The invention relates to a lamp unit (100) for a motor-vehicle lighting device, comprising: a dipped-beam module (101), a main-beam module (102), an imaging optical element (103, 503) connected downstream of the dipped-beam module (101) and the main-beam module (102), having an optical axis (104, 204, 404, 504) and a focal surface (116) orientated normal to the optical axis (104, 204, 404, 504), and a diaphragm (105, 405), which has a diaphragm edge (106, 206, 306) and extends essentially up to the focal surface (116) of the imaging optical element (103, 503) for generating the horizontal cut-off line in a light image generated by the lamp unit (100), wherein the diaphragm (105, 405) has an opaque diaphragm region (107, 407) and has a transparent diaphragm region (108, 408) with a geometric structure (109, 409) made from a transparent material at the diaphragm edge (106, 206, 306) in the region of the focal surface (116), the geometric structure (109, 409) has at least one prism body (110, 210, 310, 410, 510) with a triangular cross-sectional area, which is elongated and the longitudinal extent runs substantially transversely to the optical axis (104, 204, 404), the at least one prism body (110, 210, 310, 410, 510) has a first, a second and a third prism surface, the second prism surface (112, 212, 312, 512) encloses an internal angle α1≥θ with the first prism surface (111, 211, 311), and the third prism surface (113, 213, 313, 513) encloses an internal angle α2≥θ with the first prism surface (111, 211, 311), wherein θ is the critical angle of total internal reflection of the transparent material, the internal angles α1 and α2 are the same or different, and with the proviso that the internal angle α1 or the internal angle α2 is not 45°.