The invention relates to the field of the protection of security documents such as for example banknotes and identity documents against counterfeit and illegal reproduction. In particular, the invention relates to magnetic-field-generating devices which produce positively curved magnetic field lines in a concave fashion. The invention also relates to the use of these magnetic-field-generating devices for producing optical effect layers OEL which exhibit the optical impression of a positive rolling bar effect and to processes using these magnetic-field-generating devices, e.g. in the field of document security.