tested:

[0019] ;

[0020] Where Δψ is the phase difference between two points on the lens to be measured, , , is the phase of two points on the lens to be measured, Δd is the thickness difference between the two points on the lens, , ,and are the thickness differences of the above two points on the lens to be tested. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0022] Figure 1 This is a flow chart of a lens refractive index measurement method based on digital holographic microscopy according to an embodiment of the present disclosure.

[0023] Figure 2 The figure is a schematic diagram of the working principle of the lens refractive index measurement system based on digital holographic microscopy according to an embodiment of the present disclosure.

[0024] Figure 3 It is a schematic diagram of obtaining different phases of the lens to be measured in the measurement method of the embodiment of the present disclosure.

[0025] Figure 4 Schematic diagram of the refractive index calculation principle in the measurement method of an embodiment of the present disclosure. DETAILED DESCRIPTION

[0026] The present disclosure provides a lens refractive index measurement method based on digital holographic microscopy. In view of the fact that the detection of micro plastic lenses is a key issue in the design, manufacture and quality control of optical lenses, the measurement method proposed in the present disclosure can obtain the relative phase distribution of the micro lenses with the help of digital holographic microscopy imaging, and with the thickness data obtained by CT, the substrate refractive index of the processed optical lens can be accurately measured without secondary processing.

[0027] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0028] In an embodiment of the present disclosure, a method for measuring the refractive index of a lens based on digital holographic microscopy is provided, such as Figure 1 As shown, including operations S1-S5:

[0029] Operation S1: collecting a digital holographic micrograph of the lens to be tested;

[0030] Operation S2: Processing the digital holographic micrograph to obtain a phase diagram of the lens to be tested;

[0031] Operation S3: acquiring phase data of two points in the phase diagram of the lens to be tested;

[0032] Operation S4: acquiring thickness data of the lens to be tested at the above two points; and

[0033] Operation S5: obtaining the refractive index of the lens to be measured according to the phase data and thickness data of the two points.

[0034] According to an embodiment of the present disclosure, in operation S1, in a digital holographic process for a lens to be tested, reference light and object light are interfered at a set angle to obtain a digital holographic micrograph.

[0035] According to an embodiment of the present disclosure, operation S1 includes: building a cage-structured digital holographic microscopy system; adjusting the position of the lens to be measured so that the interference fringe imaging on the detector plane is clearly focused; and recording a hologram when the interference fringe contrast is high and the fringes are dense.

[0036] According to the embodiment of the present disclosure, in the digital holographic process, the reference light and the object light are tilted at a set angle. Interference is performed to form a digital holographic micrograph distribution The mathematical expression is as follows:

[0037] ;

[0038] S represents the object light distribution, R represents the reference light distribution, x represents the spatial coordinate in the x direction), y represents the spatial coordinate in the y direction, i represents a complex number, and k represents the wave number.

[0039] According to an embodiment of the present disclosure, in operation S2, the processing of the digital holographic micrograph includes Fourier transform, window function filtering, inverse Fourier transform, and phase unwrapping.

[0040] Specifically, the phase of the object light is reconstructed through Fourier transform, inverse Fourier transform and window function filtering; and the phase of the object light is calculated through phase unwrapping.

[0041] According to the embodiment of the present disclosure, operation S2 includes: intercepting the portion of the digital holographic micrograph where the lens is located; performing Fourier transform on the intercepted digital holographic micrograph to obtain a spectrum graph; selecting the spectrum graph using a window function One of the stages is filtered; an inverse Fourier transform is performed on the filtering result and a phase part is extracted to obtain a wrapped phase; and the wrapped phase is unwrapped.

[0042] It can be seen that the above operations S1 and S2 involve the acquisition and restoration of digital holographic micrographs. The first and second terms in the above formula (1) are zero-level DC components, which only contain the intensity information of the object light and the reference light. The third and fourth terms are mutually conjugated phase difference components. When the reference light is a quasi-plane wave, the spectrum is taken The level term can be regarded as the physical light Multiplying with the exponential constant term, it is expressed as a two-dimensional frequency shift of the object light in the frequency domain. In order to reconstruct the hologram using the Fourier filter algorithm, it is necessary to adjust the appropriate object parameter light angle To avoid zero-level terms and spectrograms in the frequency domain Level terms are aliased in the frequency domain. When the level terms are properly separated in the frequency domain, the phase of the object light can be reconstructed by Fourier transform and frequency domain filtering. :

[0043]

[0044] in and They represent Fourier transform and its inverse transform respectively. It is a window function in the Fourier domain, used to filter out the +1 level term, u represents the frequency coordinate in the horizontal direction, and v represents the frequency coordinate in the vertical direction.

[0045] According to an embodiment of the present disclosure, in operation S3, the relationship between the phase ψ and the lens thickness d and the refractive index n is obtained by the optical path calculation formula:

[0046] ;

[0047] Where λ represents the wavelength.

[0048] According to an embodiment of the present disclosure, in operation S4, thickness data at two points on the lens to be tested is acquired by computer tomography.

[0049] According to an embodiment of the present disclosure, in operation S5, the refractive index n of the lens to be tested is obtained by restoring the numerical values ​​of two points on the phase of the lens to be tested:

[0050]

[0051] Where Δψ is the phase difference between two points on the lens to be measured, , , is the phase of two points on the lens to be measured, Δd is the thickness difference between the two points on the lens, , ,and are the thickness differences of the above two points on the lens to be tested.

[0052] The above operations S4 and S5 involve the measurement of the phase.

[0053] The above measurement method is performed by constructing a measurement system to which the above measurement method is applied:

[0054] Build a digital holographic microscope (cage type), its working principle is as follows Figure 2 As shown:

[0055] a) Couple the laser output light to a one-to-two fiber coupler via a fiber coupler instead of a beam splitter; adjust the coupler to maximize the light intensity at the two fiber exits;

[0056] b) Building a digital holographic microscope optical path through a cage structure, such as Figure 2 shown.

[0057] c) Adjust the sample position so that the image on the detector plane is clearly focused;

[0058] d) Adjust the inclination angle of the beam combiner or the position of the converging lens and the fiber interface, and observe the recorded interference fringes. The denser the fringes, the more separated the items in the spectrum of the hologram. Try to make the fringes tilted to make full use of the bandwidth.

[0059] e) The hologram is recorded when the interference fringe contrast is high and the fringes are dense.

[0060] Combination Figure 3 (a)-3(e) The phase measurement may include:

[0061] a) Cut out the part of the hologram where the lens is located, such as Figure 3 (a) shown.

[0062] b) Perform Fourier transform on the hologram to obtain a spectrum diagram, such as Figure 3 (b) shown.

[0063] c) Use the window function to select one of the ±1-level spectra for filtering, such as Figure 3 (c) shown.

[0064] d) Perform inverse Fourier transform on the filtering result and extract the phase part to obtain the wrapped phase, such as Figure 3 (d) shown.

[0065] e) Unwrap the wrapped phase, such as Figure 3 (e) shown.

[0066] Combination Figure 4 As shown, when calculating the refractive index:

[0067] a) Use a resolution plate for calibration. Look up the resolution plate stripe spacing on the Internet and record it as , the number of pixels between the two corresponding stripes in the restored image is .

[0068] a) If Figure 4 As shown, the center position data of the unwrapped phase image is recorded as , the corresponding thickness is . Distance from center in CT image The lens thickness at . Start from the center position and move in any direction in the unwrapped phase recovery result pixels, where the phase data is recorded as .in The calculation formula is:

[0069]

[0070] The refractive index is calculated as:

[0071]

[0072] The present invention discloses a method for measuring the refractive index of a lens based on digital holographic microscopy. Optical holography uses the interference of reference light and object light to obtain an interference pattern of two beams of light, called a hologram. The hologram contains all the information about the amplitude and phase of the object. In optical holography, a hologram is equivalent to a complex two-dimensional grating. When irradiated with a reference light or a conjugate light of the reference light, the object can be reproduced at the conjugate position or the original position of the object. Holography is divided into coaxial holography and off-axis holography according to the angle between the reference light and the object light. When the reference light and the object light are incident in the same direction, it is coaxial holography, but the recovery result of the coaxial holography will be affected by the twin image. Off-axis holography is when the reference light and the object light are incident at a certain angle, and multi-level images can be separated in the frequency domain. Optical holography uses an optical device to generate a hologram and uses optical recovery. Digital holograms use photoelectric coupling devices (CCD or CMOS) as detectors to record holograms, replacing the holographic dry plate in optical holography. Digital holography is characterized by discretization and digitization, which greatly simplifies the technical process. In the reconstruction process, computer simulation is used to restore the amplitude and phase of the object. Digital holography has a fast reconstruction speed and is easy to process later.

[0073] Optical lenses, especially small-sized, irregular spherical lenses, are limited by the actual conditions of surface shape (non-planar, with specific curvature or even complex surface shape), surface coating, etc. Traditional refractive index methods and equipment are basically unable to directly measure the refractive index of lenses (such as prism coupling instrument, which requires the sample to be processed into a triangular prism; ellipsometer measurement results are affected by the sample coating and require a certain flatness of the measurement point). Therefore, a new method is needed to accurately measure the refractive index of the substrate of the processed optical lens without secondary processing.

[0074] The lens refractive index measurement method based on digital holographic microscopy disclosed in the present invention has strong applicability: when installing a digital holographic microscopy device, an objective lens conversion disk is used to install the objective lens, so that an objective lens with a suitable magnification can be selected for lenses of different sizes for microscopic imaging. It can be used to measure lenses of tiny sizes (less than 5 mm in diameter), and the surface shape of the measured sample has no effect on the measurement result.

[0075] The lens refractive index measurement method based on digital holographic microscopy disclosed in the present invention can realize multi-wavelength measurement: the input laser and other parts of the digital holographic microscopy device are connected by optical fiber, and the connection between the optical fiber and the flange is threaded, so the optical fiber is replaceable. By using input lasers and optical fibers of different wavelengths, the refractive index of the same lens at different wavelengths can be measured, and it can be operated at any time, and the replacement method is simple and convenient.

[0076] The lens refractive index measurement method based on digital holographic microscopy disclosed in the present invention has high measurement accuracy: the phase difference value obtained by unwrapping the reconstructed hologram is within 5.5% of the actual phase difference value.

[0077] So far, the embodiments of the present disclosure have been described in detail in conjunction with the accompanying drawings. It should be noted that the implementation methods not shown or described in the drawings or the body of the specification are all forms known to ordinary technicians in the relevant technical field and are not described in detail. In addition, the above definitions of each element and method are not limited to the various specific structures, shapes or methods mentioned in the embodiments, and ordinary technicians in the field can simply change or replace them.

[0078] In this document, unless otherwise specified, the so-called feature A "or" or "and / or" (and / or) feature B means that A exists alone, B exists alone, or A and B exist at the same time; the so-called feature A "and" (and) or "and" (and) or "and" (and) feature B means that A and B exist at the same time; the so-called "include", "comprise", "have" and "contain" mean including but not limited to these.

[0079] In addition, unless the steps are specifically described or must occur in sequence, the order of the above steps is not limited to the above list, and can be changed or rearranged according to the required design. And the above embodiments can be mixed and matched with each other or with other embodiments based on design and reliability considerations, that is, the technical features in different embodiments can be freely combined to form more embodiments.

[0080] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above description is only a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A method for measuring the refractive index of a lens based on digital holographic microscopy, comprising: Operation S1: collecting a digital holographic micrograph of the lens to be tested; Operation S2: Processing the digital holographic micrograph to obtain a phase diagram of the lens to be tested; Operation S3: acquiring phase data of two points in the phase diagram of the lens to be tested; Operation S4: acquiring thickness data of the lens to be tested at the above two points; and Operation S5: obtaining the refractive index of the lens to be measured according to the phase data and thickness data of the two points.

2. The measurement method according to claim 1, in operation S1, in a digital holographic process for the lens to be measured, the reference light and the object light are interfered at a set angle to obtain a digital holographic micrograph.

3. The measuring method according to claim 2, wherein operation S1 comprises: Build a cage-structured digital holographic microscopy system; Adjust the position of the lens to be tested so that the interference fringe imaging on the detector plane is clearly focused; and recording holograms when the interference fringe contrast is high and the fringes are densely packed.

4. According to the measurement method of claim 2, in the digital holographic process, the reference light and the object light are tilted at a set angle Interference is performed to form a digital holographic micrograph distribution The mathematical expression is as follows: ； in, S represents the object light distribution, R represents the reference light distribution, x represents the spatial coordinate in the x direction), y represents the spatial coordinate in the y direction, i represents a complex number, and k represents the wave number.

5. According to the measurement method of claim 1, in operation S2, the processing performed on the digital holographic micrograph includes Fourier transform, window function filtering, inverse Fourier transform, and phase unwrapping.

6. The measurement method according to claim 5, reconstructing the phase of the object light through Fourier transform, inverse Fourier transform and window function filtering; and calculating the phase of the object light through phase unwrapping.

7. The measuring method according to claim 5, operation S2 comprises: Cut out the portion of the digital holographic micrograph where the lens is located; Performing Fourier transformation on the intercepted digital holographic micrograph to obtain a spectrum graph; Use the window function to select one of the ±1 levels of the spectrum graph for filtering; Performing inverse Fourier transform on the filtering result and extracting the phase part to obtain the wrapped phase; and Unwrap the wrapped phase.

8. The measurement method according to claim 5, in operation S3, the phase ψ and the lens thickness are obtained by the optical path calculation formula and refractive index The relationship is: ； Where λ represents the wavelength of the test light. 9 . The measuring method according to claim 1 , wherein in operation S4 , thickness data at two points on the lens to be measured are obtained by tomography.

10. The measuring method according to claim 9, in operation S5, the refractive index n of the lens to be measured is obtained by restoring the numerical values ​​of two points on the phase of the lens to be measured: ； in, Δψ is the phase difference between two points on the lens to be measured, , , is the phase of two points on the lens to be measured, Δd is the thickness difference between the two points on the lens, , ,and are the thickness differences of the above two points on the lens to be tested.

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