A lightguide defines an optical axis and has an optical absorption coefficient of at least 0.02 cm−1 at a wavelength of 500 nm. The light guide includes a plurality of discrete light extracting structures, each with a first surface configured to extract light propagating in a first direction along the light guide, a second surface configured to extract light propagating in a second direction along the light guide, and a light extraction efficiency of less than 2%. The first surface and the second surface in each of the light extracting structures making an angle with each other larger than 90 degrees; the angle between the first and second surfaces of at least one of the light extracting structures being different from the angle between the first and second surfaces of at least another one of the light extracting structures.