JP2012181060A - Spectral characteristic measuring apparatus and calibration method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spectral characteristic measuring apparatus which allows spectral characteristics to be precisely measured even in wide-field spectroscopic imaging, and a calibration method thereof.SOLUTION: In a measurement mode, light beams emitted from each bright spot of an object to be measured enter a first and a second reflection parts through a splitting optical system. An imaging optical system allows the light beams reflected therefrom to form an interference image. The movement of the first reflection part elongates or contracts the relative difference in optical path length between the first and the second reflected light beams proceeding from the splitting optical system through the first and the second reflection parts toward the imaging optical system. An interferogram of each bright spot of the object to be measured is obtained based on the light intensity change of the interference image on this occasion. In a calibration mode, the light beams emitted from the bright spot with a prescribed wavelength λ enter the splitting optical system so as to form a plurality of interference images corresponding to the bright spot. The movement distance of the fist reflection part corresponding to one cycle of the light intensity change is obtained based on the light intensity change on this occasion. The installation angle of the first reflection part is obtained from the wavelength λ of the bright spot, the movement distance of the first reflection part, and the field angle θ of the interference image.

Description

  The present invention relates to a spectral characteristic measuring apparatus useful for wide-field imaging and a calibration method thereof.

As a technique for measuring spectral characteristics, a technique using a spectral technique called wavelength dispersion spectroscopy or Fourier spectroscopy has been proposed (see Non-Patent Document 1).
In wavelength dispersion spectroscopy, light transmitted through the measurement sample or reflected from the measurement sample surface (hereinafter referred to as object light) is irradiated onto a diffraction grating or an acousto-optic tunable filter (AOTF). This is a spectroscopic method using a principle in which the diffraction angle of diffracted light sometimes obtained differs depending on the wavelength of the object light.

  On the other hand, Fourier spectroscopy (FTIR (Fourier Transform Infrared Spectroscopy)) is a spectroscopic measurement technique based on phase shift interference using a Michelson-type two-beam interference optical system. The object light is branched into two by a beam splitter such as a half mirror, the respective light beams are reflected by the mirror, reach the half mirror again, and the two light beams are merged to observe the interference phenomenon. A mirror that reflects one of the two branched light beams (reference light) is called a reference mirror. In Fourier spectroscopy, the reference mirror is moved with high accuracy at a resolution shorter than the wavelength of light to change the interference light intensity, so-called interferogram is detected, and this interferogram is mathematically Fourier transformed. Obtain spectral characteristics.

The direction of the light beam of the object light emitted from the measurement sample surface is varied depending on scattering, refraction, reflection, and the like. When light components in various directions are irradiated on the diffraction grating and the reference mirror in this way, the spectral accuracy is lowered.
Therefore, to increase the spatial coherency (coherence) of the object light in any spectroscopic method, only a light component in a specific direction of the object light using a pinhole or slit having a minute aperture is used as a diffraction grating or a reference. The mirror is illuminated. Although depending on the required spectral performance, a pinhole having a hole diameter of about several tens of microns is used in the dispersion type spectroscopy, and a slit having an opening width of about several millimeters is used in the Fourier spectroscopy.

  When pinholes and slits are used in this way, most of the object light does not pass through the pinholes and slits and is not used for measurement, so the light utilization efficiency is low. On the other hand, the scattered light transmitted or reflected through the inside of the biological membrane is extremely weak light, and it has been difficult to evaluate the spectral characteristics of the scattered light and the like generated from the inside of the biological membrane by the conventional spectroscopic technique.

Therefore, the present inventor has proposed a method for obtaining an interferogram of an object to be measured by utilizing an interference phenomenon of an object light beam generated from each bright spot that optically configures the object to be measured (see Patent Document 1). .
In the method described in Patent Document 1, object light such as transmitted light and diffused / scattered light generated from each bright spot is guided to a fixed mirror part and a movable mirror part of a phase shifter via an objective lens, and these two mirror parts The object light beam reflected from the light forms an interference image on the imaging surface. The movable mirror part is moved by a piezo element or the like, and the interference image intensity changes with the movement of the movable mirror part to form a so-called interferogram. Therefore, the spectral characteristic (spectrum) of the object light can be acquired by Fourier transforming the interferogram.

  Thus, in the method described in Patent Document 1, since all of the light that has passed through the split optical system (objective lens, fixed mirror unit, movable mirror unit) can be used for analysis, the light use efficiency is high, The spectral characteristics can be measured even with weak light.

JP 2008-309706 A

Jiro Hiraishi, "Fourier Transform Infrared Spectroscopy" Society Publishing Center, November 1985

  In the technique described in Patent Document 1, a movable mirror is mechanically moved to give a phase difference corresponding to the amount of movement to an object beam reflected by the fixed mirror and the movable mirror. For this reason, when a light beam having an optical axis of various angles with respect to the moving axis of the movable mirror unit is incident, even if the moving amount of the movable mirror unit is the same, the amount of phase difference given to each object beam is the incident angle. It will be different. In this way, the object light given different amounts of phase difference causes an interference phenomenon on the light receiving surface and is collectively detected as an interferogram. Therefore, if the phase difference has an error corresponding to the inclination of the optical axis (that is, the incident angle), the spectral characteristics are deteriorated. In particular, when performing wide-field spectroscopic imaging, the change in the inclination of the object light with respect to the optical axis at each measurement position becomes large, so that the spectral characteristics are greatly deteriorated.

  The problem to be solved by the present invention is to provide a spectral characteristic measuring apparatus capable of measuring spectral characteristics with high accuracy even when wide-field spectral imaging is performed, and a calibration method thereof.

The spectral characteristic measuring apparatus according to the present invention, which has been made to solve the above problems,
a) a splitting optical system that guides light emitted from the measurement point of the object to be measured to the first reflecting portion and the second reflecting portion;
b) an imaging optical system that guides the light reflected by the first and second reflectors to the same point to form an interference image;
c) by moving the first reflecting part, the first reflected light from the splitting optical system to the imaging optical system through the first reflecting part and the splitting optical system through the second reflecting part, Optical path length difference expansion / contraction means for expanding / contracting the optical path length difference between the second reflected light toward the image optical system;
d) a movement amount detection unit for detecting a movement amount of the first reflection unit;
e) a light detection unit in which a plurality of detection elements are two-dimensionally arranged to detect the light intensity of the interference image;
f) Obtaining an interferogram of the measurement point of the object to be measured based on the light intensity change detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, and this interferogram A spectral characteristic measuring apparatus comprising: a processing unit that obtains a spectrum by Fourier transforming
Further, calibration light emitting means for emitting calibration light having a known wavelength from a bright spot having a known angle of view;
The amount of movement of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, An installation angle calculation unit for obtaining an installation angle of the first reflection unit based on the angle of view of the bright spot and the wavelength of the calibration light;
An angle-of-view detector that obtains the angle of view of the measurement point of the object to be measured;
The processing unit obtains the optical path length difference corresponding to the movement amount of the first reflecting unit from the installation angle of the first reflecting unit and the angle of view of the measurement point, and calibrates the spectrum based on the optical path length difference. It is characterized by doing.

In the spectral characteristic measuring apparatus of the present invention, the installation angle calculation unit may be an xyz coordinate system having an origin at the point of incidence of the calibration light on the first reflection unit, and the reflection surface of the first reflection unit A calibration optical coordinate system is defined in which the plane including the normal line is the xz plane and the axis perpendicular to the xz plane is the y axis, and the installation angle φy in the calibration optical coordinate system (where the installation angle φy is the first angle) The rotation angle around the y-axis of the normal of the reflecting surface of the reflecting portion is shown.) Is expressed by the following equation: φy = (90 ° + θy) + cos ⁻¹ {(λ / 2M _λ ) × θx}
(In the above equation, θx and θy are the angles of view of the interference image of the bright spot, the angle between the line connecting the interference image and the origin of the calibration optical coordinate system, and the x-axis and y-axis, and λ is the above-mentioned wavelength calibration light, the M _lambda indicates the movement amount of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot.)
It is characterized by calculating from.

Furthermore, in the spectral characteristic measuring apparatus of the present invention, the installation angle calculation unit calculates the installation angle φy in the calibration optical coordinate system in which the z-axis is a line connecting the interference image of the bright spot and the origin. And
In the above configuration, the angles of view θx and θy of the bright spot are θx = 0 and θy = 0, respectively, and the installation angle φy can be easily calculated.

Further, the processing unit calculates the optical path length difference L by the following formula: L = {2M cos (φy−θ1′y)} / cos (θ1x) (where θ1′y = θ1y + 90 °)
(In the above equation, θ1x and θ1y are the angle of view of the measurement point, and M is the amount of movement of the first reflecting portion.)
It may be calculated from

  It is also possible to configure the calibration light emitting means to form a plurality of discrete bright spots by causing light emitted from a monochromatic light source having a known wavelength to enter the fly-eye lens.

The calibration method of the spectral characteristic measuring apparatus of the present invention is:
a) a splitting optical system that guides light emitted from the measurement point of the object to be measured to the first reflecting portion and the second reflecting portion;
b) an imaging optical system that guides the light reflected by the first and second reflectors to the same point to form an interference image;
c) by moving the first reflecting part, the first reflected light from the splitting optical system to the imaging optical system through the first reflecting part and the splitting optical system through the second reflecting part, Optical path length difference expansion / contraction means for expanding / contracting the optical path length difference between the second reflected light toward the image optical system;
d) a movement amount detection unit for detecting a movement amount of the first reflection unit;
e) a light detection unit in which a plurality of detection elements are two-dimensionally arranged to detect the light intensity of the interference image;
f) Obtaining an interferogram of the measurement point of the object to be measured based on the light intensity change detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, and this interferogram A processing unit that obtains a spectrum by Fourier transforming
In a spectral characteristic measuring apparatus comprising: an angle-of-view detection unit for obtaining an angle of view of a measurement point of the object to be measured;
Calibration light having a known wavelength emitted from a bright spot having a known angle of view is incident on the splitting optical system, and the first light beam corresponding to one period of a change in light intensity of an interference image formed by the imaging optical system at that time. 1 Acquire the movement amount of the reflection part from the movement amount detection unit,
Based on the amount of movement of the first reflecting part, the angle of view of the bright spot, and the wavelength of the calibration light, the installation angle of the first reflecting part is obtained,
The optical path length difference corresponding to the amount of movement of the first reflective part is obtained from the installation angle of the first reflective part and the angle of view of the measurement point, and the spectrum is calibrated based on the optical path length difference. To do.

The calibration method of the present invention is an xyz coordinate system in which an installation angle of the first reflection unit is an origin at an incident point of the calibration light to the first reflection unit, and the reflection of the first reflection unit. The installation angle φy in the calibration optical coordinate system in which the surface including the normal of the surface is the xz plane and the axis perpendicular to the xz plane is the y-axis (where the installation angle φy is the normal of the reflection surface of the first reflecting portion) And the installation angle φy is expressed by the following formula: φy = (90 ° + θy) + cos ⁻¹ {(λ / 2M _λ ) × θx}
(In the above equation, θx and θy are the angles of view of the interference image of the bright spot, the angle between the line connecting the interference image and the origin of the calibration optical coordinate system, and the x-axis and y-axis, and λ is the above-mentioned wavelength calibration light, the M _lambda indicates the movement amount of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot.)
It is characterized by calculating from.

  In this case, if the z-axis is a line connecting the interference image of the bright spot and the origin, the field angles θx and θy of the bright spot are θx = 0 and θy = 0, respectively. The installation angle φy can be easily calculated.

The optical path length difference L is expressed by the following equation: L = {2M cos (φy−θ1′y)} / cos (θ1x) (where θ1′y = θ1y + 90 °)
(In the above equation, θ1x and θ1y are the angle of view of the measurement point, and M is the amount of movement of the first reflecting portion.)
It is characterized by calculating from.

  According to the present invention, the installation angle of the first reflection unit is obtained in advance, and the optical path length difference for each angle of view is obtained from the installation angle, the amount of movement of the first reflection unit, and the field angle of each measurement point of the object to be measured. Therefore, it is possible to measure the spectral characteristics at each measurement point of the object to be measured with high accuracy without causing a decrease in detection sensitivity. In addition, since the true optical path length difference can be obtained regardless of the angle of view of the measurement point of the object to be measured, it is possible to provide a spectral characteristic measuring apparatus useful for wide-field spectral imaging.

1 is a schematic system configuration diagram showing a measurement mode of a spectral characteristic measurement apparatus according to an embodiment of the present invention. The block diagram of a spectral-characteristics measuring apparatus. FIG. 6 is an operation explanatory diagram of a phase shifter. Interferogram (a) and a waveform diagram (b) of a spectrum obtained by Fourier transforming the interferogram (a). Explanatory drawing of the generation principle of an interferogram. The schematic system block diagram which shows the calibration mode of a spectral characteristic measuring apparatus. The figure which shows the positional relationship of a calibration optical coordinate system and a phase shifter. The figure which shows the positional relationship of a calibration optical coordinate system and a phase shifter when a calibration optical coordinate system is defined so that xz plane may include the normal line of the reflective surface of a movable mirror part. The figure for demonstrating the relationship between a phase shift amount and a phase difference. The figure for demonstrating how to obtain | require a phase difference in case the angle of view θx ≠ 0. The figure for demonstrating the law of reflection. The figure which shows the spectral characteristic (b) obtained by performing the Fourier-transform of the interferogram (a) obtained by the movement of a phase shifter, and an interferogram. Explanatory drawing of how to obtain | require a calibration optical coordinate system. Explanatory drawing of how to obtain | require field angle (theta) x and (theta) y of an interference image. The figure which shows the experimental result which calibrated the whole measurement surface (imaging surface) by the calibration optical coordinate system constructed | assembled using the discrete luminescent point image, and a view angle correction | amendment geometric model.

Hereinafter, specific embodiments of the present invention will be described.
As will be described in detail later, the spectral characteristic measurement apparatus according to the present embodiment includes a measurement mode and a calibration mode. The measurement mode is used when measuring the spectral characteristics of the object to be measured. First, the apparatus configuration in the measurement mode will be described with reference to FIGS. The spectral characteristic measuring apparatus 10 in the measurement mode includes an optical system 20, a detection unit 22, and a control device 40.
The optical system 20 includes an objective lens 24, a phase shifter 25, and an imaging lens 26 provided on an optical path connecting the light source 21 and the detection unit 22. The objective lens 24 constitutes a splitting optical system, and the imaging lens 26 constitutes an imaging optical system. The detection unit 22 corresponds to a light detection unit.

  The control device 40 includes a control unit 41, an interferogram acquisition unit 42, an arithmetic processing unit 43, a storage unit 44, a display unit 45, an operation unit 46, and the like. The arithmetic processing unit 43 performs arithmetic processing such as calculation of the installation angle of the phase shifter 25, correction of the spectrum, and Fourier transform of the interferogram, which will be described later. Therefore, in this embodiment, the arithmetic processing unit 43 functions as a processing unit and an installation angle calculation unit. Note that the entity of the arithmetic processing unit 43 is a personal computer mainly composed of a CPU, and arithmetic processing is achieved by executing a predetermined program on the computer.

  The light emitted from the light source 21 irradiates the object S to be measured, and thereby light rays (also referred to as “object light”) such as scattered light and fluorescent light emission from the bright spot of the object S to be measured in various directions. Radiates radially. A light beam emitted from the object to be measured S enters the objective lens 24 and is converted into a parallel light beam, and then enters the imaging lens 26 via the phase shifter 25 and is condensed on the light receiving surface 22a of the detection unit 22. To do.

  The objective lens 24 is configured to be movable in the optical axis direction by a lens driving mechanism 27. The lens driving mechanism 27 is for scanning the in-focus position of the objective lens 24, and can be constituted by, for example, a piezo element.

  The light beam after passing through the objective lens 24 does not have to be a perfect parallel light beam. As will be described later, it is sufficient that the light beam generated from one bright spot can be expanded to such a degree that it can be divided into two or more. However, if the light beam is not a parallel light beam, an error is likely to occur in the phase difference amount generated according to the phase shift amount described later. Therefore, in order to obtain higher spectroscopic measurement accuracy, it is desirable to use a parallel beam as much as possible.

  The parallel light beam that has passed through the objective lens 24 reaches the phase shifter 25. The phase shifter 25 includes a movable mirror unit 251, a fixed mirror unit 252 disposed on the left side thereof, and a drive stage 253 that moves the movable mirror unit 251. The surfaces (reflective surfaces) of the movable mirror unit 251 and the fixed mirror unit 252 are optically flat and are optical mirror surfaces that can reflect the wavelength band of the light to be measured by the apparatus 10.

In this embodiment, the phase shifter 25 corresponds to the optical path length difference expansion / contraction means of the present invention, and the movable mirror portion 251 and the fixed mirror portion 252 correspond to the first and second reflecting portions, respectively. In addition, although the reflected light is used here, transmitted light may be used.
In the following description, among the light beams that have reached the phase shifter 25, the light beam that has reached the reflection surface of the movable mirror unit 251 and reflected is the movable light, and the light beam that has reached the reflection surface of the fixed mirror unit 252 and is reflected. Is also called fixed light.

  The drive stage 253 is composed of, for example, a piezoelectric element having a capacitance sensor, and receives the control signal from the control device 40 to move the movable mirror unit 251 in the direction of arrow A with accuracy according to the wavelength of light. . Although it depends on the spectroscopic measurement ability, for example, in the visible light region, highly accurate position control of about 10 nm is required.

  The phase shifter 25 is arranged so that the reflecting surfaces of the movable mirror unit 251 and the fixed mirror unit 252 are inclined by 45 degrees with respect to the optical axis of the parallel light flux from the objective lens 24. The drive stage 253 moves the movable mirror unit 251 while maintaining the inclination of the reflecting surface of the movable mirror unit 251 with respect to the optical axis at 45 degrees. With such a configuration, the amount of movement of the movable mirror unit 251 in the optical axis direction is √2 times the amount of movement of the drive stage 253. The optical path length difference that gives a relative phase change between the two light beams of the fixed light and the movable light is twice the amount of movement of the movable mirror 251 in the optical axis direction.

  The fixed light and the movable light that reach the phase shifter 25 and are reflected by the reflecting surfaces of the movable mirror unit 251 and the fixed mirror unit 252 are converged by the imaging lens 26 and received by the light receiving surface (imaging surface) of the detection unit 22. to go into. The detection unit 22 is composed of, for example, a two-dimensional CCD camera in which a number of CCD image sensors are arranged in a two-dimensional matrix. The reflecting surface of the movable mirror unit 251 and the reflecting surface of the fixed mirror unit 252 are configured in parallel with an accuracy that does not shift the condensing position of the two light beams on the imaging surface of the detecting unit 22.

[Measurement principle]
Next, the measurement principle of the spectral characteristic measuring apparatus 10 according to the present embodiment will be described with reference to FIGS. Here, a description will be given based on an optical model in which light rays such as fluorescence and scattered light are focused on one point on the imaging surface of the detection unit 22 via the objective lens 24 and the imaging lens 26 and form an interference image.

As described above, the object light emitted from one bright spot of the measurement object S reaches the surfaces of the movable mirror portion 251 and the fixed mirror portion 252 of the phase shifter 25 through the objective lens 24. At this time, the object light reaches the movable mirror unit 251 and the fixed mirror unit 252 after being divided.
In addition, the movable mirror unit 251 and the fixed mirror with respect to the objective lens 24 are set so that the light beam reaching the surface of the fixed mirror unit 252, that is, the fixed light, and the light beam reaching the surface of the movable mirror unit 251, that is, the amount of the movable light are substantially equal. The position of the unit 252 is set, but it is also possible to equalize the light quantity by adjusting the relative light quantity difference by installing a neutral density filter in one or both of the fixed light and the movable light. .

  Light reflected by the surfaces of the movable mirror unit 251 and the fixed mirror unit 252 enters the imaging lens 26 as movable light and fixed light, respectively, and forms an interference image on the imaging surface of the detection unit 22. At this time, since light of various wavelengths is included in the light beam emitted from the object S to be measured, the optical path length difference between the movable light and the fixed light is changed by moving the movable mirror 251 to change the optical path length of FIG. A waveform of an imaging intensity change (interference light intensity change) called an interferogram as shown in a) is obtained. FIG. 4A is an interferogram in one pixel of the detection unit 22. 4A, the horizontal axis indicates the optical path length difference between the movable light and the fixed light accompanying the movement of the movable mirror unit 251, and the vertical axis indicates the imaging intensity at one point on the imaging surface.

  By performing a Fourier transform on the interferogram, it is possible to acquire spectral characteristics that are relative intensities for each wavelength of light emitted from one bright spot of the object S to be measured (see FIG. 4B). In the present embodiment, since a two-dimensional CCD camera is used for the detection unit 22, two-dimensional spectroscopic measurement of the object S can be performed.

  FIGS. 5A to 5C are explanatory diagrams of the principle of generating the interferogram. First, the relationship between the optical path length difference and the interference light intensity when the measurement wavelength is a single wavelength will be described. In FIG. 5, the horizontal axis indicates the relative optical path length difference between the fixed light and the movable light accompanying the movement of the movable mirror unit 251, and the vertical axis indicates the imaging intensity at one pixel of the detection unit 22. .

  FIGS. 5A to 5C show the relationship between the optical path length difference and the imaging intensity of three types of monochromatic light (λa> λb> λc) having different wavelength lengths. The phase shift origin shown in the vicinity of the center in FIG. 5 (indicated by the alternate long and short dash line in the figure) is in the state shown in FIG. 3B (the reflecting surface of the movable mirror unit 251 matches the reflecting surface of the fixed mirror unit 252). State). When the reflecting surfaces of the movable mirror unit 251 and the fixed mirror unit 252 coincide, there is no relative phase difference between the fixed light and the movable light. That is, since these two light beams arrive at the same phase on the imaging surface, they strengthen each other. For this reason, bright bright spots are formed on the imaging surface, and the imaging intensity is increased.

On the other hand, the movable mirror portion 251 is moved from the position shown in FIG. 3B to the position shown in FIGS. 3A and 3C, and the relative optical path length difference between the fixed light and the movable light is obtained. If this occurs, the image forming intensity is reduced because the interference condition becomes destructive when this optical path length difference becomes an odd multiple of the half wavelength (λ / 2). When the optical path length difference is an integral multiple of one wavelength, the interference condition between the two light beams is intensified, and the imaging intensity is increased.
Accordingly, when the movable mirror unit 251 is moved to sequentially change the optical path length difference, the imaging intensity due to the interference phenomenon between the two light beams changes periodically. As shown in FIGS. 5A to 5C, the cycle of the imaging intensity change is long for light having a long wavelength, and is short for light having a short wavelength.

  On the other hand, in a spectral characteristic measuring device that measures multi-wavelength light, changes in the intensity of interference light with various lengths of wavelengths are detected as a change in luminance value. This is the interferogram shown in FIG. At the phase shift origin where there is no relative optical path length difference between the movable light and the fixed light, the two light beams intensify without depending on the wavelength. Become. However, when the optical path length difference is increased, the period of intensity change of each wavelength does not match, so that the imaging intensity does not increase even when the intensity changes of multiple wavelengths are added. For this reason, in the interferogram, a change in imaging intensity is observed in which the luminance value gradually decreases as the optical path length difference increases. Thus, since the interferogram is a waveform in which single-wavelength imaging intensity changes of a single wavelength are added, this waveform data is subjected to Fourier transform to obtain a waveform for each wavelength as shown in FIG. Spectral characteristics that are relative intensities can be acquired.

[Calibration method]
Next, a calibration method for the spectral characteristic measuring apparatus 10 of the present embodiment will be described.
As described above, in the spectral characteristic measuring apparatus 10 of this embodiment, the movable mirror unit 251 is gradually moved to generate a relative optical path length difference (phase difference) between the fixed light and the movable light. A waveform of an imaging intensity change called an interferogram is obtained. Then, the interferogram is subjected to Fourier transform to obtain a spectral characteristic which is a relative intensity for each wavelength of light emitted from the bright spot of the object to be measured. In order to obtain the spectral characteristics with high accuracy, it is necessary to accurately obtain the phase difference between the fixed light and the movable light. Even if the moving amount of the movable mirror unit 251 is the same, the amount of phase difference between the movable light and the fixed light is the phase shifter 25 of the light emitted from each bright spot of the object to be measured S (the movable mirror unit 251 and the movable mirror unit 251). It depends on the incident angle with respect to the fixed mirror portion 252). The incident angle depends on the angle of view of the optical axis of the object beam at each bright spot position and the installation angle of the phase shifter 25 (the angles of the reflecting surfaces of the movable mirror unit 251 and the fixed mirror unit 252 with respect to the optical axis of the object beam). Determined. Thus, in the present apparatus 10, an angle-of-view correction geometric model is constructed to obtain the installation angle of the phase shifter 25, and the phase difference for each angle of view is obtained using this installation angle. The calibration mode refers to a device configuration for obtaining the installation angle of the phase shifter 25.

  As shown in FIG. 6, in the calibration mode, a parallel light beam emitted from a monochromatic light source having a known wavelength passes through the fly-eye lens 23 and then enters the objective lens 24. That is, the fly-eye lens 23 functions as a calibration light emitting unit. The fly-eye lens 23 is composed of a plurality of small lenses arranged in a grid pattern. The light beam emitted from the light source 21 is incident on the fly-eye lens 23, and then a condensing point is formed by each small lens. Thereafter, the light beam spreads again and enters the objective lens 24. It reaches the surfaces of the movable mirror part 251 and the fixed mirror part 252. Then, after being reflected by the surfaces of the movable mirror unit 251 and the fixed mirror unit 252, the light enters the imaging lens 26 as movable light and fixed light, and interferes with the imaging surface of the detection unit 22. In the calibration mode, a plurality of discrete bright spots are formed by using the fly-eye lens 23 in the illumination optical system. Since interference images are formed on the imaging surface corresponding to the bright spots, a plurality of discrete interference images corresponding to a plurality of discrete bright spots are formed on the imaging plane in the calibration mode.

Next, a calibration optical coordinate system defined in constructing the view angle correction geometric model will be described.
In order to define the calibration optical coordinate system, the installation angle of the phase shifter 25 has three degrees of freedom (φx, φy, φz), and the angle of view of light emitted from the bright spot and incident on the phase shifter 25 through the objective lens 24 ( It is necessary to consider a total of 5 degrees of freedom of 2 degrees of freedom as θx, θy). However, if there are many parameters to be considered, it is difficult to construct a view angle correction geometric model. Therefore, in this embodiment, the calibration optical coordinate system is defined by paying attention to the law of reflection. That is, the installation angles φx, φy, and φz of the phase shifter 25 are defined by the normal lines of the reflecting surfaces that are the surfaces of the movable mirror portion 251 and the fixed mirror portion 252 of the phase shifter 25.

FIG. 7 is a diagram showing the positional relationship between the calibration optical coordinate system and the phase shifter. As shown in FIG. 7, the installation angle φx is an elevation angle from the xz plane (that is, the angle formed by the normal of the reflecting surface of the movable mirror portion 251 and the xz plane), and the installation angle φy is a rotation angle (here) Then, the installation angle φz indicates a rotation angle around the normal to the reflection surface. Since the installation angle φz is a rotation angle around the normal, it can be ignored in the law of reflection.
As shown in FIG. 8, when a calibration optical coordinate system is defined in which the plane including the normal line is the xz plane, the installation angle φx is 0 deg. In other words, the installation angle to be considered in this model is only φy.

Thus, in the calibration optical coordinate system in which the xz plane is defined, when light emitted from each small lens of the fly-eye lens 23 enters the phase shifter 25 through the objective lens 24, these incident lights are reflected by the phase shifter 25. Reflected by the surface, an interference image is formed on the imaging surface. Further, it is assumed that the movable mirror portion 251 of the phase shifter 25 mechanically moves by a phase shift amount M along a translational movement axis (that is, a normal line) inclined by the installation angle φy from the main axis (x axis). At this time, considering the light emitted from the bright spot located on the xz plane and entering the phase shifter 25, that is, the incident light having the angle of view θx = 0, the phase difference L given to the wavefront of the incident light is As shown in FIG. 9, the phase difference L1 given to the wavefront by the phase shift amount M and the distance along the incident light from the reflection point of the movable mirror unit 251 at the initial position to the reflection point of the movable mirror unit 251 after movement. This is a difference from L2. From the above, when θ′y = θy + 90 °, the following equations (1) to (3) are obtained.
L = L1-L2 (1)
L1 = M / cos (φy−θ′y) (2)
L2 = −1 × L1 × cos (2 (φy−θ′y) (3)
From Expressions (1) to (3), the phase difference L is L = M / cos (φy−θ′y) {1 + cos (2 (φy−θ′y)}
= M / cos (φy-θ'y) x 2cos ² (φy-θ'y)
= 2Mcos (φy-θ'y) (4)
It becomes.

  The phase shift amount M of the phase shifter 25 indicates the amount of movement in the direction along the movement axis of the movable mirror unit 251. For example, a movement amount detector 255 (see FIG. 2) such as a linear encoder is provided on the drive stage 253 of the phase shifter 25. By providing, it can obtain | require from the detection value of this detector 255. FIG. Therefore, in this embodiment, the movement amount detector 255 corresponds to a movement amount detection unit.

Equation (4) is the phase difference L of the incident light from the bright spot with the angle of view θx = 0, but in the case of the incident light from the bright spot with the angle of view θx ≠ 0, as shown in FIG. An xz plane inclined by the angle of view θx with respect to a previously defined xz plane may be considered. In this case, the phase difference obtained using the calibration optical coordinate system described above is a length obtained by projecting the phase difference L in the actual xz plane onto the xy plane defined in advance. That is, the actual phase difference L is obtained by dividing the phase difference obtained by the calibration optical coordinate system by cos θx, and can be expressed by the following equation (5).
L = {2Mcos (φy−θ′y)} / cos (θx) (5)
From the equation (5), it can be seen that if the installation angle φy of the phase shifter 25 is obtained, the phase difference amount L corresponding to the angles of view θx and θy can be calculated from the phase shift amount M.

  Conversely, if the phase shift amount M, the phase difference amount L, the field angle of the bright spot, and the field angles θx and θy are known, the installation angle φy of the phase shifter 25 can be obtained from the above equation (5). However, the installation angle φy can be obtained from the equation (5) based on the premise that a calibration optical coordinate system in which the plane including the normal line is the xz plane can be defined as described above. The data that can be obtained experimentally is only the spectral characteristics of the discrete interference image on the image plane, and the normal cannot be visually recognized. Therefore, the axis passing through one of the plurality of discrete interference images on the imaging plane is defined as the z-axis, and the installation angle φy is calculated with the angles of view θx = 0 and θy = 0. Although this angle of view is the angle of view of the interference image, the angle of incidence and the angle of reflection with respect to the phase shifter 25 are the same due to the law of reflection. In the case of a plane, the angle of view of the discrete interference image and the angle of view of the discrete bright spot image are the same.

Hereinafter, a procedure for calculating the installation angle φy of the phase shifter 25 will be described.
When the phase shifter 25 is moved, the imaging intensity of the interference image changes according to the phase shift amount M of the phase shifter 25 on the imaging plane. For example, a graph P ₀ indicated by a broken line in FIG. 12A shows a change in imaging intensity of the wavelength λ when the field angle θx = 0, θy = 0, and the installation angle φy = 0. In the graph P ₀ , one period of the imaging intensity change is the wavelength λ. This is the theoretical value. Imaging intensity change in the wavelength λ when the angle of view and installation angle other than the theoretical value whereas next graph P ₁ indicated by the solid line, image intensity is changed at a pitch different from the theoretical value. This is because the movement axis of the phase difference L given to the wavefront for one period according to the angle of view is tilted with respect to the mechanical movement axis of the phase shifter 25, so that the phase difference amount substantially given to the wavefront. This is because they are different. When the phase shift amount at this time is M _lambda, when the angle of view and installation angle other than the theoretical value, so that the amount of phase shift is given a phase difference L is equal to the wavelength lambda when M _lambda. As shown in FIG. 12B, M _λ can be obtained from the spectral characteristics obtained by Fourier transforming the image intensity change data (interferogram) accompanying the phase shift. When using a light source of a single wavelength, emission line spectrum is obtained by Fourier transform of the interferogram, it is possible to obtain the M _lambda as the peak value of the emission line spectrum.

Therefore, the wavelength lambda in the phase difference amount L of formula (5), by substituting M _lambda experimentally determined phase shift amount M, be determined calculation formula of the installation angle φy shown below (6) it can.
φy = cos ⁻¹ (λ / 2M _λ cos θx) + θ′y (6)

In Expression (6), the angles of view θx and θy are also unknown. However, as described above, when one of a plurality of interference images on the imaging surface is selected and the axis passing through this interference image is assumed to be the z-axis, the field angles θx and θy are both 0, and the wavelength If λ and the phase shift amount M _λ are known, the installation angle φy can be calculated from the equation (6).

  As shown in FIG. 13, the xz plane constituted by the normal line and the z axis is rotated around the normal line. At this time, if the installation angle φy of the phase shifter 25 is not changed, the tangent line of the circle becomes the y axis of the spatial coordinate system (calibration optical coordinate system). This is because the tangent of the circle is perpendicular to the A line (see FIG. 13), which is the projection of the xz plane with respect to the imaging plane, that is, the line that always passes through the center of the circle. The acquired two-dimensional data is a part of the contour line of the installation angle φy. Therefore, the installation angle φy can be obtained by determining that the z-axis passes through one arbitrary point of the plurality of discrete interference images and defining the calibration optical coordinate system.

If the installation angle φy can be obtained as described above, the true position of all the interference images on the imaging plane can be calculated from the field angles θx, θy and the phase shift amount M using the above-described equation (5). The phase difference L can be obtained. The field angles θx and θy of the interference image in this case can be obtained by the following equations (7) and (8) from the focal length of the imaging lens and the formation position of the interference image, as shown in FIG.
θx = tan ^-1 (b / f) (7)
θy = tan ^-1 (a / f) (8)
Here, f is the focal length of the joint lens, a is the distance in the A-axis direction of FIG. 14A, and b is the distance in the B-axis direction. In a CCD camera in which a plurality of pixels are arranged at predetermined intervals on the image plane, a and b can be obtained from the positions of the pixels. Therefore, the CCD camera (detection unit 22) and the arithmetic processing unit 43 constitute an angle of view detection unit of the present invention.

  FIG. 15 shows the result of an experiment in which the entire measurement surface (imaging plane) is calibrated using the calibration optical coordinate system constructed using the discrete bright spot image formed on the object plane and the field angle correction geometric model. In this experiment, a discrete bright spot image was formed using a fly-eye lens (focal length: 10 mm, diameter: 2.3 mm, center-to-center distance: 1.7 mm), and a CCD camera (model number: XC-75, pixel size: 8.4 μm). (H) × 9.8 μm (V)) was observed at an optical magnification of 1/2. The field of view of this experimental system is 12.9 mm. A He—Ne laser (wavelength: 632.8 nm) having an emission line spectrum was used as a light source. As a result, the installation angle φy was 30 deg.

  The spectral characteristic of each interference image can be calibrated by obtaining the phase difference L from Equation (5) from the two-dimensional data of all the interference images on the imaging plane using the installation angle φy thus obtained. FIG. 15 shows the relationship between each pixel of the CCD camera, that is, the position where the interference image is formed on the imaging surface, the spectral characteristic error before and after calibration, and the spectral characteristic error after calibration. FIG. 15 shows that there is almost no difference between the spectral characteristics before and after calibration when the angle of view is small, but the difference between the spectral characteristics before and after calibration increases when the angle of view increases. Specifically, for example, in the case of an interference image at the positions of the angle of view θx = 0.56 deg. And θy = −0.92 deg., The error of the acquired spectral characteristics was reduced by 97% at maximum from 20.2 mm to 0.70 mm. The entire measurement surface could be calibrated in the range of ± 3mm.

  As described above, according to the present embodiment, the installation angle φy of the phase shifter 25 is experimentally obtained in advance, and the phase difference for each angle of view is calculated using the installation angle φy. can do.

The present invention is not limited to the above-described embodiment, and various expansions and modifications can be made without changing the gist of the present invention.
For example, in the above-described embodiment, the calibration optical coordinate system is defined for any one of the discrete interference images and the installation angle of the phase shifter is obtained. However, as shown in FIG. A calibration optical coordinate system may be defined for each of a plurality of discrete interference images that are the same, and the installation angle of the phase shifter may be obtained, and a phase difference for each angle of view may be obtained using an average value of these. In this way, measurement errors can be kept small.

  In addition, a data table representing the relationship between the amount of movement of the wavelength and phase shifter and the amount of phase difference is created for each angle of view from the change in intensity of all the discrete interference images, and this data table is used for each angle of view. The spectral characteristics may be calibrated.

Discrete bright spots can be formed using a number of light sources arranged in a two-dimensional matrix in addition to using a fly-eye lens.
The installation angle of the phase shifter can be obtained if the image intensity change data is known for at least one interference image. Therefore, in the calibration mode, one bright spot may be formed on the object surface, and the installation angle of the phase shifter may be obtained from the spectral characteristics of the interference image corresponding to this bright spot.

DESCRIPTION OF SYMBOLS 10 ... Spectral characteristic measuring apparatus 20 ... Optical system 21 ... Light source 22 ... Detection part 22a ... Light-receiving surface (imaging surface)
DESCRIPTION OF SYMBOLS 23 ... Fly eye lens 24 ... Objective lens 25 ... Phase shifter 251 ... Movable mirror part 252 ... Fixed mirror part 253 ... Drive stage 255 ... Movement amount detector 26 ... Imaging lens 27 ... Lens drive mechanism 40 ... Control apparatus 41 ... Control Unit 43 ... arithmetic processing unit

Claims (9)

a) a splitting optical system that guides light emitted from the measurement point of the object to be measured to the first reflecting portion and the second reflecting portion;
b) an imaging optical system that guides the light reflected by the first and second reflectors to the same point to form an interference image;
c) by moving the first reflecting part, the first reflected light from the splitting optical system to the imaging optical system through the first reflecting part and the splitting optical system through the second reflecting part, Optical path length difference expansion / contraction means for expanding / contracting the optical path length difference between the second reflected light toward the image optical system;
d) a movement amount detection unit for detecting a movement amount of the first reflection unit;
e) a light detection unit in which a plurality of detection elements are two-dimensionally arranged to detect the light intensity of the interference image;
f) Obtaining an interferogram of the measurement point of the object to be measured based on the light intensity change detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, and this interferogram A spectral characteristic measuring apparatus comprising: a processing unit that obtains a spectrum by Fourier transforming
Further, calibration light emitting means for emitting calibration light having a known wavelength from a bright spot having a known angle of view;
The amount of movement of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, An installation angle calculation unit for obtaining an installation angle of the first reflection unit based on the angle of view of the bright spot and the wavelength of the calibration light;
An angle-of-view detector that obtains the angle of view of the measurement point of the object to be measured;
The processing unit obtains the optical path length difference corresponding to the movement amount of the first reflecting unit from the installation angle of the first reflecting unit and the angle of view of the measurement point, and calibrates the spectrum based on the optical path length difference. Spectral characteristic measuring device characterized by that. The installation angle calculation unit is an xyz coordinate system having an origin at an incident point of the calibration light to the first reflection unit, and a plane including a normal line of the reflection surface of the first reflection unit is an xz plane, Define a calibration optical coordinate system having an axis perpendicular to the xz plane as the y-axis, and the installation angle φy in the calibration optical coordinate system (where the installation angle φy is the y of the normal of the reflection surface of the first reflecting portion) The rotation angle around the axis.) Is expressed by the following equation: φy = (90 ° + θy) + cos ⁻¹ {(λ / 2M _λ ) × θx}
(In the above equation, θx and θy are the angles of view of the interference image of the bright spot, the angle between the line connecting the interference image and the origin of the calibration optical coordinate system, and the x-axis and y-axis, and λ is the above-mentioned wavelength calibration light, the M _lambda indicates the movement amount of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot.)
The spectral characteristic measuring apparatus according to claim 1, wherein the spectral characteristic measuring apparatus is calculated from: The spectral characteristic according to claim 2, wherein the installation angle calculation unit calculates the installation angle φy in the calibration optical coordinate system in which the z-axis is a line connecting the interference image of the bright spot and the origin. measuring device.

The installation angle calculation unit calculates the installation angle φy by setting the angles of view θx and θy of the bright spot to θx = 0 and θy = 0, respectively. The processing unit calculates the optical path length difference L by the following equation: L = {2Mcos (φy−θ1′y)} / cos (θ1x) (where θ1′y = θ1y + 90 °)
(In the above equation, θ1x and θ1y are the angle of view of the measurement point, and M is the amount of movement of the first reflecting portion.)
The spectral characteristic measuring apparatus according to claim 2, wherein the spectral characteristic measuring apparatus is calculated from:   5. The spectroscopic method according to claim 1, wherein the calibration light emitting means forms a plurality of discrete bright spots by causing light emitted from a monochromatic light source having a known wavelength to enter a fly-eye lens. Characteristic measuring device. a) a splitting optical system that guides light emitted from the measurement point of the object to be measured to the first reflecting portion and the second reflecting portion;
b) an imaging optical system that guides the light reflected by the first and second reflectors to the same point to form an interference image;
c) by moving the first reflecting part, the first reflected light from the splitting optical system to the imaging optical system through the first reflecting part and the splitting optical system through the second reflecting part, Optical path length difference expansion / contraction means for expanding / contracting the optical path length difference between the second reflected light toward the image optical system;
d) a movement amount detection unit for detecting a movement amount of the first reflection unit;
e) a light detection unit in which a plurality of detection elements are two-dimensionally arranged to detect the light intensity of the interference image;
f) Obtaining an interferogram of the measurement point of the object to be measured based on the light intensity change detected by the light detection unit by expanding / contracting the optical path length difference by the optical path length difference expansion / contraction means, and this interferogram A processing unit that obtains a spectrum by Fourier transforming
In a spectral characteristic measuring apparatus comprising: an angle-of-view detection unit for obtaining an angle of view of a measurement point of the object to be measured;
Calibration light having a known wavelength emitted from a bright spot having a known angle of view is incident on the splitting optical system, and the first light beam corresponding to one period of a change in light intensity of an interference image formed by the imaging optical system at that time. 1 Acquire the movement amount of the reflection part from the movement amount detection unit,
Based on the amount of movement of the first reflecting part, the angle of view of the bright spot, and the wavelength of the calibration light, the installation angle of the first reflecting part is obtained,
The optical path length difference corresponding to the amount of movement of the first reflective part is obtained from the installation angle of the first reflective part and the angle of view of the measurement point, and the spectrum is calibrated based on the optical path length difference. Calibration method for spectral characteristic measuring apparatus. An installation angle of the first reflecting part is an xyz coordinate system having an origin at an incident point of the calibration light to the first reflecting part, and a plane including a normal line of the reflecting surface of the first reflecting part is xz The installation angle φy in the calibration optical coordinate system having a plane and an axis perpendicular to the xz plane as the y-axis (where the installation angle φy is the rotation angle around the y-axis of the normal line of the reflection surface of the first reflecting portion) The installation angle φy is expressed by the following equation: φy = (90 ° + θy) + cos ⁻¹ {(λ / 2M _λ ) × θx}
(In the above equation, θx and θy are the angles of view of the interference image of the bright spot, the angle between the line connecting the interference image and the origin of the calibration optical coordinate system, and the x-axis and y-axis, and λ is the above-mentioned wavelength calibration light, the M _lambda indicates the movement amount of the first reflecting portion corresponding to one period of the light intensity change of the interference image of the bright spot.)
The spectral characteristic measuring apparatus calibration method according to claim 6, wherein the spectral characteristic measuring apparatus is calculated from:   8. The method for calibrating a spectral characteristic measuring apparatus according to claim 7, wherein the calibration optical coordinate system has a z-axis that is a line connecting the interference image of the bright spot and the origin. The optical path length difference L is expressed by the following equation: L = {2Mcos (φy−θ1′y)} / cos (θ1x) (where θ1′y = θ1y + 90 °)
(In the above equation, θ1x and θ1y are the angle of view of the measurement point, and M is the amount of movement of the first reflecting portion.)
9. The method for calibrating a spectral characteristic measuring apparatus according to claim 7, wherein the calibration method is calculated from:

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