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Q: OCR for License Plates ( Answered 3 out of 5 stars,   3 Comments )
Subject: OCR for License Plates
Category: Computers
Asked by: dustymack-ga
List Price: $50.00
Posted: 05 Jan 2005 09:46 PST
Expires: 04 Feb 2005 09:46 PST
Question ID: 452412
I'm interested in seeing the various techniques people have used to
perform OCR (Optical Character Recognition) on License Plates and
their respective accuracy rates (both potential accuracy and actual
empirical accuracy).

Such techniques would include various template matching methods and
neural network architectures.

Of particular interest are links to research papers explaining
particular methods and software proof-of-concepts.
Subject: Re: OCR for License Plates
Answered By: maniac-ga on 09 Jan 2005 18:58 PST
Rated:3 out of 5 stars
Hello Dustymack,

The general approach apparently taken by several vendors can be
summarized as follows:
 - capture image(s) and select the best (or several good) frame(s)
 - select the "license plate" portion of image / rectify
 - isolate the characters for recognition
 - identify the characters 
followed by some post processing steps where the identified characters
are used (usually a database look up). For an example of this, see
and scroll down a little for a flow chart and several samples of the
steps used in this student project.
A tutorial on license plate recognition by one of the vendors.
Includes how license plate recognition can be used for a variety of

Specific methods used include:
 - template matching; match each letter to a specific example. Noted
in several papers that this can be quite difficult to apply in
 - structural matching; you look at the shape and connections of a
letter. For example, comparing between B, D, 9, and 6 you could
recognize the loops and relative position (top, bottom, center) and
size but have difficulty distinguishing betwen B and 8.
 - neural networks; trained with several samples can be resistant to
noise but may require a large investment in time / training with
different character shapes (font).

A very good summary of the issues with automatic recognition of license plates at
Scroll to the bottom of this reference as well for an extensive list
of suppliers. If you are interested further in the use of photography
in trafic enforcement, I suggest starting at the home page
and checking several other links as well. Some describe methods used
to obscure or work around the use of photo systems in police
enforcement. There is also another list of companies (with links) that
provide more turn key solutions.

A Dutch site, using video capture plus speed measurement to help
enforce speed limits and fining the car owner.
Scroll up / down for several good sections of information. In
particular note the claimed system accuracy of over 99% on
"identification" (apparently is a vehicle present) but only 78% on
license plates. Also note this takes images at three locations (start,
800 meter, 3000 meters) for the speed measurement so there is a need
in this system to recognize the "same vehicle" in each image.

Apparently the Dutch also found it necessary to change the font of
their license plates to improve recognition. See
for an example.

Somewhat short on details but does talk about resolution, types of
cameras, and related issues at

A look at the more general problem of identifying text in a video and
extracting that text:
Also has references to license plate capture from video (1997 IEEE
conference paper).

A recent conference (October 2004) with several OCR / vehicle license
plate recognition papers

A couple hundred references to techincal papers found with the search phrase
  license plate recognition citeseer
For example, a couple clicks from the second reference brought me to
A paper titled "New Methods for Automatic Reading of VLP's (Vehicle
License Plates)" which includes several references to / from this
paper as well. I strongly suggest using citeseer for technical
references if you have specific concepts or phrases to research.
Google also has a recent service at
where I entered the phrase
  vehicle license plate recognition
and had another 800+ references. Note that in this case, the
references are free but the full document may require a fee for

A FEW vendor sites:
Adaptive Recognition Hungary, producer of both hardware and software
for license plate recognition.
SAIC provides turnkey and custom systems for recognition as well as
integration with with related systems (e.g., recognize loads on
trucks, driver photo identification). SAIC claims over 95% accuracy on
identification numbers with higher rates if backup data is available.
Also indicates a rate of vehicles that can flow through their
recognition site (roughly 100 to 300).
Not specific to license plates, but this vendor of Neural Network
software includes an OCR data sample to train / recognize if you want
to get a feel for the issues involved.
High Tech Solutions, provides several related vehicle license plate
systems. Claims use at 2004 Olympics in Greece.
A more restricted system, recognizing license plate numbers during
manufacture of the plates (but very high speed - 1000 per hour).

Free and for fee software / algorithms are available from links at
for a variety of computer vision applications (including license plates).

Search phrases used include:
  ocr automobile license plate
  automatic recognition automobile license plate
  license plate recognition

Request for Answer Clarification by dustymack-ga on 14 Jan 2005 09:15 PST
Some of those are very good links.  Thank you.

I'm specifically interested in the accuracy ratings of specific
methods, the links are just a bonus.

Clarification of Answer by maniac-ga on 17 Jan 2005 17:47 PST
Hello Dustymack,

I am sorry the answer was not completely satisfactory. Let me clarify
the accuracy values I had previously found and provide some additional

The Dutch system claims only 78% accuracy on license plate recognition (LPR).
The SAIC system claims about 95% accuracy for LPR and when combined
with a data base lookup, higher rates [presumably by comparing "close"
matches with a threshold].
This gives you a range of accuracies - a review of the system
implementations should give you an idea of how these accuracies are

Some additional accuracy references are available through searches such as 
  LPR license accuracy
  VLP license accuracy

including sites such as:
over 90% accuracy using an automobile sensor, flash unit, rapid camera
speed, and interface to government plate databases. Addresses all US
and Mexican states, and all Canadian provinces.
A system in place in Singapore with 99.7% accuracy of "good plates".
Limited to plates issued in Singapore.
Description of a system in place at Sky Harbor Airport in Phoenix.
Reads 92% of all "readable" plates and within 2 digits 99% of the
time. Note this method of doing matches with less than perfect data.

Please note there is little information relating the specific methods
to achieve the accuracy indicated. I may be able to produce a table
like this
  Method A + B + C => Accuracy X
  Method A + D + C => Accuracy Y
  Method E + D + C => Accuracy Z
or is the information provided sufficient?


Request for Answer Clarification by dustymack-ga on 19 Jan 2005 16:31 PST
Such a table would be absolutely perfect.
In particular, when referencing a method I'm interested in the
specific method used and not so much the implementation.

Clarification of Answer by maniac-ga on 23 Jan 2005 18:13 PST
Hello Dustymack,

The sites that describe specific methods and accuracy appear to be
even more rare than I expected. The following summarizes what I could
find for specific systems.

[1] IPMS Vehicle License Plate Recognition System
Methods used for image capture:
 - standard CCTV, using a single frame (even or odd) sized so
characters are at least 18 pixels high
 - lighting, options for continuous or strobed, visible or near IR
 - single or multiple images captured
Methods for OCR:
 - breadth first AI for locating plate location in image
 - no real specifics on OCR method except it allows for "fuzzy"
matches, most likely a neural network with confidence levels
Sample plates read / accuracy:
This vendor claims over 99.7% accuracy for Singapore plates.

[2] US Customs Example
Methods used for image capture:
 - high speed "video camera"; says sensor is sensitive enough for
1/10000 second image but does not state it captures at that rate
 - lighting appears to be near visibile IR strobe
 - image capture of both front and rear of vehicle
Methods for OCR:
 - text implies it categorizes plates by issuing agency (e.g., US
state, Canadian province, presumably to aid in recognition
 - no detail on method for individual character recognition
 - has back end interface to national databases for verification; may
be used to "correct" recognition of plates
Sample plates read / accuracy:
Article claims 90% accuracy for Canadian, Mexican, and US plates.

The other online references I found would either describe methods or
accuracy but not both in the same context.

Another reference I happened to find while searching that you may find interesting.
Shows a series of images captured in natural light and comparing to
use of near IR lighting and cameras (scroll down to section starting
with "Optimizing License Plate Design..."

dustymack-ga rated this answer:3 out of 5 stars
Excellent research; but does not include originally requested data.

Subject: Re: OCR for License Plates
From: jerome0001-ga on 05 Jan 2005 14:40 PST
don't know that much about it, but here in ontario, canada hwy 407
uses OCR for billing.

Maybe you can contact some one there for more info.
Subject: Re: OCR for License Plates
From: johnfrommelbourne-ga on 05 Jan 2005 22:52 PST
Citylink in Melbourne Australia have just started a service whereby
they now advertise that their latest technology will mean that object
we stick on inner windscreen of car that is read by scanning
technology when we go through one or more sections of tollroad is now
no longer required( although still preferred) as latest technology can
simply read licence plate( here, registration plate, with  great
accuracy and record the $1.50 (or whatever fee was) against owner of
that registration plate.

 Is that of any interestor relevance?? If so just advise

 John From Melbourne
Subject: Re: OCR for License Plates
From: javisv-ga on 08 Jan 2005 02:44 PST
From sourceforge:
License: GNU
OOCR (Open OCR) is a open source character recognition program, it is
used to convert images to editable text.

License: GNU
wxOCR - an application to recognize and extract text from a given image

From planet-source-code.
Free Proyect
Experimental Optical Charachter Recognation using Neural Nets
"recognises numbers from 0 to 9
The net needs to be trained atleast 250 times on the font of your choice.
If not trained it will give undesirableresults."
Visual Basic

Free Proyect
Ocr Analyser
divide a letter in regions and check the average pixel diference.

Free Proyect
OCR a screen capture
Works with Screen captures, detects Text Matches with a specific font.

Free Proyect
A Artificially Intelligent Pattern Recognizer OCR
Works with Data Areas | Visual Basic 6

Free Proyect
Character Recognition Software v1.0
Works with Text Patterns C++

To more ocr proyects

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