Here are the key factors to consider when choosing a camera for ANPR:
Resolution
While high resolution is often synonymous with better image quality, ANPR systems do not necessarily require extremely high-resolution cameras. A camera with a resolution of 5 megapixels (MP) is typically sufficient for most ANPR applications. Higher resolutions might offer more detail, but can result in larger file sizes and increased processing times, which can hinder system performance.
Chromaticity
In the context of ANPR systems, chromaticity, or the quality of color, plays a crucial role in image clarity and accuracy. Monochrome cameras are often preferred due to their higher sensitivity to light. This makes them particularly effective in low-light conditions. They can also offer better contrast, aiding in the clear distinction of license plate characters. This can significantly improve accuracy.
However, color information may be necessary. For example, identifying vehicle types through color-coded plates or capturing additional contextual information during daylight. In some advanced systems, hybrid approaches that combine monochrome and color cameras are employed to leverage the strengths of both types.
Also read: Monochrome Camera vs. Color Camera: All You Need to Know
Dynamic Range
Dynamic range refers to a camera’s ability to capture details in both very bright and very dark areas of an image simultaneously. For ANPR systems, a high dynamic range (HDR) is essential to ensure that license plates are readable under varying lighting conditions, such as direct sunlight or shadows.
HDR prevents images from being washed out in bright light or obscured in deep shadows, maintaining clear and accurate plate information. Thus, by choosing cameras with HDR capabilities, ANPR systems can operate effectively in diverse environments – from bright urban streets to dimly lit rural roads.
Exposure Time
Exposure time is a critical factor that affects how well a camera performs in varying light conditions. ANPR cameras should ideally have auto exposure capabilities. This allows the camera to adjust its exposure settings automatically based on the surrounding light.
During the daytime, the environment is typically bright. So, a lower exposure time is needed to prevent overexposure and ensure the details of the license plate are captured clearly. Conversely, at night or in low-light conditions, a higher exposure time is necessary to capture enough light and detail. Auto exposure ensures that the camera can adapt to these changes in lighting conditions seamlessly.
Shutter Type
The type of shutter a camera uses can significantly impact its performance, especially when capturing fast-moving objects. For ANPR systems, a global shutter camera is recommended in cases where vehicles are moving fast. Unlike rolling shutters, which capture an image sequentially from top to bottom, global shutters capture the entire image at once.
A global shutter eliminates the distortion that can occur with a rolling shutter at high speeds, ensuring that the license plate is captured accurately and without any artifacts. This is essential for the system to correctly read and interpret the license plate data, even when the vehicle is moving rapidly.
Additional Considerations
Apart from these primary factors, there are other considerations that can enhance the performance of ANPR cameras:
Weatherproofing: Cameras used in outdoor ANPR applications should be weatherproof to withstand various environmental conditions such as rain, snow, and dust. Ensuring that the camera is rated for outdoor use (e.g., IP66 or IP67) will enhance its longevity and reliability.
Infrared (IR) Capability: The infrared or near-infrared capability of a camera is beneficial for capturing clear images at night. Cameras equipped with IR LEDs can illuminate the license plate without visible light. This becomes especially useful for nighttime surveillance and monitoring.
Mounting and Positioning: The placement and mounting of the camera can also affect its performance. The camera should be positioned to capture the license plates at an optimal angle – typically around 30 degrees to the vehicle’s direction of travel. Proper mounting can reduce glare and reflections, further improving image quality.
Connectivity and Integration: Ensure that the camera can integrate seamlessly with the ANPR software and other system components. Compatibility with network protocols and ease of integration are crucial for the overall functionality of the ANPR system.