anpr oct13_000_Benchmark_jul13 10/09/2013 15:07 Page 3
Masterclass
nothing wrong with this approach – or indeed with using conventional cameras for ANPR capture – but a number of caveats exist with regard to the specifications of the cameras, and how they can be applied. A general rule of thumb is that where processing or settings are automatic, often the parameters for self-adjustment will be created with typical wide area surveillance in mind, rather than specialised use such as ANPR. Beware!
The right angles
unable to capture suitable images of any vehicle travelling at speeds in excess of a few miles per hour. Often, ANPR will be used at points where traffic slows or temporarily halts. However, this does not mean that shutter speed can be ignored. The philosophy of expecting the unexpected holds true. It is recommended by Dacolian, an ANPR supplier, that minimum shutter speeds of 1/600 for 30mph traffic, 1/750 for 40mph traffic, 1/900 for 50mph traffic and 1/1300 for 70mph traffic are used. Obviously, the figures need to be rounded upwards to meet the camera settings. Note that the settings should be fixed; automatic shutter speed with a high maximum figure cannot be guaranteed to deliver the right results. Obviously, as shutter speed increases, so illumination levels need to increase too! The preferred option is to use infrared illumination (along with an IR corrected lens to prevent focus shift). Infrared lighting works well with retroreflective plates, and many dedicated ANPR cameras also include light filters or pulsed illumination to eradicate or compensate for headlight glare which can obscure numberplates. Also, infrared lighting will not dazzle drivers! There is a fine line when it comes to defining what an ANPR camera is and isn’t! There are some units which have been specifically designed to address many of the issues associated with numberplate recognition. However, there are also a growing number of conventional cameras which have been packaged as ‘ANPR’ units, often by simply including IR illumination. There is 48
www.benchmarkmagazine.com
Once you have a camera capable to delivering the right level of performance, the next task is to ensure that it is mounted correctly. Also, the right lens must be specified. Typically, ANPR system recommendations are that the numberplate should fill 20 per cent of the image width at full resolution. Cameras should be positioned with a vertical angle of no more than 30 degrees, and a horizontal angle of up to 20 degrees, although this should allow for deviations in vehicle approach. The plate should preferably be in the centre of the screen, and its angle should be such that if a horizontal line was drawn it would pass through both sides of the numberplate. Most UK numberplates are retroreflective, which means light is reflected directly back towards the source.
The final part The final part of the ANPR solution is an appropriate software package. Mirasys recommends making several considerations about the choice of software. If you have a preferred VMS, it probably has an ANPR plug-in. Solicit feedback from colleagues and peers. Asking for reference site details is not always easy – manufacturers often cannot disclose that information. Consider the supplier’s support. Will they assist with the installation? Do they offer training? How detailed is the documentation? Also what is the total cost of installation, maintenance and ownership? Expect there to be a few days (and nights) of configuration and testing. ANPR has to be set up to meet the site’s requirements. Configuring ANPR for reliability does take time. Remember that no matter how good the software itself, poor image capture can only result in errors!