JP2011033406A - Two-dimensional birefringence measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a more inexpensive two-dimensional birefringence measuring device that attains high-speed operation, using a simple configuration. <P>SOLUTION: The two-dimensional birefringence measuring device includes a polarization microscope, including a CCD camera 7; a liquid crystal phase modulator 3, disposed in an optical path of the optical system of the polarization microscope; a drive circuit 8 for driving the liquid crystal phase modulator 3; and a PC 9 for performing four-step phase shift for shifting the phase four times by π/2, each time, by the liquid crystal phase modulator 3, by controlling the drive circuit 8; acquiring four polarization interference images for a sample 4 from the CCD camera 7; and generating a phase distribution image indicating birefringence distribution characteristics of the sample 4 from a combination of the four polarization interference images. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a two-dimensional birefringence measuring apparatus that measures a distribution of minute birefringence due to residual strain of an optical disc or a distribution of minute birefringence of a living body such as plankton.

  As a conventional first birefringence measurement method, there is a method of measuring the birefringence of light transmitted through a medium to be measured by combining a polarization microscope with a Belek compensator. This is a method in which an observer performs measurement manually.

  On the other hand, as a conventional second birefringence measurement method, a method of observing birefringence by realizing an element that changes the polarization state electronically with liquid crystal has been proposed. The method and application examples are described in Non-Patent Document 1.

  On the other hand, as a conventional third birefringence measurement method, there is a method using phase shift interference for highly sensitive and highly accurate birefringence measurement. When this phase shift interferometry is introduced to achieve high sensitivity, products that use mechanical motions by piezoelectric actuators are commercially available. The technique is described in Non-Patent Document 2.

  As a conventional fourth birefringence measuring method, a system for measuring a fine line width using a phase shift interferometry using a polarization interferometer and a liquid crystal phase shifter has been proposed. The technique is described in Non-Patent Document 3.

M. Shribak and R. Oldebourg: `` Techniques for fast and sensitive measurements of two-dimensional birefringence distribution: Applied Optics, Vol. 42, No. 16 (2003), pp. 3009-3017 '' Ryutaro Shimada: “Special Feature: Latest Trends in High-precision Birefringence Measurement and Polarization Measurement: O Plus E, January 2007, pp. 30-35” M.Totzeck and H.J.Tiziani: `` Phase-shifting polarization interferometry for microstructure linewidth measurement: Optics Letters, Vol.24, No.5 (1999), pp.294-296 ''

  However, in the first birefringence measurement method, it is necessary to perform a manual operation, and there is a limit in terms of measurement accuracy. The second birefringence measurement method (Non-Patent Document 1) requires a large-scale device and complicated data processing.

  Further, in the third birefringence measurement method (Non-Patent Document 2), the phase shift interferometry is introduced to increase the sensitivity. However, since a mechanical operation using a piezo element is required, There are problems in terms of driving voltage, speed, hysteresis, and the like.

  In the fourth birefringence measurement method (Non-Patent Document 3), a liquid crystal cell is introduced for phase shift, and birefringence measurement is performed without requiring mechanical operation, but a fine structure is detected. Therefore, it is not suitable for quantitative measurement of birefringence. Since the change amount of the liquid crystal phase shifter is required to be at least 2λ or more, and a thick liquid crystal layer is required, there is a problem in terms of response speed. In addition, since the fourth birefringence measurement method handles as many as eight pieces of data, it requires a large amount of data to be processed and requires a large scale memory, which is disadvantageous in terms of speeding up.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a two-dimensional birefringence measuring apparatus that realizes an inexpensive and high-speed operation with a simple configuration.

  A two-dimensional birefringence measuring apparatus according to an aspect of the present invention includes a polarization microscope provided with an imaging device, a liquid crystal phase modulator disposed in an optical path of an optical system of the polarization microscope, and a drive for driving the liquid crystal phase modulator. By controlling the circuit and the drive circuit, four polarization interference images are obtained from the imaging device while performing a four-step phase shift in which the phase is shifted four times by π / 2 by the liquid crystal phase modulator. And an information processing device for generating a phase distribution image showing the birefringence distribution characteristic of the sample from a combination of the four polarization interference images.

  In the two-dimensional birefringence measuring apparatus, the liquid crystal phase modulator may be a liquid crystal phase shifter that performs phase shift using a bend alignment mode.

  In the two-dimensional birefringence measuring apparatus, the information processing apparatus shows the birefringence distribution characteristic of the sample while performing a four-step phase shift at a speed of 4 times or more of a normal video rate by the liquid crystal phase modulator. You may perform the real-time measurement which repeatedly produces | generates a phase distribution image.

  In the two-dimensional birefringence measuring apparatus, the information processing apparatus may measure a birefringence magnitude relationship or positive / negative of an arbitrary part of the sample when the installation angle of the sample is changed. .

  According to the present invention, it is possible to provide a two-dimensional birefringence measuring apparatus that realizes an inexpensive and high-speed operation with a simple configuration.

The figure which shows an example of a structure of the two-dimensional birefringence measuring apparatus which concerns on one Embodiment of this invention. The block diagram which shows an example of a function structure of the two-dimensional birefringence measurement function with which PC is equipped in FIG. The conceptual diagram which shows the relationship between the polarization direction of two polarizing plates, and the optical axis direction of a liquid crystal phase modulator. The flowchart which shows an example of the operation | movement by a two-dimensional birefringence measurement function. The figure which shows the example of four images obtained from a CCD camera by performing 4-step phase shift in a liquid crystal phase modulator. The figure which shows the example of the phase distribution image obtained by calculating phase distribution from four images. FIG. 7 is a diagram illustrating examples of phase distribution profiles of two parts indicated by broken lines in FIG. 6. The figure which shows the example of two phase distribution images which can discriminate | determine the positive / negative of birefringence. The figure which shows the example of two conventional phase distribution images which cannot discriminate | determine the positive / negative of birefringence.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram showing an example of the configuration of a two-dimensional birefringence measuring apparatus according to an embodiment of the present invention.

  1 includes a light source 1, a polarizing plate 2, a liquid crystal phase modulator 3, a sample 4, an objective lens 5, a polarizing plate 6, a CCD camera 7, a drive circuit 8, a personal computer 9 ( Hereinafter, it is referred to as “PC9”.

  In this two-dimensional birefringence measuring apparatus, a commercially available polarizing microscope can be used for optical systems other than the liquid crystal phase modulator 3 (light source 1, polarizing plate 2, objective lens 5, polarizing plate 6 and the like). That is, in the two-dimensional birefringence measuring apparatus of the present embodiment, the liquid crystal phase modulator 3 is arranged in the optical path of the optical system of a commercially available polarization microscope, and the CCD camera 7 as the image pickup apparatus and the liquid crystal phase modulator 3 are driven to drive. The circuit 8 and a PC 9 as an information processing apparatus are provided. Although not shown, the PC 9 includes an input device for performing various input settings, a display device for displaying measurement results, and a storage device for storing measurement results.

  Light emitted from the light source 1 is incident on the liquid crystal phase modulator 3 after passing through the polarizing plate 2. After appropriate phase modulation is applied to one polarized light in the liquid crystal phase modulator 3, the light is incident on the sample 4. The light that has passed through the sample 4 includes information on birefringence, and is imaged on the light receiving sensor of the CCD camera 7 by the objective lens 5. In the middle of this, information on intensity change due to polarization interference is obtained by the polarizing plate 6. It is like that.

  The liquid crystal phase modulator 3 is a liquid crystal phase shifter that performs phase shift using a thin liquid crystal cell having a bend alignment mode (hereinafter referred to as “bend alignment cell”) known as an OCB (Optically Compensated Bend) liquid crystal cell. . The bend alignment cell changes the phase shift amount according to the voltage applied from the drive circuit 8.

  The drive circuit 8 functions as an analog output interface circuit of the PC 9 and can sequentially switch the drive voltage applied to the liquid crystal phase modulator 3 in accordance with a control signal from the PC 9. If the driving voltage is about 10 V at the maximum, the liquid crystal phase modulator 3 can be driven without any problem.

  The PC 9 obtains four polarization interference images for the sample 4 from the CCD camera 7 while performing a four-step phase shift in which the phase is shifted four times by π / 2 by the liquid crystal phase modulator 3 by controlling the drive circuit 8. And a two-dimensional birefringence measurement function for generating a phase distribution image showing the two-dimensional birefringence distribution characteristic of the sample 4 from the combination of the four polarization interference images.

  For example, the PC 9 has a two-dimensional birefringence measurement function 10 as shown in FIG. 2, and includes an input setting processing unit 11, a phase shift drive processing unit 12, an image acquisition processing unit 13, a calculation processing unit 14, and an output processing unit. Various functions such as 15 are provided. The input setting processing unit 11 is a function for setting various parameters in accordance with an input operation from the input device. The phase shift drive processing unit 12 has a function of generating a control signal for driving the liquid crystal phase modulator 3 to the drive circuit 8. The image acquisition processing unit 13 has a function of sequentially acquiring four polarization interference images obtained through four-step phase shift driving from the CCD camera 7. The calculation processing unit 14 has a function of calculating the birefringence distribution (phase distribution) of the sample 4 from the combination of the four polarization interference images acquired from the CCD camera 7. The output processing unit 15 has a function of outputting a measurement result such as an image or profile of the birefringence distribution (phase distribution) of the sample 4 calculated by the calculation processing unit 14 to a display device or a storage device.

  Here, the 4-step phase shift method applied to the liquid crystal phase modulator 3 will be described in more detail.

An image (two-dimensional light intensity distribution) observed with a polarizing microscope is expressed by equation (1).

  Here, δ (x, y) is the phase distribution of the observation sample, that is, the measurement target. Here, since a transparent sample is assumed, the light intensity is a constant in principle, but usually some noise component is included, so that an uneven distribution or bias such as I ′ or I ″ is applied. For example, it may be caused by intensity distribution of the light source, dirt of the lens or window material, non-uniformity of the CCD sensor sensitivity, unnecessary light intrusion, and the like.

Therefore, a four-step phase shift method is adopted that removes the influence of the above-mentioned noise by adding a simple procedure and drastically improves the measurement accuracy. That is, in order to introduce a uniform phase shift φ, a phase shifter is inserted into the optical system, and φ is changed to 0, π / 2, 2π, and 3π / 2 to obtain four data I1 to I4. At this time, φ is added to the phase portion of the equation (1) in the acquired data to obtain the equation (2).

Therefore, the four data measured when the value of the phase shift amount φ is 0, π / 2, 2π, 3π / 2 are expressed as follows.

When these are used to perform the calculation as shown in the equation (7), the noises I ′ and I ″ are all erased and only tan δ remains, but further δ is as shown in the equation (8). It can be obtained with a simple calculation.

  Note that this method has no effect on the result even if the light source, other optical elements, or the CCD sensor is not highly accurate, or if some unexpected fixed disturbance is introduced. In other words, it can be said that it is an excellent technique that can perform highly accurate measurement without using expensive parts. Further, the phase modulator as a key device to which this method is applied also has the following effects because the liquid crystal phase modulator 3 that is extremely easy to handle and inexpensive is used.

・ Achieving a high-accuracy measurement system while being an inexpensive and simple system ・ Easy to drive the phase modulator and high speed, no special drive device required ・ Simple data processing and high-speed operation on a normal PC It is known that the bend alignment cell provided in the liquid crystal phase modulator 3 operates at the highest speed among those using a nematic liquid crystal material. By using the liquid crystal phase modulator 3 having such a bend alignment cell, the sample 4 can be duplicated at a speed that can be observed in a video image while maintaining the ease of use of the nematic liquid crystal and the variety of materials. It becomes possible to observe and measure the refraction distribution in real time. For example, the liquid crystal phase modulator 3 repeatedly generates a phase distribution image showing the birefringence distribution characteristics of the sample 4 while performing a four-step phase shift at a speed four times or more the normal video rate (30 frames / second). Sufficient real-time measurements can be made. Such a high-speed operation of the bend alignment cell is particularly effective in measurement of a weak phase object that does not require an unwrapping process, for example, measurement of birefringence of a marine plankton muscle.

  As shown in FIG. 3, the polarizing plates 2 and 6 are installed so that the polarization directions are orthogonal to each other. The liquid crystal phase modulator 3 has an optical axis direction that is the same as the polarization direction of the polarizing plates 2 and 6, respectively. Installed at an angle of 45 °. As a result, the light transmitted through the polarizing plate 2 propagates while being separated into light parallel to the optical axis of the liquid crystal phase modulator 3 and polarized light orthogonal thereto, and only the component of the previous parallel polarized light is phase-modulated. Receive.

  When the birefringence of the sample measured at this time has a large (positive) birefringence in a direction parallel to the phase-modulated polarized light, positive data is obtained. On the other hand, when there is a large birefringence in a direction perpendicular to the optical axis of the liquid crystal phase modulator 3, negative data is obtained. By utilizing such direction dependency, the magnitude relationship or positive / negative of birefringence at any part of the sample 4 when the installation angle of the sample 4 is changed can be measured. That is, not only the absolute value of birefringence but also the magnitude relationship and positive / negative can be easily examined. Specific examples thereof will be described later.

  Next, the operation by the two-dimensional birefringence measurement function 10 of the PC 9 will be described with reference to the flowchart of FIG. 4 and the images of FIGS.

  First, in order to obtain a bend alignment state that responds quickly in the bend alignment cell of the liquid crystal phase modulator 3, a voltage of about 10 V is applied to the liquid crystal phase modulator 3 by the drive circuit 8 for about several tens of seconds (step S1). Once the bend alignment cell of the liquid crystal phase modulator 3 is changed to the bend alignment state, there is no fear of returning to the initial state during operation.

  Next, driving is performed such that the phase shift amount φ of the liquid crystal phase modulator 3 is changed to 0, π / 2, 2π, and 3π / 2 by the drive voltages V1, V2, V3, and V4 calibrated in advance, respectively. Four image data I1 to I4 as shown in (a), (b), (c), and (d) are sequentially acquired (steps S2 to S5).

  Thereafter, based on the above-described equations (7) and (8), a phase distribution is calculated from the four image data I1 to I4, and the calculation result is output in the form of a phase distribution image or a phase distribution profile to a display device or the like. (Step S6).

  In principle, the phase distribution includes information on both the birefringence and the thickness distribution of the sample. For this reason, it is possible to extract only the birefringence by making the sample 4 have a constant thickness like a preparation. A series of processing from image data acquisition to phase calculation can be performed in real time by measuring the birefringence image by speeding up to the video rate by utilizing the high speed of the bend orientation mode. In particular, it is extremely useful for measuring biological samples that move around alive.

  FIG. 6 shows a phase distribution image obtained by calculating the phase distribution from the four image data I1 to I4. Moreover, the phase distribution profile of the (a) part and (b) part shown with the broken line in FIG. 6 is shown to FIG. 7 (a), (b), respectively. The black-and-white contrast of the image represents a quantitative distribution of the phase, and it can be clearly seen that the two thick muscle portions of the image show large birefringence. In particular, FIG. 4B shows a phase distribution picked up in a cross section of a black dot portion visible on both sides of a thick muscle. The black part shows a negative phase distribution corresponding to negative birefringence. This represents a state in which the birefringence of the muscle at the base of the leg extending in the lateral direction is clearly measured.

  Here, a method for discriminating the sign of birefringence from the phase distribution image will be described using a specific example.

  Examples of ocean plankton observation are shown in FIGS. 8 (a) and 8 (b). In the figure, birefringence of two thick muscles running in the longitudinal direction of the body is imaged. In FIG. 8A, it can be seen that the sample is placed vertically and white streaks, that is, positive birefringence occurs. When the same sample is placed horizontally, the two streaks change to black streaks as shown in FIG. 8 (b), indicating negative birefringence. In this measurement apparatus, the difference between (value in the vertical direction) − (value in the horizontal direction) is measured as birefringence, so these changes are reasonable results. As described above, when the arrangement angle of the sample is changed, not only the difference in birefringence but also the positive / negative and magnitude relationship of birefringence can be distinguished. Comparing both FIGS. 8A and 8B, it can be seen that the longitudinal direction of the muscle has a birefringence greater than the lateral direction.

  For comparison with FIGS. 8A and 8B, examples of general polarization micrographs are shown in FIGS. 9A and 9B. In general polarization microscope observation, since the sign of birefringence cannot be distinguished, the same image is observed even when the sample is placed horizontally, and it is understood that the two cannot be distinguished.

  As described above, the two-dimensional birefringence measuring apparatus according to the present embodiment uses the phase shift interferometry, and therefore can measure birefringence with extremely high accuracy, and as a phase modulator that is a key device. Since a liquid crystal phase modulator is used, installation and drive are extremely simple, and a compact, low-cost and compact two-dimensional birefringence measuring apparatus can be realized.

  In addition, the liquid crystal phase modulator is small and compact, and does not require a large driving device. Therefore, it can be incorporated into various types of microscope systems or small optical systems and has specialized performance for various measuring objects. A two-dimensional birefringence measuring apparatus can be easily constructed.

  The liquid crystal phase modulator has a great feature in its high speed. In the conventional technology, although a higher-speed phase modulator is realized by using a special material such as a ferroelectric liquid crystal, it does not reach the nematic liquid crystal in terms of the variety of materials including the price. In addition, since the measurement speed at the video rate is sufficiently possible, even a measurement object with movement can be measured in real time.

  Moreover, since the liquid crystal phase modulator uses a bend alignment mode for phase shift, high-speed phase modulation is possible, and real-time measurement of a moving sample such as a living organism is possible.

  Moreover, the thin liquid crystal cell (OCB liquid crystal cell) provided in the liquid crystal phase modulator can be driven with low voltage and low power with a small size and light weight, so that it can be easily incorporated into a microscope system. The operation by the interface circuit of the PC is possible without requiring a special driving device. Further, since the thickness of the liquid crystal cell can be reduced, the speed can be further increased.

  In addition, the liquid crystal phase modulator can be easily incorporated into an existing polarizing microscope and can measure birefringence at a video rate. Useful for refraction measurements.

  Also, since a 4-step phase shift is applied to drive the liquid crystal phase modulator, a phase shift amount of about 2π is sufficient, and the number of data to be processed is relatively small, which is advantageous in terms of speeding up. Yes, the PC memory can be small.

  In addition, since the liquid crystal phase modulator has various features as described above, it is possible to easily develop various systems according to the measurement object using these features.

  Note that the functions and processing procedures of the PC 9 described in the above embodiment are stored as a computer program in a computer-readable storage medium (for example, a magnetic disk, an optical disk, or a semiconductor memory), and this is processed as necessary. May be read out and executed. Such a computer program can also be distributed by transmitting from one computer to another computer via a communication medium.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Polarizing plate, 3 ... Liquid crystal phase modulator, 4 ... Sample, 5 ... Objective lens, 6 ... Polarizing plate, 7 ... CCD camera, 8 ... Drive circuit, 9 ... Personal computer (PC), 10 ... Two-dimensional birefringence measurement function, 11 ... input setting processing unit, 12 ... phase shift drive processing unit, 13 ... image acquisition processing unit, 14 ... calculation processing unit, 15 ... output processing unit.

Claims (4)

A polarizing microscope equipped with an imaging device;
A liquid crystal phase modulator disposed in the optical path of the optical system of the polarizing microscope;
A driving circuit for driving the liquid crystal phase modulator;
By controlling the drive circuit, four polarization interference images of the sample are acquired from the imaging device while performing a four-step phase shift in which the liquid crystal phase modulator shifts the phase four times by π / 2. An information processing apparatus for generating a phase distribution image indicating a birefringence distribution characteristic of the sample from a combination of a plurality of polarization interference images.   2. The two-dimensional birefringence measurement apparatus according to claim 1, wherein the liquid crystal phase modulator is a liquid crystal phase shifter that performs a phase shift using a bend alignment mode.   3. The two-dimensional birefringence measuring apparatus according to claim 1 or 2, wherein the information processing apparatus performs a four-step phase shift with the liquid crystal phase modulator at a speed four times or more of a normal video rate. A two-dimensional birefringence measurement apparatus that performs real-time measurement that repeatedly generates a phase distribution image showing refraction distribution characteristics.   3. The two-dimensional birefringence measuring apparatus according to claim 1, wherein the information processing apparatus measures a magnitude relationship or positive / negative of birefringence of an arbitrary part of the sample when the installation angle of the sample is changed. A two-dimensional birefringence measuring apparatus characterized by the above.

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