An illumination module (40) for emitting light beams includes a substrate (20) having a main inner surface (21), a plurality of pairs of holders (30) integrally formed on the surface and a plurality of linear light sources (40). Each holder defines a pair of adjoining slots (32, 33), each slot having a guiding portion (321, 331) and an adjoining retaining portion (322, 332). The retaining portions of each holder elastically hold one end of a corresponding linear light source.