JP6385288B2 - Spectral characteristic measuring device - Google Patents

Description

  The present invention relates to a spectral characteristic measuring apparatus.

  As a technique for measuring spectral characteristics, a technique using a spectral technique called wavelength dispersion spectroscopy or Fourier spectroscopy is known (see Non-Patent Document 1). Wavelength-dispersion spectroscopy uses light that has passed through the measurement sample or reflected from the measurement sample surface (hereinafter also referred to as “object light”) as a diffraction grating or an acousto-optic tunable filter (AOTF). Is a spectroscopic method that uses a principle in which the diffraction angle of the diffracted light obtained when the light is irradiated 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 using phase shift interference using a Michelson type two-beam interference optical system. In this method, the object light is split into two by a beam splitter such as a half mirror, the respective light fluxes are reflected by the mirror, reach the half mirror again, and these two light fluxes 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 resolution at a resolution shorter than the wavelength of light to change the intensity of interference light to obtain a so-called interferogram. Then, spectral characteristics are obtained by mathematically Fourier transforming the interferogram.

  The object light emitted from the measurement sample is composed of various light beams such as scattered light, refracted light, reflected light, and the directions of these light beams are various. 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. However, when a pinhole or slit is used, most of the object light does not pass through the pinhole or slit and is not used for measurement, so that the light use efficiency is low.

  On the other hand, in Patent Document 1, an omnidirectional light beam emitted from a measurement sample is converted into a parallel light beam by an objective lens, and then condensed on an image formation surface by an imaging lens, and the two-dimensional image of the measurement sample is collected. A technique for forming a conjugate image is disclosed. In this method, a measurement sample is irradiated with light from a multi-wavelength light source, and the transmitted light or reflected light is incident on an objective lens to form a parallel light flux. Further, as shown in FIG. 1, a phase variable filter comprising an annular fixed mirror and a disk-shaped movable mirror disposed inside the fixed mirror causes a part of the parallel light beam to be separated from the remaining parallel light beam. Arbitrary phase difference is given. Then, by detecting the multi-wavelength interference intensity at each pixel of a two-dimensional array device such as a CCD camera arranged on the imaging plane, an interferogram that is a change in interference intensity is acquired.

  The movable mirror is mechanically moved with high accuracy by a piezoelectric element or the like. Thereby, the imaging intensity change (interferogram) due to the phase shift interference between the object lights can be simultaneously observed in all the pixels. A two-dimensional spectral characteristic distribution can be obtained by mathematically Fourier transforming these interferograms.

  Patent Document 2 discloses a technique called a spatial phase shift method. Furthermore, spectroscopic measurement techniques based on phase shift interference using a Michelson interferometer include, for example, Argo's FTIR infrared spectral image spectroscopy system for infrared mid- and long-wavelength regions, Zygo's vertical scanning low-coherence interferometry (Coherence Scanning). A three-dimensional shape measuring device using Interferometry, a tomographic method called full-field OCT (Optical Coherence Tomography), and the like are known.

However, the above-described conventional spectroscopic measurement technique has the following problems.
First, each bright spot on the object plane is optically modeled and considered as an ideal point light source having no area. These ideal luminescent spots form a conjugate luminescent spot image consisting of concentric bright and dark stripes called an Airy diffraction image on the image plane by Fraunhofer diffraction by a lens (see FIG. 2). . The circular bright region at the center of the Airy diffraction image is called an Airy disk, which is twice the distance from the center of the Airy disk to the center in the width direction of the first dark ring of the diffraction image (first dark ring), that is, It is known that the diameter R of one dark ring is obtained by the following formula (1).
R = 1.22λ / N. A. (1)
Here, λ is the wavelength of light, and NA (Numerical Aperture) is the numerical numerical aperture (NA = n × sin θ) of the lens. Further, n is a refractive index, and θ is an angle of an effective diameter from the lens center.

  The Airy diffraction image is physically understood as multi-beam interference fringes emitted from one bright spot. That is, at the center of the Airy disk, the phases of all the multi-beams are aligned, and the light beams strengthen each other, resulting in a bright area. Further, the dark ring becomes a dark region because the light beams weaken each other due to the phase difference according to the optical path length difference geometrically determined according to the angle of each light beam from the center of the Airy disk.

  When the phase difference π [rad.] Is given to half of the multiple rays (object luminous flux) emitted from one bright spot, the center of the bright Airy disk changes darkly in the initial state (phase difference 0 [rad.]). The dark ring part that was dark changes brightly. Changes in light and dark (interference light intensity change) at the center of the Airy disk caused by changing the phase difference given to the object beam (hereinafter also referred to as “phase shift operation”) can be detected by a photodetector such as a photodiode array detector. What is detected is the principle of acquiring an interferogram by a conventional method.

  However, as described above, since the object surface is composed of an infinite number of bright spots, there may be cases where a change in brightness cannot be detected even if a phase shift operation is performed. This can be explained by considering the innumerable bright spots constituting the object plane as a group of pairs (hereinafter referred to as “bright spot pairs”) consisting of two bright spots existing at a distance called the Rayleigh criterion.

  As shown in FIG. 3, the Rayleigh criterion refers to a condition in which the distance between the centers of the Airy discs of the two Airy diffraction images corresponds to the radius of the first dark ring (r = 0.61λ / NA). Under this condition, the two Airy diffraction images formed from the bright spot pair have a dark ring and the center of the Airy disk overlapping each other and cancel each other. Since this relationship does not change even when the phase shift operation is performed, a change in interference intensity at the center position of the Airy disk accompanying the phase shift operation cannot be detected. That is, when a pixel that detects the luminance of the central region of one Airy disk of two Airy diffraction images formed from a bright spot pair is a pixel of interest (see FIG. 3), In the initial state where the center of the Airy disk is bright (phase difference 0 [rad.]), The dark ring of the other Airy disk overlaps the center of one Airy disk. When the phase difference π [rad.] Is given, the center of one Airy disk becomes dark, and the dark ring area of the other Airy disk that has changed to a bright area overlaps this. For this reason, the luminance value detected by the pixel hardly changes in any state.

  Since the above phenomenon occurs in two Airy diffraction images formed from any pair of bright spots, considering that there are innumerable bright spot pairs as described above on the object plane, the phase shift operation is accompanied. The change in interference intensity cannot be detected, and the clarity of the interferogram deteriorates.

  Therefore, the present inventor arranges a wire mesh on the surface of an object (hereinafter referred to as “black body”) that absorbs almost all light when irradiated with light in the mid-infrared light region (wavelength: 8 μm to 14 μm). An experiment was conducted to measure the spectral characteristics. FIG. 4A shows a captured image (optical magnification: 1 ×) of a wire mesh by a digital camera, and FIG. 4B is an infrared camera when light in the mid-infrared region is irradiated on the black body surface on which the wire mesh is arranged. A photographed image (mid-infrared image) is shown. FIG. 4C shows an interferogram of a black body surface (a region indicated by (b-1) in FIG. 4B) and a spectrum obtained by Fourier-transforming this, FIG. d) shows the interferogram of the region of the edge part of the wire mesh (the region indicated by (b-2) in FIG. 4B) and the spectrum obtained by Fourier transforming this.

  As can be seen from FIG. 4B, since the surface of the black body has a uniform light intensity distribution, the mid-infrared image has almost no pattern (low spatial frequency) and the entire surface is in a shining state. Such a low spatial frequency region is considered to be composed of a large number of bright point pairs, and the sharpness of the interferogram is extremely low (FIG. 4C). On the other hand, since there is no bright point pair that cancels out the light intensity at the edge portion of the wire mesh, an interferogram is acquired with high definition (FIG. 4D). That is, even if it is a black body, by arranging a wire mesh on the surface, a uniform light intensity distribution varies, and it is possible to eliminate bright spot pairs that cancel light intensity.

JP 2008-309706 A JP 2012-058068 A WO2014 / 054708

Jiro Hiraishi, "Fourier Transform Infrared Spectroscopy" Society Publishing Center, November 1985 Ichiro Ishimaru “High-definition Fourier spectral tomographic imaging using conjugate-resolution super-resolution grating”, Abstracts of Annual Meeting of Optical Society of Japan 2012 (Optics & Photonics Japan 2012)

  Based on the above findings, the present inventor has proposed a method of forming a lattice image on a conjugate plane as a method for improving the sharpness of an interferogram in a low spatial frequency region (Patent Document 3). In this method, an image of an object plane is once formed on an image plane optically conjugate with the object plane by a conjugate imaging optical system. Then, by installing an amplitude type diffraction grating on this conjugate plane and thinning out the bright spots, the number of bright spot pairs that cancel out the light intensity is reduced. The amplitude type diffraction grating is formed by alternately arranging transparent portions and opaque portions with respect to incident light. For example, a thin film that blocks one or more lights is formed on the surface of a plate member made of a material that transmits light. And a plurality of minute wires arranged at regular intervals. The effect of improving the clarity of interferograms by this method has been reported, for example, at the Optical Society & Photonics Japan 2012 held in 2012 (see Non-Patent Document 2).

  However, Patent Document 3 only teaches the use of an amplitude type diffraction grating, and specific conditions that can reduce the number of bright spot pairs that cancel the light intensity, such as the width of the transparent portion of the amplitude type diffraction grating, are disclosed. Patent Document 3 does not disclose the relationship between the interval and pitch (pitch) and the like and the pixel pitch of the camera for detecting the interference light intensity. Therefore, when designing a spectroscopic measurement device, prepare a plurality of amplitude type diffraction gratings with different widths and pitches of the transparent portion, and actually acquire an interferogram and evaluate the sharpness of the amplitude type diffraction grating. Trial and error such as determining appropriate values such as the width and pitch of the transparent part of the film was necessary.

  The problem to be solved by the present invention is to provide a spectral characteristic measuring apparatus capable of acquiring a clear interferogram.

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 divides measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured into first measurement light and second measurement light;
b) Optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
c) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied, on an imaging surface;
d) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals on a straight line;
e) A processing unit that obtains an interferogram of the measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram When,
f) A conjugate that is arranged between the measurement region of the object to be measured and the divided optical system and has a conjugate plane in common with the divided optical system and forms an image of the measurement light from the measurement point on the conjugate plane. A surface imaging optical system;
g) an amplitude-type diffraction grating having a periodically arranged light-transmitting part and a light-shielding part arranged on the conjugate plane,
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (1) and (2)
W = (p × 2) / (m + 1) (1)
D = (p × 2) / m (2)
Respectively.

In addition, the spectral characteristic measuring apparatus according to the present invention is
a) a conjugate plane imaging optical system for converging measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured on the first image plane;
b) an amplitude-type diffraction grating that is disposed on the first image plane and includes a periodically arranged light-transmitting portion and a light-shielding portion;
c) a splitting optical system that splits the measurement light that has passed through the light-transmitting portion of the amplitude-type diffraction grating into first measurement light and second measurement light;
d) optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
e) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied on the imaging surface;
f) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals;
g) A processing unit that obtains an interferogram of a measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram And
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (1) and (2)
W = (p × 2) / (m + 1) (1)
D = (p × 2) / m (2)
Respectively.

For example, a laser light source in the visible light region, an elongated slit several times the wavelength of the visible light (5 × 10 ⁻⁷ m), and a screen are arranged side by side, and monochromatic light from the laser light source is placed in the slit. When the light is allowed to pass, striped light (interference fringes) slightly wider than the width of the slit appears on the screen. This is because a part of monochromatic light and the remaining monochromatic light interfere with each other and are strengthened or weakened due to diffraction generated when the monochromatic light passes through the slit. FIG. 5A shows the relationship between the intensity distribution of interference fringes appearing on the screen and the slits. At this time, the relationship between the diffraction angle θ, the slit width w, and the light wavelength λ, where the interference fringes are bright (light spots) and dark (dark spots), is expressed by the following equation.
Bright spot condition: w · sinθ = 0, (N + 1/2) λ
Dark spot condition: w · sinθ = N · λ (N is a natural number)
For example, as shown in FIG. 5B, an angle θ1 that satisfies w · sin θ1 = λ is a diffraction angle at which the optical path length difference is one wavelength.

On the other hand, in the present invention, a conjugate plane imaging optical system for imaging the measurement light from the measurement point is provided on the conjugate plane, and the amplitude type diffraction is provided on the conjugate plane. A grid was placed. An amplitude type diffraction grating is an optical component having a plurality of light transmitting portions and light shielding portions arranged periodically. The light-transmitting portion may be anything that allows light to pass through, and is basically an opening. Therefore, in the following description, the translucent part is described as an opening.
By arranging the amplitude type diffraction grating on the conjugate plane, an image (aperture image) of light passing through the opening of the amplitude type diffraction grating is formed on the imaging plane. FIG. 6 shows the intensity distribution of the aperture image on the image plane when the phase difference between the first measurement light and the second measurement light is 0 (rad.) And π (rad.), And the aperture of the amplitude type diffraction grating. It is a figure which shows arrangement | positioning of a pixel. When the pixels are arranged so that the intensity distribution from the center of the aperture image to the first dark ring zone is detected by three pixels, the intensity associated with the change in the phase difference between the first measurement light and the second measurement light. Changes can be detected by each pixel. However, since the luminance value of the first dark ring zone when the phase difference is π is lower than the luminance value of the central portion when the phase difference is 0, in this arrangement, the luminance value changes due to the change of the phase difference. The remaining two pixels are smaller than the central pixel.

  FIG. 7 shows the intensity of two aperture images when the interval between the apertures of the amplitude type diffraction grating is set so that the first dark ring zones of two adjacent aperture images formed on the imaging plane overlap. The distribution and the arrangement of the amplitude diffraction grating and the pixel when this intensity distribution is detected by five pixels are shown. FIG. 8 shows an interferogram obtained from the three pixels at that time. In this configuration, since the first dark ring zone of both aperture images overlaps the pixel located at the center, the detection value of the center pixel is the sum of the intensities of the first dark ring zones of the two aperture images. . Therefore, when the first dark ring zone changes brightly (when the phase difference is π), the detected luminance value of the pixel is equivalent to the luminance value of the Airy disk detected by the pixels on both sides. Further, as shown in FIG. 8, an interferogram having a phase shift of π for each pixel column is obtained.

  At this time, if the aperture width of the amplitude type diffraction grating is W, the width of the aperture image formed on the image plane is 2W. This is because the direction θ in which the first dark spot of the aperture image is generated is obtained by sin θ = λ / W. That is, as shown in FIG. 9 and FIG. 10, the theoretical spatial resolution d of the imaging optical system is determined by d = λ / NA, but the diffracted light generated from one aperture has a large amount up to the first dark spot. Since there is an amount of light, it can be regarded as equivalent to NA≈sin θ. This is called effective N.A. In this case, the theoretical spatial resolution d = λ / N.A.≈λ / sin θ = W. That is, the diameter of the Airy disk by the diffracted light generated from the opening width W can be regarded as being equal to the opening width W. By the way, based on the idea of convolution, the aperture width W on the object plane becomes aperture width W × optical magnification m + theoretical spatial resolution d on the image plane. As described above, since the theoretical spatial resolution is d = aperture width W, the width of the aperture image can be obtained by aperture width W × (optical magnification m + 1). That is, the width of the aperture image when the optical magnification is 1 is 2W.

  In the case of an infinity correction optical system, the NA of the lens must be considered for the objective lens and the imaging lens. Since the theoretical spatial resolution is determined by the smaller NA, the NA of the lens uses the smaller one of the two NAs of the objective lens and the imaging lens.

  The aperture width W and the optical magnification m are set so that the width of the aperture image becomes an interval (pixel pitch) between two adjacent pixels. For example, when the pixel pitch p is 30 μm and the optical magnification m is 1, 30 μm × 2 = W × (1 + 1), and the aperture width W may be equal to the pixel pitch 30 μm. Further, since the distance between the centers of the openings is also a pitch of 2 pixels on the imaging surface, it can be calculated by the distance between the centers of the openings = pixel pitch × 2 / optical magnification. Therefore, when the pixel pitch p is 30 μm, the distance between the centers of the openings = 30 μm × 2/1 = 60 μm can be calculated. Further, the width of the light shielding part between the openings is 30 μm. That is, when an interference light detection unit having a pixel pitch of 30 μm is used, the aperture width W of the amplitude type diffraction grating may be set to 30 μm and the center distance D of the apertures may be set to 60 μm.

  However, the above explanation applies when the effective NA is smaller than the physical lens NA, but when the wavelength λ is long and the effective NA is larger than the lens NA as in the case of mid-infrared light. Is not true. This will be described with reference to FIG.

Here, a case where a microbolometer (pixel pitch: 23.5 μm) having a measurement wavelength band of 8 μm to 14 μm is used as an interference light detection unit will be described as an example.
In the measurement wavelength band, the direction θ that generates the first dark spot of the diffracted light with the longest wavelength is the largest, so in this case, the effective NA of the diffracted light with the wavelength of 14 μm does not fit within the NA of the lens. must not. However, for example, when the optical magnification is 1 and the pixel pitch is 23.5 μm, the aperture width W is 23.5 μm (D = 23.5 × 2 / (1 + 1) = 23.5 μm). When the effective NA of the longest wavelength of the aperture width is expressed using the direction θ,
NA = n · sinθ (where n is the refractive index)
Since the refractive index in the atmosphere is 1, and sinθ = 14 / 23.5≈0.60, a lens having an NA of 0.60 or more may be used. However, in some cases, a lens having an NA of about 0.30 may be used. .

  Conversely, in order to reduce the effective numerical aperture NA to 0.30 which is half of 0.60, the aperture width W may be set to 47 μm, which is twice 23.5 μm. The width is 47 μm × 2 = 94 μm, which is four times the pixel pitch 23.5 μm (that is, the width of 4 pixels). In this case, for example, one aperture image must be detected by being divided by 5 pixels instead of the conventional 3 pixels. However, the two pixels (shown in gray in the figure) at the outermost periphery are much weaker than the two pixels at both ends whose phases are shifted by π from the center pixel. Therefore, a layout is required to increase the overlap with the adjacent aperture image to two more pixels instead of the conventional one pixel. With such a layout, it is possible to acquire a change in imaging intensity substantially by the center pixel and the two pixels at both ends.

That is, in the present invention, when the numerical aperture NA of the objective lens constituting the split optical system is smaller than the effective numerical aperture NA, the width W of the aperture of the amplitude type diffraction grating, and the center of the aperture The distance D is expressed by the following equations (3) and (4).
W = (p × 4) / (m + 1) (3)
D = (p × 3) / m (4)
May be set to the values specified by.

For example, when the optical magnification is 1 and the pixel pitch is 23.5 μm, it is as follows.
Opening width = 23.5 × 4 / (1 + 1) = 47μm
Center distance between openings = 23.5 x 3/1 = 70.5μm
In the amplitude type diffraction grating having this opening pattern, the width of the light shielding portion is 23.5 μm.

  As described above, according to the present invention, the amplitude type diffraction grating is arranged on the plane conjugate with the measurement point (object plane), and the interference light is obtained by the light given the spatial periodic change there. . Therefore, interference light can be obtained from any sample, and a clear interferogram can be obtained. In addition, according to the present invention, the configuration of the amplitude type diffraction grating (the width W of the translucent part, the distance D between the centers of the adjacent translucent parts) is the interval between the plurality of pixels (pixel pitch) p, Since the calculation can be performed using the optical magnification m, the configuration of the spectral characteristic measuring apparatus can be designed easily.

Explanatory drawing of the conventional method of giving an arbitrary phase difference between a part of parallel light flux and the remaining parallel light flux by a phase variable filter, and acquiring an interferogram. Explanatory drawing of the change of the brightness | luminance in an Airy disc accompanying a phase shift operation. Explanatory drawing of an optical model when thinking the innumerable luminescent spot which comprises an object surface as a group of the pair which consists of two luminescent spots which keep the distance called Rayleigh reference | standard. It is a figure which shows the experimental result which investigates the sharpness of an interferogram when arrange | positioning a metal mesh on the surface of a black body and irradiating the light of a mid-infrared area | region, (a) is a picked-up image of a metal mesh, (b) is A mid-infrared image of a black body with a metal mesh on the surface, (c) is a region interferogram and spectrum of the black body surface where no metal mesh is present, and (d) is an interferometer near the edge of the metal mesh on the black body surface. Gram and spectrum. Explanatory diagram (a) of the light intensity distribution of a diffraction image (aperture image) formed by single-wavelength light that has passed through an extremely narrow aperture, and (b) an explanatory diagram of the diffraction angle at which the optical path length difference is one wavelength. . Explanatory drawing in the case of detecting the intensity distribution of one aperture image formed on an image plane with three pixels. Explanatory drawing in the case of detecting the intensity distribution of two aperture images formed on an image plane with five pixels. The interferogram observed by three pixels corresponding to one aperture image when the intensity distribution of the two aperture images formed on the imaging plane is detected by five pixels. Explanatory drawing of the relationship between the width | variety of the opening part of an amplitude type diffraction grating, and the width | variety of the aperture image formed on an image formation surface. Another explanatory view of the relationship between the width of the opening of the amplitude type diffraction grating and the width of the aperture image formed on the imaging surface. Explanatory drawing in the case of detecting the intensity distribution of two aperture images formed on an image plane with eight pixels. 1 is a top view and perspective view schematically showing a spectral characteristic measuring apparatus according to a first embodiment of the present invention. FIG. The figure which shows the change of the imaging intensity distribution accompanying a phase shift. The figure which shows the dimension of each part of a multiple slit. Spectral imaging image of the omnidirectional landscape of the laboratory. Spectral imaging image and spectrum of omnidirectional scenes in the laboratory. Explanatory drawing of the structure of the multiple slit which shows 2nd Example of this invention. Explanatory drawing of the inclination adjustment mechanism of a multiple slit. The figure which shows the result when spectral imaging is performed by irradiating the light of a mid-infrared area | region to the black body on the design conditions calculated | required in 3rd Example of this invention. The figure which shows the result of the radiation light spectroscopy imaging of the whole face. The figure which shows schematic structure and the structure of a multiple slit of the spectral characteristic measuring apparatus which concerns on 4th Example of this invention. The figure explaining the relationship between a multiple slit and a pixel pitch. FIG. 5 is another diagram illustrating the relationship between multiple slits and pixel pitch. The figure explaining the relationship between a multiple slit and a pixel pitch. The figure which shows the spectral-characteristics measuring apparatus which concerns on 5th Example of this invention. The figure which shows the multiple slit used in 5th Example. An optical system schematic diagram of a sixth example in which the present invention is applied to an FTIR spectroscopic image apparatus using a Michelson interferometer is shown. 1 is a schematic diagram of an optical system in which the present invention is applied to a three-dimensional shape measuring apparatus.

The present invention eliminates the interference intensity cancellation between bright spots, and can obtain a clear interferogram, specifically various optical wavelengths such as a measurement wavelength band, an optical magnification, and a pixel pitch of a camera to be used. The present invention provides a condition for easily calculating the optimum design value of the width of the light transmitting part or the width of the light shielding part of the amplitude type diffraction grating for each observation condition.
For example, if the width of the light-shielding part is widened, the bright spot interval becomes wide and cancellation can be eliminated. However, if the width of the light-shielding part is excessively wide, a pixel whose luminance value cannot be detected is generated. Further, when the width of the light shielding portion is narrowed, the sharpness deteriorates due to cancellation between adjacent bright spots. On the other hand, if the width of the light-transmitting portion is too narrow, the amount of light is insufficient, and if it is too wide, cancellation between bright spots occurs in one light-transmitting portion. The inventor obtained appropriate conditions by sequentially verifying such events. Specific examples will be described below.

  As shown in FIG. 12, the spectral characteristic measuring apparatus of the present embodiment is composed of a conjugate plane imaging optical system and an imaging type two-dimensional Fourier spectroscopy optical system. In the conjugate plane imaging optical system, an image of the object to be measured (object plane) is optically conjugated to the object plane using a lens according to the viewing range and magnification, which are observation conditions, such as a wide-angle lens and a micro objective lens. Form. This conjugate plane becomes the object plane of the imaging type two-dimensional Fourier spectroscopic optical system, and multiple slits are arranged on the conjugate plane. The multiple slits correspond to the amplitude type diffraction grating of the present invention, and have openings (corresponding to the light transmitting portions of the present invention) arranged periodically. A portion between the opening and the opening serves as a light shielding portion. The imaging type two-dimensional Fourier spectroscopic optical system is an infinitely corrected imaging optical system using an objective lens and an imaging lens, and a phase variable filter is provided in the vicinity of an optical Fourier transform plane.

The phase variable filter is installed with an inclination of about 45 degrees with respect to the main axis, and has a fixed mirror portion and a movable mirror portion arranged vertically in the vertical direction. The movable mirror unit is moved in the direction indicated by the arrow by a drive mechanism (not shown). In addition, although a fixed mirror part and a movable mirror part may be arrange | positioned in any of upper and lower sides, here, the movable mirror part is arrange | positioned at the upper side and the fixed mirror part is arrange | positioned at the lower side.
It is also possible to arrange a fixed mirror part and a movable mirror part on the left and right, but in this configuration, when the movable mirror part is moved greatly, the fixed mirror part becomes a shadow of the movable mirror part and the light beam is emitted. Since it may be interrupted, it is preferable to arrange in the vertical direction.

  In FIG. 12, the direction in which the fixed mirror portion and the movable mirror portion of the phase variable filter are vertically overlapped is defined as the vertical axis, and the direction orthogonal to the vertical axis is defined as the horizontal axis. On the conjugate plane and the imaging plane (two-dimensional light receiving array device), directions corresponding to the vertical axis and the horizontal axis are defined in the same manner. The multiple slits arranged on the conjugate plane are arranged such that the longitudinal direction of the opening is located on the horizontal axis and the width direction of the opening is located on the vertical axis. The imaging type two-dimensional Fourier spectroscopic optical system can be designed with an arbitrary magnification. In this embodiment, an example in which the optical magnification is set to 1 will be described.

Measurement wavelength band 0.4μm to 0.8μm, CCD camera with optical magnification of 1x with objective lens (focal length: 100mm, NA = 0.24) and imaging lens (focal length: 100mm, NA = 0.24) for infinite correction optical system An image is formed on (pixel pitch: 11 μm × 11 μm). The lens of the conjugate plane imaging optical system is an omnidirectional spectroscopic imaging apparatus using a hyperboloidal mirror (maximum angle of view 50 [deg.]). In the case of this optical condition, the width of the opening of the multiple slit (hereinafter also referred to as opening width) and the distance between the centers of the openings are obtained as follows.
Aperture width W = pixel pitch × 2 / (optical magnification + 1)
= 11 μm × 2 / (1 + 1) = 11 μm
Opening center distance D = pixel pitch × 2 / optical magnification
= 11 μm × 2/1 = 22 μm
From the above, it can be seen that a multiple slit having an opening width of 11 μm and a light shielding width of 11 μm may be designed.
In this case, since the effective NA is required to be smaller than the NA of the lens, it is confirmed.
In this embodiment, effective NA = sin θ = longest wavelength / aperture width = 0.8 μm / 11 μm≈0.073.
On the other hand, since the NA of the objective lens and the imaging lens are both 0.24, it was confirmed that the effective NA was sufficiently small and the condition was satisfied.

Next, a specific layout of the multiple slits in the above example will be described with reference to FIGS.
When the movable mirror part and the fixed mirror part are arranged vertically, anisotropy is generated depending on the direction of change of the interference image accompanying the phase shift. In the cross section passing through the vertical axis, half of the light beam is irradiated to the fixed mirror part and the movable mirror part, but in the cross section passing through the horizontal axis, the light beam is irradiated only to the fixed mirror part or the movable mirror part. Because it will be. In other words, when viewed from a cross section passing through the vertical axis, the interference intensity changes as multi-beam interference with the phase shift operation, but when viewed from a cross section passing through the horizontal axis, no relative phase difference occurs in the object beam. The interference intensity change due to the phase shift operation does not occur. In consideration of such anisotropy, the vertical axis direction requires a combination of an opening and a light-shielding part to cancel the phase cancellation between the bright spots. Since no phase cancellation occurs, the light-shielding portion is not necessary. That is, as shown in FIGS. 13 and 14, the multiple slits only need to have an opening extending in the horizontal direction. In the phase variable filter having a concentric layout described later, the rectangular openings must be arranged two-dimensionally, and the multiple slits according to the present embodiment use light more than the area of the light-shielding portion is increased. It turns out that efficiency is high.

  15 and 16 show the results of spectral imaging of the scenery in the laboratory in all directions using the spectral characteristic measuring apparatus according to this example. FIG. 15A is a color image, and FIG. 15B is a monochrome image. In these images, the fluorescent lamp on the ceiling above the laboratory, the optical vibration isolation table below, and the white board on the side are photographed. FIG. 16A is a visible light image, and FIG. 16B is a pseudo color image showing the sharpness of an interferogram, which is a change in imaging intensity accompanying a phase shift operation. FIG. 16C shows the emission line spectrum of the fluorescent lamp, and FIG. 16D shows the spectrum obtained from the pseudo color image. In this way, sufficient interference sharpness could be obtained over the entire area, and the effect of this example could be verified. Moreover, it was confirmed that the bright line spectrum unique to the fluorescent lamp on the ceiling, which is also the illumination light source, exists at the positions of wavelengths 550 nm and 610 nm of the spectrum obtained from the pseudo color image.

17 and 18 are diagrams for explaining the characteristics of the spectral characteristic measuring apparatus according to the second embodiment of the present invention. Since the configuration of the spectral characteristic measuring apparatus according to this embodiment is substantially the same as that of the first embodiment, illustration and description thereof are omitted.
In the spectral characteristic measuring apparatus shown in FIG. 12, as shown in FIG. 17, it is preferable that the center of the aperture image of the multi-slit is positioned at both the center of the left end pixel and the center of the right end pixel. The sharpness of the aperture image is increased. In other words, in order to increase the sharpness of the aperture image, it is necessary to be able to position the central axis in the longitudinal direction of the opening of the multiple slit and the central axis of the pixel column with high accuracy. Therefore, in the spectral characteristic measuring apparatus according to the present embodiment, an adjustment mechanism for adjusting the inclination of the multiple slits is provided. This adjustment mechanism is a simple adjustment that pays attention to the fact that the maximum movement amount when moving the opening of the multiple slit in the vertical axis direction for positioning is an amount corresponding to the width of one opening at most. Mechanism.

  Specifically, as shown in FIG. 18, the adjustment mechanism 20 includes a rectangular frame portion 201 that surrounds the multiple slits 10, two screws 202 and 203 disposed on the rectangular frame portion 201, and a rectangular shape. The elastic member 204 is disposed between the lower side of the frame portion 201 and the multiple slit 10. The elastic member 204 is made of synthetic rubber such as urethane rubber or silicone rubber. By independently adjusting the screwing amounts of the two screws 202 and 203 into the rectangular frame portion 201, the deformation amount of the elastic member 204 changes, and the inclination of the multiple slit 10 can be adjusted. In order to prevent the opening from being deformed when the inclination of the multiple slit 10 is adjusted, the width of the light shielding portion at the lower end of the multiple slit 10 may be narrowed to facilitate elastic deformation.

The spectral characteristic measurement apparatus according to the third embodiment is an example in which a microbolometer (pixel pitch: 23.5 μm) having a measurement wavelength band of 8 μm to 14 μm is used as the interference light detection unit. Here, it is assumed that the effective NA is larger than the NA of the physical lens.
As described above, the relationship between the aperture width of the multiple slits, the distance between the centers of the apertures, and the pixel pitch in this case is expressed by the following equation.
Aperture width W = pixel pitch p × 4 / (optical magnification m + 1)
Opening center distance D = pixel pitch p × 3 / optical magnification m
Further, in this example, a germanium material mid-infrared objective lens (focal length: 20 mm, NA: 0.30) and an imaging lens (focal length: 20 mm, NA: 0.30) are used, and the optical magnification is 1 infinite. An adaptive imaging optical system was constructed. Further, in the conjugate plane imaging optical system, the optical magnification is set to 0.02.

At this time, when the optical magnification is 1 and the pixel pitch is 23.5 μm, the opening width W and the center distance D of the opening are as follows.
Aperture width W = 23.5 × 4 / (1 + 1) = 47μm
Distance between center of opening D = 23.5 × 3/1 = 70.5μm
In this layout, the width of the light shielding portion of the multiple slit is 23.5 μm.

  FIG. 19 shows a spectral imaging result when a black body is irradiated with light in the mid-infrared region under the above conditions. It can be seen that by using multiple slits, a high-definition interferogram can be obtained over the entire black body surface without using a wire mesh as described above. It was also confirmed that the spectral characteristics obtained by the Fourier transform were in accordance with Planck's law. Therefore, as shown in FIG. 20, radiation spectrum imaging of the entire face was performed. This shows the spectral characteristic distribution in the mid-infrared region of the radiation emitted from the body. Thus, it has been demonstrated that a high-definition interferogram can be obtained for all faces at low spatial frequencies without structural illumination such as texture projection.

  21 to 24 show a fourth embodiment of the present invention. FIG. 21A is a schematic configuration diagram of the spectral characteristic measuring apparatus according to the fourth embodiment, FIG. 21B is a front view of a multiple slit, and FIG. 21C is a front view of a phase shifter. In this embodiment, a phase shifter in which the movable mirror portion and the fixed mirror portion are arranged concentrically is used. In such a phase shifter, since bright points are canceled on both the vertical axis and the horizontal axis, a light shielding portion is required around the opening. Therefore, in this embodiment, a multiple slit having a plurality of rectangular openings arranged two-dimensionally is used.

  The pixel pitch of the two-dimensional light receiving array device is usually the same size in the vertical axis direction and the horizontal axis direction, but may be different as shown in FIG. In this case, using the pixel pitches in the vertical axis direction and the horizontal axis direction, the lengths (widths) of the openings of the multiple slits in the vertical axis direction and the horizontal axis direction satisfy the expressions shown in paragraph [0040], respectively. Should be designed as follows. At first glance, it seems that the distance between the opening and the oblique direction, that is, the length of the light shielding part in the oblique direction is increased, but the length (width) of the opening in the oblique direction is also as shown in FIG. Since it becomes larger, the value satisfies the design guidelines. That is, when the pixel pitch in both the vertical axis and the horizontal axis is equal, the pixel pitch in the oblique direction is √2. On the other hand, the width of the opening in the oblique direction and the distance between the centers of the openings are also doubled in the same manner, so that the design guideline is satisfied. From the above, in this embodiment, the layout of the pixels and the layout of the openings of the multiple slits may be similar.

  For ease of manufacture, a pinhole array in which circular openings are two-dimensionally arranged as multiple slits may be used. However, in this case, since the opening width in the oblique direction is determined by the radius, it is the same in both the horizontal axis and the vertical axis, and is shorter than the design guideline. That is, in FIG. 24, the sharpness of the pixel around the center of the light-shielding portion indicated by the star symbol is deteriorated. Accordingly, whether to select a rectangular opening or a circular opening may be determined in consideration of ease of manufacture and uniformity of in-plane sharpness to be secured.

FIG. 25 shows a spectral characteristic measuring apparatus according to the fifth embodiment of the present invention. The present embodiment is an example in which a conjugate plane imaging optical system is added to the spectral characteristic measuring apparatus described in Patent Document 2. Multiple slits shown in FIG. 26 are arranged on the conjugate plane of this conjugate plane imaging optical system. This multiple slit has four rectangular openings arranged in a straight line.
In this embodiment, the spectral distribution on one straight line is acquired as an interferogram that is a spatial interference fringe by one pixel. The axis in the imaging line direction (vertical direction in the figure) is referred to as the vertical axis, and the axis in the spatial phase shift direction that is orthogonal to the axis is referred to as the horizontal axis.

  In the vertical axis direction, phase shift cancellation between luminescent spots occurs, so the aperture width and the distance between the centers of the apertures are determined by the method described above. The opening width in the horizontal axis direction is referred to as the visual field opening width. This may be set in accordance with the measured visual field width or spatial resolution. If the field width is narrowed and the spatial resolution is increased, the field opening width is narrowed. As a result, the amount of light is reduced and the measurement sensitivity is lowered. If the field opening width is increased in order to improve the measurement sensitivity, the spatial resolution decreases. This trade-off is determined and set according to the measurement target and optical conditions.

  27 and 28 show a sixth embodiment of the present invention. In this embodiment, the present invention is applied to a phase shift interference optical system using a general Michelson interferometer. This includes, for example, FTIR (Fourier Transform Infrared Spectroscopy) type spectroscopic image equipment using a Michelson interferometer, and three-dimensional shape measurement equipment using vertical scanning type low coherence interferometry (Coherence Scanning Interferometry). Furthermore, it is an effective technique for an optical coherence tomography apparatus called full-field OCT (Optical Coherence Tomography).

  A conjugate plane imaging optical system is newly installed in the conventional Michelson type phase shift interference optical system, and multiple slits are installed on the conjugate plane. Similar to the fourth embodiment, the Michelson interference system does not have optical anisotropy, and therefore, a multiple slit having a plurality of rectangular openings arranged two-dimensionally as shown in FIG. 21B is used. .

The present invention may be applied to a vertical scanning type low coherence interferometry used in a three-dimensional shape measuring apparatus and a full field OCT as shown in FIG. In FIG. 28, a conjugate plane imaging optical system is newly installed in the Michelson type phase shift interference optical system, and a rectangular aperture array is similarly installed in the conjugate plane in this example.
In this way, by preventing the cancellation of the phase between the bright spots, it is possible to improve the clearness of the interference intensity on the entire surface of the measurement object having a particularly low spatial frequency pattern.

DESCRIPTION OF SYMBOLS 10 ... Multiple slit 20 ... Adjustment mechanism 201 ... Rectangular frame part 202, 203 ... Screw 204 ... Elastic member

Claims (5)

a) a splitting optical system that divides measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured into first measurement light and second measurement light;
b) Optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
c) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied, on an imaging surface;
d) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals on a straight line;
e) A processing unit that obtains an interferogram of the measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram When,
f) A conjugate that is arranged between the measurement region of the object to be measured and the divided optical system and has a conjugate plane in common with the divided optical system and forms an image of the measurement light from the measurement point on the conjugate plane. A surface imaging optical system;
g) an amplitude-type diffraction grating having a periodically arranged light-transmitting part and a light-shielding part arranged on the conjugate plane,
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (1) and (2)
W = (p × 2) / (m + 1) (1)
D = (p × 2) / m (2)
The spectral characteristic measuring device is defined by a) a conjugate plane imaging optical system for converging measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured on the first image plane;
b) an amplitude-type diffraction grating that is disposed on the first image plane and includes a periodically arranged light-transmitting portion and a light-shielding portion;
c) a splitting optical system that splits the measurement light that has passed through the light-transmitting portion of the amplitude-type diffraction grating into first measurement light and second measurement light;
d) optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
e) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied on the imaging surface;
f) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals;
g) A processing unit that obtains an interferogram of a measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram And
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (1) and (2)
W = (p × 2) / (m + 1) (1)
D = (p × 2) / m (2)
The spectral characteristic measuring device is defined by a) a splitting optical system that divides measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured into first measurement light and second measurement light;
b) Optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
c) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied, on an imaging surface;
d) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals on a straight line;
e) A processing unit that obtains an interferogram of the measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram When,
f) A conjugate that is arranged between the measurement region of the object to be measured and the divided optical system and has a conjugate plane in common with the divided optical system and forms an image of the measurement light from the measurement point on the conjugate plane. A surface imaging optical system;
g) an amplitude-type diffraction grating having a periodically arranged light-transmitting part and a light-shielding part arranged on the conjugate plane,
When the numerical aperture NA of the objective lens constituting the split optical system is smaller than the effective numerical aperture NA,
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (3) and (4)
W = (p × 4) / (m + 1) (3)
D = (p × 3) / m (4)
The spectral characteristic measuring device is defined by a) a conjugate plane imaging optical system for converging measurement light emitted from a plurality of measurement points located in the measurement region of the object to be measured on the first image plane;
b) an amplitude-type diffraction grating that is disposed on the first image plane and includes a periodically arranged light-transmitting portion and a light-shielding portion;
c) a splitting optical system that splits the measurement light that has passed through the light-transmitting portion of the amplitude-type diffraction grating into first measurement light and second measurement light;
d) optical path length difference providing means for providing a continuous optical path length difference distribution between the first measurement light and the second measurement light;
e) an imaging optical system that forms interference light by causing interference between the first measurement light and the second measurement light to which a continuous optical path length difference distribution is applied on the imaging surface;
f) a detection unit for detecting the light intensity of the interference light arranged on the imaging surface, the interference light detection unit having a plurality of pixels arranged at equal intervals;
g) A processing unit that obtains an interferogram of a measurement point of the object to be measured based on the light intensity of the interference light detected by the interference light detection unit, and obtains a spectrum by Fourier transforming the interferogram And
When the numerical aperture NA of the objective lens constituting the split optical system is smaller than the effective numerical aperture NA,
When the interval between the plurality of pixels of the interference light detection unit is p, the optical magnification is m, the width of the light transmission part of the amplitude diffraction grating is W, and the distance between the centers of two adjacent light transmission parts is D, W And D are the following formulas (3) and (4)
W = (p × 4) / (m + 1) (3)
D = (p × 3) / m (4)
The spectral characteristic measuring device is defined by The optical path length difference providing means is such that a first transmission part whose incident surface and output surface are parallel, and one of the incident surface and the output surface is on the same plane as the incident surface or the output surface of the first transmission part, A transmissive optical member comprising a wedge-shaped second transmissive portion, the other of which is inclined with respect to one of the entrance surface and the exit surface;
The splitting optical system has an objective lens that collimates the measurement light and makes it incident on the first transmission part and the second transmission part,
The imaging optical system includes the first transmission unit and the second transmission unit in which the first measurement light transmitted through the first transmission unit and the second measurement light transmitted through the second transmission unit are incident. A cylindrical lens having an axis parallel to a line of intersection between the boundary surface of the transmission part and the incident surface of the first transmission part;
The interference light detection unit detects an intensity distribution of interference light of the first measurement light and the second measurement light incident on the cylindrical lens;
The processing unit obtains an interferogram of a measurement point of the object to be measured based on an intensity distribution of the interference light detected by the interference light detection unit, and obtains a spectrum by performing a Fourier transform on the interferogram. The spectral characteristic measuring apparatus according to any one of claims 1 to 4.