Three-dimensional shape measurement method

Abstract

To provide a three-dimensional shape measurement system and method capable of speedily measuring a three-dimensional appearance shape or internal structure of a measured object with high depth resolution.SOLUTION: A three-dimensional shape measurement system includes: a first light source 11 for applying an ultra-short light pulse train Pt having a two-dimensional distribution in which light intensity can be regarded to be uniform; a second light source 12 for outputting an ultra-short light pulse train Ps having a two-dimensional distribution in which a repetition period differs from that of the ultra-short light pulse train Pt for inspection, pulse width is narrower than that of the ultra-short light pulse train Pt, and light intensity can be regarded to be uniform; an interference section 13 for multiplexing an ultra-short pulse train Pt' reflected by a measured object and the ultra-short light pulse train Ps; a two-dimensional photo director array 14 for receiving interference light Pi; and a calculation section 15 for measuring a change in time of a two-dimensional distribution of light intensity and measuring a three-dimensional shape of a measured object for the ultra-short light pulse train Pt' reflected by the measured object from the interference light Pi.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a three-dimensional shape measurement system for measuring the three-dimensional external shape or internal structure of a measurement object, and a method thereof.

Background Art

[0002] With the rise of Industry 4.0, the importance of smart factories is increasing. As part of the realization of such an autonomous production system, there is a process in which machines perform appearance inspections of industrial products, and the use of three-dimensional (3D) shape laser measurement is expanding in this process. In addition, in order to evaluate high-quality and delicate shapes, surfaces, etc., high-definition and highly reliable inspection equipment is required, and such a role is also expected to be played by 3D laser measurement.

[0003] Particularly in the appearance inspection of industrial products that require particularly high measurement accuracy, a contact method using calipers, micrometers, etc. has been conventionally used. However, these methods rely on human labor, involve long measurement times, and are inefficient. Therefore, in recent years, shape measurement has shifted to a non-contact method using ultrasonic waves, high frequencies, light waves, etc. Among these, the measurement method using light waves has the ability to acquire a high-definition 3D shape, and with the increasing academic interest, it has attracted attention in the industry as a technology contributing to the realization of a super smart society.

[0004] Among 3D laser measurements, two-dimensional (flash) LiDAR does not have a movable scanning unit, irradiates the entire object, directly measures the distance corresponding to each pixel by a 2D photodetector array, and outputs a 3D image. Different from other LiDARs, this technology can cover all positions in the measurement range in a single measurement, and overall mapping and environmental recognition can be achieved with each detection. Therefore, it can be expected to quickly perform appearance inspections of parts within the tact time of the production line.

[0005] The aforementioned LiDAR is a technology for inspecting the appearance of objects. However, there is naturally a need to inspect the interior of the object to be measured. OCT is an abbreviation for Optical Coherence Tomography (for example, see Non-Patent Documents 4-8). It is a technology that uses the interference property of light to capture the internal structure of a sample with high resolution and high speed. Since it can irradiate near-infrared light and perform non-contact and non-invasive imaging, it is used for tomography of various organs (fundus of the eye, subcutaneous tissue) of the human body without the worry of radiation exposure (for example, see Non-Patent Documents 5-7). In recent years, its application areas have expanded significantly and it is used in various fields such as industry, medicine, and biology (for example, see Non-Patent Document 8).

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

[0007] However, while there is a demand for detecting defects in appearance on the order of μm in the visual inspection of industrial products, there is no flash LiDAR with such depth resolution. The depth accuracy of flash LiDAR is limited by the response speed of the image sensor array (see, for example, Non-Patent Documents 1 and 2). Recently, a flash LiDAR with a response speed of 130 MHz has been reported in the research stage (see, for example, Non-Patent Document 3), but the depth resolution is expected to be on the order of sub-meter. In other words, flash LiDAR has the problem that it is difficult to obtain depth resolution.

[0008] In addition, OCT basically constructs a reference path and is a combination of single-pixel imaging and laser beam scanning. It scans the target area over time to obtain 3D information on the internal structure. In the case of biological measurement, the biological object needs to be stationary during the measurement, and in industrial applications, the tact time of the production line is limited by this measurement time. That is, OCT has the problem that it is difficult to speed up the measurement.

[0009] In order to solve the above problems, an object of the present invention is to provide a three-dimensional shape measurement system and a three-dimensional shape measurement method capable of quickly measuring the three-dimensional external shape or internal structure of an object to be measured with high depth resolution.

Means for Solving the Problems

[0010] In order to achieve the above object, the three-dimensional shape measurement system according to the present invention extends the linear optical sampling (two-beam interference) technology to two dimensions.

[0011] Specifically, the three-dimensional shape measurement system according to the present invention a first light source that irradiates an object to be measured with an ultrashort optical pulse train for inspection having a two-dimensional distribution that can be regarded as having uniform light intensity; a second light source that outputs an ultrashort optical pulse train for sampling having a repetition period different from that of the ultrashort optical pulse train for inspection, a pulse width narrower than that of the ultrashort optical pulse train for inspection, and a two-dimensional distribution that can be regarded as having uniform light intensity; an interference unit that multiplexes the ultrashort optical pulse train for inspection reflected by the object to be measured and the ultrashort optical pulse train for sampling; a two-dimensional photo detector array that receives the interference light multiplexed by the interference unit; an arithmetic unit that measures the temporal change of the two-dimensional distribution of the light intensity of the ultrashort optical pulse train for inspection reflected by the object to be measured from the signal of the interference light output by the two-dimensional photo detector array and calculates the three-dimensional shape of the object to be measured; and comprises.

[0012] In addition, the three-dimensional shape measurement method according to the present invention Irradiating an object to be measured with an ultrashort optical pulse train for inspection having a two-dimensional distribution that can be regarded as uniform in light intensity; Outputting an ultrashort optical pulse train for sampling having a repetition period different from that of the ultrashort optical pulse train for inspection, a pulse width narrower than that of the ultrashort optical pulse train for inspection, and a two-dimensional distribution that can be regarded as uniform in light intensity; Combining and interfering the ultrashort optical pulse train for inspection reflected by the object to be measured and the ultrashort optical pulse train for sampling to obtain an interference light; Receiving the interference light with a two-dimensional photodetector array; and Measuring a temporal change in the two-dimensional distribution of light intensity for the ultrashort optical pulse train for inspection reflected by the object to be measured from the signal of the interference light output by the two-dimensional photodetector array, and calculating the three-dimensional shape of the object to be measured. Performing the above.

[0013] FIG. 1 is a diagram for explaining the measurement principle of a three-dimensional shape measurement system. The linear sampling method (two-comb interference method) is a technique for measuring the instantaneous complex amplitude of a signal optical pulse by using coherent correlation detection between the signal optical pulse (FIG. 1(A)), which is an ultrashort optical pulse and is reflected by an object to be measured, and a sampling pulse (FIG. 1(B)). By slightly detuning the repetition period of the signal optical pulse and the repetition period of the sampling pulse and detecting the instantaneous amplitude α while shifting the interference point, the waveform of the entire signal optical pulse can be observed with a time resolution of subpicoseconds (FIG. 1(C)).

[0014] Since the time resolution of the observable signal is determined only by the sampling pulse width, it can handle ultra-high-speed signals and perform high-speed measurements. The electronics (two-dimensional photodetector array and arithmetic unit) to be used only needs to be able to follow the repetition period of the sampling pulse, and those with a bandwidth of about several MHz can be used.

[0015] Therefore, the present invention can provide a three-dimensional shape measurement system and a three-dimensional shape measurement method capable of measuring the three-dimensional external shape or internal structure of an object to be measured at high depth resolution and at high speed.

[0016] The three-dimensional shape measurement system and method according to the present invention are characterized in that the interference light is separated by polarization, and each polarization is received by the two-dimensional photo-director array.

[0017] FIG. 2 is a diagram for explaining the measurement principle of the three-dimensional shape measurement system. This three-dimensional shape measurement system applies the linear sampling method to the signal light having a two-dimensional distribution. The signal light having a two-dimensional spatial distribution has time-of-flight information corresponding to the appearance or internal structure of the object to be measured. This signal light is combined with a sampling pulse having a spatially uniform amplitude distribution, separated into x-polarization and y-polarization by polarization separation, and then each is imaged on a two-dimensional photo-detector array. The interference signal generated there is acquired by a clock synchronized with the sampling pulse. That is, this three-dimensional shape measurement system performs complex electric field amplitude measurement by separating the signal light for each polarization.

[0018] When the polarization state of the signal light is determined, by matching the polarization state of the sampling pulse to the signal light, the polarization separation step can be omitted, and the three-dimensional appearance shape or internal structure of the object to be measured can be measured with one two-dimensional photo-detector array.

[0019] The three-dimensional shape measurement system and method according to the present invention set T, ΔT, and Δt so as to satisfy Equation (C1), where the desired measurement distance range is Δz, the speed of light is c, the repetition period of the ultrashort optical pulse train for inspection is T, the repetition period of the ultrashort optical pulse train for sampling is T + ΔT, and the difference between the pulse width of the ultrashort optical pulse train for inspection and the pulse width of the ultrashort optical pulse train for sampling is Δt.

Equation

[0020] The arithmetic unit of this three-dimensional shape measurement system can also be realized by a computer and a program, and it is also possible to record the program on a recording medium or provide it through a network.

[0021] In addition, the above inventions can be combined as much as possible.

Advantages of the Invention

[0022] The present invention can provide a three-dimensional shape measurement system and a three-dimensional shape measurement method capable of rapidly measuring the three-dimensional external shape or internal structure of a measurement object with high depth resolution.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0024] Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In this specification and the drawings, components with the same reference numerals indicate the same components.

[0025] (Embodiment 1) In this embodiment, a three-dimensional shape measurement system 301 that realizes a flash LiDAR having a depth resolution of submillimeter level will be described. FIG. 3 is a diagram for explaining the three-dimensional shape measurement system 301. The three-dimensional shape measurement system 301 includes a first light source 11 that irradiates a measurement object with an inspection ultrashort optical pulse train Pt having a two-dimensional distribution that can be regarded as having a uniform light intensity, A second light source 12 that outputs an ultrashort optical pulse train Ps for sampling, which has a repetition period different from that of the ultrashort optical pulse train Pt for inspection, a pulse width narrower than that of the ultrashort optical pulse train Pt for inspection, and a two-dimensional distribution that can be regarded as having uniform light intensity. An interference unit 13 that multiplexes the ultrashort optical pulse train Pt' for inspection reflected by the object to be measured and the ultrashort optical pulse train Ps for sampling. A two-dimensional photodetector array 14 that receives the interference light Pi multiplexed by the interference unit 13. An arithmetic unit 15 that measures the temporal change of the two-dimensional distribution of the light intensity of the ultrashort optical pulse train Pt' for inspection reflected by the object to be measured from the signal of the interference light Pi output by the two-dimensional photodetector array 14 and calculates the three-dimensional shape of the object to be measured. It is provided with.

[0026] The three-dimensional shape measurement system 301 inspects the appearance of industrial products conveyed by a belt conveyor with high precision. The first light source 11 is composed of a pulse light source and a lens, and outputs an ultrashort optical pulse train Pt for inspection having a two-dimensional distribution that can be regarded as having uniform light intensity. The pulse interval of each optical pulse of the ultrashort optical pulse train Pt is T (sec). The second light source 12 is composed of a pulse light source and a lens, and outputs an ultrashort optical pulse train Ps for sampling having a two-dimensional distribution that can be regarded as having uniform light intensity. The pulse interval of each optical pulse of the ultrashort optical pulse train Ps is T + ΔT (sec). Also, the pulse width of the optical pulse of the ultrashort optical pulse train Ps needs to be narrower than the pulse width of the optical pulse of the ultrashort optical pulse train Pt.

[0027] In this embodiment, the interference unit 13 is a beam splitter. The ultrashort optical pulse train Pt output from the first light source 11 is reflected by the interference unit 13 and irradiated onto the industrial product that is the object to be measured. The ultrashort optical pulse train Pt' reflected by the object to be measured has a flight time changed by the depth shape (surface state) of the object to be measured and has information on the surface of the object to be measured. The ultrashort optical pulse train Pt' passes through the interference unit 13, but at this time, it is multiplexed with the ultrashort optical pulse train Ps output from the second light source 12 and interferes (interference light Pi).

[0028] The two-dimensional photo-director array 14 has a plurality of photodiodes arranged two-dimensionally and receives the interference light Pi over an area. In order to accurately obtain the shape of the object to be measured, it is necessary to increase the density of the photodiodes.

[0029] Note that before causing the interference light Pi to be received by the two-dimensional photo-director array 14, the interference light Pi may be polarization-separated, and a polarization separation unit (not shown) for causing the interference light Pi to be received by the two-dimensional photo-director array for each polarization may be further provided. In this case, two two-dimensional photo-director arrays for X polarization and Y polarization are required.

[0030] The arithmetic unit 15 receives the signal of the interference light Pi output by the two-dimensional photo-director array 14 (when there is a polarization separation unit, the signals output by the two-dimensional photo-director arrays for X polarization and Y polarization respectively), measures the temporal change (shift in flight time) of the two-dimensional distribution of the light intensity on the light-receiving surface of the photo-director array 14 for the ultrashort optical pulse train Pt', and calculates the three-dimensional shape of the object to be measured. The three-dimensional shape may be displayed on a monitor.

[0031] The operation of the three-dimensional shape measurement system 301 will be described. The ultrashort optical pulse train Pt is irradiated onto the object to be measured. The ultrashort optical pulse train Pt' reflected by the object to be measured arrives at the two-dimensional photodetector array 14 with a unique flight time for each space according to the shape of the object to be measured. By sampling the signals of each photodetector by linear sampling and measuring the flight time, it becomes possible to grasp the three-dimensional shape of the object to be measured.

[0032] Note that in order to avoid the ambiguity of distance measurement, for the measurement distance range Δz of the object to be measured, the pulse period T of the ultrashort optical pulse train Pt is set to satisfy the following equation.

Equation

[0033] Note that the depth resolution is also affected by the deviation ΔT between the pulse intervals of the ultrashort optical pulse train Pt and the ultrashort optical pulse train Ps. Specifically, the time resolution (depth resolution) of the sampling data is approximately ΔT / T 2 and this becomes the case. Comparing this time resolution with δz in Equation 2, the larger one becomes the depth resolution of the three-dimensional shape measurement system 301. Therefore, as shown in the following equation, by setting ΔT so that δz becomes larger, the depth resolution can be determined by Equation 2. [Equation]

[0034] The three-dimensional shape measurement system 301 can grasp the three-dimensional shape without requiring any movable scanning unit such as mechanical beam scanning.

[0035] (Embodiment 2) In this embodiment, a three-dimensional shape measurement system 302 that realizes a flash-type OCT capable of higher tomography imaging speed than conventional OCT and further expanding applications such as measurement of internal structures of flowing biological tissues such as body fluids containing blood, or industrial paints and thin films will be described. FIG. 4 is a diagram for explaining the three-dimensional shape measurement system 302. The structure of the three-dimensional shape measurement system 302 is the same as that of the three-dimensional shape measurement system 301 described in FIG. 3.

[0036] The three-dimensional shape measurement system 302 measures the internal structure of a biological tissue, industrial paint, thin film, etc. as a measurement object non-destructively or non-invasively. For this purpose, the first light source 11 outputs an ultrashort optical pulse train Pt having a wavelength with a certain transmittance with respect to the measurement object. For example, the ultrashort optical pulse train Pt is infrared light.

[0037] The operation of the three-dimensional shape measurement system 302 will be described. The measurement object is irradiated with an ultrashort optical pulse train Pt having a certain transmittance. The ultrashort optical pulse train Pt' reflected inside the measurement object reflects the reflectance from each point inside the measurement object. That is, the ultrashort optical pulse train Pt' has information on the reflectance of each point inside the measurement object. The ultrashort optical pulse train Pt' arrives at the two-dimensional photodetector array 14 with a unique flight time according to the position inside the measurement object. By measuring the flight time by sampling the signals of each photodetector by linear sampling, the reflectance of each point inside the measurement object can be measured, and the internal structure of the measurement object can be grasped three-dimensionally from the reflectance of each point.

[0038] The relationships of the parameters of the three-dimensional shape measurement system 302 are the same as those of the three-dimensional shape measurement system 301 described with reference to FIG. 3 (see FIGS. 1, 2, and 3). For this reason, the three-dimensional shape measurement system 302 can not only achieve a high refresh rate but also realize a depth resolution on the order of submillimeters.

[0039] The three-dimensional shape measurement system 302 can measure the internal structure of the measurement object non-destructively or non-invasively.

Explanation of Signs

[0040] 11: First light source 12: Second light source 13: Interference unit 14: Two-dimensional photodirector array 15: Arithmetic unit 301, 302: Three-dimensional shape measurement system

Claims

1. A first light source that irradiates a measurement object with an ultrashort optical pulse train for inspection having a two-dimensional distribution that can be regarded as uniform light intensity; A second light source that outputs an ultrashort optical pulse train for sampling having a repetition period different from that of the ultrashort optical pulse train for inspection, a pulse width narrower than that of the ultrashort optical pulse train for inspection, and a two-dimensional distribution that can be regarded as uniform light intensity; An interference unit that multiplexes the ultrashort optical pulse train for inspection reflected by the measurement object and the ultrashort optical pulse train for sampling; A two-dimensional photo-director array that receives the interference light multiplexed by the interference unit; An arithmetic unit that measures the temporal change of the two-dimensional distribution of the light intensity of the ultrashort optical pulse train for inspection reflected by the measurement object from the signal of the interference light output by the two-dimensional photo-director array and calculates the three-dimensional shape of the measurement object; A three-dimensional shape measurement system comprising the above.

2. The three-dimensional shape measurement system according to claim 1, further comprising a polarization separation unit that separates the interference light into polarized waves and causes each polarized wave to be received by the two-dimensional photo-director array.

3. When the desired measurement distance range is Δz, the speed of light is c, the repetition period of the ultrashort optical pulse train for inspection is T, the repetition period of the ultrashort optical pulse train for sampling is T + ΔT, and the difference between the pulse width of the ultrashort optical pulse train for inspection and the pulse width of the ultrashort optical pulse train for sampling is Δt, T, ΔT, and Δt are set so as to satisfy formula (C1). The three-dimensional shape measurement system according to claim 1 or 2. 【Number C1】

4. Irradiating a measurement object with an ultrashort optical pulse train for inspection having a two-dimensional distribution that can be regarded as uniform light intensity; Outputting an ultrashort optical pulse train for sampling having a repetition period different from that of the ultrashort optical pulse train for inspection, a pulse width narrower than that of the ultrashort optical pulse train for inspection, and a two-dimensional distribution that can be regarded as uniform light intensity; Combining and interfering the ultrashort optical pulse train for inspection reflected by the measurement object and the ultrashort optical pulse train for sampling to obtain interference light; Receiving the interference light with a two-dimensional photo-director array; and Measuring the temporal change of the two-dimensional distribution of the light intensity of the ultrashort optical pulse train for inspection reflected by the measurement object from the signal of the interference light output by the two-dimensional photo-director array and calculating the three-dimensional shape of the measurement object. A three-dimensional shape measurement method for performing the above.

5. The three-dimensional shape measurement method according to claim 4, characterized in that the interference light is polarization-separated, and each polarization is received by the two-dimensional photodetector array.

6. When a desired measurement distance range is Δz, the speed of light is c, the repetition period of the ultrashort optical pulse train for inspection is T, the repetition period of the ultrashort optical pulse train for sampling is T + ΔT, and the difference between the pulse width of the ultrashort optical pulse train for inspection and the pulse width of the ultrashort optical pulse train for sampling is Δt, T, ΔT, and Δt are set so as to satisfy formula (C1). The three-dimensional shape measurement method according to claim 4 or 5, characterized by the above. 【Number C1】

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