Needless to say, license plate recognition is seeing more deployment in projects ranging from law enforcement to public/residential parking. To make LPR work effectively, setting up the system, especially the camera, is of utmost importance.Needless to say, license plate recognition is seeing more deployment in projects ranging from law enforcement to public/residential parking. To make LPR work effectively, setting up the system, especially the camera, is of utmost importance.
Mobile vs. fixed
It’s important to point out an LPR system can be mobile or fixed. In the former, the camera is installed on top of, for example, a patrol car, whereas in the latter the camera is fixed to a fixture. How the camera is installed, then, varies. “In a mobile environment, you will typically place your cameras at a 45-degree angle on the roof. This will allow to read multiple lanes of traffic with one camera,” said Robert Amante, Product Line Manager at Genetec.
In a fixed environment, there are various factors to consider when installing the camera, including its position, the width of the lane, the read distance as well as the camera angle.
When positioning the camera next to a barrier, it should be positioned behind the barrier, not in front of it. “In the latter case the vehicle might pass through the field of view before stopping. If, for any reason, the plate is mis-read, then it can never be read again,” said Lawson Noble, CTO of Vaxtor Recognition Technologies. “If the barrier rises, note that the boom arm could obscure a plate on an approaching vehicle.”
According to Noble, if the client site has a wide entrance – then an HD camera must be used. “A 1280 resolution ANPR camera, no matter how good, is not going to read plates across a 6 meter-wide lane. This would need 6 x 256 = 1536, so 1600, or 1920 for EU plates and 1920 for US sized plates,” he said.
As a general rule, the capture distance should be about three or more times the height of the camera, Noble said. “For example, if the camera has to be 3 meters high to prevent vandalism, then the capture distance should be 9 meters or more away. The shallower the angle the better,” he said.
“An ALPR camera can read to distances of up to 120 feet but the farther you try to read a plate, the more variables can come into play such as cars stacking up,” Amante said.
In a fixed environment, the user should have the camera as close as possible to the lane they want to monitor, but the camera should still have a bit of an angle. “The rule of thumb is to be within 30 degrees of pan and tilt angles,” Amante said.
According to Noble, the size of the plate captured is very important and the best way to measure this is to use the height in pixels of each character. “A good engine will read plate characters down to 12 pixels in height but generally optimum recognition is obtained with a bigger size, say 20-30 pixels high,” he said. “It is therefore critical that the correct lens be used to achieve this character height.”
He further mentioned that in some sites traffic may be crossing between lanes or approaching a site entrance at crazy angles. “If the camera cannot be positioned further back giving the traffic more time to straighten then two things can be tried: Use two cameras pointing across the entrance to cater for the extreme angles, and use traffic management, for which there are two methods, speed bumps and chicanes and lane markings,” he said. “Sometimes just marking out the lane edges with white or yellow paint can encourage the traffic to approach from the correct angle.”