JP2006053055A - Laser measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify apparatus configuration and improve laser light utilization efficiency. <P>SOLUTION: The laser measuring apparatus comprises a light source 4 for an emitting laser light B, a detector 24 for detecting scattered light of the laser light B by an object to be measured, and a separator 23 comprising a non-reflective passage section 27 for allowing the laser light B emitted from the light source 4 to pass through in a nearly non-reflective state and a reflective section 28 for reflecting the scattered light toward the detector 24. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser measuring device that measures the relative concentration distribution of suspended fine particles in the atmosphere using laser light.

  As a laser measuring device that performs various measurements using laser light, short-pulse high-power laser light is irradiated toward the atmosphere, and the laser light is scattered by particles such as dust floating in the atmosphere. Laser radar devices that measure the relative concentration distribution of dust and the like by receiving (receiving) scattered light (back scattered light) in the direction of the laser radar (see, for example, Patent Document 1 and Patent Document 2), or similar to laser radar devices A technology that measures the distance to an irradiation target with high accuracy by irradiating a solid object (measurement target) such as a vehicle, building, tree, or rock with laser light and receiving scattered light from the irradiation target. A distance measuring device (see, for example, Patent Document 3) is widely used.

In this type of laser measurement apparatus, an in-line radar measurement apparatus is also provided in which a transmission telescope and a reception telescope are integrated so that the transmission beam axis and the reception beam axis coincide with each other in order to reduce the size of the system. In the in-line type radar measurement apparatus, a configuration in which a polarization beam splitter is used to branch (separate) a reception beam from a transmission beam is also employed.
JP 2001-337029 A JP 2002-196075 A Japanese Patent Laid-Open No. 10-153417

However, the conventional laser measuring apparatus as described above has the following problems.
For example, in the case of a laser measuring device using a reflective telescope, a beam expanding optical system that expands the laser beam diameter and a light receiving telescope with a large aperture are required in order to realize eye-safety and long distance measurement for the eyes. This leads to an increase in size and complexity of the apparatus, resulting in a high price. In addition, in the case of an in-line type laser measuring apparatus using a reflective telescope, there is a problem that the energy transmission efficiency is poor because the center part of the transmission beam having a large energy intensity is blocked by the secondary mirror of the reflective telescope.
In addition, an in-line type laser measuring apparatus requires many optical parts such as a polarizing beam splitter and a wave plate, and the structure becomes complicated, so that it may be complicated to use in the field.

Further, when a beam splitter is used, the reflectance of the beam splitter is R, the transmittance is 1-R, the light output generated from the light source is Po, and the transmitted light output of the received light that is scattered from the measurement object and returns to the apparatus. If the proportionality coefficient is a, the light output Pd emitted from the light source, transmitted through the beam splitter, scattered by the measurement object, reflected by the beam splitter, and incident on the photodetector is expressed by the following equation.
Pd = a * R * (1-R) * Po (1)
Here, the transmission light utilization efficiency η is defined by the following equation.
η = Pd / (a × Po) (2)
Using equation (1), equation (2) becomes:
η = R × (1-R) (3)
In Expression (3), the reflectance R that maximizes the efficiency η is 0.5, and the utilization efficiency η at this time is 0.25.
That is, when a beam splitter is used, only 25% of the transmitted light contributes to the received signal intensity, and there is a problem that the utilization efficiency of the laser light is low.

  The present invention has been made in consideration of the above points, and an object of the present invention is to provide an inline-type laser measuring apparatus that can simplify the apparatus configuration and contribute to an improvement in the utilization efficiency of laser light.

In order to achieve the above object, the present invention employs the following configuration.
The laser measurement apparatus of the present invention is an inline-type laser measurement apparatus having a light source that emits laser light and a detector that detects scattered light from the measurement target of the laser light, and the laser light emitted from the light source Is provided with a separator having a non-reflective passing part that passes the light in a substantially non-reflective state and a reflective part that reflects the scattered light toward the detector.

Therefore, in the laser measurement device of the present invention, the laser light emitted from the light source passes through the separator in a non-reflecting state, and thus it is possible to suppress a decrease in output of the transmitted light. For this reason, it is possible to suppress a decrease in the reception efficiency of scattered light, which can contribute to an improvement in the utilization efficiency of laser light.
As a non-reflective passage part, it has the structure made into the hole provided in the said separator, and the dielectric non-reflective film provided in the surface by the side of the said laser beam in the said separator, and the surface by the side of emission A configuration is preferable.
In the case where the non-reflective passage portion is a hole, since the separator is emitted without passing through an optical element or the like, it is possible to greatly suppress the output reduction of the transmission light. Moreover, when the non-reflective passage part has a dielectric non-reflective film, the output fall of the transmission light by reflection can be suppressed. In this case, it is preferable that the reflecting portion includes a polarizing beam splitter film provided around the dielectric non-reflective film on the emission side surface.
Further, in the present invention, since the emitted laser light and scattered light can be separated by a separator having a simple configuration having a hole, a dielectric non-reflective film, and a reflective portion, the apparatus configuration is prevented from becoming complicated and high. It becomes possible to prevent pricing.

It is preferable that the optical axis of the laser beam and the optical axis of the scattered light to be received are substantially on the same axis in order to reduce the size of the apparatus.
In this configuration, it is preferable that the non-reflective passage portion is formed substantially coaxially with the optical axis of the emitted laser light.
As a result, the energy distribution of the laser light that has passed through the non-reflective passage portion of the separator becomes axially symmetric with respect to the optical axis of the laser light, and the use efficiency of the laser light can be prevented from being lowered due to the deviation of the optical axis.

The aperture of the non-reflective passage part is preferably set based on the light intensity distribution of the laser light and the light intensity distribution of the scattered light received.
The light intensity distribution of the laser beam is high in the central part of the optical axis and low in the peripheral part. On the other hand, the light intensity distribution of the scattered light received is almost uniform. However, it is possible to improve the reception efficiency of scattered light by setting the aperture of the non-reflective passage part to a size with little decrease (loss) in light intensity.

Refractive optical elements that adjust the sizes of the laser light and the scattered light are disposed. As in the case of using a reflective optical element, the laser light is blocked by the secondary mirror and receives the scattered light. It is preferable because efficiency can be prevented from deteriorating.
In this case, it is preferable that an enclosing member that encloses an optical path of the laser beam that has passed through the separator in a light-shielded state is provided between the separator and the refractive optical element.
In this configuration, it is possible to prevent the laser light that has passed through the separator from being reflected by the refractive optical element, and to prevent the reflected light from entering the detector as stray light, and the detection accuracy of the detector can be improved.

Moreover, in this invention, the structure which has a damper which attenuates the said laser beam reflected with the said separator can also be employ | adopted suitably.
In this configuration, the laser light reflected by the separator can be prevented from entering the detector as stray light, and the detection accuracy of the detector can be improved.

Moreover, in this invention, the structure which has a signal processing apparatus which derives | leads-out the distribution between the said measuring object and the distance between the said measuring objects based on the signal which the said detector detected is also employable suitably.
In this configuration, it is possible to measure the distribution of dust and the like and the distance from the measurement object with a single device, and improve the versatility of the device.

  In the present invention, the device configuration can be simplified, the power loss of the emitted laser light can be greatly reduced, the utilization efficiency of the laser light can be improved, and a semiconductor laser device or the like as a light source, It becomes possible to use a pulse light source having a relatively low output, which can contribute to downsizing and cost reduction of the apparatus.

(First embodiment)
Hereinafter, a first embodiment of the laser measurement apparatus of the present invention will be described with reference to FIGS.
FIG. 1 is a schematic configuration diagram of a laser measurement apparatus 1.
A laser measuring apparatus 1 shown in this figure is mainly composed of a transmission / reception telescope 2, a scanning apparatus 3, a laser transmitter (light source) 4, a signal processing apparatus 5 and a control apparatus 6.
The transmission / reception telescope 2 includes a cylindrical main body 21, a refractive optical element 22 including a lens and the like, a separator 23 for branching received light, and a photodetector (detector) 24.

  The main body 21 is supported by the scanning device 3, the laser transmitter 4 is disposed at the base end in the main body 21, and the refractive optical element 22 is provided at the front end with the separator 23 interposed therebetween. The refractive optical element 22 converts the laser beam B emitted from the laser transmitter 4 and incident on the optical element 22 into substantially parallel light.

  In the main body 21, a photodetector 24 is arranged on one side (upper side in FIG. 1) in a direction orthogonal to the optical axis AX of the laser beam of the separator 23, and the other side (lower side in FIG. 1). A beam damper (damper) 25 is arranged in the front. Further, an sighting device 26 having an imaging device such as a CCD camera is provided at the upper portion on the distal end side of the main body 21.

  The scanning device 3 rotates the direction of the main body 21 in a scanning manner around the vertical axis under the control of the control device 6. The laser transmitter 4 is composed of a small-sized transmitter such as a pulsed semiconductor laser, another semiconductor laser pumped microchip laser, or a fiber laser, and generates a laser pulse (laser light) B having a predetermined repetition period and refracts it. The system optical element 22 and the optical axis AX are made coincident and emitted toward the separator 23. The laser oscillator 4 is configured to detect the output of its own laser pulse B and output it to the signal processing device 5 as a synchronization signal.

  As shown in FIG. 2, the separator 23 is configured by a mirror body that is disposed, for example, by 45 degrees with respect to the optical axis AX of the laser beam B, and the laser beam B emitted from the laser transmitter 4. And a reflecting surface (reflecting part) 28 for reflecting the backscattered light of the laser beam B from the measuring object toward the photodetector 24.

The hole 27 is formed coaxially with the optical axis AX of the laser beam B, and its aperture BS is slightly smaller than the beam diameter of the laser beam B at the position of the separator 23 and is incident via the optical element 22. It is set smaller than the beam diameter of the backscattered light. Therefore, although the laser beam B incident on the separator 23 is slightly reflected, most of it passes through the hole 27.
The aperture BS of the hole 27 is set based on the light intensity distribution of the emitted laser beam B and the light intensity distribution of the backscattered light received through the optical element 22, and details thereof will be described later. To do.

  The signal processing device 5 derives the distribution of the measurement object or the distance to the measurement object based on the signal detected by the light detector 24. As shown in FIG. 3, the gain (gain) adjustment is performed. The signal conditioners 31, 32, 33 having functions, CFD (Constant Fraction Discriminator) 34, 35 for detecting rising edges of signal pulses including noise adjusted by the signal conditioners 31, 32, respectively, have two frequencies or An external clock 36 having a variable frequency, an A / D converter 37 operated by the external clock 36, and a counter (TDC: Time to Distance Converter) 38 are included.

  The counter 38 operates at the frequency of the external clock 36, starts counting the clock signal using the light emission signal of the laser beam B input from the CFD 34 as a start pulse, and receives a signal (backscattered light) from the measurement target input from the CFD 35. In this configuration, the clock signal between them is measured and output to the control device 6 together with the clock frequency f1.

The A / D converter 37 operates with the frequency of the external clock 36 being f2 (f2 <f1), and the gain adjustment is performed by the signal adjusters 32 and 33 using the light emission signal of the laser beam B input from the CFD 34 as a start pulse. The input signal from the measurement target (backscattered light reception signal) is digitally converted and output to a control device (computer) 6 having a display screen.
The control device 6 uses the signal input from the signal processing device 5 to derive the distance to the measurement target and the distribution of the measurement target.

Next, the aperture BS of the hole 27 in the separator 23 described above will be described.
In FIG. 2, when the separator 23 is not disposed, the laser beam B is transmitted (transmitted) with the aperture DLo via the refractive optical element 22. When the separator 23 is disposed, the laser beam B is transmitted (transmitted) with the aperture DL (DL <DLo) through the refractive optical element 22.
On the other hand, the backscattered light from the measurement object enters the entire effective diameter DT of the optical element 22.

Here, the relationship between the aperture BS of the hole 27 in the separator 23 and the beam efficiency of the laser beam B will be described.
First, the output P _out actually transmitted via the refractive optical element 22 with respect to the output Po of the laser beam B emitted from the laser transmitter 4 is given by the following equation (4).
Here, the light intensity distribution of the laser beam B is assumed to be a Gaussian beam having a spot size DLoo (DLoo <DLo) for easy understanding, and the loss of the optical system such as the optical element 22 is not considered. And

  Next, the power Pdo of the backscattered light scattered from the measurement object and returning to the apparatus follows the lidar equation, but here, for easy understanding, the received light scattered from the measurement object and returned to the apparatus. If the proportionality coefficient to the transmitted light output is a, it can be expressed by the following equation (5).

  Furthermore, the power Pd of the received light incident on the photodetector 24 follows the following formula (6).

  Therefore, the utilization efficiency η of the transmitted laser beam (transmitted light) B is expressed by the following equation (7) when the above equation (2) is used.

  As another form, the efficiency when the light intensity distribution of the laser beam B is assumed to be a square beam having a constant diameter DLo is expressed by the following equation (8).

FIG. 4 shows the result of calculating the beam efficiency using the equations (7) and (8).
Here, numerical values of DT = 145 mm, DLo = 100 mm, and DLoo = 50 mm were used. The spot size DLoo is set to a value that includes almost all the optical power with the aperture DLLo.

As shown in this figure, there is a condition in which the efficiency of use A and B in the case of either a Gaussian beam or a square beam is higher than the efficiency of use C in the case of using a beam splitter. For example, when the Gaussian beam A has DL = 70, the light utilization efficiency (Pd / Po) is about 67%. For the square beam B, DL = 105 and the efficiency is about 56%. The utilization efficiency C is more than twice.
Since the actual laser beam B has a light intensity distribution deviated from an ideal distribution (Gaussian beam), the aperture BS of the hole 27 is preferably set using the actually measured light intensity distribution. The beam efficiency using the actual laser light is also within the range of the Gaussian beam utilization efficiency A and the square beam utilization efficiency B, and the utilization efficiency is higher than in the case of actually using the beam splitter.

Subsequently, a measurement process performed by the laser measurement apparatus 1 will be described.
The laser light B emitted from the laser transmitter 4 is shielded by the separator 23 at the peripheral portion where the light intensity is low, and the central portion where the light intensity is high passes through the hole portion 27 of the separator 23. That is, in terms of light intensity, the laser beam B passes through in a substantially non-reflecting state. Since the laser beam B that has been shielded and reflected by the separator 23 is attenuated by the beam damper 25, the laser beam B is diffused in the main body 21 and enters the light detector 24 as stray light (noise component) so as not to hinder measurement. Instead of providing the beam damper 25, a member or a film that absorbs the laser beam B may be provided on the surface of the separator 23 that faces the laser transmitter 4 (the surface opposite to the reflecting surface 28). In this case, for example, a configuration in which a light shielding plate formed of chromium or the like is provided, or a configuration in which the light shielding film is formed by chromium plating or the like can be employed.

The laser beam B that has passed through the hole 27 of the separator 23 is emitted from the laser measurement device 1 in a state where it is converted into parallel light having a diameter DL by the refractive optical element 22.
Then, the backscattered light from the measurement object is incident at the effective diameter DT of the optical element 22 along the same optical axis AX as the laser beam B, and reflected by the reflecting surface 28 outside the hole 27 in the separator 23 to be reflected in the donut. The light is condensed on the light detector 24 with a light distribution.

  Here, when measuring the distance to the measuring object, the counter 38 starts counting the clock signal of the external clock 36 with the rising edge of the emission signal of the laser beam B input via the signal adjuster 31 and the CFD 34 as a start pulse. Then, the counting of the clock signal is stopped using the received light signal of the backscattered light input from the light detector 24 via the signal conditioner 32 and the CFD 35 as a stop pulse. The control device 6 uses the clock signal measured by the counter 38 between the start pulse and the stop pulse and the clock frequency f1 until the laser beam B is emitted and received as backscattered light. A time difference is obtained, and since this time difference is the time required for a round trip to the measurement target, the distance to the measurement target is derived and displayed on the display screen.

  On the other hand, when dust or the like is to be measured, it is necessary to measure a weak signal. Therefore, the gain of the light detector 24 and the signal adjuster 32 is increased to detect the signal of the backscattered light. The gain-adjusted light reception signal is input to the A / D converter 37 via the signal adjuster 33. A light emission signal of the laser beam B is input to the A / D converter 37 via the signal conditioner 31 and the CFD 34, and the A / D converter 37 starts operation using this light emission signal as a start pulse. ing. Here, information such as dust is included in an analog signal incident on the photodetector 24, and the A / D converter 37 digitally converts the analog signal waveform and outputs it to the control device 6. The control device 6 analyzes the signal input from the A / D converter 37 and displays information on the measurement target (for example, dust floating state) on the display screen according to the position of the main body 21 driven by the scanning device 3.

  As described above, in the present embodiment, the separator 23 is configured to pass the laser beam B through the hole 27 and reflect the backscattered light toward the photodetector 24 at the reflecting surface 28. The configuration can be simplified, and the power loss in the separator 23 of the laser beam B emitted from the transmitter 4 can be greatly reduced, and the utilization efficiency of the laser beam B can be improved. Therefore, in this embodiment, it is possible to use a pulse light source having a relatively low output, such as a semiconductor laser device, as a light source, which can contribute to downsizing and cost reduction of the device. In this embodiment, since the telescope using the refractive optical element 22 is used instead of the reflective telescope, laser light is not shielded by the secondary mirror or the like, and deterioration of transmission and reception efficiency can be prevented.

  In particular, in the present embodiment, although the optical axis of the laser beam B and the optical axis of the backscattered light are substantially the same in-line type device configuration, the transmitted light can be transmitted without using a polarization beam splitter or a wave plate. Since backscattered light can be easily separated from a certain laser beam B, the configuration of the apparatus can be simplified, the apparatus can be miniaturized, and can be easily used in the field.

Further, in the present embodiment, the aperture 27 is set based on the light intensity distribution of the laser beam B and the light intensity distribution of the backscattered light. In addition, it is possible to improve the utilization efficiency of the laser beam B.
In addition, in this embodiment, since the hole portion 27 of the separator 23 is formed substantially coaxially with the optical axis AX of the laser light B, it is possible to prevent a decrease in the utilization efficiency of the laser light due to the deviation of the optical axis. .

  Furthermore, in this embodiment, it is possible for the signal processing device 5 to measure both the distribution of the measurement target such as dust and the distance to the measurement target with high accuracy using the result of receiving the backscattered light. In addition, it is possible to realize a highly functional laser measuring device having functions of a laser radar device and a distance measuring device. Further, in this signal processing device 5, since it is not necessary to reproduce an analog signal (analog waveform) at the time of measuring the distance to the measurement object, the amount of data is reduced and the storage memory increases or counts as the number of data increases. An increase in time can be prevented.

  In the present embodiment, since the beam damper 25 for attenuating the laser beam B reflected by the separator 23 is provided, the light intensity of the stray light incident on the photodetector 24 as a noise component can be reduced, and measurement is performed. It can contribute to improvement of accuracy and reliability.

(Second Embodiment)
Next, a second embodiment of the laser measurement device according to the present invention will be described with reference to FIG. In this figure, the same reference numerals are given to the same elements as those of the first embodiment shown in FIG. 2, and the description thereof is omitted.
The difference between the second embodiment and the first embodiment is the configuration of the separator 23.

  In the separator 23 shown in FIG. 5, a dielectric non-reflective film 47 is formed on the entire surface on the incident side of the laser beam B, and slightly smaller than the beam diameter of the incident laser beam B on the exit side surface. A dielectric non-reflective film 48 having a small diameter (aperture) BS is formed substantially coaxially with the laser beam B. These dielectric non-reflective films 47 and 48 constitute a non-reflective passage part according to the present invention. A polarizing beam splitter film (reflecting part) 49 for reflecting the backscattered light of the laser beam B by the measurement target toward the photodetector 24 is provided around the non-reflective film 48 on the emission side surface, for example. It is formed in a donut shape by vapor deposition or the like.

In the laser measuring apparatus having the above configuration, for example, the linearly polarized laser beam B is emitted from the separator 23 with the aperture BS in a substantially non-reflecting state. Then, the backscattered light from the measurement object is incident at the effective diameter DT of the optical element 22 with the same optical axis AX as the laser light B, reflected by the polarization beam splitter film 49 in the separator 23, and the donut-shaped light distribution. Then, the light is condensed on the light detector 24.
Subsequent measurement processing is the same as in the first embodiment.
Also in this embodiment, the same actions and effects as those in the first embodiment can be obtained.
Further, in this embodiment, the background light is halved by the effect of the polarization beam splitter film 49, and it is possible to perform measurement with higher sensitivity than the configuration of the first embodiment.

(Third embodiment)
Next, a third embodiment of the laser measurement device according to the present invention will be described with reference to FIG. In this figure, the same reference numerals are given to the same elements as those of the first embodiment shown in FIG. 2, and the description thereof is omitted.
The difference between the second embodiment and the first embodiment described above is that a hood for preventing stray light is provided.

  As shown in FIG. 6, a conical tube-shaped hood (enclosure member) 50 that surrounds the optical path of the laser beam B that has passed through the separator 23 is provided between the separator 23 and the refractive optical element 22. Yes. The hood 50 is formed of a light shielding material or has a light shielding surface on the inner surface, and is configured to shield the laser light B reflected by the refractive optical element 22 so as not to be emitted from the optical path of the laser light B. It has become.

  In the laser measuring apparatus of the present embodiment, in addition to obtaining the same operation and effect as the first embodiment, the laser beam reflected by the refractive optical element 22 is separated from the transmitted light and the received light. Since the reflected light of B can be prevented from entering the light detector 24 as stray light, the detection accuracy of scattered light can be improved.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but it goes without saying that the present invention is not limited to such examples. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

It is a figure which shows embodiment of this invention, Comprising: It is a schematic block diagram of a laser measuring apparatus. It is the elements on larger scale of the principal part of the laser measuring device which concerns on 1st Embodiment. It is a block diagram of the signal processing part and control apparatus which comprise a laser measuring apparatus. It is a figure which shows the relationship between the aperture of a separator, and beam efficiency. It is the elements on larger scale of the principal part of the laser measuring device which concerns on 2nd Embodiment. It is the elements on larger scale of the principal part of the laser measuring device which concerns on 3rd Embodiment.

Explanation of symbols

BS ... aperture, B ... laser pulse (laser light), 1 ... laser measurement device, 4 ... laser transmitter (light source), 5 ... signal processing device, 22 ... refractive optical element (lens), 23 ... separator, 24 ... Photodetector (detector) 25. Beam damper (damper) 27. Hole (non-reflective passing part) 28. Reflecting surface (reflective part) 47, 48. Dielectric non-reflective film (non-reflective passing part) ), 49... Polarizing beam splitter film (reflective portion), 50. Hood (enclosure member)

Claims (11)

A laser measuring device having a light source that emits laser light and a detector that detects scattered light from the measurement target of the laser light,
A laser measuring apparatus comprising: a separator having a non-reflective passage that allows laser light emitted from the light source to pass in a substantially non-reflective state; and a reflector that reflects the scattered light toward the detector. . The laser measurement device according to claim 1,
The non-reflective passage portion is a hole provided in the separator. The laser measurement device according to claim 1,
The non-reflective passage part has a dielectric non-reflective film provided on the laser light incident side surface and the emission side surface of the separator. The laser measurement apparatus according to claim 3, wherein
The laser measuring apparatus according to claim 1, wherein the reflecting portion includes a polarizing beam splitter film provided around the dielectric non-reflective film on the emission side surface. In the laser measuring device according to any one of claims 1 to 4,
An optical axis of the laser beam and an optical axis of the scattered light to be received are substantially on the same axis. The laser measurement apparatus according to claim 5, wherein
The non-reflective passage part is formed substantially coaxially with the optical axis of the emitted laser light. In the laser measuring device according to any one of claims 1 to 6,
The aperture of the non-reflective passage part is set based on the light intensity distribution of the laser light and the light intensity distribution of the scattered light received. In the laser measuring device according to any one of claims 1 to 7,
2. A laser measuring apparatus comprising: a refractive optical element that adjusts the size of the laser light and the scattered light. The laser measurement apparatus according to claim 8,
A laser measuring apparatus comprising: an enclosing member that encloses an optical path of the laser light that has passed through the separator in a light-shielded state between the separator and the refractive optical element. In the laser measuring device according to any one of claims 1 to 9,
A laser measuring apparatus comprising a damper for attenuating the laser beam reflected by the separator. In the laser measuring device according to any one of claims 1 to 10,
A laser measurement device comprising a signal processing device for deriving a distribution of the measurement object and a distance between the measurement object based on a signal detected by the detector.

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