150 | TRAFFIC MANAGEMENT
The right lenses for new CMOS sensors The latest CMOS global shutter image sensors offer a step change in reducing blur in readouts of vehicles traveling at speed. For camera manufacturers and users to enjoy the full benefits of this new technology, however, the sensors must be matched with the correct lenses Words | Dr Ronald Mueller, Tamron, Japan
Modern traffic systems using automatic license plate recognition (ALPR, also known as automatic number plate recognition, or ANPR), for example in tolling, law enforcement or route tracking, require CMOS (complementary metal oxide semiconductor) sensors with global shutter technology. These are now available with larger optical formats and higher resolutions than previous chips.
New sensors, better performance Unlike previously available CMOS sensors with rolling shutter, the new chips read out all pixels at the same time. Thus, the blur of images due to the motion of a vehicle during readout is cut to a minimum. CCD (charge-coupled device) sensors, on the other hand, have always provided this feature. The new CMOS global shutter technology, however, provides much higher frame rates, lower readout noise, higher
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dynamic range and digital image data, right from the sensor output. While this allows for a simpler and lower-cost camera design, the new sensors are, or will be, produced in much higher quantities than CCDs ever were, leading to significantly reduced unit prices. Since these sensors come with larger formats and resolutions, they demand new camera lens designs.
Benefits for traffic applications Tamron is one of the first major lens manufacturers to release new products specifically developed for the new global shutter CMOS sensors. The new M112FM series with fixed focal length offers a cost-effective solution for all C-mount cameras, for example with sensors from the Pregius sensor family by Sony. These sensors are especially well suited to traffic applications with and without artificial lighting, for several reasons…
Firstly, their global shutter and high sensitivity allows for short exposure times of just a few microseconds, which ensures blur-free images independent of the velocity of vehicles. Secondly, low readout noise, down to two electrons (the second Pregius generation), provides crisp images even under low-light conditions. Finally, their outstanding saturation capacity of up 30,000 electrons (the first Pregius generation) produces captures of high dynamic range (HDR) with clear grayscale resolution of details in both bright areas (e.g. number plates) and dark areas (e.g. drivers’ faces). The M112FMs support all sensors of the first and second generation, such as: IMX174/249/302 (1st gen., 2.4MP, 1/1.2in), IMX250/264 (2nd gen., 5.1MP, 2/3in) and IMX252/265 (2nd gen., 3.2MP, 1/1.8in). Despite the differences in sensor formats of up to 1/1.2in and pixel sizes as small as 3.45µm, the lenses of this series are suitable
