Importance of cut - off line in automotive headlamps aiming

The basic element of passing beam headlights is the "cut-off line". It was intended in European beam pattern as tool for correct adjustment of passing beam according good road illumination distance and oncoming drive eyes protection of excessive glare. Cut-off line was established on the one hand as a natural part separating bright and dark area in the conventional passing beam, on the other assigned its essential function of the visual aiming of headlights. It is used in laboratory practices as well as in vehicles factories and in services and technical periodical inspection. The most important is that cut-off line was defined visually and subjectively over 50 years ago. But needed aiming precision according guaranteed road visibility and glare protection is much higher than historically moulded practical cut-off performance. In paper is analysed definition of cut-off line, their performance and meaning, historical evolution and influence for technical development of headlamps design. Also, mathematical approximation and modelling of cut-off line visual perception is described. The important is analysis of practical examples of real cut-off lines, their connections with significant beam pattern characteristics and possible ambiguous aiming during type approval and during vehicle exploitation. Also, analyzed are perspectives of future cut-off line use, development or - may be replacement by other means better serving this very important task to guarantee correct adjustment beam pattern according good road illumination and protection against excessive glare.