Abstract:
Provided are an imaging method and device for imaging using far infrared light that make it possible to quickly image a subject without producing damage or a non-linear phenomenon in the subject. A variable-frequency coherent light source is used, illumination light from the light source is irradiated onto a linear area on an imaging subject, transmitted or reflected light is used to form an image of the imaging subject, a non-linear optical crystal is used for wavelength conversion, and a one-dimensional or two-dimensional array sensor is used to image the imaging subject while the imaging subject is moved in at least one direction.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a far-infrared imaging apparatus for capturing an image of a specimen by using light in a far-infrared region in testing steps such as analysis of component distribution of chemical substances in a specimen or testing of a different component or a foreign substance and relates to an imaging method using the same. 
       BACKGROUND ART 
       [0002]    Electromagnetic waves in a far-infrared region from about 25 μm to 4 mm in wavelength are also referred to as terahertz waves and have both transmittance of a radio wave and straightness of light, and many substances have inherent peaks in absorption spectra in this region. Therefore, the electromagnetic waves are expected to be effective in identifying substances. However, conventionally, there have been no small and easy-to-use light sources for emitting light in this region, and a detector therefor needs to be cooled by liquid helium or the like and is therefore difficult to handle. Thus, the electromagnetic waves have been used only for limited research use. 
         [0003]    In 1990s, light sources and detectors using femtosecond lasers, which have a small size and do not need to be cooled, were implemented, and research and development for implementation thereof have been eagerly performed. At present, general-purpose spectrometry devices based on time-domain spectroscopy are commercially available, and research and development are being performed for application to various fields such as security, biosensing, medical/pharmaceutical, industrial, and agricultural fields (for example, see NPL 1). Since about 2000, compact coherent light sources capable of tuning a frequency in a broad band have also been eagerly researched, which causes increase in power thereof (for example, see NPL 2), Furthermore, a technique for performing detection using a non-linear optical crystal at high SN has also been developed (for example, see NPL 3). 
         [0004]    For industrial application, it is required to acquire an image of a specimen in many fields. As means for achieving this, there has conventionally been known a method of acquiring an image by placing a specimen on an XY stage and repeatedly measuring the specimen while moving the specimen by using a spectroscopic analysis device for point detection (for example, see NPL 1). Further, a method using a two-dimensional array far-infrared light detector (for example, see PTL 1) and a method of acquiring an image by using an electrooptical crystal and a two-dimensional array CCD camera for visible light (for example, see PTL 2) are also proposed. Furthermore, a method using a one-dimensional array far-infrared light detector is also proposed (for example, see NPL 4). 
       CITATION LIST 
     Patent Literatures 
       [0005]    PTL 1: JP Patent Publication (Kokai.) 2003-075251 A 
         [0006]    PTL 2: JP Patent Publication (Kohyo) 2000-514549 A1 
       Non Patent Literatures 
       [0007]    NPL 1: Handbook of Terahertz Technology edited by Terahertz Technology Forum, pp. 426-456, published by NGT corporation on Nov. 29, 2007 
         [0008]    NPL 2: Shikata et. al THz-Wave Parametric Generation and Its Linewidth Control, the Institute of Electronics, information and Communication Engineers Transactions C, Vol. J85-C, No. 2, pp. 52-63 (2002) 
         [0009]    NPL 3: S, Hayashi. et, al., “High-peak-power and tunable terahertz-wave generation and sensitive detection by using nonlinear parametric conversion”, Proc. of 37th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Mon-B-1-2, Sep. 24 (2012) 
         [0010]    NPL 4: Michael Herrmann et. al., “Multi-channel Signal Recording with Photoconductive Antennas for THz Imaging”, Proc. of 10th IEEE International Conference on THz Electronics (THz2002), pp. 28-31 (2002) 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0011]    For industrial application., testing of many specimens and testing of a large specimen are demanded, and therefore speedy image acquisition is required. However, in a method based on conventional point measurement, in which an image is formed by moving a specimen in X and Y directions, it takes a few hours to acquire one image in some cases. This is a cause of delaying implementation. 
         [0012]    For speeding-up, it is necessary to use a high-power light source to irradiate a measurement point, with larger optical energy to thereby reduce a measurement time for each point and speed up scanning in the X and Y directions. However, when a measurement point is irradiated with high-power light in point measurement, the specimen may be damaged due to heat generated by absorption of optical energy or a non-linear effect may be exerted due to electric field intensity of light, and therefore a measurement result may be changed. 
         [0013]    Meanwhile, the method using a two-dimensional array detector is suitable for speeding-up because scanning of a specimen in the X and Y directions is unnecessary or frequency thereof can be remarkably reduced. However, the methods needs to irradiate a large area while maintaining illuminance of illumination light, which requires a much higher-power light source. In the case where the power of the light source is insufficient, an exposure time to acquire an image at one position becomes long and a sufficient speeding-up effect cannot be exerted. 
         [0014]    In the case where the light source using a femtosecond laser is used, it is necessary to acquire various data while changing an optical path length of detection light in order to acquire spectrometry measurement data for one point. Therefore, it takes time for measurement even. in the case where a high-power light source and a two-dimensional array detector are used. 
         [0015]    An object of the invention is to provide an imaging method and an imaging apparatus which can perform imaging at a high speed in imaging using far-infrared light without causing damage to or a non-linear phenomenon in a specimen serving as an imaging target. 
       Solution to Problem 
       [0016]    In order to achieve the above object, in the invention, a frequency-tunable coherent light source having higher power is used, and a line on an imaging target is irradiated with illumination light from a light source by using an illumination optical system, an image of the imaging target is formed by using transmitted light or reflected light, wavelength conversion is performed by using a non-linear optical crystal, the imaging target is imaged by using a. one-dimensional or two-dimensional array sensor while being moved in at least one direction. 
       Advantageous Effects of Invention 
       [0017]    According to the invention, it is possible to perform imaging at a high speed by using a high-power light source without causing damage to or a non-linear Phenomenon in a specimen serving as an imaging target. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  illustrates a configuration example of an imaging apparatus using light in a far-infrared region in Example 1. 
           [0019]      FIG. 2  illustrates an example of a pump beam shaping optical system for detection in Example 1. 
           [0020]      FIG. 3  illustrates a second example of the pump beam shaping optical system for detection in Example 1. 
           [0021]      FIG. 4  illustrates an example of a non-linear crystal for detection in Example 1. 
           [0022]      FIG. 5  illustrates a configuration example of a non-linear crystal for detection (conventional example). 
           [0023]      FIG. 6  illustrates a configuration example of an imaging apparatus using light in a far-infrared region in Example 2. 
           [0024]      FIG. 7  illustrates a configuration example of an imaging apparatus using light in a far-infrared region in Example 3 (Example 3). 
           [0025]      FIG. 8  illustrates a configuration example of an imaging apparatus using light in a far-infrared region (conventional example). 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Hereinafter, examples of the invention will be described with reference to the attached drawings. 
       Example 1 
       [0027]      FIG. 1  illustrates an example of the whole configuration of an imaging apparatus in Example  1 . FIG. I illustrates a configuration example of an apparatus for imaging an image of a specimen  200  by using light transmitted through the specimen  200 . This apparatus includes a wavelength-tunable far-infrared light source  100 , an illumination optical system  155 , a stage  202 , a far-infrared light imaging optical system  182 , a non-linear optical crystal  132  for detection, a detection optical system  245 , a light detector  290 , a pump light irradiation optical system  220 , a control unit  500 , and a signal processing unit  400 . 
         [0028]    As the wavelength-tunable far-infrared light source  100 , there is used a far-infrared light source for generating far-infrared light due to difference frequency generation or parametric generation by causing two kinds of laser beams having different wavelengths to enter a non-linear optical crystal. For example, when MgO:LiNbO3 is used as a non-linear optical crystal  130 , a Q-switch YAG laser with a short pulse is used as a light source  110  of pump light  115 , and light from a wavelength-tunable light source  120  is caused to enter the non-linear optical crystal  130  as seed light  125 , it is possible to obtain far-infrared light due to parametric generation. When a Si prism  140  is attached to the non-linear optical crystal  130 , it is possible to efficiently extract the generated far-infrared light. When a wavelength of the seed light  125  is changed between about 1066 nm to 1076 nm and an incident angle thereof to the non-linear optical crystal  130  is adjusted, it is possible to change a frequency of the generated far-infrared light in a range from about 0.5 THz to 3 THz. 
         [0029]    A linear illumination region  205  on the specimen  200  is irradiated with the far-infrared light obtained as described above by using the illumination optical system  155 . As the illumination optical system  155 , an imaging optical system including at least two lenses  150  and  170  is used. Specifically, a light source of far-infrared light is arranged in a front focal plane of the lens  150 , and the lens  170  is arranged so that a rear focal plane of the lens  150  and a front focal plane of the lens  170  correspond to each other, and further the specimen.  200  is arranged in a rear focal plane of the lens  170 . When an aperture stop is provided in the rear focal plane of the lens  150  (that is, which is also the front focal plane of the lens  170 ), a double telecentric optical system is formed, but the aperture stop is not essential herein. Far-infrared light emitted from the wavelength-tunable far-infrared light source  100  is a beam-like light source along beams of the pump light  115  and is diverged in  FIG. 1( a )  and is a parallel luminous flux in  FIG. 1( b ) . In  FIG. 1( a ) , light is converted into a parallel luminous flux by the lens  150  and is converged on the line of the specimen through a polarization rotation optical element  160  for rotating a polarization direction of far-infrared light at 90 degrees and the lens  170 . Meanwhile, in  FIG. 1( b ) , light is concentrated by the lens  150  once, is transmitted through the polarization rotation optical element  160  for rotating a polarization direction of far-infrared light at 90 degrees, and then becomes a parallel luminous flux at the lens  170  again, and thus the specimen is irradiated with the light. When the illumination optical system is an afocal system (that is, the lens  170  is arranged so that the rear focal plane of the lens  150  and the front focal plane of the lens  170  correspond to each other), it is possible to irradiate the specimen  200  with substantially parallel far-infrared light in  FIG. 1 b   ), which is generated in the non-linear optical crystal  130 , as a parallel luminous flux as it is. This makes it possible to efficiently introduce far-infrared light transmitted through the specimen into the far-infrared light imaging optical system  182 . 
         [0030]    Because the illumination optical system  155  is the imaging optical system as described above, it is possible to obtain stability of illumination when a wavelength of far-infrared light emitted from the wavelength-tunable far-infrared light source  100  is changed. In order to change the wavelength of the far-infrared light, the incident angle is adjusted by changing the wavelength of the seed light  125 . However, in that case, an emission direction of generated far-infrared light changes in an in-plane direction of  FIG. 1( b )  (for example, θ 1  to θ 2  in  FIG. 1 ). When the illumination optical system  155  is the imaging optical system and a light source of far-infrared light and a surface of the specimen  200  have a conjugate relationship (imaging relationship), it is possible to prevent a spot of far-infrared light from moving even on the surface of the specimen  200 . Because a position is not changed even in the case where a wavelength of far-infrared light is changed, an illumination light amount is also not changed, and therefore stable illumination can be obtained. On the contrary, in the case where the illumination optical system  155  is not an imaging optical system, an illumination position of illumination light may be shifted to a completely different position, and therefore stable imaging is difficult. However, this shall not apply where imaging is performed at a fixed wavelength or Where a wavelength is changed in a small range and a. change in a direction of far-infrared light is sufficiently small. 
         [0031]    The polarization rotation optical element  160  converts a polarization plane of far-infrared light that is emitted from the Si prism  140  and vibrates in a direction (x direction) vertical to the sheet of  FIG. 1( b )  into polarized light that vibrates in a y direction in the sheet of  FIG. 1( b ) . Herein, the reason why the polarization plane is rotated is that a polarization direction of far-infrared light to be incident on the non-linear optical crystal  132  for detection is changed to a direction needed for detection in accordance with a direction of the non-linear optical crystal  132 . In the case where a wavelength of far-infrared light is changed in a wide range, a polarization rotation system having low wavelength dependency, such as an achromatic wave plate, is needed. In the case there the wavelength is changed in a narrow range and wavelength dependency of polarization rotation performance is ignorable, a half-wave plate using birefringence of quartz is preferably used as the polarization rotation element  160 . Because a phase difference is used, the wavelength dependency remains in the polarization rotation performance, but a size of the optical system can be reduced. 
         [0032]    The specimen  200  serving as an imaging target is placed on the stage  202  and is movable in at least one direction. When data of the line is acquired while the specimen  200  is being moved in a direction (x direction) orthogonal to a longitudinal direction of the linear illumination region  205 , it is possible to acquire data (image) of the surface of the specimen  200 . In the case where a region to be imaged on the specimen  200  is longer than a length of the linear illumination region  205  in the longitudinal direction, an xy stage is used as the stage  202  to combine scanning in the x direction with feeding in the y direction, and therefore imaging can be performed in a wider region. 
         [0033]    Detection of far-infrared light transmitted through the specimen  200  is performed by performing wavelength conversion in the linear optical crystal  132  from far-infrared light into near-infrared light having a wavelength of around 1066 nm to 1076 nm, introducing the near-infrared light into the light detector  290  by using the detection optical system  245 , and performing photoelectric conversion in the light detector  290  having sensitivity to near-infrared light. As a near-infrared detector, a 1D or 2D array detector is suitable for industrial application because the array detector is comparatively easily available, has a high response speed, and can be used at a room temperature, 
         [0034]    Far-infrared light transmitted through the specimen  200  is introduced into the non-linear optical crystal  132  for detection by using the far-infrared light imaging optical system  182 . The far-infrared light imaging optical system  182  is a double telecentric imaging optical system including at least two lenses  180  and  190  and an aperture stop  185  and forms an image of the surface of the specimen  200  in the non-linear optical crystal  132  through the Si prism  140 . On the surface of the specimen  200 , the region  205  on the line is irradiated with far-infrared light, and therefore this part is imaged on a line in the non-linear optical crystal  132 . As the non-linear optical crystal  132 , LiNbO3 or MgO:LiNbO3 is preferably used. 
         [0035]    As pump light  235  needed for wavelength conversion, pump light used for generating far-infrared light in the wavelength-tunable far-infrared light source  100  is incident on the non-linear optical crystal  132  through the pump light irradiation optical system  220  and a half-wave plate  230 . As illustrated in  FIG. 2 , the pump light irradiation optical system  220  includes a half-wave plate  227 , a delay optical system  228  for matching a timing of a light pulse, a diffractive optical element  226  for dividing a beam into a plurality of beams, and a lens  224  for restoring the divided beams to a line of parallel beams. Each of the divided beams is a pump light beam for detection for use in a single channel detection system in a conventional example illustrated in  FIG. 8 . The divided beams are adjusted by the half-wave plate  230  so that a polarization direction thereof corresponds to an axis direction of the non-linear optical crystal  132  (y axis direction in  FIG. 1 .) and are introduced into the non-linear optical crystal  132 . 
         [0036]      FIG. 3  illustrates another example of the pump light irradiation optical system  220 .  FIG. 3  is different from  FIG. 2  in that beams are enlarged in one direction to form elliptical beams by using a cylindrical lens  222  instead of the diffractive optical element  226  for dividing a beam into a plurality of beams and using a cylindrical lens  225  instead of the lens  224  for restoring the beams to a line of parallel beams. Pump light is formed into not isolated beams but continuous beams, and therefore it is expected that near-infrared light subjected to wavelength conversion is also continuous. This makes it possible to achieve detection with high lateral resolution. 
         [0037]    Note that, herein, an example where the half-wave plate  227  is arranged in the pump light irradiation optical system  220  and the half-wave plate  230  is arranged behind the pump light irradiation optical system  220  has been described, but any one of the half-wave plates may be omitted. When both the half-wave plates are used, a degree of freedom in design of an optical system layout can be high. However, in the case where a certain layout taken into consideration of restriction of a polarization direction can be employed, any one of the half-wave plates can be omitted. Because the half-wave plate  2 . 27  of the pump light irradiation optical system  220  is arranged before a beam is divided, it is possible to use a half-wave plate having a small effective aperture. In the ease where the half-wave plate  230  is arranged behind the pump light irradiation optical system  220 , a polarization direction thereof is not disrupted because no optical element is provided therebehind, and therefore it is possible to introduce pump light having optimal polarization into the non-linear optical crystal  132 . 
         [0038]    Herein, an example where pump light used for generating far-infrared light in the wavelength-tunable far-infrared light source  100  is reused as the pump light  235  needed for wavelength conversion has been described. However, in the case where the power of the light source  110  of the pump light is sufficient, the pump light  115  may be divided into two parts in front of the non-linear optical crystal  130 , and one part may be used to generate far-infrared light and the other part may be used to perform wavelength conversion at the time of detection. With this, beams having a clear profile can be used to perform wavelength conversion at the time of detection. This makes it possible to improve efficiency of wavelength conversion and improve detection sensitivity. 
         [0039]      FIG. 4  illustrates an example of the non-linear optical crystal  132  for detection used in this example.  FIG. 5  illustrates an example of the conventional non-linear optical crystal  132  for comparison. The Si prism  140  is provided on the nonlinear optical crystal  132 , and far-infrared light  195  transmitted through the specimen  200  is incident therethrough. The pump light  235 , which is obtained by shaping beams into elliptical beams or a line of isolated beams in the pump light irradiation optical system  220 , is introduced into the crystal through an end surface of the non-linear optical crystal  132 . Wavelength conversion occurs due to interaction of those two kinds of beams, and near-infrared light  237  corresponding to a wavelength of the far-infrared light incident thereon is emitted from the end surface of the crystal. In this example, as the nonlinear optical crystal  132 , a crystal that causes an incident end of the pump light  235  to be in parallel to an incident optical axis of the far-infrared light  195  is used. Because such a shape is employed, it is possible to reduce a region where the pump light  235  exists and far-infrared light does not exist in the crystal. Generally, when intensive pump light is incident on the non-linear optical crystal, light having a wavelength different from that of the pump light is spontaneously emitted from atoms excited by the pump light even in the case where far-infrared light does not exist, and the light is amplified to be detected as a light noise in some cases. In the conventional example ( FIG. 5 ), a part of a distance d along an optical path of the pump light  235  is a noise source. This unnecessary part of the crystal which can be a noise source is reduced in this example, and therefore it is possible to reduce noise components and detect far-infrared light signals at high SN. Note that inclination of a surface of the incident end of the nonlinear optical crystal  132  does not need to be strictly in parallel to the incident optical axis of the far-infrared light  195 . It is only necessary that a region where the pump light  235  exists and far-infrared light does not exist in the crystal can be substantially reduced. In this example, it is necessary to propagate the pump light  235  in substantially parallel to upper and lower surfaces of the nonlinear optical crystal  132  in  FIG. 4( a ) , and therefore, in this example, it is preferable to cause the pump light  235  to be obliquely incident on the upper and lower surfaces of the nonlinear optical crystal  132  in consideration of refraction on an incident surface thereof. 
         [0040]    Near-infrared light generated by wavelength conversion in the non-linear optical crystal  132  is formed into elliptical beams or a line of isolated beams depending on a shape of the pump light  235  for detection. Those beams are introduced into the light detector  290  by the detection optical system  245  in which two orthogonal cross sections passing through an optical axis have different imaging relationships. 
         [0041]    The detection optical system  245  is preferably configured so that at least the non-linear optical crystal  132  side is telecentric in an A-A′ section of  FIG. 1( a )  (that is, cross section along linear illumination) and beams of near-infrared light generated by wavelength. conversion are concentrated on the light detector  290  in the plane of  FIG. 1( a )  (that is, cross section orthogonal to linear illumination) orthogonal to the A-A′ section. FIG. I illustrates the example where. the A-A′ section is double telecentric, and the example is preferably configured by using cylindrical lenses  250  and  280  and an aperture stop  270  as illustrated in  FIG. 1 . In  FIG. 1( a ) , beams only need to be simply concentrated on the light detector  290 , and therefore it is only necessary that the cylindrical lens  260  is provided and the light detector  290  is arranged in the vicinity of a focal plane thereof. The near-infrared light  237  generated by wavelength conversion, which is illustrated in the A-A′ section of  FIG. 1( a ) , is an example of a luminous flux of near-infrared light emitted from one point deviating from the optical axis. This near-infrared light travels in parallel to the optical axis along the pump light  235  in the A-A′ section, and therefore it is possible to efficiently introduce the near-infrared light into the light detector  290  by forming the detection optical system  245  so that the non-linear optical crystal  132  side is telecentric. Further, stray light is blocked by the aperture stop  270 , and therefore it is possible to detect near-infrared light at a high SN ratio. Note that the remainder of the pump light  235  is also emitted from an emission end of the non-linear optical crystal  132 . Because light intensity thereof is high, it is desired to appropriately perform termination processing by using a beam dump  240  so as to prevent the remainder of the pump light from becoming stray light. 
         [0042]    As the light detector  290 , a one-dimensional array or two-dimensional array having sensitivity to a wavelength of about 1066 nm to 1076 nm is used. A back-illuminated CCD sensor made of Si or an InGaAs sensor is preferably used. 
         [0043]    In the configuration in this example, in the ease where a wavelength of far-infrared light is changed, a spot of near-infrared light emitted from the non-linear optical crystal  132  and concentrated on the light detector  290  moves in an in-plane direction in  FIG. 1( a )  depending on the wavelength of the far-infrared light. Therefore, when a two-dimensional array sensor is used as the light detector  290 , it is possible to measure a wavelength of far-infrared light detected from a spot position, and therefore an absorption spectrum can be accurately measured. Meanwhile, in the case where measurement of the wavelength is unnecessary, a one-dimensional array sensor may be used, in this case, the one-dimensional array sensor is arranged so that pixels are arrayed in the A-A′ section of  FIG. 1( a ) . The pixels are arranged in a direction corresponding to the longitudinal direction of the linear illumination region  205  on the specimen  200 , and therefore it is possible to image the linear illumination region  205  at once. Note that, in order that the linear illumination region is within a region of the pixels even in the case where the spot moves, it is preferable to use a one-dimensional array sensor having wide pixels whose pixel width in a direction orthogonal to a pixel array is larger than a pixel pitch. When the one-dimensional array sensor is used, an amount of information output from the sensor is reduced to reduce information processing, and therefore it is possible to perform processing at a high speed or processing in a simple circuit. 
         [0044]    Note that the detection optical system  245  may be configured so that at least the non-linear optical crystal  132  side is telecentric in the A-A′ section of  FIG. 1( a )  (that is, cross section along linear illumination) and may be a simple imaging optical system in the plane of  FIG. 1( a )  (that is, cross section orthogonal to linear illumination) orthogonal to the A-A′ section. This can be achieved by only arranging the cylindrical lens  260  so that an emission surface of the non-linear optical crystal  132  and a surface of the light detector  290  have an imaging relationship in the configuration illustrated in  FIG. 1 . When an emission end surface of the non-linear optical crystal  132  is imaged on the surface of the light detector  290  in the  FIG. 1( a ) , it is possible to fix near-infrared light emitted from the non-linear optical crystal  132  in the same position of the light detector  290  even in the case where a wavelength of far-infrared light is changed. Therefore, it is possible to form a simple system by using a one-dimensional array sensor as the light detector  290 . Further, an amount of information output from the sensor is reduced to reduce information processing, and therefore it is possible to perform processing at a high speed or processing in a simple circuit, 
         [0045]    The control unit  500  controls the whole apparatus and functions as a user interface. The control unit  500  controls the wavelength-tunable far-infrared light source  100 , the stage  202 , the pump light irradiation optical system  220 , and the signal processing unit  400  and displays signals and data processed in the signal processing unit  400 . In the case where imaging is performed while a wavelength is being fixed, the wavelength-tunable far-infrared light source  100  is controlled to generate specified far-infrared light, and synchronization between move of the stage  202  and data acquisition in the light detector  290  is control led. In the case where data is acquired while a wavelength is being changed, setting of the wavelength and synchronization between move of the stage  202  and data acquisition in the light detector  290  are controlled. Further, when an optical path length in the delay optical system  228  of the pump light irradiation optical system  220  is controlled depending on a thickness of the specimen  200  and a timing of the far-infrared light  195  transmitted through the specimen  200  is matched with a timing of the pump light  235 , it is possible to achieve detection at high SN. 
         [0046]    The signal processing unit  400  receives a signal subjected to photoelectric conversion in the light detector  290  and generates an image of the specimen  200  on the basis of positional information on the stage  202  obtained at the time of receiving the signal. In the case where a one-dimensional array sensor is used as the light detector  290 , wavelength information specified by the wavelength-tunable far-infrared light source  100  is integrated, and therefore an image for each wavelength or an image obtained by integrating spectroscopic information is generated. In the case where a two-dimensional array sensor is used as the light detector  290 , wavelength information on illumination light and information on transmittance or reflectance of the specimen  200  corresponding thereto are extracted from output data from the light detector  290 , and therefore an image for each wavelength or an image obtained by integrating spectroscopic information is generated. By comparing the above image with spectroscopic image data (reference data) obtained when no specimen is placed, the spectroscopic image data being stored in a storage area of the signal processing unit  400 , an absorption spectrum is calculated, and therefore two-dimensional distribution (absorption spectrum image) of the absorption spectrum can be obtained. 
         [0047]    Note that, in this example, an example where a -switch YAG laser with a short pulse is used as the light source  110  of the pump light  115  of the wavelength-tunable far-infrared light source  100  has been described. However, a line width of a basic spectrum only needs to be small, and therefore a mode-locked laser may be used. Because a repetition rate thereof is high, it is possible to perform measurement at a higher speed. 
       Example 2 
       [0048]      FIG. 6  illustrates a configuration example of an imaging apparatus in Example 2. In this configuration, imaging is performed by using reflected light of the specimen  200 . The directions of the optical path from the wavelength-tunable far-infrared light source  100  to the illumination optical system  155  and the optical path from the far-infrared imaging optical system  182  and thereafter are changed at the surface of the specimen  200 , far-infrared light is caused to be obliquely incident on the specimen  200  and reflected light thereof is detected. With this, it is possible to measure a specimen having low transmittance and measure a spectroscopic property of a surface of the specimen. When a mechanism system capable of changing incident angles of an illumination system and an imaging optical system is constructed, incident angle dependency of the spectroscopic property can be also measured. 
       Example 3 
       [0049]      FIG. 7  illustrates a configuration example of an imaging apparatus in Example 3. In this configuration, imaging is performed by causing far-infrared light to be vertically incident on the specimen  200  and using reflected light thereof When the wavelength-tunable far-infrared light source  100  to the illumination optical system  155  and the far-infrared imaging optical system  182  and thereafter are overlapped by a beam splitter  206  around the surface of the specimen  200 , far-infrared light is caused to be vertically incident on the specimen  200  and reflected light thereof is detected. In the case where a polarization beam splitter is used as the beam splitter  206 , it is possible to omit the polarization rotation optical element  160 . Further, it is also possible to form a system for polarizing and separating incident light and reflected light by using a polarization beam splitter as the beam splitter  206  and providing a quarter-wave plate therebehind. With this, even in the case Where a specimen has large incident angle dependency of a spectroscopic property, it is possible to measure a basic property thereof without considering the incident angle dependency. 
         [0050]    Note that, in this example, there has been described an example where the linear illumination region  205  on the specimen  200  is irradiated with far-infrared light by using the illumination optical system  155  and detection is performed by using a line of isolated beams as pump light for detection in order to perform imaging. However, a part of the linear illumination region  205 , which is a region where no specimen exists, may be irradiated to acquire reference data for absorbance measurement. It is possible to simultaneously acquire the reference data and light transmitted through a specimen or reflected light thereof, and therefore it is possible to reduce an influence of disturbances such as a change in output of a laser light source and an electric noise entering a circuit of a detection system. Thus, measurement can be achieved with high accuracy. 
       Reference Signs List 
       [0000]    
       
           100  wavelength-tunable far-infrared light source 
           200  specimen 
           155  illumination optical system 
           205  linear illumination region 
           182  far-infrared light imaging optical system 
           185 ,  270  aperture stop 
           235 ,  115  pump light 
           130 ,  132  non-linear optical crystal 
           245  detection optical system 
           290  light detector, array sensor 
           202  stage 
           110 ,  120  laser light source 
           115 ,  125  laser beam 
           140  Si prism 
           160  polarization rotation element 
           270  pump light irradiate optical system 
           226  diffractive optical element 
           227 ,  230  half-wave plate