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The right lenses for CMOS
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 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
for all models. Thanks to their diameter of just 29mm, they are the smallest lenses of their kind on the market and integrate easily into compact weatherproof housings.
Tamron also offers a solution for applications such as multi-lane highway monitoring, which require higher resolutions, or scenarios demanding sensors with a larger optical format. The M111FM series delivers distortion and shading free images with C-mount cameras based on the all-new Sony Pregius sensors IMX255/267 (8.9MP, 1in) and IMX253/304 (12.3MP, 1in) as well as for CMOSIS CMV4000 (4MP, 1in) or ON Semi 5000 (5.3MP, 1in) cameras.
In machine vision applications in particular, where camera images need to be processed automatically, for instance for license plate recognition, distortion-free optical imaging by the camera lens is essential. As one of the market leading suppliers of machine vision lenses, Tamron’s optical devices are optimized for low distortion and high light transmission. Furthermore, all lenses provide several threaded holes to fix focus and aperture, including finger screws as well as headless screws. To accommodate the varying requirements of different imaging systems in traffic, the M112FM series is available at focal lengths of 75mm, 50mm, 35mm, 25mm, 16mm, 12mm and 8mm.
Conclusion
The new CMOS global shutter sensors provide revolutionary imaging performance, which is especially beneficial for machine vision applications in intelligent traffic systems.
In the future, most new system designs – and retrofits – will feature cameras with these sensors, but it is essential that the sensors have optical camera lenses matching their format and pixel size.
The Tamron M112FM series is universally applicable to all Sony Pregius CMOS sensors, with a global shutter up to 5MP, making the choice an easy one for any system engineer. n
Below: Tamron
