EZ Thoughts

It wasn’t that long ago, that the idea of capturing License Plates and storing them in a database meant spending what most companies would consider to be their entire security budget.  Over the past 10 years, the use of these systems has increased dramatically and as technology has improved, the costs of having such a system have come down also.

The process of capturing vehicle plates is often referred to as ANPR (or Automatic Number Plate Recogntion), an ANPR Solution consists of 3 main elements:

1.  ANPR Camera

Specifically designed to capture car license plates due to its high TV Line resolution, ANPR Cameras are often monochrome (why need colour when you only need to see the plate), have Infra Red LED’s built in to capture at night and have a high shutter speed to cope with a moving vehicle and still keep the license plate in focus.

ANPR Camera

ANPR cameras also typically come pre-setup for capturing plates at certain distances e.g. 10m LPR Camera or 20m LPR Camera.

Once you’ve got the camera, you then need to consider the installation of the ANPR camera (see later in this article for more information on camera positioning).

2. ANPR System

This would be an embedded ANPR or PC Based ANPR DVR designed to capture the vehicle plates and store them on a database.  License plates are typically stored in JPEG image format.  ANPR Systems often include more advanced features such as the ability to create your own database, open gates or barriers using I/O connectivity and send email or other alerts upon the detection of certain license plates.

ANPR System

3. The Installation Environment

Not technically a component but a vital part of any ANPR System is the actual environment of the installation.  Many things need to be addressed including:

• Camera Position (including Angle of the camera)
• Size of the license plate on the ANPR System
• Capturing Type (Motion or some form of alarm input e.g. a button)
• Speed of the Vehicle
• Weather and Lighting Conditions

The points above all need to be taken into consideration as failure to do so could leave you with some very expensive non functioning equipment!

Camera Position

As a rule, the camera should not point down to the vehicle at an angle of more than 30 degrees.  And the camera should not be more than 10 degrees to the side.

Although some ANPR Systems might be able to capture at angles beyond these, these are the recommended optimal settings.

Position of the ANPR Camera Is Critical

Size of License Plate on screen

It’s not the case big is best in this case.  Typically, the license plate appearing on your ANPR System should be between 1/3 and 1/5 width of the screen.

Capture Type

There are 3 main ways to trigger your ANPR system to actually “capture” the  license plate:

• I/O Trigger – This is the preferred choice by using either a button, beam or sensor that acts as an alarm input signal.  This creates the most accurate form of capture as you can setup where the vehicle will be exactly, each time the trigger occurs.  So you are almost guaranteed that the vehicle will appear say 10m away at the perfect angle everytime.

• Motion Trigger – This is often used for motorways or any installation where it is not possible to use any form of alarm input signal.  Recogition success might be reduced as the vehicles could trigger at slightly different distances and angles meaning the ANPR system and Camera might miss a character or even the entire plate.

• Manual Trigger – This is used when you require some form of human interaction before allowing a vehicle access.  Some ANPR Systems have the option to specify a manual trigger (e.g. a certain key on a keyboard) in addition to using Motion or I/O.

Speed of the Vehicle

Another factor to consider is the actual speed the vehicle is travelling at.  It may be that the ANPR System can only capture up to a certain speed (e.g. 100mph) or that the ANPR Camera shutter speed needs adjusting to cope with higher speeds.

Weather & Lighting

The weather can play a part in affecting license plate capture.  Rain, sleet and snow might render even the most intelligent ANPR system useless if the camera cannot ignore these elements.  Likewise, direct sunlight can also interfere with the capturing success and where possible the position of capture should avoid direct sunlight if possible.

Of course, even when all of these factors are taken into account if the license plate is dirty, you’ll struggle to get any recognition at all!