The present invention discloses, inter alia, an authentication item manufactured on product units for encoding the product units as being authentic. The authentication item is an intensity-modulating surface comprising a plurality of adjacent protrusions such that the intensity-modulating surface has alternating height. Each of the protrusions has at least two slopes of respective different orientation. When being in an authenticating position with respect to incident electromagnetic radiation employed for authentication, the authentication item modulates the intensity of the electromagnetic radiation in a manner that generates an intensity profile which is characteristic of the authentication item. In the authentication position, the propagation direction of the incident electromagnetic radiation is at least approximately parallel to one of the at least two slopes of the plurality of protrusions.