Target detection imaging system and method based on fiber laser phased array

Abstract

The present invention provides a target detection imaging system and method based on a fiber laser phased array, comprising a fiber laser phased array light source, for generating an array fiber laser with a high-speed variable intensity distribution, wherein the array fiber laser is composed of a plurality of phase-controllable fiber lasers; a light splitting unit divides the array fiber laser into an emission light and a reference light; an emission optical system projects the emission light onto the target surface; a reference optical system collects the intensity distribution of the reference light; a receiving optical system receives the return light reflected from the target and collects its intensity distribution; a computing control system implements phase control on each fiber laser in the array fiber laser, so that its intensity distribution changes at a high speed, and reconstructs the target image based on the intensity distribution of the reference light and the intensity distribution of the return light. The present invention has the characteristics of high emission power, fast refresh rate, and convenient control of the field of view of the illumination light field, and can significantly improve the target detection distance and the correlation imaging speed.

Description

Technical Field

[0001] The invention relates to the technical field of optical detection and imaging, and in particular to a target detection and imaging system based on a fiber laser phased array. Background Art

[0002] Correlation imaging is a new active imaging technology for imaging objects based on high-order correlation (intensity correlation) of light fields. Its basic principle is to divide the light field with intensity fluctuations into two beams. One beam is recorded by a camera after free propagation to obtain the light field intensity distribution, which is called the reference arm light. The other beam is called the object arm light. The scattered light or transmitted light after irradiating the object is collected by a point detector without spatial resolution. The object arm light and the reference arm light are measured multiple times according to the same time sequence signal, and then the two-dimensional or three-dimensional image of the object is reconstructed based on the correlation algorithm. Correlation imaging is a non-localized imaging method, often also called "ghost" imaging.

[0003] Compared with traditional imaging, correlation imaging has many advantages due to its special imaging mechanism. First, correlation imaging uses point detectors to collect return light. Compared with array detectors, point detectors have higher sensitivity and can achieve imaging under weak light conditions. In addition, in non-visible light bands such as infrared and X-rays, point detectors are cheaper than ordinary array detectors. Secondly, correlation imaging requires multiple sampling and uses statistical averaging to reconstruct images, so it can suppress noise that is not related to light source statistics. Finally, correlation imaging reconstructs images based on algorithms, and can be combined with advanced algorithms such as compressed sensing and deep learning to optimize images and improve imaging quality.

[0004] Due to its strong anti-noise performance and high detector sensitivity, correlation imaging has important application potential in the field of long-distance target detection. When performing long-distance detection and imaging of targets in an atmospheric environment, increasing the detection distance, improving the imaging speed, and improving the imaging quality are the three main research directions. The anti-noise characteristics of correlation imaging can effectively overcome the impact of atmospheric turbulence on imaging quality, while the high sensitivity characteristics are conducive to imaging under weak light conditions, which can effectively increase the detection distance.

[0005] However, in the specific application process, the correlation imaging technology also faces some technical problems. At present, the methods for obtaining intensity fluctuation light fields in correlation imaging systems usually include laser plus rotating frosted glass, spatial light modulator or digital micromirror array. Rotating frosted glass can obtain a changing speckle field by scattering laser at different positions of the frosted glass. The frequency of change depends on the speed of the rotating motor, so the modulation frequency of the changing speckle field is low; the spatial light modulator can obtain a light field with controllable intensity distribution through amplitude and phase modulation, but its modulation rate is also low (usually tens of Hz); the digital micromirror array obtains a light field with controllable intensity distribution through binary amplitude modulation, and its maximum modulation rate is only tens of kHz. In order to further increase the modulation frequency, LED arrays have also been widely used in correlation imaging technology in recent years to generate rapidly changing intensity fluctuation light fields, and their maximum modulation rate can reach the MHz level. However, due to the heat dissipation problem of LED arrays, their transmission power is low and they cannot be currently applied to the field of long-distance target detection.

[0006] In summary, in order to further expand the application of correlation imaging technology in the field of long-distance target detection, developing a light source with high transmission power and high modulation rate has become an important research direction for the development of this technology field. Summary of the invention

[0007] In view of the defects in the prior art, the present invention proposes a target detection imaging system and method based on a fiber laser phased array.

[0008] In order to achieve the above technical objectives, the technical solution proposed by the present invention is:

[0009] In one aspect, the present invention provides a target detection and imaging system based on a fiber laser phased array, comprising:

[0010] A fiber laser phased array light source is used to generate an interference speckle field with a high-speed variable intensity distribution, wherein the interference speckle field is composed of multiple phase-controllable fiber lasers;

[0011] A spectroscopic unit, which divides the interference speckle field into emission light and reference light;

[0012] A transmitting optical unit projects the transmitting light onto a target surface;

[0013] A reference optical unit for collecting the intensity distribution of the reference light;

[0014] A receiving optical unit receives the return light reflected from the target and records the return light intensity distribution;

[0015] The computing control unit is used to implement phase control on each fiber laser in the array fiber laser so that its intensity distribution changes at a high speed, and at the same time reconstruct the target image based on the intensity distribution of the reference light and the intensity distribution of the returned light.

[0016] On the other hand, using the above-mentioned target detection and imaging system based on fiber laser phased array, the present invention provides a target detection and imaging method based on fiber laser phased array, specifically:

[0017] The branch fiber light sources of the fiber laser phased array light source interfere to form a speckle field, and the spatial distribution of the interference speckle field is refreshed by implementing phase control through the calculation control unit;

[0018] The interference speckle field is split by the spectroscopic unit, one path is used as reference light to enter the reference optical unit to record the spatial intensity distribution of the interference speckle field, and the other path is used as reflected light to enter the transmitting unit to project the interference speckle field onto the target object.

[0019] The return light generated by the target object is received by the receiving optical unit and the recorded return light intensity distribution is transmitted to the calculation control unit;

[0020] The calculation control unit runs a reconstruction algorithm to reconstruct the target object image based on the spatial intensity distribution of the interference speckle field recorded by the reference optical unit and the intensity distribution of the return light recorded by the receiving optical unit. The reconstruction algorithm can be an intensity correlation algorithm, a differential correlation algorithm or a normalized correlation algorithm.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] 1. The present invention provides a method for correlation imaging based on a fiber laser phased array. Compared with the existing correlation imaging system based on rotating ground glass, spatial light phase modulator, and digital micromirror array, the imaging system provided by the present invention has the characteristics of high transmission power and fast modulation frequency, and can be applied to long-distance target detection to achieve rapid imaging of long-distance targets;

[0023] 2. The fiber laser phased array of the present invention can use a high-bandwidth phase modulator to achieve a phase modulation frequency of MHz or even GHz, and achieve rapid changes in the interference light field morphology. At the same time, the laser power is amplified by the fiber amplifier in each fiber link, which can achieve a high transmission power output of more than 100 watts and improve the detection distance;

[0024] 3. The interference light field of the fiber laser phased array in the present invention can realize convenient conversion of the field of view through phase control, can realize the tracking of moving targets, and can set the optimal speckle shape through return light feedback to achieve high-quality imaging.

[0025] In summary, the active correlation imaging system based on fiber laser phased array provided by the present invention has great application value in the field of long-distance target detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0027] Figure 1 is a structural diagram of an embodiment of the present invention;

[0028] Figure 2 is a light path structure diagram of an embodiment of the present invention;

[0029] Figure 3 FIG. 1 is a flow chart of an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention more clearly understood, the following will be used with drawings and detailed descriptions to clearly illustrate the spirit of the contents disclosed by the present invention. After understanding the embodiments of the contents of the present invention, any person skilled in the art can change and modify the techniques taught by the contents of the present invention without departing from the spirit and scope of the contents of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention.

[0031] In one embodiment, referring to Figure 1 , provides a target detection imaging system based on fiber laser phased array, including:

[0032] The fiber laser phased array light source 100 is used to generate an interference speckle field with a high-speed variable intensity distribution, wherein the interference speckle field is composed of multiple phase-controllable fiber lasers;

[0033] A light splitting unit 200, which splits the interference speckle field into emission light and reference light;

[0034] The emission optical unit 300 projects the emission light onto the target surface;

[0035] A reference optical unit 400 for collecting the intensity distribution of the reference light;

[0036] A receiving optical unit 500 receives the return light reflected from the target and records the return light intensity distribution;

[0037] The calculation control unit 600 is used to implement phase control on each fiber laser in the array fiber laser so that its intensity distribution changes at a high speed, and reconstruct the target image based on the intensity distribution of the reference light and the intensity distribution of the return light.

[0038] Compared with the existing correlation imaging system based on rotating frosted glass and spatial light phase modulator, the correlation imaging system of the present invention has the characteristics of high transmission power and fast modulation frequency, which can significantly improve the detection distance and imaging speed of correlation imaging; at the same time, it has the potential for single-arm computational correlation imaging.

[0039] Reference Figure 2 In one embodiment, a target detection and imaging system based on a fiber laser phased array is provided, which includes a fiber laser phased array light source, a spectroscopic unit, a transmitting optical unit, a reference optical unit, a receiving optical unit and a calculation control unit.

[0040] The fiber laser phased array light source includes a single-frequency fiber laser seed source 1, a fiber laser preamplifier 2, a beam splitter 3, a phase modulator array 5, a fiber amplifier array 6, a fiber collimator array 7, and a beam expander array 8. The single-frequency fiber laser seed source 1 is first injected into the fiber laser preamplifier 2 for power amplification. The amplified fiber laser is split by the beam splitter 3. Each fiber laser after the splitting corresponds to a phase modulator in the phase modulator array 5. Each phase modulator controls the phase change of the single fiber laser. Each fiber laser after phase modulation is re-amplified by a corresponding fiber amplifier in the fiber amplifier array 6. Each fiber laser after amplification is collimated and output by a corresponding collimator in the fiber collimator array 7, and then expanded by a corresponding beam expander in the beam expander array 8 to reduce the divergence angle. The single-frequency fiber laser seed source 1 can use a near-infrared fiber laser seed source, and the fiber amplifier array 6 uses a near-infrared fiber amplifier array.

[0041] The spectroscopic unit comprises a focusing lens 9, a field lens 10, a first reflector 11, and a spectroscope 12. The focusing lens 9 converges and interferes the multi-path fiber lasers output by the fiber laser phased array light source to form an interference speckle field. The field lens 10 images the laser beam waist on the first reflector 11, which is reflected to the spectroscope 12 by the first reflector 11. The spectroscope 12 divides the incident interference speckle field into transmitted light and reflected light, and its transmittance is about 1%. The transmitted light enters the reference optical unit as the reference light, and the reflected light enters the emission optical unit as the emission light.

[0042] The reference optical unit includes a second reflector 13, a reference lens 14, an attenuation plate 15 and a CCD camera 16 arranged in sequence. The reference light is reflected by the reflector 13 to the reference lens 14. The reference lens 14 is used to image the reference light into the CCD camera 16. The attenuation plate is used to adjust the intensity of the light field entering the CCD camera 16 and protect the light-sensitive surface of the CCD camera 16. The CCD camera 16 collects the intensity distribution of the reference light.

[0043] The transmitting optical unit includes a transmitting lens 17 , through which the transmitted light is projected onto a target object 18 , and the reflected light of the target object 18 is received by the receiving optical unit.

[0044] The receiving optical unit receives the return light reflected from the target object 18 and records the return light intensity distribution. The receiving optical unit is provided with a receiving lens 19, a third reflector 20, a collimating lens 21, a filter 22, a first imaging lens 23, an aperture 24, a second imaging lens 25 and a single pixel detector 26 in sequence along the optical path direction. The receiving lens 19 receives the return light reflected from the target object 18, and the return light is imaged onto the single pixel detector 26 after passing through the third reflector 20, the collimating lens 21, the filter 22, the first imaging lens 23, the aperture 24, and the second imaging lens 25, so as to realize the collection of the target reflected return light intensity distribution. The filter 22 is used to eliminate the influence of stray light, and the aperture 24 is used to adjust the field of view size and the signal size to improve the signal-to-noise ratio. The first imaging lens 23 and the second imaging lens 25 image the entrance pupil of the receiving lens group onto the single pixel detector to realize signal collection.

[0045] The calculation control unit includes a computer 28, a phase control unit 4 and a data acquisition unit 27. The computer 28 is combined with the phase control unit 4 to apply a phase modulation signal to each phase modulator in the phase modulator array 5 in the fiber laser phased array light source. The computer 28 is combined with the data acquisition unit 27 to collect the return light intensity signal and the intensity distribution of the reference light, and convert them into electrical signals and digital signals for data analysis and reconstructing the image of the target object.

[0046] In the fiber laser phased array light source of one embodiment, the 1064nm single-frequency laser generated by the single-frequency fiber laser seed source 1 is amplified by the fiber pre-amplifier 2 to obtain sufficient seed power, and then enters each branch fiber through the fiber beam splitter 3. The computer 28 combines with the phase control unit 4 to apply a set voltage signal to each phase modulator in the phase modulator array 5, thereby performing phase control on each branch fiber laser. The phase modulator in the phase modulator array 5 adopts a LiNbO3 electro-optical phase modulator, and the modulation bandwidth can reach the MHz level or even the GHz level. After phase modulation, each fiber laser is amplified by a corresponding fiber amplifier in the fiber amplifier array 6. Specifically, the fiber amplifier can adopt an ytterbium-doped fiber amplifier, and the fiber amplifier performs corresponding power amplification according to the detection distance requirements. After amplification, it is collimated and output by the collimator array 7, and then the spot size is increased and the divergence angle is reduced by the beam expander array 8. The light field after beam expansion passes through the focusing lens 8 to generate an interference speckle field on the focal plane. The light beam output by the collimator can be regarded as a Gaussian beam, and each fiber laser output from the output end of the fiber laser phased array light source can be expressed as:

[0047]

[0048] where a n and (x n ,y n ) are the amplitude and center coordinates of the nth fiber laser in the output plane (xy plane), w0 is the beam waist, Ф n,p represents the phase of the pth modulation cycle applied by the phase modulator to the nth fiber laser, Ф in is the initial phase of the nth fiber laser, which changes at a low speed and can be regarded as a constant when the modulation rate is high.

[0049] When the observation area is in the far field and meets the Fraunhofer diffraction condition, according to the Fraunhofer approximation theory, the light field intensity distribution on the observation plane (ξ-η plane) is the Fourier transform of the light field intensity distribution on the xy plane. On the ξ-η plane, the speckle field in the pth modulation period can be expressed as:

[0050]

[0051] Where k is the spatial wave number and D is the observation distance, so the intensity distribution of the pth speckle field can be expressed as:

[0052]

[0053]

[0054] Among them I EN(ξ，η) is the light field intensity envelope, I (ξ，η，p) The rest of is closely related to the array arrangement of the fiber lasers and the phase applied to each fiber laser.

[0055] In the transmitting optical unit, the field lens 10 is located at the focal plane of the focusing lens 9. Its function is to image the interference speckle field at the focal plane onto the transmitting mirror 17. The function of the reflector 11 is to change the direction of the light path and reduce the size of the optical system. Before the interference speckle field enters the transmitting mirror 17, the spectrometer 12 is inserted to split the intensity distribution information of the speckle field. The splitting ratio is 99:1. The reflected light with the main energy component enters the transmitting optical unit for illuminating the long-distance target object, and the transmitted light enters the reference optical unit.

[0056] In the reference optical unit, the reference lens 14 images the interference speckle field into the CCD camera 16. The magnification of the reference lens 14 should be designed and matched according to the speckle size and the pixel size of the CCD camera. An attenuation plate 15 is introduced into the imaging optical path of the reference lens 14 to adjust the intensity of the interference speckle field light entering the CCD camera 16 to protect the photosensitive surface of the CCD camera.

[0057] The transmitting mirror 17 in the transmitting optical unit images the interference speckle field onto the target object 18, and the target object 18 reflects the return light through the receiving lens 19 and enters the receiving optical unit. In the receiving optical unit, the reflector 20 is used to adjust the direction of the optical path. The return light is collimated by the collimating lens 21 and then filtered out by the filter 22 to remove stray light other than 1064nm. The collimating lens 21 is used to reduce the angle at which the return light enters the filter 22, enhance the filtering effect, and improve the imaging signal-to-noise ratio. The filtered return light passes through the imaging lens group composed of the first imaging lens 23 and the second imaging lens 25 and is recorded by the single-pixel photodetector 26 for intensity. The field of view aperture 24 is inserted between the first imaging lens 23 and the second imaging lens 25 to adjust the signal size and field of view size of the receiving system, and can also shield the stray strong light spots in the field of view of the return light to improve the imaging quality. The single-pixel detector 26 can be a photomultiplier tube or an avalanche photodiode.

[0058] The computer 28 and the data acquisition unit 27 collect the spatial intensity distribution of the interference speckle field recorded by the reference optical unit and the intensity distribution of the return light reflected by the target object recorded by the receiving optical unit, and reconstruct the image of the target object 18 based on them.

[0059] In one embodiment of the present invention, an imaging method based on the above imaging system is provided, referring to Figure 3 , the specific process is:

[0060] (S1) The branch fiber light sources of the fiber laser phased array light source interfere in the far field to form a speckle field I (ξ，η，p) , the spatial distribution of the interference speckle field is refreshed by implementing phase control through the computational control unit;

[0061] (S2) The interference speckle field is split by the spectroscopic unit and one path is used as reference light to enter the reference optical unit to record the spatial intensity distribution of the interference speckle field. (ξ，η，p) , and enters the transmitting unit as reflected light to project the interference speckle field onto the target object;

[0062] (S3) the return light T(p) generated by the target object is collected by the receiving optical unit and converted into an electrical signal by the single-pixel photodetector and transmitted to the computing control unit;

[0063] (S4) The calculation control unit constructs a reconstruction algorithm to reconstruct the target object image based on the spatial intensity distribution of the interference speckle field recorded by the reference optical unit and the intensity distribution of the return light recorded by the receiving optical unit.

[0064] In step (S3) of this embodiment, the return light T(p) generated by the target object is the interference speckle field I (ξ，η，p) The inner product with the reflectivity function W(ξ,η) of the target object satisfies the following formula:

[0065]

[0066] Where N and M are the number of pixels in the ξ and η directions of the far-field observation plane, respectively.

[0067] In step (S4) of this embodiment, the target object is reconstructed using an intensity association algorithm, and a total of K samplings are performed. The reconstructed object image satisfies:

[0068]

[0069] in is the mean value of the intensity distribution of the returned light collected by the single-pixel photodetector when the interference speckle field with different intensity distributions is sampled K times.

[0070] From the above examples, it can be seen that the correlation imaging optical system based on the laser phased array provided by the present invention has the characteristics of high transmission power and fast modulation frequency, and can achieve the effect of rapid correlation imaging of long-distance targets. At the same time, through precise phase control, it also has the potential for single-arm computational correlation imaging, providing a new and reliable technical approach for realizing long-distance target detection and imaging.

[0071] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0072] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A target detection and imaging system based on a fiber laser phased array, characterized in that: include: A fiber laser phased array light source is used to generate an interference speckle field with a high-speed variable intensity distribution, the interference speckle field is composed of a multi-channel phase-controllable fiber laser, the fiber laser phased array light source includes a single-frequency fiber laser seed source, a fiber laser pre-amplifier, a beam splitter, a phase modulator array, a fiber amplifier array, a fiber collimator array and a beam expander array, the single-frequency fiber laser seed source is first injected into the fiber laser pre-amplifier for power amplification, the amplified fiber laser is split by the beam splitter, each channel of fiber laser after the splitting corresponds to a phase modulator in the phase modulator array, each phase modulator controls the phase change of the single fiber laser, each channel of fiber laser after phase modulation is re-amplified in power by a corresponding fiber amplifier in the fiber amplifier array, each channel of fiber laser after amplification is collimated and output by a corresponding collimator in the fiber collimator array, and then expanded by a corresponding beam expander in the beam expander array to reduce the divergence angle; A spectroscopic unit, which separates the interference speckle field into emission light and reference light. The spectroscopic unit comprises a condensing lens, a field mirror, a first reflector, and a spectroscope. The condensing lens converges and interferes multiple fiber lasers output by a fiber laser phased array light source to form an interference speckle field. The field mirror images the laser beam waist on the first reflector, which is reflected to the spectroscope by the first reflector. The spectroscope separates the incident interference speckle field into transmitted light and reflected light. The transmitted light enters the reference optical unit as the reference light, and the reflected light enters the emission optical unit as the emission light. The emission optical unit projects the emission light onto the target surface. The reference optical unit includes a second reflector, a reference lens, an attenuation plate and a CCD camera arranged in sequence. The reference light is reflected by the reflector onto the reference lens. The reference lens is used to image the reference light into the CCD camera. The attenuation plate is used to adjust the intensity of the light field entering the CCD camera. The CCD camera collects the intensity distribution of the reference light. A reference optical unit for collecting the intensity distribution of the reference light; A receiving optical unit receives the return light reflected from the target and records the intensity distribution of the return light. The receiving optical unit is provided with a receiving lens, a third reflector, a collimating lens, a filter, a first imaging lens, an aperture, a second imaging lens and a single-pixel detector in sequence along the optical path direction. The receiving lens receives the return light reflected from the target object, and the return light is imaged onto the single-pixel detector after passing through the third reflector, the collimating lens, the filter, the first imaging lens, the aperture and the second imaging lens, so as to collect the intensity distribution of the return light reflected from the target. The computing control unit is used to implement phase control on each fiber laser in the array fiber laser so that its intensity distribution changes at a high speed, and at the same time reconstruct the target image based on the intensity distribution of the reference light and the intensity distribution of the returned light.

2. The target detection and imaging system based on fiber laser phased array according to claim 1, characterized in that: The control signal of one phase modulator in the phase modulator array is generated by a calculation control unit and applied to each phase modulator through a phase control unit.

3. The target detection and imaging system based on fiber laser phased array according to claim 2 is characterized in that: The optical fiber collimators in the optical fiber collimator array are arranged in a plane perpendicular to the optical path according to a certain rule, and the optical axes of each optical fiber laser output light are parallel to each other after passing through the optical fiber collimator array.

4. The target detection and imaging system based on fiber laser phased array according to claim 3 is characterized in that: The transmitting optical unit comprises a transmitting lens, the transmitting light is projected onto the target object through the transmitting lens, and the reflected light of the target object is received by the receiving optical unit.

5. The target detection and imaging system based on fiber laser phased array according to any one of claims 1 to 4, characterized in that: The calculation control unit includes a computer, a phase control unit and a data acquisition unit. The computer is combined with the phase control unit to apply a phase modulation signal to each phase modulator in the phase modulator array in the fiber laser phased array light source. The computer is combined with the data acquisition unit to collect the return light intensity signal and the intensity distribution of the reference light, and convert them into electrical signals and digital signals for data analysis to reconstruct the image of the target object.

6. A target detection and imaging method using a target detection and imaging system based on a fiber laser phased array as claimed in any one of claims 1 to 4, characterized in that: include: The fiber optic light sources of each branch of the fiber laser phased array light source interfere to form a speckle field, and the spatial distribution of the interference speckle field is refreshed by implementing phase control through the calculation control unit; The interference speckle field is split by the spectroscopic unit, one path is used as reference light to enter the reference optical unit to record the spatial intensity distribution of the interference speckle field, and the other path is used as reflected light to enter the transmitting unit to project the interference speckle field onto the target object; The return light generated by the target object is received by the receiving optical unit and the recorded return light intensity distribution is transmitted to the calculation control unit; The calculation control unit runs a reconstruction algorithm to reconstruct the image of the target object based on the spatial intensity distribution of the interference speckle field recorded by the reference optical unit and the intensity distribution of the return light recorded by the receiving optical unit.

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