Electro-Optical Technologies, Inc.

License Plate Identification/Recognition (LPI/R) Technology
Definition of Terms

Most currently-issued North American license plates utilize a reflective aluminum blank (12 × 6 inch • 304.8 × 152.4 mm) from the 3M Company (Saint Paul, Minnesota US). Although proper display of license plates on all “road-legal” motor vehicles (passenger cars, municipal vehicles, police units, fire engines, ambulances, taxicabs, limousines, hearses, trucks, vans, apportioned vehicles, transporters, motor homes, and buses) is centrally mandated, the program typically is administered by an agency at the state, provincial, commonwealth, republic, or territorial level.

The major objective of any LPI/R system is to interpret license plate alphanumerics. Along with the letters and numbers, many applications require the jurisdiction which issued the plate to be included with that information. The latter task is an onerous one because not all US States, for example, have the complete state name on the plate. Even when the name is present, usually it is too small to be captured by the image acquisition subsystem and, therefore, cannot be identified automatically. (One innovative approach is shown below.)

Sometimes, its distinctive features can be employed to help ascertain a plate’s political origin. Those are: font, syntax, number of character cells, special characters, excluded characters, and fabrication process.

Font refers to an alphanumeric character’s shape and style (typeface). All plates are not made with the same font set. There are hundreds of fonts; a standard text on typesetting (“Linotron 202 Type Specimen Book One”, Southern New England Typographic Service, Inc., Hamden, Connecticut US, 1983) illustrates most.
Syntax is any consistent ordering of characters, defined by systematic placement of letters and/or numbers. Horizontal character spacing also is a component of syntax. When fewer than the maximum number of characters appears on the plate, justification (flush left, flush right, centered, or full) is another attribute that can designate a particular issuing authority. With a “custom-ordered” or “personalized” plate (sometimes termed a “vanity” or “prestige” plate), syntactic rules and regulations generally do not apply. Though, in a majority of such cases, the plate’s character allowance (number of cells) is unchanged.
Character Cells
Every letter or number occupies a single cell. A cell nominally is three inches (76.0 mm) high; that measurement can vary slightly depending on the cell’s vertical placement. Unusual “truncated” character cells are only two inches (51.0 mm) in height. Each cell’s width and syntax (whether restricted to a letter or a number) act to determine the maximum number of cells which -- on North American plates -- is 8½.

Blank spaces, short hyphens, periods, and bullets are narrower than most characters and occupy a half-cell. Half-height characters can be stacked (in the vertical) so two are accommodated within a single cell. On occasion, three may be stacked vertically as is done by the State of Maryland, or diagonally as on some US Federal Government-issued plates. Even four diagonal characters are found on some State of Nevada dealer plates.

Special Characters
Special characters form a superset of the typical ones. Those may be a true letter (Ñ), numeral (Ø), symbol (&), or icon (like the “keystone” used on Commonwealth of Pennsylvania plates).
Excluded Characters
An excluded character is absent from the set of standard letters and numbers (A through Z; zero through nine). On State of Wyoming plates, for example, the letters “I”, “O”, and “Q” are not used when all other characters are numbers. The number “1” may not be substituted for the letter “I” nor the number “0” for the letter “O”. A four-character plate cannot contain more than two letters “W” nor two letters “M”.
“Embossing” is the most commonly encountered means of plate manufacture. A metal blank is placed face-down on an anvil and the character die is struck into it. After righting the plate, the letters and numbers appear in relief and are roll-coated.

“Incusing” (also called debossing) is similar to embossing except the plate is face-up on the anvil. Viewed from the front, the characters seem to be pressed inward. Plates from Vermont are incused.

In a single-step technique used by Alabama, Alaska, Arizona, Arkansas, Colorado, Delaware, the District of Columbia, Georgia, Hawaii, Idaho, Indiana, Iowa, Maryland, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, South Carolina, South Dakota, Tennessee, Texas, Washington, West Virginia, Wisconsin, Wyoming, the US Federal Government, and the Department of State (on some older issues), the plate is not hammered at all. A graphic lamination process applies the materials so foreground characters remain flush with the background. Other locales are transitioning to the newer methodology.

Diversity among US plates is dramatic. Frequently, different plate designs are furnished within the same jurisdiction. An elective plate style is referred to as a variety. (The phrases “special” or “special design” are synonymous.) Numerous varieties, for example, are offered by the States of Florida, Illinois, and West Virginia. The vehicle registrant may accept the “standard-issue” plate or select an alternate (such as local interest or Alma Mater commemoratives, bicentennial issues, etc.), customarily at extra cost. Most optional varieties retain the font from the standard issue; however, there are exceptions.
LPI/R technology is finding novel uses in an enormous yet continually growing number of scenarios; spanning the obvious in law enforcement to more pedestrian but nonetheless exciting applications. As the number of license plates on North American roadways reaches staggering proportions, it has become imperative to “read” each and every character on the plate. For most applications, no longer is there the luxury of ignoring half-cell or half-height characters. Indeed, difficult-to-image, small letters, numbers, and special characters are showing up with ever greater frequency. Here is one vendor’s impressive solution:

Optimizing License Plate Design for Intelligent Character Recognition
The graphic lamination process affords the opportunity to specify retro-reflectivity of foreground versus background, individually. A partially-reflective plate can be manufactured where the background -- made of 3M’s Scotchlite® Reflective Sheeting -- is retro-reflective, while the alphanumerics’ pigment is absorptive. Digital license plate technology supports custom background graphics and a rich palette of foreground colors. When illuminated for machine readability (in near-infrared, for example), the foreground becomes highly contrasted against a uniform background, shadows virtually are non-existent (because the plate is unembossed), and alphanumerics are crisp, clear, and well-defined.

Full-Color Image                     Monochrome Image                   Infrared Image (simulated)                 Segmented Image

Commercial Suppliers of Core LPI/R Technology

The following seventy-seven companies are manufacturers/vendors (versus system integrators or third party resellers) of software, hardware, and firmware-based recognition engines. They are listed at the sole discretion of E-OTEK. Application-specific product endorsements neither are expressed nor implied as each potential use is individual and unique in its requirements. (Click a plate icon for a high-resolution image.)


A copy of our Pictorial Guide to Standard-Issue Passenger Car License Plates (including Canada’s 10 Provinces and 3 Territories, México’s 31 States and the Distrito Federal, plus the 50 United States and the District of Columbia) is yours for the asking. Simply direct a request to us per the below contact details. We regret that failure to include your complete postal address will result in the lack of a response.

Contact Us
We hope the foregoing provides meaningful insight into the many complexities encountered when automating video-based LPI/R. It would be our sincere pleasure to discuss the details of your project. Please contact us for a copy of our complete statement of capabilities and qualifications or download it here.
Electro-Optical Technologies, Inc.
1960 Gallows Road, Suite 220
Vienna, Virginia 22182-3827-70
 Telephone: 1-703-893-0744
  Facsimile: 1-443-587-0303
 Skype: leejnelson
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