In intelligent traffic systems, VGA resolution cameras based on Charge-Coupled Device (CCD) technology have long been the standard. However, with continuous improvements in Complementary Metal-Oxide-Semiconductor (CMOS) image sensors, CMOS-based cameras are now a viable option for demanding license plate recognition applications.

System Requirements

The latest system requirements demand higher resolution (HDTV), frame rates of 30 to 60 frames per second, and support for color, Near-Infrared (NIR) sensitivity, high dynamic range, and low noise at high gain. Traffic imaging systems must also handle extreme lighting conditions and temperature variations that can affect image quality.

CCD vs. CMOS Comparison Overview

Advantages of CCD

  • Accepted technology
  • Perfect global shutter
  • Good sensitivity in NIR
  • Low noise

Advantages of CMOS

  • No smear or blooming
  • Very good dynamic range within images
  • Faster frame speeds
  • Low power consumption

Advantages and Limitations of CCD-Based Cameras

CCD-based cameras have been the traditional choice for many years, making them an accepted technology. They offer several benefits:

  • Perfect global shutter: Essential for capturing fast-moving objects without motion blur.
  • High quantum efficiency (QE): Excellent sensitivity in low-light conditions, particularly with NIR illumination.
  • Low read noise: Reduces image degradation in challenging lighting scenarios.

However, CCDs have limitations:

  • Dark current increases at high temperatures, potentially adding noise to images. While CCDs generally have lower dark current than CMOS sensors, this remains a concern in extreme environments.

Advantages and Limitations of CMOS-Based Cameras

CMOS-based cameras offer significant advantages:

  • No blooming or smear: Prevents unwanted artifacts from bright objects.
  • High frame rates (up to 30–60 frames per second): Ideal for real-time processing.
  • Low power consumption: Beneficial for systems requiring extended operation.

However, CMOS sensors also have drawbacks:

  • Higher total noise levels compared to CCDs. Noise can increase with temperature and impact image quality if not properly managed.
  • Fixed Pattern Noise (FPN): A common artifact in CMOS sensors that may require advanced processing techniques to mitigate.

Conclusion

In the near future, CMOS image sensors are expected to outperform CCD-based solutions for high-speed traffic applications. However, market adoption will take time as systems undergo necessary upgrades.

CMOS technology also holds value in non-high-speed applications due to its improved inter-scene dynamic range and energy efficiency. As sensor improvements continue, we can expect further enhancements in noise reduction and performance.

Related Article

For additional insights on image sensor advancements, see:
http://www.vision-systems.com/articles/print/volume-16/issue-10/features/leading-edge-views-machine-vision-gets-moving-part-ii.html

Last Updated: 2025-09-04 18:51:01