What is a license plate recognition camera, and where is it used?

How does license plate recognition work?

When a vehicle enters the detection area, the camera captures one or several frames. The software finds the plate in this frame, separates the characters and reads them. The result is saved as text: which plate, from which camera, at what time it passed. In some models this is done inside the camera, and in some systems on the recorder or in separate software.

The plate that is read can be compared with a list. If a resident's vehicle is on the registered list, the barrier opens; when a vehicle that is not on the list arrives, a notification goes to the staff. In a car park, entry and exit times are matched and the time the vehicle stayed inside is calculated.

Where is it used?

License plate recognition is useful at any entrance where vehicle traffic needs to be controlled. It is installed at many different scales, from a single apartment building car park to the weighbridge entrance of a large factory.

Expectations also change depending on where it is used. At a housing estate the aim is to let residents in without making them wait; a delay of a few seconds is acceptable, but a vehicle that cannot be read needs to get through quickly by a backup method. At a factory weighbridge the aim is the record: which truck came in at what time and left at what time. Here, keeping a complete record of every vehicle matters more than the speed of the barrier. In a car park, entry and exit records must match; a plate that cannot be read at exit makes the vehicle appear to be still inside.

• Housing estate and residence entrances: automatic barrier opening for residents' vehicles.
• Paid and free car parks: tracking entry and exit times and occupancy.
• Factory, warehouse and weighbridge entrances: logging delivery vehicles.
• Hospitals, schools and public institutions: separating staff and visitor vehicles.
• Hotel and shopping mall car parks.
• Vehicle-focused businesses such as gas stations and car washes.

How it differs from a normal security camera

A high-resolution security camera can also record a plate legibly in daylight. The problem is doing this for every vehicle, at night and while moving. License plate recognition cameras are tuned for this job.

With a narrow-angle lens it sees only the lane, and the plate takes up a large part of the image. With a short exposure time it captures the plate of a moving vehicle without blurring it. Infrared light bounces back from the plate's reflective surface, so the plate stands out clearly at night too. Settings that suppress headlight glare let the characters be made out even when the headlights of an oncoming vehicle hit the camera.

Instead of forcing a general-view camera to read plates, think of two cameras at the entrance: one focused on the lane for the plate, and the other seeing the vehicle, the driver and the surroundings in wide view. After an incident, the recordings of both together give more information.

Expert note: Aim the plate camera at the point where the vehicle stops or slows down, that is, a few meters in front of the barrier. On a straight road where vehicles pass quickly, the reading rate drops.

Mounting: angle, distance and height

In license plate recognition, mounting determines the result more than the choice of device. The straighter the camera sees the plate, the more reliable the reading. A camera looking from the side or from too high sees the characters at a slant; similar characters such as 0, O, 8 and B get confused.

Plan a separate camera for each lane. Trying to read two lanes with one camera widens the angle and shrinks the plate. The camera's lens and the distance to the plate must match; the manufacturer states how many pixels wide the plate should appear in the image, and the installation is adjusted to this value.

If the rear plate also needs to be read, a separate camera is placed on the exit lane. This matters for vehicles that have only a rear plate, such as motorcycles and trailers.

Factors that reduce reading accuracy

No license plate recognition system reads every vehicle without errors. Knowing the factors that reduce reading is necessary for setting up the system and for keeping expectations realistic afterwards.

Muddy, bent, faded plates or plates with a non-standard typeface are read incorrectly. Mounts and frames fitted over the plate can cover characters. In Şanlıurfa, summer dust and winter mud dirty the plate as well as the camera glass; lens cleaning is needed more often on a plate camera than on a normal camera. Foreign plates and some special plate types may not be among the formats the software recognizes.

The angle of the sun during the day also has an effect. If the late-afternoon sun is directly in front of the camera, the reading rate can drop for a few hours. When choosing the installation spot, check which direction the camera faces and where the sun passes during the day.

Together with the barrier and car park system

License plate recognition often works together with a barrier or car park software. Decide at the start who will update the registered vehicle list: a resident who changes vehicles will wait at the barrier if they do not tell the management their new plate. For guest vehicles, define a route such as staff approval, a temporary registration or an intercom.

Also plan what will happen when a plate cannot be read. A system installed without a backup method such as a card, a remote control or the staff opening by hand locks the entrance on the first muddy day.

Because plate records can point to a vehicle and therefore to its owner, they may be treated as personal data. Decide how long the records will be kept and who will have access; where you are unsure, consult a lawyer.

Questions to ask when getting a quote

When comparing license plate recognition quotes, looking only at the camera's resolution is misleading. Two quotes may state the same megapixels, but one reads inside the camera while the other may require a separate software license. Ask the quoting party for written answers to the questions below.

In the first weeks after installation, look at the log of plates that could not be read. If the same vehicles keep going unread, the problem is the plate; if different vehicles go unread at certain hours, the sun angle, headlights or focus setting should be reviewed. This fine-tuning is a normal part of a license plate recognition installation.

• Is plate reading done inside the camera or in separate software?
• Is there a separate license or annual fee for the software?
• How many vehicle records and how many users can be defined?
• How does it connect with the barrier and car park software?
• Can a log and report be produced for plates that could not be read?
• Who will adjust the angle and focus after installation?

Frequently asked