Optical Filters in License Plate Recognition
With the development of the national economy, the number of highways, urban roads, and parking lots is constantly increasing, posing increasingly higher demands on traffic control and safety management. Intelligent Transportation Systems (ITS) have become the main direction of current traffic management development, and license plate recognition technology, as the core of the ITS, plays a pivotal role.
Due to the complexity of vehicle license plates, which come in similar shapes, varying backgrounds, and damaged or illegible characters, and given that they are moving and are typically captured outdoors, obtaining high-resolution, low-noise original images becomes particularly crucial. These images play a decisive role in subsequent license plate character extraction and recognition. Typically, in license plate recognition scenarios, the environmental light source is highly complex, significantly affecting the clear imaging of license plates. As a component that isolates external interference light, the importance of narrowband filters is self-evident, and the choice of their performance is also crucial.
Characteristics of Interference Light During License Plate Capture
The interference light during license plate capture mainly includes sunlight, both direct and diffused or scattered, as well as headlights from cars, primarily halogen, xenon, and LED lights. Additionally, there are some road lighting sources such as high-pressure sodium or incandescent lamps. The interference light sources are complex, covering a wide wavelength range, even exceeding the CCD spectral response range of 400~1100nm. Furthermore, the interference intensity is high. Therefore, removing interference light during license plate capture is not an easy task.
Selection of Filter
When processing license plate images, it is common to convert the image into a black-and-white image with grayscale. The light source used during shooting can be either visible light or near-infrared light. Currently, near-infrared light sources with wavelengths of 850nm and 940nm are predominantly used, as they are invisible light and can avoid dazzling the driver during shooting.
When Selecting a Filter, the Following Points Should be Considered:
Determine the passband range of the filter based on the adopted light source for photography
The cut-off range of the filter must cover the spectral response range (400~1100nm) of the CCD camera with a silicon substrate
Considering the relatively strong signal of the interference light, it is more appropriate to choose a cut-off depth of T<0.1% for the narrowband filter
The bandwidth of a narrowband filter should be between 20 and 30nm
The peak transmittance of the filter should be higher, generally requiring a value greater than 80%.
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Hanzhong Brisun Optics Co., Ltd. Is the high precision optical element manufacturer provides customized production of Various optical lenses, including spherical lens, cylindrical lens, optical window, mirror, prism, filter, metal base mirror and other high-precision optical elements. The base materials include various optical glass, fused quartz, calcium fluoride (CaF2), zinc selenide (ZnSe), germanium (GE), silicon (SI), sapphire, metal and other materials. And provide antireflective film, high reflection film, spectroscopic film, metal film and other optical coatings.
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