Inspection equipment and 3D tomography equipment

Abstract

To provide an inspection device and three-dimensional tomographic image formation apparatus which can greatly expand a visual field, extremely shorten a measurement time, realize a deep depth of field and realize real-time inspection to factory lines and the like.SOLUTION: A coherent light source is used as a light source 101, and the light emitted from the light source 101 is divided into two beams, an intersection is formed by crossing the two beams with a condensing optical system 102, and an inclined fringe which is inclined with respect to the optical axis is formed in the intersection. Only a straight advancing light component in the forward scattered light or the back scattered light generated in the inclined fringe is received by a light reception optical system 103. The light reception optical system 103 receives only a Doppler signal that does not include the scattered light component by a light reception element arranged at the focal position of the light reception optical system 103.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an inspection apparatus and a three-dimensional tomography apparatus for the purpose of understanding the internal structure of a medium that is transparent in the region of visible light and infrared light, and for detecting foreign matter and defects in said medium. [Background technology]

[0002] For a long time, diagnostic devices that apply the principle of optical coherence tomography (OCT) have been widely used for medical examinations such as fundus examinations and subcutaneous tissue examinations. The principle of OCT is to obtain a three-dimensional tomographic image using an optical system similar to a Michelson interferometer. This is achieved by moving a mirror for the reference light (reference light mirror) back and forth on the light-receiving surface to interfere the reference light and sample light in the optical axis direction, and using a light source (such as a laser diode (LD), superluminescent diode (SLD), or supercontinuum (SC) light source) that has coherence equal to the thickness of one "layer" in the depth direction of the object being examined. In other words, approximately half of the coherence length of the light source corresponds to the thickness of the "layer". To create multiple "layers," the reference light mirror is moved along the optical axis by the number of "layers" required. A schematic diagram of an early type of time-domain (TD) OCT is shown in Figure 1A. The TD OCT in Figure 1A includes a light source 10, a reference light mirror 12, a beam splitter 14, and a detector 16. Interference occurs only when wave packets having temporal (partial) coherence corresponding to each "layer" overlap. Conversely, the reference light mirror 12 is moved so that the wave packet reflected by the reference light mirror 12 and reaching the detector 16 overlaps with the wave packet reflected inside the object T being inspected. Alternatively, moving the reference light mirror 12 changes the position of the overlapping wave packets of the reference light and sample light along the optical axis within the sample. That is, the position of the "layer" along the optical axis within the sample shifts by approximately half the coherence length of the light source 10, which is roughly equal to the thickness of one "layer" unit.

[0003] In addition to the aforementioned medical and biological examinations that apply the principles of OCT, applications are also progressing to the inspection of freshness of fresh foods and processed foods, the detection of foreign objects mixed inside food, the detection of defects in semiconductor substrates, the detection and inspection of scratches inside plastic products, the inspection of textile products, the inspection of paint coatings, and the inspection of pharmaceuticals. Non-Patent Literature 1 is a useful reference for details on these matters. Patent Literature 1 shows an early type of time-domain OCT. Patent Literature 2 shows an improved version of Patent Literature 1, a time-domain method that uses a fiber coupler. Both are Michelson type as shown in Figure 1A, and problems remain with the aforementioned narrow field of view and long-term measurement. Non-Patent Literature 2 also explains OCT technology, starting with the time-domain method, and also explaining the spectral domain (SD) method and the swept source (SS) method. The spectral domain method eliminates mechanical movement and improves inspection speed by decomposing the spectrum using a spectrometer such as a diffraction grating and receiving the diffracted light with a line sensor. Furthermore, swept-source OCT, which rapidly changes the wavelength of the light source, achieves even higher speeds. However, in all methods, the field of view is very narrow, only a few millimeters. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-543 [Patent Document 2] Special Publication No. 2004-502483 [Non-patent literature]

[0005] [Non-Patent Document 1] Non-destructive evaluation technique for the internal structure of materials using SS-OCT, Takumi Takahashi (Materials Properties Group, Department of Mechanical and Materials Technology), Junichi Tatami (Yokohama National University), Research Report 2021 (Kanagawa Prefectural Institute of Industrial Science and Technology), 5-9) [Non-Patent Document 2] Optical Coherence Tomography (OCT), Masamitsu Haruna, Journal of the Institute of Image Information and Television Engineers, Vol. 65, No. 1, pp. 67-71 (2011) [Overview of the project] [Problems that the invention aims to solve]

[0006] The biggest problem with conventional methods is that numerous repeated measurements (averaging or smoothing) are required to improve the signal-to-noise ratio (S / N), resulting in long inspection times. Furthermore, the inspection area is extremely narrow. In recent years, miniaturization has been achieved using fiber couplers, making the devices easier to move, but because they are still reference light-based, numerous repeated measurements are necessary to improve the S / N, measuring an area of ​​a few millimeters square over several seconds. Improvements have been made to address the aforementioned problems, and the methods have changed, achieving high speeds close to television rates, such as 20 fps (frames per second), but the problems of S / N and narrow field of view still remain. In other words, the long inspection times of seconds and the extremely narrow field of view, which are not much of a problem in medical inspection equipment, become unacceptable when applied to industrial use. Furthermore, a major problem for industrial applications is that the vast majority of lines require online inspection, and most inspections are of large size.

[0007] The object of this invention is to realize an inspection device that uses a line sensor as the light-receiving element of an OCT device applicable to the aforementioned industrial inspection device, and that enables optical tomography without the use of a reference light mirror, unlike the conventional Michelson method. Needless to say, it is also applicable to medical inspection devices and analysis devices. [Means for solving the problem]

[0008] (1) The inspection apparatus according to the present invention comprises a coherent light source as a light source, a focusing optical system, and a light receiving optical system. The focusing optical system splits the light emitted from the coherent light source into two, and the split light is made to intersect to form an intersection, and at the intersection, an inclined fringe is formed that is inclined with respect to the optical axis of the light from the coherent light source. The light receiving optical system receives the straight light component of the forward scattered light or back scattered light generated at the inclined fringe. The light receiving optical system receives a Doppler signal that does not contain the scattered light component using a light receiving element placed at the focal position of the light receiving optical system.

[0009] (2) The three-dimensional tomography apparatus according to the present invention comprises the inspection apparatus, wherein in the tilt fringe formation region, a plurality of the photodetectors are used as pixel units in the main scanning direction and the sub-scanning direction, and a rectangular parallelepiped consisting of the pixel units and the number of "layers" in the optical axis direction in the tilt fringe formation region is used as a unit rectangular parallelepiped. The frequency spectrum of each "layer" contained in the unit rectangular parallelepiped is obtained from the Doppler signal generated in the tilt fringe within the unit rectangular parallelepiped by discrete Fourier transform, and the density or transmittance of each coordinate in the main scanning direction, sub-scanning direction and optical axis direction of the object to be inspected at the intersection is calculated based on the frequency spectrum, and a three-dimensional tomography image is obtained from these three-dimensional coordinate values. In the present invention, "layer" means the optical structure contained in the tilt fringe, and when simply written as a single word, it represents a unit. (However, this does not apply to the claims.)

[0010] As mentioned above, conventional reference light type measuring devices have a narrow field of view and long measurement time. Therefore, the inventors of this invention propose a differential heterodyne interference method that offers overwhelmingly faster inspection time, a significantly wider field of view, and improved signal-to-noise ratio. This differential heterodyne interference method employs a method in which a sheet beam obtained by splitting the beam of a spatiotemporally coherent light source (such as a laser diode (LD), fiber laser, etc.), i.e., a light source with a long coherence length, into two beams intersects near the lens focal point and outside the Rayleigh range, instead of using a spatiotemporally coherent light source, i.e., a light source with a short coherence length, which is a reference light type. Conventional heterodyne laser Doppler velocimeters (LDVs), which use coherent light sources such as gas lasers or LDs, cross the beam waists of two beams at the lens focal point and generate interference fringes parallel to the optical axis within the Rayleigh range at the beam intersection. In contrast, the present invention crosses the beams outside the Rayleigh range near the beam waists and uses cylindrical or spherical wavefronts with substantially concentric cross-sections to produce fringes inclined with respect to the optical axis. Figure 1B shows the fringe pattern when the beams cross at the beam waists as in an LDV. Within the Rayleigh range, the waves can be considered almost plane waves, so the fringe pattern is also parallel. Unlike conventional reference optical coherence tocometry (OCT), the OCT of the present invention is a differential OCT.

[0011] Furthermore, since the present invention uses a laser, which is a coherent light source, there is an abundance of light sources with high optical power density. W-class semiconductor lasers are currently commercially available and easily obtainable. In other words, even when receiving backscattered light, by using a high-power laser, there is no shortage of power density even when the beam cross-sectional area is increased by shaping it into a sheet beam.

[0012] Therefore, a wide inspection area can be realized for each stage. Moreover, since the ratio of the reference light to the sample light is not constant as in the reference light type, there is a problem with the visibility of the signal (the amplitude of the AC component of the signal is small compared to the DC component), and the detector is constantly exposed to the reference light. Therefore, the reference light method has a large shot noise, while in the differential type, since the scattered light of almost each beam has an intensity ratio of approximately 1:1, the visibility of the signal is excellent and the shot noise is small. Also, since a high-power coherent light source is used, signal detection is possible up to a deep position, and it is advantageous in terms of being applicable to thicker inspection objects. Furthermore, by adopting a telecentric optical system for the light receiving optical system, almost the direct light component can be received, and by arranging the optical system and the light receiving element array alternately in the main scanning direction, a sufficient inspection width can be satisfied while maintaining a good S / N even for a wide inspection object.

Advantages of the Invention

[0013] According to the present invention, the field of view can be widened for each stage, the measurement time can be extremely shortened, and furthermore, a deep depth of field can be realized, enabling real-time inspection on factory lines and the like.

Brief Description of the Drawings

[0014] [Figure 1A] It is a schematic diagram of an initial type TD (Time Domain) OCT. [Figure 1B] It is a diagram showing mutually parallel fringes formed at the beam intersection of a conventional differential type LDV (Laser Doppler Velocimeter). [Figure 2A] It is a schematic diagram showing the state when cylindrical waves are crossed to form "tilted fringes". [Figure 2B] It is a schematic diagram showing "tilted fringes" three-dimensionally. [Figure 3] It is a schematic diagram simplifying and showing the method of generating a signal according to the method of the present invention. [Figure 4A] It is a diagram showing the in-plane of the main scanning direction and the sub-scanning direction of the minimum pixel unit. [Figure 4B]This is a diagram representing a unit cuboid used to obtain a three-dimensional image. [Figure 5] This is a schematic diagram of the visibility of the Doppler signal observed in a differential heterodyne detection system. [Figure 6A] This figure shows the frequency spectrum obtained by one of the photodetector arrays. [Figure 6B] This is a schematic diagram showing the discrete spectrum obtained by cutting the spectrum in Figure 6A at intervals of frequency resolution Δf. [Figure 7] This is a schematic diagram comparing the scattered light intensity in the optical axis direction of an object being inspected as it travels straight through the object. [Figure 8A] This diagram shows how the beam intersection is located downstream of the beam waist (towards the photodetector array from the focal point). [Figure 8B] This diagram shows how the beam intersection is located upstream of the beam waist (towards the light source from the focal point). [Figure 9A] This is a diagram with "layers" added to Figure 8A. [Figure 9B] This is Figure 8B with "layers" added. [Figure 10A] This diagram illustrates how cylindrical and spherical waves intersect outside the beam waist, forming "gradient fringes," and how the fringe spacing changes depending on the position of the "layers" in the optical axis direction, including the "layers" themselves, and shows the case where the layers are located downstream of the beam waist (towards the photodetector). [Figure 10B] This diagram illustrates how cylindrical and spherical waves intersect outside the beam waist, forming "inclined fringes," and how the fringe spacing changes depending on the position of the "layers" in the optical axis direction, including the "layers" themselves. It shows the case where the fringe is located upstream (towards the light source) of the beam waist. [Figure 11A] This figure shows the Mie scattered light intensity for a particle size of 5 μm, expressed as intensity in all directional angles. [Figure 11B] This figure shows the Mie scattered light intensity for a particle size of 10 μm, expressed as intensity in all directional angles. [Figure 11C] This figure shows the Mie scattered light intensity for a particle size of 15 μm, expressed as intensity in all directional angles. [Figure 11D] This figure shows the Mie scattered light intensity for a particle size of 20 μm, expressed as intensity in all directional angles. [Figure 11E] This figure shows the Mie scattered light intensity for a particle size of 25 μm, expressed as intensity in all directional directions. [Figure 11F] This figure shows the Mie scattered light intensity for a particle size of 30 μm, expressed as intensity in all directional angles. [Figure 11G] This figure shows the Mie scattered light intensity for a particle size of 0.05 μm, expressed as intensity in all directional angles. [Figure 12] This figure shows the relative ratio of forward-scattered and back-scattered light intensity for different particle sizes, when the solid angle NA of a telecentric light-receiving optical system is set to 0.02. [Figure 13A] In the Lambert-Beer law, the damping coefficient and absorption coefficient are represented by ρ, and the damping rate for ρ = 1 - 1 is shown. [Figure 13B] In the Lambert-Beer law, the damping coefficient and absorption coefficient are represented by ρ, and the damping rate for ρ = 0.1-1 is shown. [Figure 14A] This figure shows examples of forward-scattered light receiving methods and back-scattered light receiving methods employing differential "inclined fringes" in a cross-sectional view in the main scanning direction, with the forward-scattered light receiving method shown in the figure below. [Figure 14B] This diagram shows examples of forward-scattered and back-scattered light receiving methods employing differential "inclined fringes" in a cross-sectional view in the main scanning direction, with both forward-scattering and back-scattering receiving systems arranged on the optical axis. [Figure 14C] Examples of forward-scattered and back-scattered light receiving systems employing differential "inclined fringes" are shown in a cross-sectional view in the main scanning direction, with the forward-scattered light receiving system positioned on the optical axis and the back-scattered light receiving system positioned off the optical axis. [Figure 15] This diagram illustrates a pair of forward-scattering detection methods (a tilt fringe-forming optical system and a telecetric detection optical system). [Figure 16] This is an example of changing the fringe spacing by intersecting a plane wave beam and a cylindrical wave beam. [Figure 17]This is an example of changing the fringe spacing by intersecting distorted wavefronts. [Figure 18] This is a schematic diagram of several forward-scattering light detection methods (tilted fringe formation optics and telecetric light detection optics). [Figure 19] This is a schematic diagram of a focusing optical system in which three types of near-infrared wavelength light sources are split by a beam splitter, made into parallel beams by a collimator lens, and then intersected upstream of the beam waist by a focusing lens, forming a common region where the three wavelengths are superimposed. [Figure 20A] This is a schematic diagram showing a case where the beam is widened before the focusing lens, forming a beam intersection downstream of the focusing lens's inherent focal point. [Figure 20B] This is a schematic diagram showing a case where the beam is narrowed before the focusing lens, forming a beam intersection upstream of the focusing lens's intrinsic focal point. [Figure 21A] This figure shows a specific example of a case where a beam intersection is formed on the downstream side of the beam waist. [Figure 21B] This figure shows a specific example of a case where a beam intersection is formed on the upstream side of the beam waist. [Figure 22] This figure shows the power density based on the beam diameter and beam waist in the case of Figure 21A. [Figure 23] This diagram shows the error between the wavefront curvature of a Gaussian beam and the optical axis distance. [Figure 24] This diagram shows a Doppler signal with high-frequency modulation applied and the original Doppler signal (the dashed line represents the bandpass filter). [Figure 25] This is a schematic diagram showing how the fringe spacing of the "inclined fringe" changes when SS (Swept_Source) is used as the light source. [Figure 26] This is a schematic diagram showing the case where the measurement length is extended using AOD. [Figure 27A] This is a schematic diagram illustrating a method of preventing the beam waist from entering the object being inspected (by placing the intersection downstream of the beam waist). [Figure 27B]This is a schematic diagram illustrating a method of preventing the beam waist from entering the object being inspected (by positioning the intersection upstream of the beam waist). [Figure 28] This is a schematic diagram showing how the fringe can be moved back and forth in the sub-scanning direction to be applied to a stationary object being inspected. [Figure 29] This is a schematic diagram illustrating a scenario where multiple light sources are arranged as an array in the sub-scanning direction, allowing them to be applied to a stationary object being inspected. [Figure 30A] This is a schematic diagram of a forward scattering detection method using a microchannel plate. [Figure 30B] This is a schematic diagram of a light receiving method that combines a forward scattering method using a microchannel plate with a back scattering method. [Figure 30C] This is a schematic diagram of a modified light receiving method that adds a backscattering method to the forward scattering method using a microchannel plate. [Figure 31] This figure shows the case where the optical output is multiplied using a microchannel plate. [Figure 32A] This figure shows an example of the arrangement of multiple light-receiving lenses. [Figure 32B] This figure shows an example of the arrangement of multiple light-receiving lenses. [Figure 32C] This figure shows an example of the arrangement of multiple light-receiving lenses. [Figure 32D] This figure shows an example of the arrangement of multiple light-receiving lenses. [Figure 32E] This figure shows an example of the arrangement of multiple light-receiving lenses. [Modes for carrying out the invention]

[0015] <Inclined fringe and solid angle of light reception> Compared to the reference light receiving method, where reference light is constantly incident on the photodetector, the differential method, where only scattered light is incident on the photodetector, has lower shot noise and improved signal-to-noise ratio. In the backscattering method, a coherent backscattering peak appears, and the received light intensity is said to increase by several tens of percent. However, because it is backscattering, the physically received scattered light intensity is much smaller than the forward scattered light intensity. In contrast, by using forward scattering, the scattered light intensity increases dramatically, and therefore the observation area in the optical axis direction, such as the depth and thickness of the object being inspected, is increased. Conventionally, the forward scattering method has been considered difficult to distinguish between multiple scattered light and straight-traveling light in high-density light-scattering media. However, in the present invention, only the straight-traveling light component with coherency of forward-scattered light is received using a differential heterodyne method. As a result, only coherent light interferes, and a Doppler signal with a highly visible AC component is obtained. In addition to the forward scattering method, a telecentric optical system is used in the photodetector optical system. By reducing the aperture diameter, the telecentric optical system can narrow the receiving solid angle (Numerical Aperture, hereinafter sometimes referred to as "NA") to a state close to that of a parallel beam of light. Therefore, in addition to obtaining a Doppler signal, it is possible to restrict the light to only the straight-traveling light component with superior coherence. Thus, the present invention has the advantage of eliminating the scattered light component and receiving only the straight-traveling light component with high coherence.

[0016] Furthermore, in order to discriminate the Doppler signal for each "layer" in the depth direction, "gradient fringes (gradient interference fringes)" are used to simultaneously receive signal light of different frequencies, and the frequency spectrum is replaced with the position of each "layer" in the depth direction based on the relationship between frequency and depth. Therefore, it has high spatial resolution in the depth direction (thickness direction), and the deep depth of field of a telecentric photodetector optical system of 10 mm or more is divided into regions in the optical axis direction as "layers" with a thickness of tens to hundreds of "several tens of μm or less", and by adding the optical axis direction component corresponding to the frequency spectrum to the two-dimensional components in the main scanning direction and sub-scanning direction, the frequency spectrum can be plotted in three-dimensional space and a three-dimensional image of the object to be inspected can be obtained. Moreover, by combining a staggered arrangement of photodetector arrays and a telecentric optical system, the inspection width in the main scanning direction can be made significantly wider than in conventional methods, and the application fields are diverse, including not only industrial use but also medical use and the field of physical and chemical science. The method of generating "gradient fringes" is shown in Figure 2A.

[0017] Figure 2A is a schematic diagram showing the formation of a "gradient fringe" when cylindrical waves are intersected. Figure 2B is a schematic diagram showing the "gradient fringe" in three dimensions. A beam emitted from a light source such as a highly coherent LD (not shown) is split into two beams with extremely small path difference using a beam splitter or the like, and incident on a focusing lens (not shown). A laser including a linearly polarized LD may be used as the light source. A cylindrical lens with a beam profile formed into a sheet may be used as the focusing lens. The cylindrical lens must have extremely low aberration performance. Therefore, as the cylindrical lens, a lens with extremely low wavefront aberration is used so that the wavefront of the "gradient fringe" does not deviate from the cylindrical or spherical wave. The sheet-shaped beam (incident beam) emitted from the focusing lens is intersected near the beam waist and at a position approximately outside the Rayleigh range. The reason for intersecting the beams near the beam waist is to ensure sufficient light reception intensity, that is, to avoid reducing the optical power density as much as possible. When intersecting sheet beams at a position away from the beam waist, a wedge prism or the like can be used on the photodetector array side (downstream side) of the focusing lens to appropriately change the intersection angle of each sheet beam, so that they intersect at a position away from the focusing lens's inherent focal point. Furthermore, branching of the beam emitted from the light source is necessary when using a light source with a relatively short coherence length, but this is not the case when using a light source with a long coherence length.

[0018] Besides the method using a wedge prism, it is also possible to achieve this by pre-dividing the bifurcated beams that enter the focusing lens from parallel beams to non-parallel beams before they enter the focusing lens, causing them to intersect at a position away from the beam waist. Various other methods are also conceivable.

[0019] The method for generating the "inclined fringe" used in the present invention will now be explained. First, by intersecting sheet-shaped beams near the beam waist and at a position approximately outside the Rayleigh range, cylindrical waves or spherical waves are formed at a position outside the beam waist and outside the Rayleigh range region. Furthermore, when sheet-shaped beams intersect, the wavefront, which was approximately a plane wave (or a wavefront with a very large radius of curvature) at the beam waist, begins to change to a cylindrical wave from a position beyond the Rayleigh range centered on the beam waist. This cylindrical wave then spreads in a substantially concentric manner within a cross section perpendicular to the main scanning direction and the optical axis, forming an "inclined fringe." This "inclined fringe" makes it possible to obtain Doppler signals of different frequencies in the thickness direction (optical axis direction) of the object to be inspected (not shown in the figure). Obtaining different frequency signals in the thickness direction (optical axis direction) means that it becomes possible to discriminate the structure of the object to be inspected in the thickness direction.

[0020] Thus, a sloped fringe is formed by generating cylindrical or spherical waves at the intersection (beam intersection) where light intersects outside the beam waist. However, the sloped fringe may also be formed by a combination of cylindrical and plane waves, or a combination of spherical and plane waves, or by intersecting wavefronts other than plane waves, cylindrical waves, and spherical waves.

[0021] <Frequency information contained in a "layer" and the discrete Fourier transform (DFT)> Figure 3 is a schematic diagram illustrating a simplified signal generation method according to the present invention. Figure 4A shows the in-plane dimensions of the smallest pixel unit in the main scanning direction and sub-scanning direction. Figure 4B shows a unit rectangular parallelepiped for obtaining a three-dimensional stereoscopic image. In Figure 4A, the length of one side constituting the smallest unit in the main scanning direction is denoted as Drm, and the length in the sub-scanning direction is denoted as Drs. Therefore, Drm × Drs equals the two-dimensional smallest unit (μm) based on the main scanning direction and sub-scanning direction. 2This represents the minimum three-dimensional unit (μm) based on the main scanning direction, sub-scanning direction, and depth of field direction. In Figure 4A, Drm = Drs. In Figure 4B, in addition to Drm and Drs, Dl is defined as the depth of field length of one side constituting the smallest unit. Thus, Drm × Drs × Dl represents the minimum three-dimensional unit (μm) based on the main scanning direction, sub-scanning direction, and depth of field direction. 3 This represents the smallest unit of a rectangular parallelepiped (unit cuboid) represented by Drm × Drs × Dl, which is the smallest unit for obtaining a three-dimensional image.

[0022] Therefore, the volume Vr(μm) of a unit rectangular parallelepiped is 3 The Doppler signal is represented as Dr × Drs × Dl, and the photodetector arrays contained in the cross-section perpendicular to the main scanning direction and the cross-section perpendicular to the sub-scanning direction of the unit rectangular parallelepiped observe the unit rectangular parallelepiped extending along the depth of field direction of the telecentric photodetector optical system. The unit rectangular parallelepiped contains the entire frequency band of the observed Doppler signal. Furthermore, the Doppler signal generated in the "tilting fringe" within the unit rectangular parallelepiped is output as a low-frequency signal by the photodetector array picking up minute vibrations in the detection system, even when the object being inspected is almost stationary. Therefore, a bandpass filter is set to obtain a signal only in the fringe frequency band that matches the transport speed of the object being inspected. The bandpass filter should be set to a band that satisfies the sampling theorem described later and does not generate aliasing noise. Also, if a digital filter is used for the bandpass filter, it is easier to accommodate changes and switches in transport speed.

[0023] Frequency analysis is performed for each unit cuboid. Well-known methods such as the Discrete Fourier Transform (DFT) and the Fast Fourier Transform (FFT) are used for frequency analysis. Furthermore, in order to perform the frequency analysis accurately and to satisfy the DFT processing conditions, a photodetector array smaller than the unit pixel is used in the cross-sections of the main and sub-scanning directions of the unit cuboid. In addition, oversampling may be performed in the main scanning direction of the photodetector array. Multiple lines may also be provided in the sub-scanning direction. In Figure 4A, oversampling is performed on the photodetector array, and multiple lines of photodetector arrays are provided in the sub-scanning direction.

[0024] As described above, in this embodiment shown in Figure 3, light emitted from a light source 101, which is a coherent light source, is focused by a focusing optical system 102. At this time, the focusing optical system 102 splits the light from the light source 101 into two, and the split light beams intersect to form an intersection (beam intersection), and at this intersection, an inclined fringe is formed that is inclined with respect to the optical axis of the light from the light source 101. Specifically, when the light split into two by the beam splitter 121 is focused by the cylindrical lens 122 acting as a focusing lens, the angle is changed by the wedge prism 123 and the beams intersect at the intersection. The wedge prism 123 is an example of an optical element that has power in a direction perpendicular to the plane containing the optical axis of the light from the light source 101, and forms an intersection by intersecting the light at a position on the optical axis other than the intrinsic focal position of the cylindrical lens 122.

[0025] Of the forward-scattered or back-scattered light generated at the tilt fringe, the straight-traveling light component is incident on the photodetector optics 103 and is received by each photodetector in the photodetector array 131 included in the photodetector optics 103. The photodetector optics 103 receives the Doppler signal, which does not contain the scattered light component, using a photodetector positioned at the focal point of the photodetector optics 103. Each photodetector in the photodetector array 131 is arranged in a single-row array by being aligned in a direction perpendicular to the optical axis of the light from the light source 101.

[0026] In this embodiment, the light-receiving optical system 103 includes a telecentric light-receiving optical system. The telecentric light-receiving optical system may be a bilateral telecentric optical system or a unilateral telecentric optical system. In this example, a bilateral telecentric optical system is used in which an aperture 133 is positioned between two light-receiving lenses 132.

[0027] Next, the specific processing method will be described. In this embodiment, the frequency spectrum is obtained from the signal obtained after A / D conversion of the signal output by the photodetector using DFT (FFT) processing. At that time, a unit cuboid included in the slope fringe is formed. The unit cuboid includes multiple photodetectors in the main scanning direction and the sub-scanning direction, as well as all "layers" within the depth of field. A digital filter may be applied to the digital signal obtained after A / D conversion of the signal output by the photodetector to limit the bandwidth to only the Doppler signal. To perform DFT (FFT) processing, N data points are used. Considering the resolution, N data points are acquired within the time that the object under inspection stays at the smallest unit pixel that constitutes the three-dimensional structure as an arbitrary position passes through the smallest unit pixel in the sub-scanning direction.

[0028] First, let the transport speed be Vc. The photodetector array 131 has rectangular pixels, and the size of the photodetectors is Psm for the length in the main scanning direction and Pss for the length in the sub-scanning direction. Therefore, the time required to travel a distance of Pss in the sub-scanning direction is Pss / Vc, and in terms of frequency, it is Vc / Pss (Hz). Now, if the photodetector array 131 consists of elements with 1 line Ns pixels, the line rate is Lr (Hz), the number of oversamplings is Nos, the number of pixels in the photodetector array 131 in the main scanning direction included in the smallest unit pixel is Nmr (=N / Nos), and the number of lines in the photodetector array 141 in the sub-scanning direction is Nsl, then in order to perform DFT with N data points, it is necessary to satisfy N = Nos × Nmr × Nsl (in Figure 4A, Nmr = Nsl). More specifically, the sampling number N is the total number of natural numbers of the digital signals output from the photodetectors contained in the unit cuboid and after A / D conversion, and satisfies the condition of being a power of 2. Nos is the number of oversamplings of the reading lines of the photodetectors. Nmr is the number of photodetectors contained in the unit cuboid in the main scanning direction. Nsl is the number of photodetectors or lines contained in the unit cuboid in the sub-scanning direction. By performing DFT (FFT) processing on a total of N data points, the frequency spectrum in the depth direction (optical axis direction) of the unit cuboid can be obtained. At the same time, the frequency spectrum is associated with each "layer". Then, this frequency spectrum can be replaced with the density at any position in the depth direction of the object being inspected. The sampling number N in the DFT (FFT) processing is determined so that the frequencies of adjacent "layers" can be separated.

[0029] We will examine N DFT (FFT) operations on the volume of a unit cuboid. N discrete data points in the time domain (d0, d1, ..., d N-1 ) into the frequency domain N discrete data (D0, D1, ..., DA N-1 The conversion to ) is performed using the following formulas (1) and (2).

number

number

[0030] Therefore, the N data points time-divided from the photodetector array 131 are converted into N divided data points in the frequency domain. These N divided data points in the frequency domain constitute the frequency spectrum. The entire frequency spectrum is the source data into which the data divided by frequency resolution is converted to the frequencies of each "layer".

[0031] The cross-sectional area Sr of a unit rectangular parallelepiped with volume Vr is given by Sr = Drm × Drs. Since the length of the photodetector array 131 in the main scanning direction is Psm and the length in the sub-scanning direction is also Psm, the photodetector area Sp is Sp = Psm × Psm. By dividing the unit rectangular parallelepiped cross-sectional area Sr by the pixel size, the number of photodetectors Ns contained in the cross-sectional area Sr of the unit rectangular parallelepiped can be obtained. That is, Ns = Nmr × Nsl.

[0032] If Ns is smaller than the number of N elements required for the DFT, that is, if N > Ns, then N must be a number obtained by multiplying Ns by a power of 2, i.e., N = Ns × 2 m (m: a natural number greater than or equal to 2), and N and Ns are adjusted under this condition, and the oversampling number Nos is multiplied by Ns. Therefore, N = Nos × Ns. Alternatively, Nos = N / Ns.

[0033] Regarding measurement accuracy, it is important to accurately measure the frequency of the signal obtained from the "slope fringe." In other words, it is necessary to increase the visibility of the signal (the ratio of the AC component to the DC component). Therefore, as mentioned above, it is preferable to make the element size of the photodetector array 131 smaller than the fringe spacing. That is, the size of the photodetector in the subscanning direction may be smaller than the fringe spacing in the slope fringe. Alternatively, the size of the photodetector array 131 in the subscanning direction may be smaller than the fringe spacing in the slope fringe. Of course, the visibility of the Doppler signal will deteriorate, but signal detection is possible even if it is slightly larger than the fringe spacing. However, if it exceeds the fringe spacing and becomes the size of several fringe spacings, it can no longer be identified as a fringe. In other words, the AC component cannot be detected, and only the DC component is detected. Therefore, the settings are determined by considering factors such as the smallest unit pixel size, the pixel size of the photodetector array 131, the carrier speed, the scanning frequency, the detection speed, the signal quality (S / N ratio and visibility), and the number of DFT (FFT) operations, i.e., the resolution and sharpness when constructing a 3D image.

[0034] Figure 4B shows the relationship between a unit cuboid and one "layer" when forming "layers" in the object being inspected during DFT (FFT) processing and obtaining a 3D image. The depth of one "layer" is basically determined by dividing the length of the unit cuboid by the length of either the main scanning direction or the sub-scanning direction of the unit cuboid's cross-section, which gives the number of "layers". The number of "layers" can also be calculated by dividing the depth of field by the number obtained by dividing the entire frequency band by the frequency resolution.

[0035] Figure 5 is a schematic diagram of the visibility of the Doppler signal observed in a differential heterodyne detection system. If visibility is good, the signal will be as shown in Figure 5(A); if visibility deteriorates, the signal will be as shown in Figure 5(B). Figure 5(B) shows that the amplitude of the AC component is smaller compared to Figure 5(A). When the amplitude of the AC component decreases, the intensity of the Doppler signal decreases, and the signal-to-noise ratio deteriorates.

[0036] Figure 6A shows the frequency spectrum obtained by one of the photodetector arrays 131. Figure 6B is a schematic diagram of the discrete spectrum obtained by cutting the spectrum from Figure 6A at each frequency resolution Δf. Figure 6A shows the frequency spectrum of one "layer" obtained. This frequency spectrum is a discrete spectrum with a resolution at least higher than the number of "layers". Figure 6B is a discrete spectrum obtained by averaging or downsampling the frequency spectrum from Figure 6A to match the number of "layers". From this spectrum, by converting it to the density of each "layer" according to the attenuation of transmittance and combining it with the position of the "layer" and the 2D data of the photodetector array 131, one of the 3D data of the object to be inspected can be obtained.

[0037] In this embodiment, the frequency of the Doppler signal generated by the tilt fringe is decomposed into arbitrary "layers," and the frequency of the Doppler signal generated from each "layer" is associated with the "layer," thereby detecting the density or transmittance of each "layer" divided along the optical axis direction of the object to be inspected. The longitudinal direction of the photodetector array 131 is the main scanning direction. The sub-scanning direction, which is orthogonal to the main scanning direction and the optical axis of the light from the light source 101, is the transport direction of the object to be inspected. In this embodiment, three-dimensional data is constructed from signals detected at arbitrary positions along the optical axis direction divided by the main scanning direction, the sub-scanning direction, and each "layer."

[0038] <Correction of optical signal intensity> The object to be inspected is a medium that is light-scattering and isotropic. In this invention, since only the straight-traveling component of scattered light is detected by the photodetector array 131, the optical path length is the same in all cases. Furthermore, the degree of attenuation of any light incident on each "layer" of the object to be inspected and scattered is the same. Therefore, the above-mentioned isotropic media of different densities are used as reference media, measurements are performed in advance using the above-mentioned reference media, and the degree of light attenuation in each "layer" is stored in memory as a correction value table. Then, using the correction value table corresponding to the actual inspection medium, corrections are performed according to the attenuation rate in each "layer" of each object to be inspected, and the actual density or transmittance in each "layer" is determined. Alternatively, at this time, the spectral intensity obtained by the discrete Fourier transform of each "layer" of the object to be inspected may be corrected using the light attenuation rate obtained by the discrete Fourier transform of the Doppler signals of each "layer" of the above-mentioned reference media in advance.

[0039] Figure 7 is a schematic diagram comparing the scattered light intensity in the optical axis direction of an object being inspected as it travels straight through the object. When the received signal is converted into a frequency spectrum, its intensity can be corrected for the power density due to the widening of the fringe spacing of the "inclined fringe" at a predetermined position where the beam is irradiated and light scattering occurs. The widening of the light beam due to the "inclined fringe" is inversely proportional to the power density of the beam forming the "inclined fringe". In other words, as the fringe spacing of the "inclined fringe" narrows, the beam power density increases, and as it widens, the power density decreases. This ratio is also equal to the ratio of detected frequencies (detection frequencies). The detection frequency may be used to correct the power density. In this case, since it is an optical system that forms a sheet beam, the correction may be made by dividing the maximum value of the frequency by the minimum value.

[0040] When a "gradient fringe" is formed downstream of the beam waist, the fringe frequency on the light-receiving optical system side of the object being inspected becomes the minimum fringe frequency. Using the frequency spectral intensity I(Ffmax) of the minimum fringe interval, i.e., the maximum fringe frequency, as a reference, the spectral intensity obtained by cutting out each "layer" can be multiplied by (f(Ff(max)) / f(Ffc)) with frequency f(fc), so Ic = Ic·(f(Ff(max)) / f(Ffc)). The corrected frequency spectral intensity becomes the density or transmittance in each "layer". Conversely, when a "gradient fringe" is formed on the light source side (upstream side) of the beam waist, the fringe frequency on the downstream side becomes larger, the opposite to when the "gradient fringe" is formed downstream of the beam waist.

[0041] <Method for generating angled fringes> Figures 8A and 8B are schematic diagrams illustrating the beam intersection formed when beams cross outside the Rayleigh range of the beam waist. Figure 8A shows the beam intersection located downstream of the beam waist (towards the photodetector array 131 from the focal point). Figure 8B shows the beam intersection located upstream of the beam waist (towards the light source 101 from the focal point). The area enclosed by the thick solid line in each figure corresponds to the beam intersection region (intersection region). The "inclined fringe" formed in the intersection region generates Doppler signals of different frequencies in each "layer".

[0042] Figure 9A is a diagram of Figure 8A with a "layer" added. Figure 9B is a diagram of Figure 8B with a "layer" added. Figures 9A and 9B also show the positional relationship between the beam intersection and the "layer". "Inclined fringes" are lined up in the intersection region as shown in Figures 8A and 8B. The number of these fringes is determined based on the distance between the optical axes of a pair of beam waists, the wavelength, and the distance from the beam waist to the "inclined fringe". The field of view (area of ​​focus) of the photodetector array 131 on the receiving lens 132 side is narrower than the width of the beam intersection shown in Figures 8A and 8B. The number of "inclined fringes" received by the area projected onto the photodetector array 131 included in the cross-section of the main scanning direction and sub-scanning direction of the unit rectangular parallelepiped after passing through the telecentric photodetector optical system of the present invention is only a few.

[0043] Figures 10A and 10B show, including the layers, how cylindrical and spherical waves intersect outside the Rayleigh range of the beam waist, forming a "gradient fringe," and how the fringe spacing changes depending on the position of the "layer" in the optical axis direction. Figure 10A shows the case where cylindrical and spherical waves intersect outside the beam waist, forming a "gradient fringe," and how the fringe spacing changes depending on the position of the "layer" in the optical axis direction, including the "layer," and is located downstream of the beam waist (on the photodetector side). Figure 10B shows the case where cylindrical and spherical waves intersect outside the beam waist, forming a "gradient fringe," and how the fringe spacing changes depending on the position of the "layer" in the optical axis direction, including the "layer," and is located upstream of the beam waist (on the light source side).

[0044] <Fringe spacing for "inclined fringe"> As described above, if approximately concentric wavefronts intersect, the fringe will spread out approximately concentrically as it moves away from the beam waist, similar to the diffraction and interference pattern in Young's double-slit experiment. For the fringe spacing DF in "inclined fringe," the beam waist is considered as an aperture, and Dbw is the distance between the two sheet-like beam waists in the sub-scanning direction, λ is the wavelength of the light source, Lbn is the distance from the sheet beam waist to the nearest intersection, and Lbf is the distance to the furthest intersection.

[0045] First, the beam waist diameter Dbw is calculated using the following equation (3), where f is the focal length of the converging lens and Dbi is the diameter of the beam incident on the converging lens.

number

number

[0046] The well-known formula for determining the fringe spacing in Young's diffraction experiments is as follows: If the beam waist is the distance between the two apertures, and the distance between the beam waists (beam waist spacing) is Wb, then the fringe spacing DF can be calculated from the following formula (5). Note that L is the distance from the beam waist.

number

[0047] Furthermore, with respect to equation (5), if we let Lbn be the distance from the beam waist of the sheet beam at the beam intersection to the closest position, and Lbf be the distance to the furthest position, then the fringe spacing DFn of the "inclined fringe" closest to the beam waist can be found using equation (6) below, and the furthest fringe spacing DFf can be found using equation (7) below.

number

number

[0048] <Wavefront of a Gaussian beam> The wavefront of the sheet beam intersection that forms the fringe is described below. The beam diameter W(Z) of the Gaussian beam in the optical axis direction (Z direction) can be obtained from equation (8) below.

number

number

[0049] On the other hand, the radius of curvature R(Z) of the wavefront in the direction of the optical axis Z is expressed by the following equation (10).

number

[0050] The length of the optical axis direction of the measuring section of the "inclined fringe" should be greater than or equal to the depth of field of the telecentric light-receiving optical system, and the width of the measuring section should be greater than or equal to the field of view dimension in the sub-scanning direction of the cross-section of the unit rectangular parallelepiped in the main scanning direction and sub-scanning direction. If the telecentric light-receiving optical system is a 1:1 optical system with a focal length of f=50mm, then if the aperture diameter is about 1mm, the depth of field and depth of focus can be about 10mm. The depth of field and depth of focus can be increased by increasing the size of the optical system itself. The telecentric light-receiving optical system may also be a reduction optical system. For example, if a f=500mm light-receiving lens on the object being inspected is used instead of an f=50mm light-receiving lens, the geometric optical NA will become 1 / 10 if the diameter of the aperture used with the original f=50mm lens is not changed. In this case, a depth of field of 100mm can be obtained. For example, with fresh vegetables, fruits, raw fish, and various packaged foods, increasing the focal length makes it theoretically possible to inspect for defects in the internal structure or the presence of foreign objects. However, since the numerical aperture (NA) of the object-side lens in a telecentric optical system becomes smaller, at f=50mm and f=500mm, the amount of light needs to be increased 100-fold when considering the solid angle ratio.

[0051] <Details of the correction for light attenuation using the Lambert-Beer law> When the object being inspected is a thick, light-scattering medium, light attenuation becomes a problem. In the present invention, the forward scattering detection method provides higher light reception intensity compared to the backscatter detection method. Scattering within living organisms, when expressed in terms of particle size, is said to range from a few micrometers to several tens of micrometers, and Mie scattering is dominant in this range. However, because it is multiple scattering, light attenuation follows the Lambert-Beer law.

[0052] The transmittance T when a parallel beam of light is incident on a light-scattering and isotropic medium can be determined by the following equation (11) based on the Lambert-Beer law. In equation (11), x is the distance, I0 is the incident light intensity, I is the exit light intensity, and ρ is the attenuation constant.

number

[0053] In a living body, when the light source wavelength λ is about 800 nm, the attenuation constant ρ is approximately 0.1 -1 ~1 mm -1 and is considered to be. Therefore, when a medium with a thickness of about 1 mm is used, the emitted light intensity I is e of the incident light intensity I0 -1 attenuates to. Assuming that the thickness at which the emitted light intensity is attenuated to 1 / 100 can be detected and ρ = 0.1 -1 is the case, the allowable measurement depth Da is about 45 mm. Also, when ρ = 1 mm -1 is the case, the allowable measurement depth Da is about 4.5 mm.

[0054] FIG. 11A is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 5 μm represented by the intensity in the omnidirectional angular direction. FIG. 11B is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 10 μm represented by the intensity in the omnidirectional angular direction. FIG. 11C is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 15 μm represented by the intensity in the omnidirectional angular direction. FIG. 11D is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 20 μm represented by the intensity in the omnidirectional angular direction. FIG. 11E is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 25 μm represented by the intensity in the omnidirectional angular direction. FIG. 11F is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 30 μm represented by the intensity in the omnidirectional angular direction. FIG. 11G is a diagram showing the Mie scattering light intensity in the case of a particle diameter of 0.05 μm represented by the intensity in the omnidirectional angular direction. In FIGS. 11A to 11F, the Mie scattering intensity with respect to the azimuth angle by spherical particles is shown. Considering the living body to be mainly composed of cytoplasm, cell wall, and cell membrane, etc. from the perspective of refractive index, the wavelength λ is 800 nm, the relative refractive index N of the cytoplasm is 1.35, and the relative refractive index N' of the cell wall is 1.46 to 1.6. Note that the relative refractive index N' of the cell wall is calculated typically as 1.46 or 1.6. In OCT, the size of the Mie scattering region is 5 μm to 25 μm.

[0055] However, since Mie scattering is a multi-particle scattering system involving a very large number of particles, it is difficult to represent living organisms solely by the intensity of Mie scattered light. Therefore, it is preferable to explain the attenuation of scattered light using the Lambert-Beer law. On the other hand, each individual light scattering is still either Mie scattering or Raylegh scattering. In the present invention, since the reception is almost entirely of the straight-line light component, the scattered light intensity distribution is treated as according to Mie theory, and the attenuation rate follows the Lambert-Beer law.

[0056] In Figure 11A, the vertical axis represents the scattered light intensity corresponding to the relative refractive index when the particle diameter is 5 μm and the wavelength λ is 800 nm. The horizontal axis represents the azimuthal angle. Also, in Figure 11A, the region enclosed by the thick solid line corresponds to the backscattering region, and the other regions correspond to the forward scattering region (in this invention, the side scattering region (90° directional scattering) is ignored). Comparing the light intensities of forward scattering and backscattering with a receiving solid angle NA of 0.02, 10 3 There is a large difference in the ratio of scattered light intensities as shown above. This difference is due to the fine structure (vibrational phenomena) specific to the Mie scattering region, but it becomes more pronounced as the particle size increases. As shown in Figure 11E, when the particle size is 25 μm, 10 4 This results in an even larger difference, as shown above. In other words, in the case of a transmissive optical system that can employ a forward scattering detection method, the amount of light received is greater than in the backscattering method. Figure 11G shows the scattered light intensity in the azimuthal direction when the wavelength λ is 0.85 μm, the particle size is 0.05 μm, the relative refractive index N of the cytoplasm is 1.35, and the relative refractive index N' of the cell wall is 1.6. As shown in Figure 11G, it can be seen that there is no difference between forward scattering and backscattering when the particle size is 0.05 μm.

[0057] In the forward-scattered light receiving method, objects that cannot be inspected in the back-scattered light receiving method, specifically objects with a thickness greater than the threshold, or in other words, objects with a depth greater than the threshold, can be inspected. This is explained below. Equation (12) below is a modified version of equation (11) above, in which the intensity of the scattered light received is increased by M times.

number

[0058] Figure 12 shows the relative ratio of forward-scattered and back-scattered light intensity for different particle sizes, when the solid angle NA of a telecentric light-receiving optical system is set to 0.02. According to Figure 12, the intensity of the forward-scattered light is approximately 100 to 1000 times greater than the intensity of the back-scattered light.

[0059] Figure 13A shows the Lambert-Beer law, where the damping coefficient, absorption coefficient, etc., are represented by ρ, and ρ = 1 -1 This shows the decay rate. Figure 13B shows the case where ρ = 0.1, representing the decay coefficient, absorption coefficient, etc. in the Lambert-Beer law. -1 This shows the attenuation rate. Also, Figure 13A shows that M in the above equation (12) is 10 3 The results of substituting into the above equation (12) to obtain the damping rate are shown in Figure 13B. 4 The results of substituting the values ​​to obtain the attenuation rate are shown. In Figures 13A and 13B, the thick solid line represents 10 -2 The points where attenuation occurs are indicated. It can be seen that the forward-scattered light receiving method has a tolerance range of 2 to 3 times greater than that of the back-scattered light receiving method.

[0060] Figure 14A shows examples of forward-scattered and back-scattered light receiving methods employing differential "tilted fringes" in a cross-sectional view in the main scanning direction, and shows only the forward-scattered light receiving method. Figure 14B shows examples of forward-scattered and back-scattered light receiving methods employing differential "tilted fringes" in a cross-sectional view in the main scanning direction, with both the forward-scattered and back-scattered light receiving systems positioned on the optical axis. Figure 14C shows examples of forward-scattered and back-scattered light receiving methods employing differential "tilted fringes" in a cross-sectional view in the main scanning direction, with the forward-scattered light receiving system positioned on the optical axis and the back-scattered light receiving system positioned off the optical axis.

[0061] Furthermore, as shown in Figure 14B, when a backscattering light-receiving method is included in the forward-scattering light-receiving method, the cylindrical lens 122, which is a condensing lens, is divided and arranged in such a way that the light-receiving element array 131 can be arranged on the optical axis for both the backscattering light-receiving method and the forward-scattering light-receiving method. In addition, when a backscattering light-receiving method is included in the forward-scattering light-receiving method, an aperture may be provided in the cylindrical lens 122 such that the backscattered light satisfies the NA of the light-receiving lens system and the length of the light-receiving element array 131.

[0062] Furthermore, Figure 14C shows modified examples of the forward scattering and backscattering detection methods in Figure 14B, in which a prism 124 is placed downstream of the cylindrical lens 122 so as not to obstruct the optical path of the branched beam, and the optical axis is bent in the backscattering detection method.

[0063] <Optical system> Figure 15 shows an optical system consisting of a focusing optical system 102 that forms a "tilted fringe," a light-receiving optical system 103 that receives scattered light generated from the "tilted fringe," and a light-receiving element array 131 as a set. In other words, Figure 15 is a diagram that shows a set of forward-scattering light-receiving systems (tilted fringe forming optical system and telecentric light-receiving optical system). As a telecentric light-receiving optical system, a double-sided telecentric optical system that can increase the depth of field is preferred.

[0064] The photodetector array 131 receives forward-scattered light scattered by the "gradient fringe" at each pixel along the main scanning direction. That is, each pixel of the photodetector array 131 receives signals with different frequencies in the optical axis direction of the "gradient fringe" within a set storage time. In order to perform frequency analysis, it is necessary to acquire signals with phase shifted by a frequency resolution shifted in the time axis direction, and to obtain data for DFT (FFT) processing. Therefore, the photodetector array 131 is made smaller than the minimum fringe spacing, and a unit cuboid is defined. The aforementioned unit cuboid is the smallest unit cuboid that satisfies the frequency resolution, in which multiple photodetector arrays 131 are assembled and the cuboid is composed of cuboids with the pixel planes in the main scanning direction and sub-scanning direction of the photodetector array 131 as its base. Then, DFT (FFT) processing is performed on multiple data shifted in the time axis direction to obtain the frequency spectrum contained in the unit cuboid.

[0065] By performing a series of processes to determine the frequency spectrum across the entire main and sub-scanning directions, the optical systems shown in Figures 14A, 14B, and 14C provide a set of three-dimensional information for a unit rectangular parallelepiped containing all "layers" whose length corresponds to the length of the "inclined fringe" in the main scanning direction and the length moved in the sub-scanning direction, and whose length is equal to the depth of field of the light-receiving optical system 103.

[0066] <Other ways to change the fringe spacing> In addition to the aforementioned "inclined fringe" method, there are other means of changing the fringe spacing in the depth direction. Figure 16 shows an example of changing the fringe spacing by intersecting a plane wave beam and a cylindrical wave beam. The fringe shown in Figure 16 is created by adding a plane wave and a cylindrical wave. The fringe shown in Figure 16 spreads out in an elliptical shape in the optical axis direction. Because the elliptical fringe spreads out as it approaches the photodetector, the fringe spacing changes in the optical axis direction. Therefore, it becomes possible to obtain Doppler signals with different frequencies in the depth direction. In other words, it is possible to obtain information in the thickness direction (depth direction) of the object being inspected.

[0067] The elliptical fringe is formed by intersecting the approximately plane wave generated within the Rayleigh range with the cylindrical wave generated at the beam waist and outside the Rayleigh range. For example, the Rayleigh range of a beam with a beam waist diameter of 70 μm is approximately 10 mm. The beam waist diameter differs between this beam and the beam used for cylindrical wave generation, with the beam waist diameter Dbw being set to Dbw = 30 μm. Furthermore, by positioning the waist of the cylindrical wave generation beam closer to the light source 101 than the beam intersection, the Rayleigh range is positioned just before it intersects with the plane wave. The Rayleigh range of the cylindrical wave generation beam is approximately 884 μm.

[0068] Thus, once the beam waist position and other parameters are determined, the cylindrical wave and plane wave will reliably intersect. While the plane wave generation beam has a diameter of 70 μm to 99 μm, the cylindrical wave generation beam has a diameter of 70 μm to 340 μm at a depth of 10 mm. The cylindrical wave generation beam expands about three times more than the plane wave generation beam. Therefore, the power density of the cylindrical wave generation beam decreases by about 1 / 10th, but the power is compensated for to a certain extent by the plane wave generation beam.

[0069] One method for changing the fringe spacing in the optical axis direction is to use the spherical aberration of the lens. Figure 17 shows an example of changing the fringe spacing by intersecting distorted wavefronts. As shown in Figure 17, by distorting the wavefront from a plane wave, the distorted wavefronts can be intersected to change the fringe spacing in the optical axis direction.

[0070] <Measuring fringe spacing> The detection frequency of the Doppler signal at the optical axis position of each "layer" within the depth of field of the light-receiving optical system 103 in the direction of the optical axis of the "inclined fringe" is determined in advance. The correction method is described below.

[0071] In this invention, a stepping motor is used as the drive motor for the turntable, and it is driven in microsteps and precisely controlled. A paramagnetic or nonmagnetic metal wire, which can be processed into a fine wire of a few micrometers, such as tungsten wire, is attached to a metal fitting having the shape of the rotating object and fixed on the turntable, with the metal fitting positioned at the center of the rotating body. The rotation angle is precisely controlled and rotated so as to cross the fringe to obtain a Doppler signal, and the fringe spacing is determined from the relationship between the rotation speed and the observed frequency.

[0072] Furthermore, to determine the fringe spacing of the "inclined fringe," the turntable itself can be moved along the optical axis, and the fringe spacing in each "layer" of the "inclined fringe" can be determined from the relationship between the rotation speed and the observed frequency. In this case, the Doppler signal of the backscattered light will be detected.

[0073] Alternatively, a metal wire can be stretched across a glass substrate with good parallelism on a stage that moves in the sub-scanning direction, and the stage can be moved back and forth at a constant speed in the main scanning direction to cross the "inclined fringe" and measure the Doppler signal. The fringe spacing can then be determined from the frequency of the Doppler signal. Alternatively, a fine wire formed on the glass substrate using semiconductor deposition technology can be placed and fixed on the stage, and the stage can be moved back and forth in the main scanning direction and sub-scanning direction to detect the Doppler signal and determine the fringe spacing in each "layer". When receiving forward scattered light, an opening can be made in a flat plate, the aforementioned metal wire can be stretched across it, and the plate can be placed on the reciprocating stage to detect the Doppler signal of the forward scattered light.

[0074] In this way, Doppler signals are detected from thin metal wires or finely vapor-deposited wires approximately the same width as the fringe spacing, and the fringe spacing in each "layer" of the "inclined fringe" is determined. The frequency spectrum obtained from the unit cuboid described later and the density (transmittance) in each "layer" of the object to be inspected are then determined. The density (transmittance) at any position within the depth of field is then combined with arbitrary positions (coordinates) in the main scanning direction and sub-scanning direction, and the density (transmittance) in the 3D coordinates is plotted in 3D space to obtain a 3D image of the object to be inspected.

[0075] In this embodiment, in the region where the "inclined fringe" is formed, multiple photodetectors are used as pixel units in the main scanning direction and the sub-scanning direction. A rectangular parallelepiped consisting of this pixel unit and the number of "layers" in the optical axis direction in the region where the "inclined fringe" is formed is used as a unit rectangular parallelepiped. The frequency spectrum of each "layer" contained in the unit rectangular parallelepiped is obtained from the Doppler signal generated in the "inclined fringe" within the unit rectangular parallelepiped by DFT (FFT) processing. Based on this frequency spectrum, the density or transmittance of each coordinate in the main scanning direction, sub-scanning direction, and optical axis direction of the object to be inspected at the beam intersection is calculated, and a three-dimensional tomographic image is obtained from these three-dimensional coordinate values.

[0076] Figure 18 shows an optical system in which multiple optical systems are arranged in the main scanning direction, using the optical system shown in Figure 15 as the basic unit. The length of the photodetector array 131 in the optical system of Figure 15 is approximately 14 mm, and for wider objects to be inspected, multiple optical systems can be arranged. For example, for an object to be inspected that requires a main scanning length of 300 mm, 21 pairs of condensing optical systems 102 and photodetecting optical systems 103 can be arranged in a staggered pattern, and overlapping portions can be provided at the ends of each photodetector array 131 aligned in the main scanning direction of each optical system to ensure the full reading width. For even wider objects to be inspected, the number of staggered optical systems can be increased, making it possible to accommodate factory lines. Furthermore, it is also fully capable of handling applications in the medical testing field, the physical and chemical field, and analytical instruments.

[0077] The light sources 101 of each optical system are arranged side by side in parallel with the reading line L, and the optical axes of the light sources 101 substantially coincide with the optical axes of the light receiving lenses 132. The light emitted from each light source 101 is branched after being collimated. Each condensing optical system 102 includes a cylindrical lens 122 having power only in the sub-scanning direction so as to form inclined fringes in a direction substantially orthogonal to the sub-scanning direction, and a wedge prism 123 that separately inclines each branched light with respect to the sub-scanning direction so as to form an intersection at a position other than the specific focal point of the cylindrical lens 122. The condensing optical system 102 and the light receiving optical system 103 are paired and arranged alternately in a staggered manner along the main scanning direction.

[0078] <Fringe pitch and refractive index> The inspection object has a specific refractive index. In the present invention, since it corresponds to the applicable range of paraxial optics when the incident angle is extremely small, it is considered within the range of the paraxial optics. First, the refraction angle θ' of the light ray incident on the inspection object becomes smaller than the incident angle θ. Also, regarding the wavelength, assuming the wavelength in the medium with refractive index N' is λ', the wavelength λ' = λ / N' becomes shorter. λ is the wavelength in vacuum (air). On the other hand, the beam waist interval Wb' becomes narrower than the beam waist interval Wb in vacuum (air) by the amount that the refraction angle θ' becomes smaller, and Wb' = Wb / N'. Therefore, the fringe pitch of the "inclined fringe" is DF = L·λ' / Wb' = L·(λ / N') / (Wb / N'), and ultimately, the fringe pitch DF of the "inclined fringe" may be set as the fringe pitch in air, which is DF = L·λ / Wb. However, since it is the case when L = L' is set, inside the inspection object, considering the elongation of L' = L·N', the position of the "layer" and the position of the depth of field of the light receiving optical system are determined. The elongation may be corrected using the inspection object sample.

[0079] <DFT (FFT) processing> For the number of samplings N and the sampling frequency fn, if the frequency resolution is Δf, there is the relationship of the following formula (1).

Equation

[0080] Next, DFT (FFT) processing is performed on the acquired signal for each unit cuboid. The DFT (FFT) processed Doppler signal becomes a frequency spectrum, and the peak values ​​of the spectrum are calculated by separating them according to the frequency range containing each predetermined "layer". Since the depth of each "layer" is known, the peak values ​​of the spectrum are corrected for the optical power density in the direction of the optical axis of the "gradient fringe". That is, the corrected peak value is the density or transmittance in the depth direction that is finally obtained, and by combining the 2D coordinates of the main scanning direction and sub-scanning direction with the coordinates in the depth direction and plotting the density (transmittance) data on a 3D coordinate system, a 3D tomographic image or 3D picture can be obtained.

[0081] Thus, in this embodiment, the intensity of the frequency spectrum obtained by DFT (FFT) processing, the coordinates on the optical axis of the "layer" in the tilt fringe from which the frequency spectrum was obtained, and the coordinates in the main scanning direction and sub-scanning direction are used to plot the frequency spectrum data corrected by the optical power density in the optical axis direction of the tilt fringe onto a 3D coordinate system to obtain 3D image data. From the obtained 3D tomographic image, or from the 3D image, a 3D video (3D tomographic image) in which the image changes over time may be obtained.

[0082] To restore the original signal and avoid aliasing noise, Nyquist's sampling theorem requires that the aforementioned fs and the Nyquist frequency fn satisfy the relationship shown in equation (14). Furthermore, it is generally known that in frequency analysis equipment, the original signal must satisfy equation (15), which is more strictly limited than equation (14).

number

number

[0083] <Pseudo-colorization> Furthermore, by using a 3-line sensor equipped with near-infrared filters for 3 wavelengths in a CMOS sensor, the near-infrared light can be appropriately converted to RGB and obtained as a pseudo-color image for each near-infrared wavelength. Figure 19 shows a focusing optical system 102 in which light from three types of near-infrared wavelength light sources 101 is made into a parallel beam by a collimator lens 125, then split by a beam splitter 126, and intersected upstream of the beam waist by a cylindrical lens 122, and which has a common part where the three wavelengths are superimposed. The darkly filled area in the plan view is the common part of the three wavelengths. Figures 20A and 20B show the beam splitter 126 in detail. In this example, the beam splitter 126 includes a wedge prism 126a and a beam stop 126b. In Figure 20A, the beam is widened in front of the cylindrical lens 122, and the beam intersection is formed downstream of the focal point specific to the cylindrical lens 122. In Figure 20B, conversely, the beam is narrowed before the cylindrical lens 122, forming a beam intersection upstream of the focal point specific to the cylindrical lens 122.

[0084] The wedge prism 126a is an optical element positioned symmetrically with respect to the optical axis on the upstream side in the optical axis direction of the cylindrical lens 122 included in the focusing optical system 102, has power in the sub-scanning direction, and is formed in a wedge shape when viewed in cross-section in the main scanning direction. However, the wedge prism 126a may be positioned not only on the upstream side in the optical axis direction of the cylindrical lens 122, but also on the downstream side.

[0085] <Increasing the output power of the light source> Although the aforementioned light source 101 was described with a CW laser in mind, efficient light reception can be achieved and the output power of the light source 101 itself can be increased by using PWM control to illuminate only for the accumulation time of the photodetector array 131. In other words, the Doppler signal may be frequency modulated by directly modulating the light source 101. Specifically, it is preferable that the frequency used to modulate the light source 101 is smaller than the clock frequency that drives one element of the photodetector array 131 which includes multiple photodetectors, that the accumulation time of the photodetector array 131 and the pulse illumination time of the light source 101 are synchronized, and that the pulse illumination time is within the accumulation time of the photodetector array 131.

[0086] <Improvement of signal-to-noise ratio through frequency modulation> Conventionally, in LDV and laser Doppler blood flowmeters, frequency shifters (acousto-optic modulators (AOMs) are often used) are utilized to modulate the Doppler signal at high frequencies of several tens of MHz, thereby improving the signal-to-noise ratio (S / N) and capturing the movement of internal structures within the object being examined. This is particularly effective in blood flowmeters, which are multi-particle systems, for improving the S / N. In the present invention, using a frequency shifter is also effective for improving the S / N. In LDV, generally, the beam emitted from the light source (laser) is split into two, one beam is Bragg-diffracted at the frequency f1 of the AOM, and the other beam is Bragg-diffracted at the frequency f2. At the beam intersection, the difference frequency Δfd is Δfd = |f1 - f2|, the fringe at the intersection moves, and a Doppler signal modulated at high frequency Δfd is observed. This method is used when the AOM needs to be driven near its original driving frequency, but the modulation frequency is to be lower than the driving frequency. In addition to the above, the S / N ratio can be improved by employing a system in which the Doppler signal is observed using a signal that is directly modulated from light from a light source as the carrier wave. Figure 24 shows a Doppler signal with high-frequency modulation and the original Doppler signal (the dashed line indicates a bandpass filter).

[0087] The above demonstrates how to obtain a 3D image of an object being inspected by processing the Doppler signal obtained from a "tilted fringe" using DFT (FFT). However, in factory line inspection equipment, it is not always necessary to observe the 3D structure of foreign objects or defects. Normally, the presence or absence of foreign objects or defects is determined, and only when it is determined that foreign objects or defects have been detected does it suffice to create a 3D image from the accumulated data and identify the type of foreign object or defect.

[0088] In the differential heterodyne detection method of the present invention, a "tilted fringe" is formed near the beam waist, and forward scattered light is received, making it possible to inspect objects with greater thickness (depth) compared to conventional OCT. Furthermore, by providing multiple "layers" that generate different frequencies in the optical axis direction (thickness direction or depth direction) from the "tilted fringe" portion, a highly visible Doppler signal is obtained through heterodyne detection that can detect only the straight-traveling light component. Combined with the small solid angle NA of the telecentric optical system, a higher signal-to-noise ratio is achieved, and by arranging multiple photodetector arrays 131 in a staggered pattern, it becomes possible to inspect wide and large objects.

[0089] <Example 1> <Formation of "inclined fringes" and fringe spacing> As mentioned above, the fringe spacing DF in the "inclined fringe" section forms an "inclined fringe" that differs from the fringe spacing formed at the beam waist in conventional LDV (Laser Doppler Velocimeter). Specifically, the beam waist is considered as an opening, the width of the short side of the sheet-like beam waist is Dbw, the light source wavelength λ is λ = 0.8 μm, the distance from the beam waist of the sheet beam to the nearest intersection is Lbn, and the distance to the furthest intersection is Lbf. The intersection angle (half-angle) θc of the sheet beam is set to θc = 1.5° and is used when determining the geometric shape of the intersection.

[0090] First, regarding the beam waist diameter Dbw, if we let f be the focal length of the focusing lens and Dbi be the diameter of the beam incident on the focusing lens, then from equation (3), when Dbi = 1.7 mm and f = 50 mm, Dbw ≈ 30 μm. Therefore, the beam divergence angle (half-angle) θd of the focused beam can be obtained from equation (4) as θd ≈ 0.97°.

[0091] Next, we determine the size of the intersection. Using equation (5), which is well known in Young's diffraction experiments for determining the fringe spacing DF, we set Wb = 0.35 mm, and the intersection angle (half-angle) θc of the two sheet beams to θc = 1.5°. Figure 21A shows a schematic diagram reflecting the values ​​of the above embodiment. The length of the beam intersection in the optical axis direction is 18.9 mm.

[0092] Figure 21B shows the case where the beam intersection is formed on the light source side of the beam waist. The advantage of placing the beam waist on the light source side is that the beam does not directly enter the photodetector lens even if the beam waist spacing is narrowed. In the case of Figure 21B, Wb = 0.1 mm. By narrowing the beam waist spacing, it becomes possible to widen the fringe spacing and increase the size of the photodetector array. Furthermore, it is possible to make the width of the beam intersection longer and make it nearly parallel to the optical axis, which has the advantage of preventing a decrease in power density at the beam intersection.

[0093] <Measurement Department Manager and Depth of Field of the Light-Receiving System> In Figure 21A, for the beam intersection to encompass a unit rectangular parallelepiped, a fringe formation section is required extending from a position Z=4.7mm away from the beam waist to a length of 14.7mm (10mm depth of field). From the beam intersection length of 18.9mm shown in Figure 21A, we select a range where the size of the photodetector satisfies within the depth of field. Lbn=4.7mm, Lbf=14.7mm, and the beam waist spacing Wb is Wb=0.35mm. Therefore, from equation (5), the fringe spacing DF(nbw) on the side of the "inclined fringe" closer to the beam waist is DF(nbw)=37.6μm, and the fringe spacing DF(fbw) on the side further from the beam waist is DF(fbw)=117.6μm. Figure 22 shows the beam diameter and power density relative to the beam waist in the case of Figure 21A. At distances between 4.7 mm and 14.7 mm from the beam waist, the beam can be used with a power density ranging from slightly over 50% to 10% of that at the beam waist.

[0094] In forward-scattered light reception, the scattered light intensity is about three to five orders of magnitude higher than the back-scattered light intensity. Therefore, even if the light intensity is reduced by about 10%, it is still possible to obtain a light intensity (strength, height) of two to four orders of magnitude higher on the photodetector, so there is no problem. Furthermore, in back-scattered light reception, it is possible to compensate for the reduced power by using a high-power light source while taking into account optical damage to the object being inspected.

[0095] Next, we determine the frequency Ff of the "inclined fringe". When the transport speed of the object being inspected is 0.5 m / sec, Ff = 4.25 KHz to 13.3 KHz, so the frequency band ΔFf is ΔFf = 9.05 KHz. Also, the fringe passage time Ft calculated from the fringe frequency is Ft = 75 μsec to 235 μsec.

[0096] <Wavefront of a Gaussian beam> Figure 23 shows the error between the wavefront curvature and optical axis distance of the Gaussian beam. According to equation (8), the Rayleigh range Zr for this embodiment 1 is Zr ≈ 884 μm. As the radius of curvature of the beam wavefront at the intersection, the radii of curvature R(Z=4.7 mm) and R(Z=14.7 mm) at the intersection of a unit rectangular parallelepiped are R(Z=4.7 mm) = 4.87 mm and R(Z=14.7 mm) = 14.75 mm. The error between the radius of curvature and distance closer to the beam waist is 3.5%. For this reason, the relationship between frequency and the "layer" contained in the measurement volume of the unit rectangular parallelepiped is corrected in advance using the method described in <Measurement of Fringe Frequency> above, and the position of the "layer" in the depth of field is accurately determined, and in addition to the 2D data in the main scanning direction and sub-scanning direction, 3D data is obtained.

[0097] As mentioned above, the transport speed of the object to be inspected is set to 0.5 m / sec. Therefore, the fringe frequency Ff observed when the object to be inspected passes over the fringe will be analyzed in the frequency range of Ff = 4.25 KHz to 13.3 KHz, and the bandwidth ΔFf will be ΔFf = 9.05 KHz. Here, the pixel size is determined when observing the fringe frequency. In this embodiment 1, a photodetector array with 4096 3.5 μm square photodetectors in the main scanning direction and 16 lines in the sub-scanning direction is used. CMOS line sensors with high scanning speeds are commercially available at this size. At the same time, it is sufficient to identify the smallest fringe spacing of the 37.6 μm "gradient fringe". Then, for the DFT processing described later, a unit pixel of 16 elements and 16 lines is set as one unit when using 3.5 μm square photodetectors. Here, if we consider a cube of the same size as one unit in the depth direction (optical axis direction) of the object being inspected, it becomes possible to obtain signals of different frequencies from 178 layers of inclined fringes within a depth of field of 10 mm. Next, we consider what frequency resolution should be set to for a depth of 56 μm × 178 layers ≈ 10 mm while the object being inspected moves 56 μm in the sub-scanning direction, and how many samples are required in that case.

[0098] <Changes in Doppler signal frequency along the optical axis due to tilted fringes and sampling frequency: It is necessary to be able to perform frequency analysis of a single signal while simultaneously discriminating between adjacent "layers"> When the measurement area with the fringe formed is projected onto the photodetector array, the pixel size area of ​​the photodetector array is 56 μm. 2 The number of fringes projected ranges from one to five. These projected fringes traverse within a single pixel of the photodetector array, causing output variations in each "layer" corresponding to the fringe spacing. Similar to the sub-scanning direction, 16 elements (equivalent to one pixel in the main scanning direction) are treated as a set, and oversampling is performed 16 times using a 16-parallel processing photodetector array (256 elements × 16 sets). Since the transport speed is 0.5 m / sec, the time required to pass a 3.5 μm square pixel in the sub-scanning direction is 7 μsec, which translates to a frequency of 143 kHz. Commercially available photodetector arrays have elements with line rates of 200 kHz or higher, making 143 kHz × 16 parallel processing entirely feasible.

[0099] 16 parallel processing is equivalent to a line rate of 2.29 MHz for a single-line sensor. Therefore, 4096 / 16 = 256 pixels are used as one set, and a total of 16 sets are used to output one reading line in parallel.

[0100] <The ability to distinguish signals originating from adjacent "layers" within the same frequency band.> To determine the sampling frequency Fs in this embodiment 1, we consider the frequency resolution required to distinguish between adjacent "layers". Due to the number of "layers" in this embodiment 1 and the fringe of the "gradient fringe" forming section, the Doppler signal detected at a transport speed of 0.5 m / sec is observed from a minimum value of Ff(min) = 4.25 KHz to a maximum value of Ff(max) = 13.3 KHz. Since the maximum fringe frequency Ff(max) = 13.3 KHz, the sampling frequency Fs required by Nyquist's sampling theorem can be obtained from equation (15) as Fs = 2.56·Ff(max) ≈ 34 KHz. Using this value, we again calculate the frequency resolution Δf(N=4096), which is Δf(N=4096) = 34 KHz / 4096 ≈ 8.3 Hz. On the other hand, the fringe frequency bandwidth ΔFf is ΔFf = 9.1 kHz. Dividing this by 178 layers gives ΔFf / 178 = 51 Hz, resulting in Δf(N=4096) = 8.3 Hz, which is 6 data points per layer, making it sufficiently distinguishable.

[0101] Then, the 4096 samples are processed using DFT (FFT) according to equation (1) to convert N data points in the time domain into N frequency data points in the frequency domain. This yields the frequency spectrum, i.e., density or transmittance, of each of the 178 layers within the "gradient fringe" of the unit cuboid. These are then combined with a pair of 2D data points in the main scanning direction and sub-scanning direction to form 3D data, thus completing a set of 3D data points for the unit cuboid.

[0102] By repeating this operation in both the main and sub-scanning directions, all 3D data of the entire object being inspected can be obtained, and therefore, the 3D data of the target object can be converted into a 3D image.

[0103] The above assumes N=4096 data points, but we will also consider the case where N=1024 in terms of frequency resolution. Substituting the sampling frequency Fs=34KHz and N=1024 into equation (13), we get Δf(N=1024)=33Hz, which is slightly smaller than the frequency interval of the "layer" of 51Hz obtained from the fringe frequency bandwidth ΔFf=9.1KHz, and satisfies the frequency resolution during detection. Therefore, N=1024 also satisfies the sampling number. The N=1024 considered here represents the case where, in the rectangle enclosed by the smaller frame of the unit cuboid shown in Figure 4B, the number of oversamplings is Nos=16, the number of pixels in the main scanning direction of the photodetector array included in the minimum unit pixel is Nmr=8, and the number of lines in the photodetector array in the subscanning direction is Nl=8. In this case, the unit rectangular prism becomes 28 μm square, and the resolution of the "layer" in the optical axis direction does not change, but the pixel density doubles in the main scanning direction and sub-scanning direction.

[0104] <Example 2> When the transport speed is 1 m / sec, the fringe frequency Ff(1 m / sec) obtained from the "inclined fringe" is twice the above, 8.5 KHz ≤ Ff ≤ 26.6 KHz, and the bandwidth ΔFf(1 m / sec) is ΔFf(1 m / sec) = 18.1 KHz. Also, the time required to pass through a photodetector with a size of 3.5 μm is 3.5 μsec, which, when converted to frequency, is 285.7 KHz. In this case, the number of parallel processes can be increased to 32 parallel processes (128 photodetectors as one set), or oversampling can be performed 8 times, so Nos × Nmr × Nsl = 8 × 16 × 16 = 2048, i.e., N = 2048. If the thickness of the "layer" of the unit rectangular parallelepiped is the same as in <Example 1>, which is 56 μm, then there are 178 "layers". To distinguish the signals of adjacent "layers", when N=2048, the maximum fringe frequency Ff is Ff(max)=26.6KHz, so Fs=2.56·Ff(max)=68.1KHz. On the other hand, the frequency resolution ΔFs is ΔFs=68.1KHz / 2048=33.3Hz. Therefore, 3 data can be assigned to the frequency interval of 101Hz between "layers" at 1m / s, making it possible to distinguish the signals of adjacent "layers".

[0105] <Example 3> In industrial applications, a spatial resolution of 500 μm is acceptable in some fields, such as the detection of foreign objects inside food. In this embodiment 3, the spatial resolution of a unit rectangular parallelepiped is set to 338.4 μm (8 pixels, 8 lines), and an 8-line CMOS sensor is used. The CMOS line sensor has a photodetector array of 400 pixels per line, equivalent to 600 dpi (42.3 μm square). Scanning is performed independently for each line, and the line rate is 125 kHz. That is, the scanning time is 8 μs / line. The transport speed is 0.5 m / s. Therefore, the time required to pass through the sub-scanning direction of the photodetector is 84.6 μs. Thus, oversampling is possible up to 10 times per line. In this embodiment 3, Nos = 8. The number of photodetectors contained in the cross-sections of a 338.4 μm unit rectangular parallelepiped in the main scanning direction and sub-scanning direction is 338.4 / 42.3 = 8 elements in the main scanning direction. Therefore, if there are 8 lines in the sub-scanning direction, N = Nos(8) × Nmr(8) × Nsl(8) = 512. In this example, the thickness of the "layer" in the optical axis direction is 8 elements × 42.3 = 338.4 μm, so it is sufficient to be able to distinguish frequency signals from 30 layers for a depth of field of 10 mm.

[0106] The parameters in Figure 21B are the same as in Figure 21A, except for the beam waist spacing, which is Wb = 0.1 mm. The detected Doppler signal frequencies are DF(4.7 mm) = 75.2 μm when the fringe spacing of the "tilted fringe" is 4.7 mm away from the beam waist, and DF(14.7 mm) = 235.2 μm when the fringe spacing of the "tilted fringe" is 14.7 mm away from the beam waist. Therefore, the fringe spacing is larger than the size of the photodetector array (42.3 μm), and the fringes are sufficiently distinguishable. Furthermore, the beam divergence angle θd is set to θd = 0.97° with a beam waist diameter Dbw of Dbw = 30 μm. In addition, the difference between the beam crossing half-angle θc (= 1.5°) and the beam divergence angle is reduced. This allows the length of the optical axis direction at the crossing point to be increased. The length of the intersection is 13.1 mm (assuming a beam waist spacing Dbw of 0.1 mm), and the width of the intersection is 338.4 μm, which is sufficient to ensure a depth of field that includes the width of a unit rectangular parallelepiped in the sub-scanning direction. The number of fringes is 1 to 4. Next, the transport speed is 0.5 m / s, and signals with Ff = 2.1 KHz to 6.7 KHz and ΔFf = 4.6 KHz are obtained. Since the maximum frequency F(Doppler_max) is F(Doppler_max) = 6.7 KHz, it is sufficient if the sampling frequency is F(Doppler_max2.56) = 17.1 KHz or higher, which is obtained by multiplying this by 2.56. A line rate of 125 KHz fully satisfies the above sampling frequency requirement. Therefore, from N=512, a frequency resolution ΔFs(Doppler_max)=34Hz is obtained, and since there are 30 "layers", and the frequency bandwidth of the Doppler signal is 3.6KHz, 3.6KHz / 30 is 153Hz. Thus, the above frequency resolution is obtained from 4 or more data points for each "layer", which fully satisfies the discrimination resolution.

[0107] Aligning the optical axis of a transmissive optical system has generally been considered difficult (particularly noticeable in confocal systems). The present invention addresses the problem of optical axis alignment inherent in transmissive optical systems. In the forward scattering method shown in Figures 14A, 14B, and 14C, a translation mechanism and a tilt adjustment mechanism may be provided so that the light-receiving optical system enclosed by the dashed line can be translated in the sub-scanning direction and the optical axis direction, and its tilt can be adjusted in the main scanning direction. In this case, the sample of the object to be inspected can be transported, and the optical axis can be adjusted using the translation mechanism and tilt adjustment mechanism in the light-receiving optical system to maximize the output of the light-receiving element array.

[0108] <Example 1> In the differential heterodyne type inspection device described above, a "gradient fringe" is formed, and the Doppler signals of the "layers" contained in the "gradient fringe" generate different frequencies in each "layer". By processing these Doppler signals with DFT (Discrete Fourier Transform), the frequency spectrum is obtained, and the density or transmittance of each "layer" is determined. Figure 25 shows the case where the light source 101 is an SS (Swept_Source), which is the light source currently used in SS_OCT. The light source 101, consisting of an SS light source, is a wavelength-swept coherent light source.

[0109] Figure 25 and equation (5) show that the fringe spacing DF of the tilted fringe changes. By acquiring Doppler signals for each wavelength in all "layers" in each unit cuboid, obtaining the frequency spectrum of each "layer" by DFT processing, converting it to density or transmittance, and repeating the above process by sweeping the wavelengths successively, more detailed information about biological tissue can be obtained. Furthermore, since the fringe spacing changes even within the Raylegh range, if there is a difference in the radius of curvature required to identify the "layers" even in the beam intersection region near the beam waist where the optical power density is higher, it may be brought closer to the beam waist. Note that in the case of Modification 1, the light source 101 consisting of an SS light source is a light source with a long coherence length, and SS (Swept_Source) with a long coherence length is already commercially available.

[0110] <Modification 2> Figure 26 shows a focusing optical system 102 in which the angle of the incident beam to the cylindrical lens 122 is varied using an AOD (Acousto-Optical Deflector) 127. Light emitted from the light source 101 is collimated so that the cross-section of the light beam relative to the optical axis is approximately circular, then split into two by a beam splitter 121 and incident on a pair of AODs 127. These AODs 127 scan the emitted light in a plane including the sub-scanning direction and the optical axis, and synchronize the scanning of the pair of AODs 127 so that the two split beams intersect at the same position on the optical axis, thereby adjusting the deflection angle.

[0111] In the wedge prism 126a shown in Figures 20A and 20B, the angle of incidence can be changed, but the angle is fixed. By using AOD127, the angle of incidence can be changed at high speed, and therefore the position of the "tilted fringe" in the direction of the optical axis can be changed at high speed. Therefore, a telecentric optical system with a long depth of field can be adopted for the light-receiving optical system 103. In other words, a longer measuring section can be formed.

[0112] As an example, we use the AOD127, which has a maximum deflection angle of 12 mrad (at: λ=800 mm). An angle of ±6 mrad becomes the intersection angle θc = 26 mrad ± 2 mrad (1.5° ± 0.34°) obtained from the original lens's focal length. From this, the intersection position on the optical axis becomes 45 mm (-6 mrad) - 50 mm - 64 mm (+6 mrad), and the intersection moves 19 mm along the optical axis. The length of the unit cuboid at the intersection was equal to the depth of field, which was 10 mm, but a measuring section twice the length can be formed, and therefore the depth of field of the light-receiving optical system 103 can also be doubled. To double the depth of field, as an example, the focal length fo of the upstream light-receiving lens 132 of the light-receiving optical system 103 should be set to fo = 100 mm, and the focal length fr of the downstream light-receiving lens 132 should be set to fr = 50 mm.

[0113] When the reduction ratio is 1 / 2, the fringe spacing projected onto the pixels of the photodetector becomes 1 / 2. Therefore, the fringe spacing of the aforementioned "inclined fringe" on the pixels of the photodetector array 131 is DF(nbw) = 37.6 / 2 = 18.8 μm, and the fringe spacing DF(fbw) on the side farther from the beam waist is DF(fbw) = 117.6 / 2 = 58.8 μm. With the aforementioned 3.5 μm square photodetector array 131, the fringe can be sufficiently distinguished. Furthermore, the light beam incident on the AOD127 is preferably circular, and the beam diameter Φb is preferably Φb ≤ 2 mm. Therefore, the light beam emitted from the light source 101 is first collimated into a circular cross-section, then split into two by a beam splitter, then incident on the AOD127 and subjected to Bragg diffraction, and then appropriately formed into a sheet beam with a cylindrical lens pair and simultaneously collimated.

[0114] <Variation 3> When the beam waist is located inside the object being inspected, the power density of the beam waist is maximized. When attempting to increase the power density at the beam intersection, the object being inspected may be damaged. To avoid damage, the beam waist is positioned outside the object being inspected. Figures 27A and 27B show examples where the beam waist formed by light emitted from the light source 101 is located outside the object being inspected. Figure 27A shows the case where the beam intersection is located downstream of the beam waist, and Figure 27B shows the case where the beam intersection is located upstream of the beam waist. By positioning the beam waist outside the object being inspected, it becomes possible to increase the power density at the beam intersection to the point where the object being inspected will not be damaged.

[0115] <Modification 4> Figure 28 shows a method that allows the beam intersection to move in the sub-scanning direction, enabling inspection even when the object being inspected is stationary. The light beam emitted from the light source is split into two, and one AOD127 is placed for each beam, deflecting each beam to the same angle. By deflecting them to the same angle, the beam intersection moves in the sub-scanning direction.

[0116] <Modification 5> Figure 29 shows a case where multiple light sources 101 are arranged as an array in the sub-scanning direction, allowing application to a stationary object to be inspected. A first collimator lens 125a and a second collimator lens 125b are positioned between the array of light sources 101 and the beam splitter 126. By using multiple light sources 101 arranged in the sub-scanning direction, which is approximately perpendicular to the main scanning direction and the optical axis, the beam intersection can be moved in the sub-scanning direction. The fringe spacing measurement method described above can be used directly for inspecting a stationary object to be inspected. That is, by placing the object to be inspected in the same position instead of the wire substrate used to measure the fringe spacing, and moving the beam intersection in the sub-scanning direction, it becomes possible to inspect the object to be inspected itself. This method is suitable for inspecting relatively small objects to be inspected when there is no transport system.

[0117] A wedge prism 126a is positioned symmetrically with respect to the optical axis on the upstream side in the optical axis direction from the cylindrical lens 122 included in the focusing optical system 102. The wedge prism 126a is an optical element that has power in the sub-scanning direction and is formed in a wedge shape when viewed in cross-section in the main scanning direction. Although not shown in Figure 29, the photodetector array 131 used has a size in the sub-scanning direction that is smaller than the fringe spacing in the inclined fringe. The frequency spectrum can be obtained from the signal after A / D conversion of the signal output by each photodetector in the photodetector array 131 using DFT (FFT) processing. However, the wedge prism 126a may be positioned downstream of the cylindrical lens 122 in the optical axis direction, not just upstream.

[0118] <Variation 6> Figures 30A, 30B, and 30C show schematic diagrams of a case where an MCP (microchannel plate; hereinafter referred to as MCP) 134 is placed upstream of the photodetector array 131. The configuration is the same as in Figures 14A, 14B, and 14C, except for the placement of the MCP 134. In the present invention, since the light beam incident on the MCP 134 is a straight-traveling light component, the incident surface of the MCP 134 is tilted with respect to the optical axis in order to further increase the Balanche effect on the photocathode of the MCP 134. In addition, a fluorescent substance (not shown) is coated on the light-receiving surface of the photodetector array 131, and by converting electrons emitted from the MCP 134 into photons, the photodetector array 131 can receive more photons than before they were incident on the MCP 134. In this configuration, the MCP134 is positioned inclined in front of the photodetector with respect to the main scanning direction or the sub-scanning direction, a fluorescent material is applied to the light-receiving surface of the photodetector, and when electrons emitted from the MCP134 strike the fluorescent material, secondary photons are generated and strike the light-receiving surface of the photodetector. Furthermore, it is preferable that the aperture size of the MCP134 is smaller than the pixel dimensions of the photodetector array 131, or smaller than the size of the cross-section perpendicular to the optical axis of the unit rectangular parallelepiped.

[0119] When the MCP134 is tilted by 10 degrees, the electron exit port moves 176 μm relative to the entrance port, assuming the thickness of the MCP134 is 1 mm (two-stage MCP134). Therefore, it is preferable to pre-position the photodetector array 131 offset by 176 μm with respect to the optical axis of the photodetector lens 132. However, this does not apply if the photodetector array 131 in the main scanning direction is sufficiently long to satisfy the amount of movement, or if there are many lines in the sub-scanning direction. By using the MCP134 as described above, the amplification factor becomes approximately 1 million to 10 million times or more, and even when receiving backscattered light, output from light-scattered light equivalent to or better than when receiving forward-scattered light without using the MCP134 can be obtained. When receiving forward-scattered light, M=10 in equation (12) 6 This means the measurement thickness of the object being inspected can be set to 30 mm or more. Figure 31 shows M=10 6 and M=10 7 The following shows the case compared to M=1. In both cases, ρ=0.5mm -1 That is the case.

[0120] <Arrangement of light-receiving lenses> Figures 32A to 32E show an example of the arrangement of multiple light-receiving lenses 132. In Figures 32A to 32E, the X direction is the main scanning direction, and the Y direction is the sub-scanning direction. The Z direction is orthogonal to the X and Y directions. The light-receiving optical system 103 comprises multiple light-receiving lenses 132 arranged along the main scanning direction. The light-receiving element array 131 comprises multiple light-receiving elements arranged in a line along the main scanning direction, and each light-receiving element receives light that has passed through the multiple light-receiving lenses 132. In the example in Figures 32A to 32E, the multiple light-receiving lenses 132 are spaced apart from each other by more than the diameter of each light-receiving lens 132, and the optical axis of the light-gathering optical system 102 and the optical axis of the light-receiving optical system 103 coincide. In addition, the multiple light-receiving elements provided in the light-receiving element array 131 form at least one row or more reading lines L.

[0121] In the examples of Figures 32A and 32E, the multiple light-receiving lenses 132 are spaced apart from each other at a distance of less than or equal to the field of view 135 of each light-receiving lens 132. In the examples of Figures 32C and 32D, the multiple light-receiving lenses 132 are spaced apart from each other at a distance greater than or equal to the field of view 135 of each light-receiving lens 132. In the example of Figure 32B, a pair of adjacent light-receiving lenses 132 in the main scanning direction are spaced apart from each other at a distance of less than or equal to the field of view 135 of each light-receiving lens 132, while a pair of light-receiving lenses 132 are spaced apart from each other at a distance greater than or equal to the field of view 135 of each light-receiving lens 132. In the example of Figure 32A, multiple light-receiving elements are arranged in a single array to form a single light-receiving element array 131, forming a single reading line L, whereas in the examples of Figures 32B to 32E, the multiple light-receiving elements form at least two or more reading lines L.

[0122] More specifically, in the example shown in Figure 32E, multiple photodetectors are arranged in two or more rows to form multiple photodetector arrays 131, and each of the multiple photodetector arrays 131 is spaced further apart from each other than the diameter of the photodetector lens 132 in a direction perpendicular to the reading line L, and also spaced further apart from each other than the dimensions of the field of view 135 of the photodetector lens 132. On the other hand, in the examples shown in Figures 32B to 32D, multiple photodetectors are arranged in two or more rows to form multiple photodetector arrays 131, and each of the multiple photodetector arrays 131 is spaced further apart from each other than the dimensions of the field of view 135 of the photodetector lens 132 in a direction perpendicular to the reading line L. In particular, in the example shown in Figure 32C, the number of photodetector lenses 132 corresponds to the number of photodetector arrays 131, and the optical axis of each photodetector lens 132 passes through approximately the center of each photodetector array 131.

[0123] Furthermore, in the examples of Figures 32B, 32C, and 32E, the multiple photodetector arrays 131 are shorter than each of the two reading lines L, and the photodetector arrays 131 located on one reading line L and the photodetector arrays 131 located on the other reading line L are arranged alternately in a staggered pattern along the main scanning direction. In particular, in the example of Figure 32E, the multiple photodetector lenses 132 are arranged in a single row parallel to the multiple photodetector arrays 131 between the two reading lines L, and the optical axis of each photodetector lens 132 passes through approximately the center in the sub-scanning direction between the two reading lines L. [Explanation of Symbols]

[0124] 101 Light source 102 Focusing Optical System 103 Light receiving optical system 121 Beam Splitter 122 Cylindrical Lens 123 Wedge Prism 124 Prisms 125 Collimator Lens 126 Beam Splitter 126a Wedge Prism 126b Beam Stop 127 AOD (acousto-optic deflector) 131 Photodetector array 132 Light-receiving lens 133 Aperture 134 MCP (Microchannel Plate) 135 Field of view L reading line

Claims

1. Coherent light sources as light sources, A focusing optical system that splits the light emitted from the coherent light source into two, intersects the split light to form an intersection, and forms an inclined fringe at the intersection that is inclined with respect to the optical axis of the light from the coherent light source, The system includes a light-receiving optical system that receives the forward-scattered light or the straight-traveling light component of the back-scattered light generated at the aforementioned inclined fringe, The inspection apparatus is characterized in that the light-receiving optical system receives a Doppler signal that does not contain scattered light components using a light-receiving element positioned at the focal point of the light-receiving optical system.

2. The inspection apparatus according to claim 1, characterized in that it decomposes the frequency of the Doppler signal generated by the tilt fringe into arbitrary layers, and corresponds the frequency of the Doppler signal generated from each layer to the respective layer, and detects the density or transmittance of each layer divided along the optical axis direction of the object to be inspected.

3. The light-receiving optical system comprises a light-receiving element array in which the light-receiving elements are arranged in a direction perpendicular to the optical axis, The inspection apparatus according to claim 2, characterized in that the longitudinal direction of the light-receiving element array is the main scanning direction, the sub-scanning direction perpendicular to the main scanning direction and the optical axis is the transport direction of the object to be inspected, and three-dimensional data is constructed from signals detected at arbitrary positions in the optical axis direction separated by the main scanning direction, the sub-scanning direction and the layer.

4. The inspection apparatus according to claim 1, characterized in that the inclined fringe is formed by generating cylindrical waves or spherical waves at the intersection where light intersects outside the beam waist.

5. The inspection apparatus according to claim 1, characterized in that the inclined fringe is formed by a combination of cylindrical waves and plane waves, or a combination of spherical waves and plane waves, or by intersecting wavefronts other than plane waves, cylindrical waves, and spherical waves.

6. The inspection apparatus according to claim 1, wherein the light-gathering optical system comprises a light-gathering lens and an optical element having power in a direction perpendicular to the plane including the optical axis, and the intersection portion is formed by intersecting light at a position on the optical axis other than the intrinsic focal position of the light-gathering lens.

7. The light-receiving optical system comprises a plurality of light-receiving lenses arranged along the main scanning direction, and a plurality of light-receiving elements arranged in a line along the main scanning direction, which receive light that has passed through the plurality of light-receiving lenses. The inspection apparatus according to claim 1, characterized in that the plurality of light-receiving lenses are spaced apart from each other at a distance greater than the diameter of the light-receiving lenses, the optical axis of the light-collecting optical system and the optical axis of the light-receiving optical system coincide, and the plurality of light-receiving elements form at least one row or more reading lines.

8. The inspection apparatus according to claim 7, characterized in that the plurality of light-receiving lenses are spaced apart from each other at a distance of less than or equal to the field of view dimension of the light-receiving lens.

9. The plurality of light-receiving lenses are arranged to be spaced further apart from each other than the field of view dimension of the light-receiving lens. The inspection apparatus according to claim 7, characterized in that the plurality of light-receiving elements form at least two rows or more of the reading lines.

10. The aforementioned plurality of light-receiving elements are arranged in a single row in an array to form a single light-receiving element array. The inspection apparatus according to claim 7, characterized in that the plurality of light-receiving lenses are spaced apart from each other at a distance greater than or equal to the diameter of the light-receiving lens, and spaced apart from each other at a distance less than or equal to the field of view dimension of the light-receiving lens.

11. The inspection apparatus according to claim 7, wherein the plurality of light-receiving elements are arranged in two or more rows to form a plurality of light-receiving element arrays, and each of the plurality of light-receiving element arrays is spaced apart from each other in a direction perpendicular to the reading line, by a distance greater than or equal to the diameter of the light-receiving lens, and spaced less than or equal to the field of view of the light-receiving lens.

12. The inspection apparatus according to claim 7, wherein the plurality of light-receiving elements are arranged in two or more rows to form a plurality of light-receiving element arrays, and each of the plurality of light-receiving element arrays is spaced apart from each other in a direction perpendicular to the reading line, to a distance greater than the field of view dimension of the light-receiving lens.

13. The inspection apparatus according to claim 11 or 12, characterized in that the plurality of light-receiving lenses are arranged in a number corresponding to the plurality of light-receiving element arrays, and the optical axis of each light-receiving lens penetrates approximately the center of each light-receiving element array.

14. The inspection apparatus according to claim 11 or 12, characterized in that the plurality of light-receiving element arrays are light-receiving element arrays shorter than each reading line, which are arranged in a plurality on each of the two rows of reading lines, and the light-receiving element arrays arranged on one reading line and the light-receiving element arrays arranged on the other reading line are arranged alternately in a staggered pattern along the main scanning direction.

15. The inspection apparatus according to claim 14, characterized in that the plurality of light-receiving lenses are arranged in a single row parallel to the plurality of light-receiving element arrays between the two rows of reading lines, and the optical axis of each light-receiving lens passes through approximately the center in the sub-scanning direction between the two rows of reading lines.

16. The coherent light source is arranged parallel to the reading line, the optical axis of the coherent light source substantially coincides with the optical axis of the light receiving lens, and the light emitted from the coherent light source is branched after collimation. The light-gathering optical system comprises a light-gathering lens having power only in the sub-scanning direction so as to form the inclined fringe in a direction substantially perpendicular to the sub-scanning direction, and an optical element that separately inclines each of the branched light beams with respect to the sub-scanning direction so as to form the intersection at a position other than the intrinsic focal point of the light-gathering lens, The inspection apparatus according to any one of claims 10 to 12, characterized in that the light-collecting optical system and the light-receiving optical system form a pair and are arranged alternately in a staggered pattern along the main scanning direction.

17. The inspection apparatus according to claim 1, characterized in that the coherent light source uses a laser including an LD having linear polarization.

18. The inspection apparatus according to claim 1, characterized in that the light source is a wavelength-swept coherent light source.

19. The inspection apparatus according to claim 1, characterized in that the cross-section of the light beam emitted from the coherent light source is collimated into a substantially circular shape in the direction of the optical axis, then split into two by a beam splitter and incident on a pair of acousto-optic deflectors, the emitted light is scanned in a plane including the sub-scanning direction and the optical axis, and the scanning of the pair of acousto-optic deflectors is synchronized so that the two split beams intersect each other at the same position on the optical axis, thereby adjusting the deflection angle.

20. The inspection apparatus according to claim 1, characterized in that the beam waist formed by the light emitted from the coherent light source is located outside the object to be inspected.

21. The inspection apparatus according to claim 1, characterized in that the intersection portion is movable in the sub-scanning direction by using a plurality of light sources arranged in the main scanning direction and a sub-scanning direction which is substantially perpendicular to the optical axis.

22. The inspection apparatus according to claim 21, further comprising a photodetector array having optical elements arranged symmetrically with respect to the optical axis, either upstream or downstream in the optical axis direction with respect to the condensing lens included in the condensing optical system, having power in the sub-scanning direction and being formed in a wedge shape in a cross-sectional view in the main scanning direction, and the size in the sub-scanning direction being smaller than the fringe spacing in the inclined fringe.

23. The inspection apparatus according to claim 21, comprising a photodetector array whose size in the sub-scanning direction is smaller than the fringe spacing in the inclined fringe, and wherein the frequency spectrum is obtained from the signal after A / D conversion of the signal output by the photodetector using a discrete Fourier transform.

24. The inspection apparatus according to claim 1, characterized in that it corrects the spectral intensity obtained by the discrete Fourier transform of each layer of the object to be inspected using the optical attenuation rate obtained by performing a discrete Fourier transform on the Doppler signals of each layer of a plurality of reference media in advance.

25. The inspection apparatus according to claim 1, characterized in that a microchannel plate is positioned inclined in front of the light-receiving element with respect to the main scanning direction or the sub-scanning direction, a fluorescent material is coated on the light-receiving surface of the light-receiving element, and when electrons emitted from the microchannel plate are incident on the fluorescent material, secondary photons are generated and incident on the light-receiving surface of the light-receiving element.

26. The inspection device is provided as described in claim 1, A three-dimensional tomography apparatus characterized in that, in the region where the inclined fringe is formed, a plurality of the photodetectors are treated as pixel units in the main scanning direction and the sub-scanning direction, a rectangular parallelepiped consisting of the pixel units and the number of layers in the optical axis direction in the region where the inclined fringe is formed is treated as a unit rectangular parallelepiped, the frequency spectrum of each layer contained in the unit rectangular parallelepiped is obtained from the Doppler signal generated in the inclined fringe within the unit rectangular parallelepiped by discrete Fourier transform, the density or transmittance of each coordinate in the main scanning direction, sub-scanning direction and optical axis direction of the object to be inspected at the intersection is calculated based on the frequency spectrum, and a three-dimensional tomography image is obtained from these three-dimensional coordinate values.

27. The three-dimensional tomography apparatus according to claim 26, characterized in that a three-dimensional tomographic image is obtained from the three-dimensional tomographic image.

28. An optical element for use in the inspection apparatus described in claim 1, characterized in that it is arranged symmetrically with respect to the optical axis on the upstream side in the optical axis direction with respect to the focusing lens included in the focusing optical system of the inspection apparatus, has power in the sub-scanning direction, and is formed in a wedge shape in a cross-sectional view in the main scanning direction.

29. An optical element for use in the inspection apparatus described in claim 1, characterized in that it is arranged symmetrically with respect to the optical axis downstream of the focusing lens included in the focusing optical system of the inspection apparatus, has power in the sub-scanning direction, and is formed in a wedge shape in a cross-sectional view in the main scanning direction.

30. A light-receiving element for use in the inspection apparatus described in claim 1, characterized in that the size of the light-receiving element in the sub-scanning direction is smaller than the fringe spacing in the inclined fringe.

31. A photodetector array for use in the inspection apparatus described in claim 1, characterized in that the size of the photodetector array in the sub-scanning direction is smaller than the fringe spacing in the inclined fringe.

32. A signal processing method for an inspection apparatus according to claim 1, characterized in that a frequency spectrum is obtained from a signal obtained by A / D conversion of a signal output by a photodetector using a discrete Fourier transform.

33. When obtaining the frequency spectrum from the signal obtained after A / D conversion of the signal output by the photodetector using the discrete Fourier transform, a unit rectangular parallelepiped is formed within the slope fringe, and this unit rectangular parallelepiped includes a plurality of the photodetectors in the main scanning direction and the sub-scanning direction, as well as all layers within the depth of field. When N is the total number of natural numbers of the digital signals output from the light-receiving elements contained in the unit rectangular parallelepiped and after A / D conversion, Nos is the number of oversamplings of the reading lines of the light-receiving elements, Nmr is the number of light-receiving elements contained in the unit rectangular parallelepiped in the main scanning direction, and Nsl is the number of light-receiving elements or lines contained in the unit rectangular parallelepiped in the sub-scanning direction, N=Nos・Nmr・Nsl The signal processing method according to claim 32, characterized in that it satisfies the condition and N is a power of 2.

34. The signal processing method according to claim 33, characterized in that the number of samples N in the discrete Fourier transform is determined such that the frequencies of adjacent layers can be separated.

35. The signal processing method according to claim 33, characterized in that the intensity of the frequency spectrum obtained by the discrete Fourier transform, the coordinates on the optical axis of the layer in the tilted fringe from which the frequency spectrum was obtained, and the coordinates in the main scanning direction and sub-scanning direction are plotted on a three-dimensional coordinate system to obtain three-dimensional image data by correcting the frequency spectrum data with respect to the optical power density in the optical axis direction of the tilted fringe.

36. The signal processing method according to claim 32, characterized in that a digital filter is applied to the digital signal obtained by A / D conversion of the signal output by the light-receiving element, thereby limiting the signal to a bandwidth containing only the Doppler signal.

37. A method for frequency modulating a Doppler signal by directly modulating the light source in order to perform the signal processing method described in claim 32, characterized in that the frequency for modulating the light source is smaller than the clock frequency for driving one element of a photodetector array including a plurality of photodetectors, the storage time of the photodetector array and the pulse lighting time of the light source are synchronized, and the pulse lighting time is within the storage time of the photodetector array.

38. A light-receiving optical system for use in the inspection apparatus described in claim 1, characterized by comprising a parallel movement mechanism in the sub-scanning direction and the optical axis direction, and a tilt adjustment mechanism in the main scanning direction.

39. A method for adjusting the optical axis of a light-receiving optical system according to claim 38, characterized in that the optical axis is adjusted using a sample of the object to be inspected, and by the parallel movement mechanism and the tilt adjustment mechanism in the light-receiving optical system.

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