Starting sheet material (1) adapted for forming a reflector (2) for a lighting device (40), the starting sheet material (1) comprising a flat reflective sheet material (14) shaped as a sector of an annulus bendable into a parabolic reflector or a truncated cone reflector, the flat reflective sheet material (1) comprising a first surface (3), a second surface (4), a first edge (5) shaped as a segment of a circle with a first radius and a second edge (6) shaped as a segment of a circle with a second radius, the second radius being smaller than the first radius, a third edge (7) and a fourth edge (8), the third edge (7) and the fourth edge (8) extending between said first edge (5) and said second edge (6), wherein optical structures (9) are provided on at least a part of one of the first surface (3) and the second surface (4), said optical structures (9) enabling collimation and beam shaping of incident light, said optical structures (9) being applied to the flat reflective sheet material (14) along a direction (11) perpendicular to the radial direction (12) of the flat reflective sheet material (1).