License plate recognition filter
With the continuous increase of highways, urban roads, and parking lots, higher and higher requirements have been put forward for traffic control and safety management. Intelligent traffic management systems have become the mainstream of traffic management development, and license plate recognition technology, as the core of this system, plays a crucial role. Due to the complexity of the license plate itself, with different backgrounds, similar shapes, and unclear damaged fonts, coupled with the fact that the car is moving and mostly photographed outdoors, obtaining high-resolution, low-noise original images is particularly important and plays a decisive role in the extraction and recognition of subsequent license plate characters. Usually, in license plate recognition, the ambient light source is very complex. It greatly affects the clear imaging of license plates. Narrowband filters are important components for isolating external interference light elements, and the selection of their performance is also crucial.
The license plate shooting device filter is usually installed in front of the lens or CCD sensor inside the camera. There is a certain angle between the camera's position and the license plate, usually less than 20 degrees. Due to the relatively large illumination range, the lighting system is generally independent.
Characteristics of interference light when taking license plate photos
The main interference lights when taking license plates are sunlight, including direct and diffuse reflection or scattered sunlight, as well as headlights of car lights, mainly halogen lamps, xenon lamps and LED lamps, and other street lights such as high-pressure sodium lamps or incandescent lamps. The interference light source is complex and has a wide wavelength range, even exceeding the spectral response range of CCD from 400 to 1100 nm. Moreover, the interference intensity is high, making it difficult to remove the interference light when shooting license plates.
Selection of Narrowband Filter
When processing license plate images, they are usually converted into black and white images. The light source used for shooting can be a visible light source or a near-infrared light source. At present, the wavelengths of near-infrared light sources are 850 nm and 940 nm, respectively. This is an invisible light that can prevent drivers from feeling dizzy while taking photos.
When choosing a filter, the following points should be considered:
1、 Determine the bandwidth range of narrowband filters based on different light sources
2、 The cut-off range of the filter should cover the spectral response range of the CCD camera on a silicon substrate (400~1100 nm).
3、 Considering the strong signal of interference light, the cut-off depth t of the narrowband filter is less than or equal to; 0.1% is more appropriate.
4、 The bandwidth of narrowband filter is suitable between 20~30nm
5、 The peak transmittance of the filter is generally required to be above 80%.
The license plate shooting device filter is usually installed in front of the lens or CCD sensor inside the camera. There is a certain angle between the camera's position and the license plate, usually less than 20 degrees. Due to the relatively large illumination range, the lighting system is generally independent.
Characteristics of interference light when taking license plate photos
The main interference lights when taking license plates are sunlight, including direct and diffuse reflection or scattered sunlight, as well as headlights of car lights, mainly halogen lamps, xenon lamps and LED lamps, and other street lights such as high-pressure sodium lamps or incandescent lamps. The interference light source is complex and has a wide wavelength range, even exceeding the spectral response range of CCD from 400 to 1100 nm. Moreover, the interference intensity is high, making it difficult to remove the interference light when shooting license plates.
Selection of Narrowband Filter
When processing license plate images, they are usually converted into black and white images. The light source used for shooting can be a visible light source or a near-infrared light source. At present, the wavelengths of near-infrared light sources are 850 nm and 940 nm, respectively. This is an invisible light that can prevent drivers from feeling dizzy while taking photos.
When choosing a filter, the following points should be considered:
1、 Determine the bandwidth range of narrowband filters based on different light sources
2、 The cut-off range of the filter should cover the spectral response range of the CCD camera on a silicon substrate (400~1100 nm).
3、 Considering the strong signal of interference light, the cut-off depth t of the narrowband filter is less than or equal to; 0.1% is more appropriate.
4、 The bandwidth of narrowband filter is suitable between 20~30nm
5、 The peak transmittance of the filter is generally required to be above 80%.