1. A lighting device comprising:a luminescent concentrator, the luminescent concentrator comprising a waveguide having a radiation input face, a radiation exit face, and a width (W) being defined by the radiation input face and an opposite face, the waveguide comprising a radiation converter element distributed in the waveguide with a radiation converter concentration, wherein the waveguide comprises a single crystal;
a solid state light source configured to irradiate the radiation input face of the waveguide with solid state light source radiation;
wherein the radiation converter element is configured to absorb at least part of the light source radiation and to convert into radiation converter element radiation, and
wherein a concentration of the radiation converter element within the waveguide is selected to be at least three times of that necessary for the radiation converter element to absorb 98% of the light source radiation over the width (W) of the waveguide.