KR20220153733A - Multiple focusing-based high-resolution optical coherence tomography apparatus for depth-of-focus enhancement - Google Patents

Abstract

According to an embodiment of the present invention, an optical interference tomography apparatus comprises: a light source; a light distributor controlling the path of light; a measuring end irradiating the light inputted through the light distributor toward a subject to be photographed, and transferring the measurement reflection light generated by being reflected from the subject; a reference end transferring a reference reflection light generated based on the light inputted through the light distributor; a detection unit receiving and analyzing the interference light formed by the reference reflection light and the measurement reflection light; and an image processing unit generating a tomography image of the subject based on the interference light analyzed by the detection unit. Accordingly, even if a sample is curved, a high-resolution tomography image for a needed point can be acquired at once through the improvement of the depth of focus even without changing the position of the focus.

Description

Multi-focal-based high-resolution optical coherence tomography device for enhancing depth of focus

The present invention relates to a multi-focal high-resolution optical coherence tomography apparatus for improving depth of focus, and more particularly, to a multi-focal high-resolution optical coherence tomography apparatus for improving depth of focus that can improve depth of focus through multiple focal points. It is about.

The optical coherence tomography system, which has recently been in the limelight in the field of medical optics research and the medical device industry, is a technology that can non-invasively perform tomographic imaging of the internal microstructure of living tissue using light in the near-infrared band. It is a method of obtaining depth information of a sample. It is divided into time domain optical coherence tomography (TD-OCT), spectral domain optical coherence tomography (SD-OCT), and variable wavelength optical coherence tomography (SS-OCT).

Among them, in the spectral domain optical coherence tomography (SD-OCT), light emitted from a near-infrared light source having a wide wavelength band is divided into a sample stage and a reference stage through a tube distributor.

In addition, the light incident on the sample is back-scattered at different depths of the internal structure of the sample, and the light incident on the reference end is reflected by the fixed mirror of the reference end. The light reflected from the sample and reference ends meets again at the optical splitter, and interference occurs when the optical path difference between the two separated lights is smaller than the coherence length of the light source. The generated interference signal is detected as a signal in the frequency domain through a spectrometer, and the depth of the sample can be imaged through inverse Fourier transform.

However, in the case of OCT according to the prior art, a plurality of spectrometers or multi-line cameras are used to compensate for the problem of low sensitivity compared to other types of tomography devices, but in this case, slow detection time, additional cost, and two spectrometers Problems such as detection errors in the high-frequency signal band caused by misalignment between the signals are occurring.

In addition, there was a problem in that the image focus had to be moved and checked when scanning a subject several times due to insufficient depth of focus or when scanning a curved sample.

Republic of Korea Patent Registration No. 10-1258557

The present invention has been made to solve the above problems, and an object of the present invention is to improve the depth of focus and obtain high-resolution tomographic imaging for the necessary points at once without changing the position of the focus point even for curved samples. An object of the present invention is to provide a high-resolution optical coherence tomography apparatus based on multi-focus for improving the depth of focus, which can shorten the measurement time through this.

Optical coherence tomography apparatus according to an embodiment of the present invention for achieving the above object, a light source for emitting light; an optical splitter that receives the light and adjusts the path of the light; a measurement stage for radiating the light input through the optical splitter toward an object to be photographed, and transmitting measurement reflection light generated by reflection from the object; a reference stage for transmitting standard reflected light generated based on the light input through the optical splitter; a detector configured to receive and analyze interference light formed by the standard reflected light and the measurement reflected light; and an image processing unit generating a tomographic image of the object to be captured based on the interference light analyzed by the detection unit, wherein the light splitter is positioned between the light source, the measurement stage, the reference stage, and the detection unit to move a first light splitter for controlling a path of light and distributing the light emitted from the light source into reference light input to the reference end and measurement light input to the measurement end; and a second optical splitter positioned between the first optical splitter and the measuring end to adjust a path of moving light and redistribute the measurement light.

Here, the measurement stage may include a first collimator that converts the first distributed light distributed from the second optical splitter into parallel light based on a direction in which the measurement light travels toward the photographing target; a first mirror positioned at a rear end of the first collimator to change a path of the first distributed light; a second collimator converting the second distributed light from the second optical splitter into parallel light; a first lens positioned at a rear end of the second collimator to change a focal position of the second distributed light; a beam splitter matching a path of the first distribution light reflected by the first mirror with a path of the second distribution light passing through the first lens; a galvanometer scanner for reflecting the first distributed light and the second distributed light; and a second lens that collects the first and second distributed light reflected from the galvanometer scanner.

Further, the second optical splitter redistributes the light distributed from the first optical splitter into the first and second distributed light, the ratio of the first divided light is greater than the ratio of the second divided light. can be set higher.

In addition, the measurement reflection light is a combination of a first measurement reflection light generated by the first distribution light and a second measurement reflection light generated by the second distribution light, and the image processing unit determines the focus position of the photographing target. A final tomography image may be generated by merging a plurality of tomographic images generated by correcting a difference in optical path length based on or averaging interference signals.

On the other hand, the light source, a first light source for emitting a first light; and a second light source that emits second light, and the light splitter may further include a third light splitter that transfers the light emitted from the first light source and the second light source to the first light splitter.

The first light source and the second light source may emit light having different optical powers or different center wavelengths.

According to one aspect of the present invention described above, by providing a multi-focus-based high-resolution optical coherence tomography imaging system for improving depth of focus, even if a sample is curved through depth of focus enhancement, a required point at one time without changing the position of the focus A high-resolution tomographic image can be obtained for , and the measurement time can be shortened through this.

1 is a block diagram for explaining an optical coherence tomography apparatus according to an embodiment of the present invention;
2 is a schematic diagram for explaining an optical coherence tomography apparatus according to an embodiment of the present invention;
3 is a conceptual diagram for explaining multi-focus by an optical coherence tomography apparatus according to an embodiment of the present invention;
4 is a view for explaining a state in which optical coherence tomography according to an embodiment of the present invention is applied to a 3D print-based dental crown manufacturing sample;
5 is a schematic diagram for explaining an optical coherence tomography apparatus according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The detailed description of the present invention which follows refers to the accompanying drawings which illustrate, by way of illustration, specific embodiments in which the present invention may be practiced. These embodiments are described in sufficient detail to enable one skilled in the art to practice the present invention. It should be understood that the various embodiments of the present invention are different from each other but are not necessarily mutually exclusive. For example, specific shapes, structures, and characteristics described herein may be implemented in another embodiment without departing from the spirit and scope of the invention in connection with one embodiment. Additionally, it should be understood that the location or arrangement of individual components within each disclosed embodiment may be changed without departing from the spirit and scope of the invention. Accordingly, the detailed description set forth below is not to be taken in a limiting sense, and the scope of the present invention, if properly described, is limited only by the appended claims, along with all equivalents as claimed by those claims. Like reference numbers in the drawings indicate the same or similar function throughout the various aspects.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the drawings.

1 is a block diagram for explaining an optical coherence tomography apparatus according to an embodiment of the present invention, FIG. 2 is a schematic diagram for explaining an optical coherence tomography apparatus according to an embodiment of the present invention, and FIG. It is a conceptual diagram for explaining multi-focus by an optical coherence tomography apparatus according to an embodiment of the present invention, and FIG. 4 shows a state in which optical coherence tomography according to an embodiment of the present invention is applied to a 3D print-based dental crown manufacturing sample. It is a drawing for explanation.

The optical coherence tomography apparatus according to the present embodiment is intended to improve the depth-of-focus of the conventional optical coherence tomography apparatus by 2 to 3 times compared to that of the conventional apparatus, and the depth-of-focus has been insufficient in the past. Thus, it is possible to solve the problem of repeatedly photographing a subject. Such an optical coherence tomography apparatus includes a light source 100, an optical splitter 200, a reference stage 300, a measurement stage 400, a detection unit 500, and an image processing unit 600.

The light source 100 in this embodiment generates and emits light as a broadband light source (BLS). The light source 100 is a light source that outputs light with a wide spectral radiation spectrum including the near-infrared region, and a super luminescent diode, a titanium sapphire laser, or a femtosecond laser may be used. can

The optical splitter 200 is provided to adjust the path of light, and is located at the rear end of the light source 100 based on the path along which the light travels. In this embodiment, the optical splitter 200 is a fiber coupler. FC), it is possible to achieve a stable and constant light distribution and output regardless of the incident angle of light, and to achieve a simple and compact configuration.

In particular, unlike the conventional optical coherence tomography apparatus including a single optical splitter for distributing the light output from the light source 100 to the reference end 300 and the measurement end 400, the optical splitter according to the present embodiment ( 200) includes a first light splitter 210 and a second light splitter 220 so that the light emitted from the light source 100 has multiple focal points in the depth direction.

As shown, the first optical splitter 210 is located between the light source 100, the reference stage 300, the measurement stage 400, and the detection unit 500, and the light emitted from the light source 100 is the measurement stage ( 400) and the reference stage 300, the optical path is adjusted to be distributed. Here, the light distributed by the first optical splitter 210 and moved to the reference stage 300 is referred to as the reference light, and the light moved to the measurement stage 400 is referred to as the reference light. It is desirable to set the distribution ratio to 50:50.

On the other hand, the second optical splitter 220 is located between the first optical splitter 210 and the measuring end 400, and among the light distributed by the first optical splitter 210, the measurement light is divided into the first and second splitters. redistribute light As shown in FIG. 2, this is to improve the depth of focus by differentiating positions F1 and F2 at which the measurement light is focused. At this time, the second optical splitter sets the distribution ratio of the first distribution light to the second distribution light to 60:40 so that the ratio of the first distribution light is higher. This is adjusted based on the position at which the light is focused. In this embodiment, when the focal position F1 of the first distributed light is focused relatively deeper than the focal position F2 of the second distributed light among multiple focal points in the depth direction, efficient efficiency is achieved. Although the optical power of the first distribution light is set higher for focusing, it may be changed as needed in order to obtain more accurate image information.

On the other hand, the reference end 300 is provided to transmit the reference reflected light generated based on the light input through the optical splitter 200, and as shown, the third collimator 310 and the third lens 320 and A second mirror 330 is included. Here, the reference light may refer to light input to the reference stage 300 through the first optical splitter 210 among the optical splitters 200 .

The third collimator 310 is located at the rear end of the first optical splitter 210 based on the direction in which the reference light input through the first optical splitter 210 travels, and can convert the reference light into parallel light. The light is transmitted to the third lens 320 .

The third lens 320 may be located at the rear end of the third collimator 310 and transmit the received reference light to the second mirror 330 .

In addition, the second mirror 330 reflects the reference light transmitted from the third lens 320 to generate reference reflected light, and the reference reflected light thus generated proceeds in the reverse direction of the reference light incident path to the first optical splitter 210. can be forwarded to

In addition, the measuring stage 400 irradiates the light input through the optical splitter 200 toward the object to be photographed, and transmits the reflected light generated by reflection from the object to be photographed. The first collimator 410 and the first mirror 415 ), a second collimator 420, a first lens 421, a beam splitter 423, a galvanometer scanner (GS) 425, and a second lens 427. Hereinafter, the position of each component will be described based on the direction in which the light output from the second optical splitter 220 moves toward the subject S to be photographed.

The first collimator 410 is located at the rear end of the second optical splitter 220 to convert the first distributed light distributed from the second optical splitter 220 into parallel light, and convert the collimated light into a first mirror ( 415).

The first mirror 415 may be positioned at the rear end of the first collimator 410 to change the path of the first distribution light and transmit it to the beam splitter 423 .

The second collimator 420 is located at the rear end of the second optical splitter 220 in parallel with the first collimator 410 to convert the second distributed light distributed by the second optical splitter 220 into parallel light. , parallel light may be transferred to the first lens 421 .

The first lens 421 may be positioned at the rear end of the second collimator 420 to change the focal position of the second distributed light and transmit the second distributed light having the changed focal position to the beam splitter 423 . Through this, as shown in FIG. 2, the focal point F2 of the second distributed light is located above the focal position F1 of the first distributed light based on the depth direction. In this embodiment, a plurality of light splitters 200 are provided and some of the light distributed therethrough is multiplexed through the first lens 421 to create several focal points in the depth direction (Depth-directional multi- focusing) It is possible to improve the existing focus depth of about 150μm to 300~450μm, which is 2~3 times more.

Therefore, unlike the prior art in which image information was obtained by repeating shooting due to insufficient depth of focus, a high-resolution image can be obtained even through a single shooting by improving the depth of focus through the above configuration.

On the other hand, the beam splitter 423 is located at the rear end of the first mirror 415 and the first lens 421, and passes through the path of the first distribution light reflected by the first mirror 415 and the first lens 421. Match the paths of the second distribution light. The beam splitter 423 transfers the first and second distributed light to the galvanometer scanner 425 .

The galvanometer scanner 425 is located at the rear end of the beam splitter 423 and can continuously reflect incident light by changing an angle. The galvanometer scanner 425 transfers the first distributed light and the second distributed light received from the beam splitter 423 to the second lens 427 .

The second lens 427 is located at the rear end of the galvanometer scanner 425, and collects the first and second distributed light transmitted from the galvanometer scanner 425 and radiates them toward the photographing target S. have.

Further, in the measurement unit 400, the first and second distributed lights are radiated toward the target S through the above configuration, and the first and second distributed lights are reflected and generated from the target S. The measurement reflected light obtained by combining the first measurement reflected light and the second measurement reflected light may be transmitted to the first optical splitter 210 by proceeding in a direction opposite to the direction in which the measurement light is transmitted.

The measurement reflected light and the reference reflected light generated by the measurement end 400 and the reference end 300 may be combined in the first optical splitter 210, and as a result, interference light is formed in the first optical splitter 210. The interfering light thus interfered may be transmitted to the detector 500 to obtain a tomographic image of the subject S to be photographed.

The detection unit 500 receives coherent light through the first optical splitter 210, detects a spectrum of the coherent light, obtains an interference spectrum image through analysis of the spectrum, and transmits the interference spectrum image to the image processing unit 600. can be conveyed To this end, a fourth collimator 510, a third mirror 520, a diffraction grating 530, a fourth lens 540, and a line scan camera (LSC) 550 may be included. Hereinafter, the direction in which the interference light is input to the detector 500 and proceeds will be described.

The fourth collimator 510 converts the input coherent light into parallel light and transmits the collimated light to the third mirror 520, the third mirror 520 being located at the rear end of the fourth collimator 510, The coherent light transmitted from the fourth collimator 510 is transferred to the diffraction grating 530 .

The diffraction grating 530 diffracts the incident interference light. In this embodiment, the diffraction grating 530 can be selected in various ways according to specifications, and the light diffracted by the diffraction grating 530 passes through the fourth lens 540. It is transmitted to the linescanner camera 550 through the

On the other hand, the image processing unit 600 measures the depth information of the imaging target S through Fourier transformation of each pixel of the interference spectrum image received from the detection unit 500, thereby determining the tomographic image of the imaging target S. image can be created.

As described above, the image processing unit 600 in the present embodiment captures the object to be photographed using multiple focal points, and as shown in FIG. 4, as two images are generated according to the focal position, the optical path length (optical path length) One image can be finally obtained through post-processing, such as combining images to match the depth of focus (DOF) part that is in focus by correcting the difference in length) or averaging the interference signal generated by the detection unit 500. have.

The optical coherence tomography device of this embodiment can be used for eardrum inspection, tooth sample (crown, implant, etc.) evaluation and 3D scanning, industrial lens and display inspection technology, as well as It is possible to compensate for the disadvantage that it is difficult to accurately analyze the part.

On the other hand, FIG. 5 is a schematic diagram for explaining an optical coherence tomography device according to another embodiment of the present invention. Unlike optical coherence tomography according to the example, a plurality of light sources 1000 are provided to output a plurality of light sources 1100 and 1200, an optical splitter 2000, a reference stage 3000, and a measurement stage ( 4000), a detection unit 5000, and an image processing unit 600.

A light source 1000 according to another embodiment includes a first light source 1100 emitting a first light and a second light source 1200 emitting a second light. In this case, the first light source 1100 and the second light source 1200 may emit light having different optical powers or different center wavelengths. In addition, the first light source 1100 includes the first light source collimator 1150 to convert the emitted light into parallel light and transmits it to the third light splitter 2300, and the second light source 2200 includes the second light source collimator ( 125), light having a different light output or a different central wavelength from that emitted from the first light source 1100 may be converted into parallel light and transmitted to the third light splitter 2300.

And the optical splitter 200 according to another embodiment is provided to adjust the paths of the first light and the second light transmitted from the first light source collimator 1150 and the second light source collimator 1250, and the first light splitter ( 2100), a second optical splitter 2200 and a third optical splitter 2300. Also, unlike the optical splitter 200 according to one embodiment implemented as a fiber coupler (FC), the optical splitter 2000 according to another embodiment is implemented as a beam splitter (Beam).

Based on the traveling direction of the light moving to the subject (S), the first optical splitter 2100 is located at the rear end of the third optical splitter 2300, and converts the light received from the third optical splitter 2300 into the reference light. And the measurement light is distributed and transmitted to the reference end 3000 and the measurement end 4000.

The second optical splitter 2200 is located at the rear end of the first optical splitter 2100, and as described above, among the light distributed by the first optical splitter 2100, the measurement light is divided into the first and second distribution lights. It is redistributed and transmitted to the measurement stage 4000.

The third light splitter 2300 is located between the first light source collimator 1150, the second light source collimator 1250, the first light splitter 2100, and the detection unit 5000, and transmits from the first light splitter 2100. The received interference light is transferred to the detector 5000.

Meanwhile, the reference end 3000 is provided to transmit reference reflected light generated based on light input through the optical splitter 2000, and as shown, the third mirror 3100 and the third lens 3200 and A fourth mirror 3300 is included. Here, the reference light may refer to light input to the reference stage 3000 through the first optical splitter 2100 among the optical splitters 2000 .

The third mirror 3100 is located at the rear end of the first optical splitter 2100 based on the direction in which the reference light input through the first optical splitter 2100 travels, and transmits the reference light to the third lens 3200.

The third lens 3200 may be located at the rear end of the third mirror 3100 and transfer the received reference light to the fourth mirror 3300 .

In addition, the fourth mirror 3300 reflects the reference light transmitted from the third lens 3200 to generate reference reflected light, and the reference reflected light generated in this way proceeds in the reverse direction of the reference light incident path to the first optical splitter 2100. can be forwarded to

The measuring end 4000 according to another embodiment is to improve the depth of focus by differentiating the positions F1 and F2 at which the measurement light is focused. 4200, a first lens 4210, a beam splitter 4230, a galvanometer scanner 4250, and a second lens 4270.

Unlike the measurement stage 400 according to another embodiment, since the optical splitter 2000 is provided as a beam splitter, the configuration of the collimator may be omitted. The first mirror 4100 receives the first distribution light from the second optical splitter 2200 and transfers it to the beam splitter 4230, and the second mirror 4200 receives the second distribution light from the second optical splitter 2200. is received and transmitted to the first lens path 4210.

The first lens 4210 may be positioned at the rear end of the second mirror 4200 to change the focal position of the second distributed light and transmit the second distributed light having the changed focal position to the beam splitter 4230 . Through this, as shown in FIG. 2, the focal point F2 of the second distributed light is located above the focal position F1 of the first distributed light based on the depth direction.

The beam splitter 4230 sets the paths of the first and second distributed lights transmitted from the first mirror 4100 and the first lens 4210 to be the same and transmits the same paths to the galvanometer scanner 4250 .

The galvanometer scanner 4250 is located at the rear end of the beam splitter 4230 and can continuously change the angle of the incident light and reflect it. The galvanometer scanner 4250 transfers the first distributed light and the second distributed light received from the beam splitter 4230 to the second lens 4270 .

The second lens 4270 is located at the rear end of the galvanometer scanner 4250, and collects the first and second distributed light transmitted from the galvanometer scanner 4250 and radiates them toward the photographing target S. have.

Further, in the measurement unit 4000, the first and second distributed lights are radiated toward the target S through the above configuration, and the first and second distributed lights are reflected and generated from the target S. The measurement reflected light obtained by combining the first measurement reflected light and the second measurement reflected light may be transmitted to the first optical splitter 2100 by proceeding in a direction opposite to the direction in which the measurement light is transmitted.

The reference reflected light and the measurement reflected light generated by the above-described reference stage 2000 and measurement stage 3000 may be combined in the first optical splitter 2100, and as a result, interference light may be formed in the first optical splitter 2100. The interfering light thus interfered may be transferred to the detector 5000 to obtain a tomographic image of the subject S to be photographed.

Meanwhile, the detection unit 5000 according to another embodiment of the present invention detects the spectrum, obtains an interference spectrum image through analysis of the spectrum, and transmits the interference spectrum image to the image processing unit 600. To this end, a fifth mirror 5200, a diffraction grating 5300, a fourth lens 5400, a beam splitter 5600, and line scanner cameras 5510 and 5520 may be included.

The fifth mirror 5200 is located at the rear end of the third optical splitter 2300 and transfers coherent light input from the third optical splitter 2300 to the diffraction grating 5300 .

The diffraction grating 5300 diffracts the incident coherent light, and the diffracted light is transmitted to the beam splitter 5600 through the fourth lens 5400.

In addition, the beam splitter 5600 transmits the diffracted light based on the first measured reflected light among the diffracted lights received through the fourth lens 5400 to the second line scanner camera 5510, and based on the second measured reflected light. The diffracted light is transferred to the first line scanner camera 5520.

On the other hand, the image processing unit 600 measures the depth information of the imaging target S through Fourier transformation of each pixel of the interference spectrum image received from the detection unit 5000, thereby determining the tomographic image of the imaging target S. image can be created.

Although various embodiments of the present invention have been shown and described above, the present invention is not limited to the specific embodiments described above, and is commonly used in the technical field to which the present invention pertains without departing from the gist of the present invention claimed in the claims. Of course, various modifications are possible by those with knowledge of, and these modifications should not be individually understood from the technical spirit or prospect of the present invention.

100, 1000: light source 1100: first light source
1200: second light source 1150: first light source collimator
1250 Second light source collimator 200, 2000: optical splitter
210, 2100: first optical splitter 220, 2200: second optical splitter
2300: 3rd optical splitter 300, 3000: reference stage
400, 4000: measurement stage 500, 5000: detection unit
600: image processing unit

Claims (6)

a light source that emits light;
an optical splitter that receives the light and adjusts the path of the light;
a measurement stage for radiating the light input through the optical splitter toward an object to be photographed, and transmitting measurement reflection light generated by reflection from the object;
a reference stage for transmitting standard reflected light generated based on the light input through the optical splitter;
a detector configured to receive and analyze interference light formed by the standard reflected light and the measurement reflected light; and
An image processing unit for generating a tomographic image of the photographing target based on the interference light analyzed by the detection unit;
The optical splitter,
It is located between the light source, the measuring end, the reference end, and the detection unit to adjust a path of moving light, and distributes light emitted from the light source into reference light input to the reference end and measurement light input to the measurement end. a first optical splitter; and
and a second optical splitter positioned between the first optical splitter and the measurement end to adjust a path of moving light and redistribute the measurement light.
According to claim 1,
The measuring end, based on the direction in which the measuring light travels toward the photographing target,
a first collimator converting the first distributed light from the second optical splitter into parallel light;
a first mirror positioned at a rear end of the first collimator to change a path of the first distributed light;
a second collimator converting the second distributed light from the second optical splitter into parallel light;
a first lens positioned at a rear end of the second collimator to change a focal position of the second distributed light;
a beam splitter matching a path of the first distribution light reflected by the first mirror with a path of the second distribution light passing through the first lens;
a galvanometer scanner for reflecting the first distributed light and the second distributed light; and
The optical coherence tomography apparatus comprising a second lens for collecting the first and second distributed light reflected from the galvanometer scanner.
According to claim 2,
The second optical splitter,
In redistributing the light distributed from the first optical splitter into the first and second distributed lights, a ratio of the first distributed light is set higher than a ratio of the second distributed light. Interference tomography device.
According to claim 2,
The measured reflected light,
A first measurement reflection light generated by the first distribution light and a second measurement reflection light generated by the second distribution light are combined;
The image processing unit,
The optical coherence tomography apparatus according to claim 1 , wherein a final tomographic image is generated by merging a plurality of tomographic images generated by correcting a difference in optical path length based on the focal position of the photographing target or by averaging interference signals.
According to claim 1,
The light source,
A first light source emitting a first light; and
A second light source emitting a second light;
The optical splitter,
The optical coherence tomography apparatus further comprises a third optical splitter for transmitting the light emitted from the first light source and the second light source to the first optical splitter.
According to claim 5,
The first light source and the second light source, the optical coherence tomography apparatus, characterized in that for emitting light having different optical power or different center wavelength (Center wavelength).

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