The present invention relates to light curtains, in particular safety light curtains, for monitoring a protective field. Furthermore, the present invention relates to optical units which are part of such a light curtain according to the present invention, an optical unit comprises a plurality of radiation emitting and/or radiation receiving elements for transmitting and/or receiving radiation beams forming said light curtain, and an elongated support element forming an outer housing of said optical unit said support element having two opposing peripheral regions which are formed to allow an abutting assembly with another identical optical unit. Said radiation emitting and/or radiation receiving elements are arranged within said support element to form a row, and wherein at least one peripheral one of said radiation emitting and/or radiation receiving elements is located directly adjacent to an outer wall of at least one of said peripheral regions of the support element.