What to know about the camera in license plate recognition
Source: William Pao, a&s International
The components in a license plate recognition
(LPR) system include a camera, an LPR engine running on the appropriate processor, storage medium, as well as other optional items such as IP relays and sound devices. Among them, the camera
can be said as the most important.
"A normal, cheap, off-the-shelf camera will not suffice. You will see demos of low-cost cameras with slow single lane or stopped traffic with good ambient light – but the system needs to work 24/7 with all qualities of plates,” said Lawson Noble, CTO of Vaxtor Recognition Technologies
. "The camera should either be a dedicated LPR camera with twin lenses or a high-end CCTV camera which must be set up correctly.
That said, features that cameras in LPR applications should have are summarized as follows.
Engine on board vs. remote
The LPR analytics can be located either in the camera, in a local processor close to the camera, or on the backend server. More and more, dedicated LPR cameras, those with LPR engines loaded, have become increasingly popular.
“Since only plate read data needs to be transferred over the network, this means decreased data load on the network and server as all the image processing and analysis is done in the unit. The cameras are not dependent on the server, hence providing uninterrupted coverage even when connectivity goes down,” said Robert Amante, Product Line Manager at Genetec
Illumination is a key element as the camera needs to capture plates at night. “All cameras need to work in the dark. Some have their own IR illumination – and if not then this must be added,” Noble said. “If a true LPR camera is used then the IR lighting is very important. The surface mount LEDs must be fitted with the correct lenses and often diffusers to evenly spread the illumination into a rectangle and not a large circle.”
Amante meanwhile advises that different illumination wavelengths must be available where plates are not standard. “For example, plates with red characters are not ideal for IR illumination,” he said.
“Some plates are non-retro-reflective (for example UAE) and require white light to read correctly. In Florida the orange/green metal pressed plates are a real challenge and 750nm is often used which can be seen by humans as a red glow,” Noble said.
According to Noble, most modern cameras are IP-based and can send data in MJPEG or H.264. “In each case the compression should be kept to a minimum. The received image may look great to a human but when zoomed in the compressed soft images cause havoc for an OCR engine if the pate is damaged, dirty or badly lit,” he said.
Noble however cautions it is important not to use too much resolution when it is not needed. “OCR on a standard EU plate needs about 256 pixels per meter lane width – so 3 meters equals 768p. If you have a brand new 4k resolution camera, looking at one lane, then set the resolution to 1020p or even 1280p – no more is needed,” he said. “Large images cause the system to run slowly processing all the unnecessary pixels which can cause plates to be missed altogether.”
The camera should be somewhat ruggedized
to be working effectively in an outside environment 24-7. According to experts, IP67 will suffice in most scenarios.
Noble adds that cameras should also have good sunshields to keep the sun from the lens and to reduce heat. “White housings and even cooled housings are sometimes used in hot environments. Longer all-round sunshields are sometimes necessary to keep road spray from covering the lens,” he said.