Electro-Optical Technologies, Inc.
License Plate Identification/Recognition (LPI/R) Technology
Definition of Terms

Many currently-issued North American license plates utilize a reflective aluminum blank (~12 x ~6 inch · 304.8 x 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, some applications require the jurisdiction which issued the plate to be included with that information. This 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, it’s usually too small to be captured by the image acquisition subsystem and, therefore, cannot be identified automatically. (One innovative approach is shown below.)

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


 
Font
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
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 (whether 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 (7.6 cm) high; this measurement can vary slightly depending on the cell’s vertical placement. Unusual “truncated” character cells are only two inches (5.1 cm) 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 (in the vertical) can be stacked so two are accommodated within a single cell. On occasion, three may be stacked vertically as done by the State of Maryland, or diagonally 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. They may be true letters (Ñ), numerals (Ø), symbols (&) or icons (like the “keystone” found on State 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”.
Fabrication
“Embossing” is the most commonly encountered means of plate manufacturing. 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.

“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, Idaho, Indiana, Iowa, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New York, Ohio, Oregon, South Carolina, South Dakota, Tennessee, Texas, Washington, 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 considering a transition to this newer methodology.

Varieties
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” also are synonymous.) Numerous varieties, for example, are offered by the States of 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, bicentennials, Y2K issues, etc.), customarily at extra cost. Most optional varieties retain the font from the standard issue; however, there are exceptions.

Applications
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, these pesky -- 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. This 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 eighty-two 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 Atlas of Standard-Issue Passenger Car License Plates (including Canada’s 10 Provinces and 3 Territories, Mexico’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. Please be certain to include your complete postal address.
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.
Box 3125, Falls Church, Virginia 22043-0125-25
United States of America
Telephone: 1-703-893-0744
   Facsimile: 1-443-587-0303
 Skype: leejnelson
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