A light source comprising: a light generating unit for generating light having a coherence length in the range of 3 to 10 μm, wherein the light generating unit is an organic light emitting diode, OLED, and an outcoupling device for coupling light out of the light generating unit in an outcoupling direction, wherein the outcoupling device comprises: a first region for facing the light generating unit, the first region being made of a first material, being optically homogenous, a second region having a refractive index being smaller than the refractive index of the first region, and an intermediate region between the first region and the second region, the intermediate region being structured and being made of said first material, wherein the intermediate region comprises particles for structuring the intermediate region, the particles having a refractive index being a) equal to or smaller than the refractive index of the second region or b) equal to or larger than the refractive index of the first region, and wherein the particles are embedded on the surface of the second region in the intermediate region so that they are embedded in said first material, wherein the organic light emitting diode comprises a cathode layer, an anode layer and several intermediate layers between the cathode layer and the anode layer, and characterized in that the first region has a thickness in the outcoupling direction being larger than 10 μm, wherein the refractive index of the first region matches with an average of the refractive indices of the intermediate layers. a light generating unit for generating light having a coherence length in the range of 3 to 10 μm, wherein the light generating unit is an organic light emitting diode, OLED, and an outcoupling device for coupling light out of the light generating unit in an outcoupling direction, wherein the outcoupling device comprises: a first region for facing the light generating unit, the first region being made of a first material, being optically homogenous, a second region having a refractive index being smaller than the refractive index of the first region, and an intermediate region between the first region and the second region, the intermediate region being structured and being made of said first material, a first region for facing the light generating unit, the first region being made of a first material, being optically homogenous, a second region having a refractive index being smaller than the refractive index of the first region, and an intermediate region between the first region and the second region, the intermediate region being structured and being made of said first material, wherein the intermediate region comprises particles for structuring the intermediate region, the particles having a refractive index being a) equal to or smaller than the refractive index of the second region or b) equal to or larger than the refractive index of the first region, and wherein the particles are embedded on the surface of the second region in the intermediate region so that they are embedded in said first material, wherein the organic light emitting diode comprises a cathode layer, an anode layer and several intermediate layers between the cathode layer and the anode layer, and characterized in that the first region has a thickness in the outcoupling direction being larger than 10 μm, wherein the refractive index of the first region matches with an average of the refractive indices of the intermediate layers.