A narrow-band light-supplementing imaging system and method

Abstract

This invention discloses a narrowband supplementary lighting imaging system. A first imaging unit is used to acquire an environmental image based on external ambient light. A second imaging unit is used to receive external light signals, process the light signals to obtain narrowband supplementary lighting, and acquire an enhanced license plate image based on the narrowband supplementary lighting. A fusion unit, connected to both the first and second imaging units, is used to fuse the environmental image and the enhanced license plate image to obtain a fused image containing a clear license plate. This invention effectively improves the recognition accuracy and output image reliability of the license plate recognition system by acquiring an environmental image and an enhanced license plate image, and then fusing the environmental image and the enhanced license plate image to obtain a fused image.

Description

Technical Field

[0001] This invention relates to the field of traffic lighting technology, and more particularly to a narrowband supplementary lighting imaging system and method. Background Technology

[0002] Clear imaging of vehicle license plates at night is a core supporting technology for intelligent transportation scenarios such as road checkpoints, speeding detection, and park entrance and exit management. In natural nighttime environments with insufficient illumination, conventional visible light cameras struggle to capture identifiable license plate details. The industry commonly employs active illumination solutions to enhance the brightness of the shooting area. This involves setting up a supplementary light source next to the camera to illuminate passing vehicles, utilizing the retroreflective properties of the license plate surface to increase reflected brightness and assist the camera in imaging the license plate. To balance the brightness difference between the bright license plate and the dark background, related technologies incorporate wide dynamic range imaging and multi-exposure fusion, attempting to preserve both license plate and background details within the same frame. Some solutions also explore using multiple cameras to independently acquire scene and license plate images, attempting to avoid the exposure issues inherent in single-channel imaging. After years of iteration, these technologies can now meet the basic usage requirements in scenarios with good lighting conditions and low vehicle speeds.

[0003] However, existing methods still face significant limitations in practical applications. Vehicle license plates are made of high retroreflectivity materials, and under supplemental lighting conditions, the reflected brightness of the license plate area is far higher than that of the vehicle body, road surface, and surrounding environment—the brightness difference can be over a hundred times. Conventional wide dynamic range sensors cannot cover such a large light ratio. When shooting high-speed vehicles, multi-exposure fusion schemes are prone to motion blur and ghosting issues due to the relative displacement between the license plate and the background, resulting in blurred and unrecognizable character edges. Existing multi-camera solutions often employ general-purpose optical designs, requiring telephoto lenses to ensure imaging resolution for distant license plates. This results in a relatively thick overall device, making it difficult to fit into the compact housing of existing standard monitoring equipment. Deployment in confined spaces such as roadside poles and gantries is also challenging. During the fusion process, images independently captured by different cameras are prone to license plate misalignment due to lens position deviations. Dark noise captured by the dedicated license plate capture channel can also contaminate the clear scene image after fusion, leading to problems such as localized black spots and obvious stitching marks in the final output image. Existing nighttime license plate imaging solutions struggle to simultaneously ensure scene integrity, license plate clarity, and equipment deployment flexibility, failing to meet the all-weather stable imaging requirements in complex road environments. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention discloses a narrowband supplementary lighting imaging system, comprising: The first imaging unit is used to acquire environmental images based on external ambient light. The second imaging unit is used to filter out external light signals to retain narrowband supplementary light, and to acquire an enhanced license plate image based on the narrowband supplementary light. The fusion unit, connected to the first imaging unit and the second imaging unit respectively, is used to fuse the environmental image and the license plate enhancement image to obtain a fused image containing a clear license plate.

[0005] Preferably, it also includes a narrowband fill light, which is used to emit narrowband fill light to the license plate of a moving vehicle, and the narrowband fill light is received by the second imaging unit after being reflected by the license plate.

[0006] Preferably, the first imaging unit, the second imaging unit, and the fusion unit are integrated inside the monitoring housing, and the narrowband fill light is disposed outside the monitoring housing.

[0007] Preferably, the second imaging unit includes a second lens, a light processing module, and a second image sensor arranged sequentially. The second lens is used to acquire external light signals; The light processing module is used to filter and calibrate the light signal acquired by the second lens to filter out light signals outside the target band and retain narrowband fill light; The second image sensor is used to process the narrowband fill light calibrated by the light processing module to obtain an enhanced license plate image.

[0008] Preferably, the light processing module includes: Lens assembly, used to calibrate light signals; An infrared filter is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in the lens group include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group adopts a folded optical path structure, the infrared filter is disposed inside the lens group and is located in front of or behind the lens.

[0009] Preferably, the light processing module includes: Lens assembly, used to calibrate light signals; An infrared filter is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in the lens group include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group adopts a non-folded optical path structure, the infrared filter is placed before or after the lens group.

[0010] Preferably, it also includes a narrowband fill light, which is used to emit narrowband fill light to the license plate of a moving vehicle, and the narrowband fill light is received by the second imaging unit after being reflected by the license plate; The second image sensor is a black and white image sensor; The narrowband supplemental lighting imaging system also includes a control unit, which is connected to the second image sensor and the narrowband supplemental lighting lamp respectively; The control unit includes: The data acquisition module is used to acquire the grayscale data of the license plate in the enhanced license plate image; A processing module, connected to the data acquisition module, is used to process the license plate grayscale data to obtain the exposure status of the license plate enhanced image; A first adjustment module, connected to the processing module, is used to adjust the exposure parameters of the second image sensor according to the exposure conditions; The exposure parameters include at least one of the gain of the second image sensor and the exposure time of the second image sensor; The second adjustment module is connected to the processing module and is used to adjust the operating parameters of the narrowband fill light according to the exposure conditions. The operating parameters include at least one of the output light intensity of the narrowband fill light and the operating timing of the narrowband fill light.

[0011] Preferably, the fusion unit includes: The candidate region determination module is used to determine the candidate regions of the license plate that contain the clear license plate in the license plate enhancement image; An enhanced region determination module, connected to the candidate region determination module, is used to obtain the first region coordinates of the license plate candidate region on the license plate enhanced image, and determine the second region coordinates on the environment image according to the first region coordinates and a preset mapping relationship; The fusion module, connected to the enhancement region determination module, is used to overlay the license plate candidate region onto the corresponding position of the second region coordinates on the environmental image to obtain the fused image.

[0012] Preferably, the fused image is obtained using the following formula:

[0013] in, F represents the fused image; M is the mask corresponding to the license plate candidate area; This refers to the candidate area for license plates; I1 is the environmental image.

[0014] This invention discloses a narrowband supplemental lighting imaging method, applied to the narrowband supplemental lighting imaging system, comprising: An environmental image is obtained by using the first imaging unit based on external ambient light; The second imaging unit filters out external light signals to retain narrowband supplementary light, and the license plate enhancement image is obtained based on the narrowband supplementary light. The environmental image and the license plate enhancement image are fused using a fusion unit to obtain a fused image containing a clear license plate.

[0015] The following beneficial effects can be obtained by using the present invention: By processing ambient light to obtain an environmental image, and processing narrowband supplementary light to obtain a license plate enhancement image, the license plate enhancement image is fused with the environmental image to obtain a fused image containing a clear license plate. This solves the problems of exposure contradiction caused by excessive brightness difference between license plate and background under nighttime supplementary lighting, motion blur caused by high-speed shooting, difficulty in compact deployment of long focal length lenses, and misalignment and noise pollution caused by dual-channel fusion. It effectively improves the recognition accuracy and reliability of the license plate recognition system. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the narrowband supplementary lighting imaging system of the present invention; Figure 2 This is a schematic diagram of the structure of the first imaging unit and the second imaging unit of the present invention; Figure 3 This is a schematic diagram of the structure of the lens group of the present invention positioned in front of the infrared filter; Figure 4 This is a schematic diagram of the structure of the lens group of the present invention positioned behind the infrared filter; Figure 5 This is a schematic diagram of the control unit of the present invention; Figure 6 This is a schematic diagram of the fusion unit of the present invention; Figure 7 This is a schematic diagram of the narrowband supplementary lighting imaging method in this invention.

[0017] In the attached diagram: 1. First imaging unit; 11. First lens; 12. First image sensor; 2. Second imaging unit; 21. Second lens; 22. Light processing module; 221. Lens group; 222. Infrared filter; 23. Second image sensor; 3. Fusion unit; 31. Candidate region determination module; 32. Enhancement region determination module; 33. Fusion module; 4. Control unit; 41. Data acquisition module; 42. Processing module; 43. First adjustment module; 44. Second adjustment module; 5. Narrowband fill light. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0021] This invention discloses a narrowband supplemental lighting imaging system, such as... Figure 1 and Figure 2 As shown, it includes: The first imaging unit 1 is used to acquire environmental images based on external ambient light. The second imaging unit 2 is used to filter out external light signals to retain narrowband supplementary light, and to acquire an enhanced license plate image based on the narrowband supplementary light. The fusion unit 3 is connected to the first imaging unit 1 and the second imaging unit 2 respectively, and is used to fuse the environmental image and the license plate enhancement image to obtain a fused image containing a clear license plate.

[0022] Specifically, the first imaging unit 1 includes a first lens 11 and a first image sensor 12. The first lens 11 is used to receive external ambient light, and the first image sensor 12 is used to process the ambient light to obtain an environmental image. The first image sensor 12 is disposed behind the first lens 11 and together with the first lens 11 constitutes the first imaging unit 1.

[0023] On one hand, the first imaging unit 1 is used to acquire environmental images, including vehicles, roads, and the surrounding environment. The first lens 11 receives ambient light reflected from the road scene, vehicles, and license plate area, and images the ambient light onto the first image sensor 12. The first image sensor 12 converts the received ambient light signal into a first image signal, forming an environmental image on the detection surface of the first image sensor 12, which is used to reflect the road environment, vehicle outlines, vehicle driving status, and the overall monitoring screen.

[0024] In one embodiment, a visible light transmittance filter or an infrared cutoff filter is provided between the first lens 11 and the first image sensor 12, so that the first image sensor 12 mainly obtains environmental images suitable for overall scene observation and reduces the impact of infrared supplementary light on the overall scene color and brightness distribution.

[0025] In applications requiring observation of black-and-white scenes at night, the first imaging unit 1 can also retain a certain infrared response.

[0026] On the other hand, the second imaging unit 2 is used to acquire an enhanced image of the license plate. The light processing module 22 is used to filter out or weaken light in non-target wavelength bands, so that the second image sensor 23 mainly receives narrowband supplementary light, thereby improving the image contrast of the license plate area.

[0027] Furthermore, the fusion unit 3 is used to fuse the clear license plate in the license plate enhancement image into the environmental image to obtain a fused image.

[0028] In a preferred embodiment of the present invention, such as Figure 5 As shown, it also includes a narrowband fill light 5, which is controlled by the second adjustment module 43 of the control unit 4; The narrowband fill light 5 is used to emit narrowband fill light to the license plate of a moving vehicle. The narrowband fill light is reflected by the license plate and then received by the second imaging unit 2.

[0029] Specifically, the narrowband fill light 5 consists of one or more infrared light-emitting devices.

[0030] The narrowband fill light 5 emits narrowband fill light towards the vehicle's license plate area for supplemental lighting. After the narrowband fill light shines on the license plate, due to the strong reflective properties of the license plate surface, part of the narrowband fill light will be reflected back from the license plate area. The second lens 21 receives the external ambient light and the narrowband fill light reflected back from the license plate. After processing by the light processing module 22, only the narrowband fill light is retained. The second image sensor 23 acquires the enhanced license plate image based on the narrowband fill light.

[0031] Furthermore, during the day or when ambient light is sufficient, the first imaging unit 1 can be primarily used to output scene images. In this state, the narrowband supplementary light 5 can be turned off, and the second imaging unit 2 can be in a switched-off, standby, or auxiliary acquisition state. The first imaging unit 1 can directly acquire images containing vehicles, license plates, and road environment, meeting the needs of conventional monitoring and license plate recognition.

[0032] In a preferred embodiment of the present invention, the first imaging unit 1, the second imaging unit 2 and the fusion unit 3 are integrated inside the monitoring housing, and the narrowband fill light 5 is disposed outside the monitoring housing.

[0033] Specifically, the narrowband supplemental lighting imaging system is mounted on license plate recognition devices installed in areas such as highways and parking lot entrances and exits. The license plate recognition device includes a monitoring housing, which houses a first imaging unit 1 and a second imaging unit 2.

[0034] The narrowband supplementary light 5 can be positioned on the same side of the monitoring housing as the first lens 11 and the second lens 21, or between the first lens 11 and the second lens 21, or at other suitable locations on the monitoring housing, to provide imaging and supplementary lighting for road vehicles and license plate areas. The illumination direction of the narrowband supplementary light 5 is selected towards the area where license plates may appear to improve the efficiency of infrared supplementary lighting. The specific number, arrangement, divergence angle, and output power of the narrowband supplementary lights 5 can be selected based on the road width, shooting distance, number of lanes, and installation height.

[0035] Furthermore, the monitoring housing can be a single-piece housing or a housing structure composed of components such as a front housing, a rear housing, a mounting bracket, and a protective cover. The first lens 11, the second lens 21, and the narrowband fill light 5 can be mounted on the same front panel or on the same rigid bracket. This rigid mounting structure ensures that the relative positions of the two imaging channels remain stable during transportation, installation, and long-term use, reducing the need for repeated calibration.

[0036] In a preferred embodiment of the present invention, such as Figure 2 As shown, the second imaging unit 2 includes a second lens 21, a light processing module 22, and a second image sensor 23, all configured in one step. The second lens 21 is used to acquire external light signals; The light processing module 22 is used to filter and calibrate the light signal acquired by the second lens 21 to filter out light signals outside the target wavelength band and retain narrowband supplementary light; The second image sensor 23 is used to process the narrowband supplementary light calibrated by the light processing module 22 to obtain the license plate enhancement image.

[0037] Specifically, both the first lens 11 and the second lens 21 are mounted on the front end or front side of the monitoring housing. The second image sensor 23 is located behind the second lens 21 and together with the second lens 21 and the light processing module 22, constitutes the second imaging unit 2.

[0038] Furthermore, the optical axes of the first lens 11 and the second lens 21 are arranged in parallel to facilitate obtaining the lens position relationship between the first lens 11 and the second lens 21. Based on the lens position relationship, the mapping relationship between the environmental image formed by the first imaging unit 1 and the license plate enhancement image formed by the second imaging unit 2 is obtained.

[0039] In a preferred embodiment of the present invention, such as Figure 3 and Figure 4 As shown, the light processing module 22 includes: Lens group 221 is used to calibrate the light signal; Infrared filter 222 is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in lens group 221 include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group 221 adopts a folded optical path structure, the infrared filter 222 is set inside the lens group 221 and is located in front of or behind the lens.

[0040] Specifically, the lens group 221 adopts a narrowband-specific compact structure, which can be set between the second lens 21 and the second image sensor 23, or directly mounted on the second lens 21. It is used to optimize narrowband supplementary lighting in the target band, so that the license plate character edges are clearer and the license plate area imaging size is larger in the license plate enhancement image formed by the second image sensor 23.

[0041] In one embodiment, the target band is a narrowband band, and the infrared filter 222 used retains narrowband supplementary light within 800 nm to 900 nm.

[0042] Specifically, the narrowband fill light emitted by the narrowband fill light 5 is within the target wavelength band, such as 850 nm. The infrared filter 222 retains the light signal within the target wavelength band, i.e., the narrowband fill light, and filters out the light signal outside the target wavelength band, thereby reducing the impact of ambient light, vehicle headlights, and other stray light on the license plate enhancement image.

[0043] Since the second imaging unit 2 mainly operates in the narrow band that matches the narrow band supplement lamp 5, the compact lens group 221 does not need to take into account the wide range of visible light chromatic aberration correction at the same time. It can focus on correcting spherical aberration, field curvature, astigmatism or distortion in the target band, thereby improving the resolution of the license plate area in the license plate enhancement image.

[0044] Furthermore, the infrared filter 222 can be disposed between the second lens 21 and the second image sensor 23, or it can be disposed at the front or rear end of the second lens 21, as long as it is located in the optical path of the second imaging unit 2 and enables the second image sensor 23 to mainly receive the narrowband supplementary light of the target band corresponding to the narrowband supplementary light lamp 5.

[0045] The infrared filter 222 can be a bandpass filter, a long-pass filter, or other filter elements that can achieve infrared band selection.

[0046] Furthermore, when employing a folded optical path structure, the lens group 221 may include one or more of the following: a polarization beam splitter, a phase delay element, a semi-transparent and semi-reflective element, a reflective element, or a folding prism, so that the narrowband supplementary light signal reflected from the license plate is folded back and propagated at least once within the lens group 221. Through this folded optical path, the second imaging unit 2 can form a longer equivalent optical path within a shorter physical length, thereby adapting to application scenarios where the thickness of the monitoring housing is limited but long-distance license plate magnification imaging is required.

[0047] Furthermore, narrowband optimized lenses and folded optical path structures such as the aforementioned aspherical lenses, freeform lenses, diffractive optical elements, and hybrid diffractive lenses can be used simultaneously.

[0048] The folded optical path structure is used to compress the physical length of the optical path in the second imaging unit 2 and extend the equivalent optical path. The lens group 221 is used to correct aberrations and improve the imaging quality of license plate characters in the target wavelength band. Through the combination of the two, a compact, telephoto-optimized, and narrowband-optimized license plate enhancement imaging channel can be formed.

[0049] In a preferred embodiment of the present invention, such as Figure 3 and Figure 4 As shown, the light processing module 22 includes: Lens group 221 is used to calibrate the light signal; Infrared filter 222 is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in lens group 221 include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group 221 adopts a non-folded optical path structure, the infrared filter 222 is placed before or after the lens group 221.

[0050] Specifically, when the infrared filter 222 is positioned before the lens group 221, the light signal is first filtered, and after retaining the narrowband supplementary light, the narrowband supplementary light is calibrated through the lens group 221. When the infrared filter 222 is positioned after the lens group 221, the light signal is first calibrated through the lens group 221, and then the calibrated narrowband supplementary light is retained through the infrared filter 222.

[0051] In a preferred embodiment of the present invention, the second image sensor 23 is a black and white image sensor; The narrowband supplemental lighting imaging system also includes a control unit 4, which is connected to the second image sensor 23 and the narrowband supplemental lighting lamp 5 respectively; like Figure 5 As shown, the control unit 4 includes: Data acquisition module 41 is used to acquire the license plate grayscale data of the license plate enhancement image in the license plate candidate region; Processing module 42, which is connected to data acquisition module 41, is used to process the license plate grayscale data to obtain the exposure of the license plate enhancement image; The first adjustment module 43 is connected to the processing module 42 and is used to process the license plate grayscale data to obtain the exposure of the license plate enhanced image, and adjust the exposure parameters of the second image sensor 23 according to the exposure. The exposure parameters include at least one of the gain of the second image sensor 23 and the exposure time of the second image sensor 23; The second adjustment module 44 is connected to the processing module 42 and is used to process the license plate grayscale data to obtain the exposure of the license plate enhancement image and control the working parameters of the narrowband fill light 5 according to the exposure. The operating parameters include the output light intensity of the narrowband fill light 5 and the operating timing of the narrowband fill light 5.

[0052] Specifically, black and white image sensors do not require color filter separation, which can improve the utilization efficiency of infrared reflected light signals and help enhance the imaging brightness and character contrast of license plate areas at night.

[0053] Meanwhile, the first image sensor 12 can be selected from color image sensors or other image sensors suitable for overall scene imaging according to actual application needs, in order to obtain images of vehicles and road environments.

[0054] The license plate enhancement image includes a license plate candidate region, which is used to represent the area where a clear license plate is located in the license plate enhancement image.

[0055] Because the narrowband fill light 5 illuminates the license plate, the reflected signal in the license plate candidate area of ​​the license plate enhancement image may be significantly stronger than that in the surrounding background area. If the second imaging unit 2 still uses a long exposure time or a high sensor gain, the license plate candidate area of ​​the license plate enhancement image is prone to large areas of whitening, bright spots, or characters being obscured. Therefore, a control unit 4 is provided to continuously acquire the license plate grayscale data in the license plate candidate area of ​​the license plate enhancement image through the data acquisition module 41.

[0056] The license plate grayscale data includes the average grayscale, peak grayscale, saturated pixel ratio within the candidate license plate area, and the grayscale difference between the characters and the background.

[0057] The processing module 42 processes the license plate grayscale data to obtain the brightness of the license plate area in the candidate area of ​​the license plate, which is used as the exposure of the license plate enhancement image.

[0058] When the brightness of the license plate area exceeds a preset brightness threshold, the first adjustment module 43 reduces the exposure time and gain of the black and white image sensor to create a suitable grayscale difference between the license plate background and the characters, thereby avoiding overall overexposure of the license plate area. This adjustment method will significantly darken the background area outside the license plate, but will result in a license plate enhancement image with clearer character edges and higher contrast.

[0059] In a preferred embodiment, the control unit 4 continuously acquires the grayscale data of the license plate within the candidate region of interest (ROI) and performs closed-loop adjustment on the second image sensor 23. For example, let the average grayscale within the ROI be Yavg, the saturation pixel ratio be Rsat, and the grayscale difference between the character and the background be ΔY. When Rsat exceeds a preset saturation threshold, the control unit 4 reduces the exposure time or gain of the second image sensor 23; when Yavg is below a preset brightness threshold, it increases the exposure time, gain, or output intensity of the narrowband fill light 5; when ΔY is within a preset recognizable range, it locks the current parameters and outputs the enhanced license plate image. This method avoids the overexposure of the license plate area caused by ordinary full-image automatic exposure which primarily controls the dark background.

[0060] In addition, the control unit 4 can also adopt a fixed parameter control method.

[0061] Specifically, the fixed parameter control method refers to preset the exposure time and gain of the second image sensor 23 based on the on-site test results in order to obtain a clearer license plate enhancement image.

[0062] In a preferred embodiment of the present invention, such as Figure 6 As shown, the fusion unit 3 includes: The candidate region determination module 31 is used to determine the candidate regions of license plates containing clear license plates in the license plate enhancement image; Enhanced region determination module 32, connected to candidate region determination module 31, is used to obtain the first region coordinates of the license plate candidate region on the license plate enhanced image, and determine the second region coordinates on the environment image according to the first region coordinates and a preset mapping relationship; The fusion module 33 is connected to the enhancement region determination module 32, and is used to cover the license plate candidate region to the corresponding position of the second region coordinates on the environmental image to obtain the fused image.

[0063] Specifically, after receiving the two images—the license plate enhancement image and the environmental image—the fusion unit 3 first performs necessary preprocessing on the two images.

[0064] Preprocessing can include, but is not limited to, noise reduction, brightness adjustment, distortion correction, cropping, scaling, time synchronization, license plate region extraction, and image format conversion.

[0065] Subsequently, the preprocessed license plate enhancement image and the environment image are transmitted to the candidate region determination module 31 to determine the license plate candidate region.

[0066] Furthermore, the first lens 11 and the second lens 21 are rigidly mounted on the same monitoring housing or the same mounting reference component. The first lens 11 and the second lens 21 are parallel or aligned in the vertical direction, resulting in a fixed horizontal offset between the two images. Because the relative positional relationship between the two images is stable, the mapping relationship between them can be obtained through pre-calibration. In actual operation, image registration is performed directly based on the mapping relationship, reducing problems caused by rotation, tilt, or complex perspective transformations due to installation errors.

[0067] The coordinates of the second region on the environmental image are obtained through the mapping relationship, which are used to represent the area covered by a clear license plate in the candidate license plate region.

[0068] A corresponding mask is generated based on the candidate license plate region. The mask is then used to cover the corresponding position of the license plate candidate region on the environmental image to obtain a fused image containing a clear license plate.

[0069] In a preferred embodiment of the present invention, the fused image is obtained using the following formula:

[0070] in, F represents the fused image; M is the mask corresponding to the license plate candidate area; This is the candidate area for vehicle license plates; I1 represents the environmental image.

[0071] This local mask fusion method enhances only the effective area of ​​the license plate, avoiding the dark background of the second imaging unit 2 from affecting the overall scene image.

[0072] This invention discloses a narrowband supplemental lighting imaging method, applied to the aforementioned narrowband supplemental lighting imaging system, such as... Figure 7 As shown, it includes: An environmental image is obtained by using the first imaging unit 1 based on external ambient light. The second imaging unit 2 filters out external light signals to retain narrowband supplementary light, and the license plate enhancement image is obtained based on the narrowband supplementary light. The fusion unit 3 is used to fuse the environmental image and the license plate enhancement image to obtain a fused image containing a clear license plate.

[0073] A preferred embodiment is now provided to help those skilled in the art understand the present invention: When operating at night or in low-light environments, the narrowband fill light 5 is activated and emits narrowband fill light towards the vehicle license plate area. The first imaging unit 1 acquires an overall nighttime scene image to provide information on the vehicle outline, road environment, and surrounding background. The second imaging unit 2 receives external light signals through the second lens 21, retains the narrowband fill light reflected from the license plate through the infrared filter 222 and lens group 221, calibrates the narrowband fill light, acquires the narrowband fill light through the second image sensor 23, and shortens the exposure time and / or reduces the sensor gain under the control of the control unit 4 to obtain a license plate enhancement image with clearer license plate characters. The fusion unit 3 fuses the license plate enhancement area in the license plate enhancement image to the corresponding position in the environmental image, and outputs the fused image through the interface, which can be used for display, storage, transmission, license plate recognition algorithm processing, or traffic law enforcement evidence collection.

[0074] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A narrowband supplemental lighting imaging system, characterized in that, include: The first imaging unit is used to acquire environmental images based on external ambient light. The second imaging unit is used to filter out external light signals to retain narrowband supplementary light, and to acquire an enhanced license plate image based on the narrowband supplementary light. The fusion unit, connected to the first imaging unit and the second imaging unit respectively, is used to fuse the environmental image and the license plate enhancement image to obtain a fused image containing a clear license plate.

2. The narrowband supplementary lighting imaging system according to claim 1, characterized in that, It also includes a narrowband fill light, which is used to emit narrowband fill light to the license plate of a moving vehicle. The narrowband fill light is reflected by the license plate and then received by the second imaging unit.

3. The narrowband supplementary lighting imaging system according to claim 2, characterized in that, The first imaging unit, the second imaging unit, and the fusion unit are integrated inside the monitoring housing, and the narrowband fill light is located outside the monitoring housing.

4. The narrowband supplementary lighting imaging system according to claim 1, characterized in that, The second imaging unit includes a second lens, a light processing module, and a second image sensor arranged sequentially. The second lens is used to acquire external light signals; The light processing module is used to filter and calibrate the light signal acquired by the second lens to filter out light signals outside the target band and retain narrowband supplementary light; The second image sensor is used to process the narrowband fill light calibrated by the light processing module to obtain an enhanced license plate image.

5. The narrowband supplementary lighting imaging system according to claim 4, characterized in that, The light processing module includes: Lens assembly, used to calibrate light signals; An infrared filter is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in the lens group include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group adopts a folded optical path structure, the infrared filter is disposed inside the lens group and is located in front of or behind the lens.

6. The narrowband supplementary lighting imaging system according to claim 4, characterized in that, The light processing module includes: Lens assembly, used to calibrate light signals; An infrared filter is used to filter out light signals outside the target wavelength band in order to retain narrowband supplementary light; The lenses in the lens group include any one of aspherical lenses, freeform lenses, diffractive optical elements, and hybrid refractive-diffractive lenses; When the lens group adopts a non-folded optical path structure, the infrared filter is placed before or after the lens group.

7. The narrowband supplementary lighting imaging system according to claim 4, characterized in that, It also includes a narrowband fill light, which is used to emit narrowband fill light to the license plate of a moving vehicle. The narrowband fill light is reflected by the license plate and then received by the second imaging unit. The second image sensor is a black and white image sensor; The narrowband supplemental lighting imaging system also includes a control unit, which is connected to the second image sensor and the narrowband supplemental lighting lamp respectively; The control unit includes: The data acquisition module is used to acquire the grayscale data of the license plate in the enhanced license plate image; A processing module, connected to the data acquisition module, is used to process the license plate grayscale data to obtain the exposure status of the license plate enhanced image; A first adjustment module, connected to the processing module, is used to adjust the exposure parameters of the second image sensor according to the exposure conditions; The exposure parameters include at least one of the gain of the second image sensor and the exposure time of the second image sensor; The second adjustment module is connected to the processing module and is used to adjust the operating parameters of the narrowband fill light according to the exposure conditions. The operating parameters include at least one of the output light intensity of the narrowband fill light and the operating timing of the narrowband fill light.

8. The narrowband supplementary lighting imaging system according to claim 1, characterized in that, The fusion unit includes: The candidate region determination module is used to determine the candidate regions of the license plate that contain the clear license plate in the license plate enhancement image; An enhanced region determination module, connected to the candidate region determination module, is used to obtain the first region coordinates of the license plate candidate region on the license plate enhanced image, and determine the second region coordinates on the environment image according to the first region coordinates and a preset mapping relationship; The fusion module, connected to the enhancement region determination module, is used to overlay the license plate candidate region onto the corresponding position of the second region coordinates on the environmental image to obtain the fused image.

9. The narrowband supplementary lighting imaging system according to claim 8, characterized in that, The fused image is obtained using the following formula: ； in, F represents the fused image; M is the mask corresponding to the license plate candidate area; This refers to the candidate license plate area; I1 is the environmental image.

10. A narrowband supplemental lighting imaging method, characterized in that, Applied to the narrowband supplemental lighting imaging system as described in any one of claims 1-9, comprising: An environmental image is obtained by using the first imaging unit based on external ambient light; The second imaging unit filters out external light signals to retain narrowband supplementary light, and the license plate enhancement image is obtained based on the narrowband supplementary light. The environmental image and the license plate enhancement image are fused using a fusion unit to obtain a fused image containing a clear license plate.

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