A Study of optoelectronic and environmental factors affecting the accuracy of vehicle-mounted licence plate recognition

Rademeyer, Martin Christian (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Across the world, licence plate recognition (LPR) technology has been used to combat vehicle-related crime in urban areas. However, commercially available LPR systems are expensive and not feasible for large scale adoption in developing countries. The development of a low-cost system will require an informed approach to selection of an appropriate camera, as well as a realistic understanding of the system's performance under various conditions. This work investigated the effect of optoelectronic and environmental factors on the ability of vehicle-mounted LPR systems to correctly identify licence plates. A theoretical LPR camera model was developed to estimate the effect of different cameras, while the effects of motion, orientation and lighting were evaluated in a series of experimental tests. The most influential optoelectronic factors were shown to be focus, focal length and image sensor resolution. Licence plates could theoretically be recognised across a large area using a fixed-focus prime lens mounted on a high-resolution image sensor. Furthermore, recognition was impaired during high-speed turn manoeuvres, as well as in cases where licence plates were orientated at more than a 45° angle to the camera. In night-time conditions, retroffective licence plates could be recognised at a distance comparable to that of daytime conditions, while oncoming headlights were shown to hinder accurate recognition. The optoelectronic model proved useful for selection of a cost-effective camera for use in an open source LPR system. Moreover, the study of environmental factors provided valuable insight into the limitations of LPR systems in various environmental and traffic conditions.

AFRIKAANSE OPSOMMING: Regoor die wêreld is nommerplaatherkennings-tegnologie (NPH) gebruik om voertuigverwante misdaad in stedelike gebiede te bekamp. Kommersiële beskikbare NPHstelsels is egter duur en nie haalbaar vir grootskaalse aanneming in ontwikkelende lande nie. Die ontwikkeling van 'n laekostestelsel vereis 'n ingeligte benadering tot die keuse van 'n gepaste kamera, sowel as 'n realistiese begrip van die stelsel se vermoë onder verskillende omstandighede. Hierdie werk het die effek van opto-elektroniese- en omgewingsfaktore op die vermoë van voertuig-gemonteerde NPH-stelsels om lisensieplate korrek te identifiseer geondersoek. 'n Teoretiese NPH-kamera model was ontwikkel om die effek van verskillende kamera tipes te bestudeer, terwyl die effekte van beweging, oriëntasie en beligting in 'n reeks eksperimentele toetse geëvalueer was. Die mees invloedryke opto-elektroniese faktore was uitgeken as fokus, brandpuntafstand en beeldsensor resolusie. Nommerplate kon teoreties oor 'n groot gebied herken word deur die gebruik van 'n vaste-fokus primêre lens saam met 'n hoë-resolusie beeldsensor. Verder, was herkenning belemmer tydens hoë-spoed draaibewegings, sowel as in gevalle waar nommerplate teen 'n hoek, groter as 45°, weg van die kamera georiënteer was. In nagtoestande kon retroflekterende nommerplate op 'n afstand herken word vergelykbaar met dié van die dagtoestande, terwyl aankomende hoofligte akkurate herkenning belemmer het. Die opto-elektroniese model was nuttig bewys vir die keuse van 'n koste-effektiewe kamera, vir gebruik in 'n oopbron NPH-stelsel. Daarbenewens het die studie van omgewingsfaktore waardevolle insig voorsien rakende die beperkings van NPH-stelsels in verskeie omgewings- en verkeerstoestande.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106016
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