License Plate Recognition, What it is and How it Works

Products using License Plate Recognition (LPR) Technology are gaining in popularity all over the world. One draw is its convenience. Since all countries and states require registered vehicles to display at least one license plate, there is no need for a transmitter or transponder to be attached to the car.

 Police departments make use of the technology for traffic law enforcement............

and for auto theft recovery.

 Government entities use it for security, restricting vehicle entry to secure areas. State highway authorities, as well as private parking facilities, are it using to extract payment for toll and parking fees. How do you think Repo men track down cars whose drivers neglect their auto loan payments? 

Yes, LPR systems. 

Since we’re all under its scrutiny, it would be nice to know how it works.

LPR is a form of Optical Character Recognition. OCR is the technology that is concerned with the electronic recognition and digital encoding of printed or handwritten characters by means of an optical scanner and specialized software. In OCR processing the entire image is analyzed for light and dark area so each individual character can be identified as such and saved as a coded binary character instead of a picture.

The components of LPR are designed to provide optical character recognition (OCR). The results depend on two factors:

·         image accessing components -  camera and illumination

·         Image recognizing components –   software and the recognition algorithms built into it.

Let’s look at a typical system

Camera

 An  IP camera will deliver a quality image. This is especially critical in the case of cameras that capture fast-moving vehicles. The best systems combine a camera lens with a short shutter speed and high power illumination to minimize any motion blur which would compromise recognition accuracy. Depending on the objectives, a camera is positioned to capture either the front or the rear license plate.

In some applications more than one camera is used, such as when the driver’s face also needs to be captured

Illumination

Infra-red flash has been found to be the best type of lighting for LPR. First of all since the human eye cannot detect it, the driver will not be startled, nor will any covert applications be compromised. Second, most states are retro-reflective license plates which reflect I-R light quite well.

 And third, it works equally well in day and night light.

Frame Grabber

This is the interface which provides connectivity between the camera and the PC running the LPR software, allowing it to read the image information. The frame grabber converts the analog signal from the camera into a digital stream of 1’s and 0’s that can be understood by the computer.

Computer

This is usually a PC running Windows or Linux. The computer is the coordinator of the system – running the LPR application, reading the images, analyzing and identifying the plate, all while interfacing with other applications and systems

Software

License plate recognition technology falls into the category, OCR or Optical Character Recognition. OCR technology uses software that reads the text of scanned images. Computers use OCR software to convert scanned text into binary coded digital documents.LPR software, in particular, reads the text from digital pictures of a license plate and translates it into a form that the computer can manipulate.

Putting it all together

Let’s take a look at how a LPR system works when it’s used for access control.

·         A vehicle is approaching a lift gate. It activates the LPR system by driving over a magnetic loop.

·         Infra-red illumination lights up the targeted area, usually the front or back of the vehicle including the license plate and the camera takes the picture.

·        The Frame Grabber allows the image to be sent to the PC

At this point, let’s take a detailed look. The targeted area must be singled out, i.e., the license plate. Localizing is an algorithmic function that determines the portion of the image that holds the license plate. The first task of localization is to disregard the headlight, mirror, grill, bumper, etc.

What the algorithm is searching for is a rectangular shape. But since a vehicle can have several rectangles, additional algorithms must be employed to seek out characteristics endemic to the plate, such as background color and particular dimensions. (Since license plate size and dimensions are not uniform worldwide, some systems are country-specific.)

·         Once the license plate has been isolated, the software takes over, using a different set of algorithms. It enhances the image, locates the license plate, extracts the string of characters, and identifies the fonts.

 Again, a closer look is in order. Now that the plate has been localized, other algorithms come into play.

An algorithm to adjust any angular skew so the plate image can be accurately sampled, corrected and recalculated to the optimal size

Another algorithm to regulate the brightness and contrast.

An algorithm to separate the alphanumeric characters so each one can be processed by OCR algorithms.

The isolated string of characters is translated by an OCR algorithm into an alpha numeric text entry.

And lastly a complex set of algorithms to verify the alpha numeric information and sequence, dictated by a specific rule set.

 These algorithms operate within millisections with each one dependent on the completion of the one preceding it.

Database

Upon completion of the sequence outlined above, the resulting entries can be stored on a local database or sent over a network. Minimum information would include license plate numbers, vehicle make, time, date, location. In some instances the image of the driver’s face might be desired.

Personal Testimony

Last summer, I had the opportunity to be the subject of LPR. I was driving a rental car in Florida en route to Key West when my directions instructed me to get on what’s known as Florida’s Turnpike. Subsequent signs informed me tolls were solely electronic which translated into no cash tolls. Cars needed to display the Sun Pass.

I was familiar with "Passes". My own car parked back at JFK airport in NY had an EZ Pass transponder anchored on the windshield.

EZ Pass is an interstate system that allows for prepayment via credit card. With EZ Pass one can travel the Eastern Seaboard  from Maine as far south as Virginia without stopping to pay tolls. Every time a car passes a toll reader, the fee is deducted from the credit card payment.

 Once it is exhausted, the card is hit and your account is flush once more.

 Problem was – Florida does not honor EZ Pass. It only has eyes for the Sun Pass. My rental did not have a Sun Pass.

 I was taken back. I had asked the rental agent in Orlando if I’d run into this problem and he’d assured me all Freeways accepted cash. Now what? An instant answer loomed ahead. The second option was that cameras would record your license plate and you would be billed for your total tolls plus an administrative fee.

 I wondered how this worked. I assumed Florida would bill my rental company. Would the company then bill me? Or since they already had my credit card information, would they just go ahead and charge the card? To make matters more complicated, I was driving a car with Georgia plates. I wondered how long it would take for the process to play out.

Not too long. In August I noticed the rental company had placed a charge for $13. What a seamless operation. And now I know exactly how it occurred. LPR to my rescue.

Whether you're seeking  information about LPR products or IP cameras, contacting Kintronics is also a seamless operation whether you choose to go to our website www.kintronics.com or call us at 800-431-1658. One of our knowledgeable sales engineers will be glad to assess  your needs and recommend a system. Or better yet, just fill out this information form for a quick response.