As vehicles pass through the field of view of the ALPR camera a picture is taken of license plate and the vehicle. A series of algorithms are performed on the image to isolate the plate and render the alphanumeric characters into an electronically readable format. The sophistication and complexity of each of these algorithms determines the accuracy of the system. ALPR systems can be deployed in a variety of ways, including mobile ALPR systems, fixed ALPR systems, and portable ALPR systems.
There are six primary algorithms that the software requires for identifying a license plate:
- Plate localization – Finding and isolating the plate on the picture
- Plate orientation and sizing – Compensates for the skew of the plate and adjusts the dimensions to the required size
- Normalization – Adjusts the brightness and contrast of the image
- Character segmentation – Finds the individual characters on the plates
- Optical character recognition (OCR) – Translation of images of text into an electronically readable format
- Syntactical/Geometrical analysis – Check characters and positions against state-specific rules to identify the state of issuance for the license plate
Fixed ALPR Systems
Fixed and portable ALPR systems require an installation design plan that includes infrastructure to support the camera system.33 This infrastructure includes power for the system and any networking that provides the ability to transmit data between the camera and the command/information center.
Some common considerations for fixed systems are:
• Existing physical infrastructure
• Site location
• Available power
• Available network infrastructure
• Number of cameras
• Dispatch requirements
Existing Physical Infrastructure. A great deal of physical infrastructure already exists at key locations along roadways or potential targets (e.g. sports stadium or power plant). Utilizing established infrastructure can offer a number of advantages such as reduction in costs associated with setting up a site, ease of access, and existing power connections. Consideration should be given however to the agency responsible for the infrastructure as special permits and ongoing maintenance may be required.
Site location. When choosing site locations for fixed and portable ALPR units, consideration should be given to whether officers will be routinely stationed nearby and their possible response times.
Available Power. Fixed and portable systems require power at the location of the camera. The need for power may limit the possible locations for mounting or require additional resources.
Available network infrastructure. Fixed and portable systems require network connectivity between the ALPR system’s computer processor (generally located with the camera) and the server receiving database updates. The updates enable the processors at the camera location to identify vehicles of interest that have been recently entered into the databases. Agencies should consider how this network connectivity will be accomplished to ensure successful updates are received and how the information will be secured.
Dispatch requirements. Fixed and portable systems typically provide alert notifications to the communications/operations center. This increases the workload for the dispatch personnel. Depending on the system configuration, the ALPR system may require an additional computer screen for the dispatcher to monitor. Dispatch personnel need to be effectively trained and be able to include the associated actions into their existing responsibilities. It is also important to ensure that the dispatch facility has sufficient power and space for any additional computers or servers the ALPR system may require.
Number of cameras. A fixed system typically requires the installation of one camera for each lane of traffic being monitored. Multiple cameras at one location may improve the ability to locate a suspect or wanted vehicle.