A light generating system comprising: - a plurality of light sources configured to provide light source light; - an elongated luminescent body comprising an elongated support and a coating layer, wherein the elongated luminescent body further comprises a body axis, and a length parameter P of a body dimension perpendicular to the body axis, wherein the length parameter P is selected from height, width and diameter, wherein: - the elongated support comprises a support material, a support material index of refraction n1, wherein the support material index of refraction n1 is at least 1.4, a support surface, and a support length; - the coating layer is configured on at least part of the support surface over at least part of the support length (L 1), wherein the coating layer comprises a coating layer material, a coating layer index of refraction n2, wherein coating layer index of refraction n2 is at least 1.4, and a coating layer thickness, wherein the coating layer material has a composition different from the support material, wherein the coating layer material comprises a luminescent material configured to absorb one or more of UV radiation and visible light, and to convert it into luminescent material light having a wavelength at a spectral maximum of the luminescent material light in one or more of the visible and the infrared; - the support material is transmissive for the luminescent material light, and -0.2≤n1-n2≤0.2 and d1/P≤0.25 apply, - wherein the coating layer, or an outer layer configured over the coating layer, has a root mean square height Sq of at maximum 1/10 of a wavelength at a spectral maximum of the luminescent material light, and - wherein the elongated luminescent body comprises one or more side faces, wherein the elongated luminescent body comprises a radiation input face and a radiation exit window, wherein the radiation input face is configured in a light receiving relationship with the plurality of light sources, wherein the luminescent material configured to absorb at least part of the light source light and convert it into the luminescent material light, and wherein the radiation input face is configured perpendicular to the radiation exit window. - a plurality of light sources configured to provide light source light; - an elongated luminescent body comprising an elongated support and a coating layer, wherein the elongated luminescent body further comprises a body axis, and a length parameter P of a body dimension perpendicular to the body axis, wherein the length parameter P is selected from height, width and diameter, wherein: - the elongated support comprises a support material, a support material index of refraction n1, wherein the support material index of refraction n1 is at least 1.4, a support surface, and a support length; - the coating layer is configured on at least part of the support surface over at least part of the support length (L 1), wherein the coating layer comprises a coating layer material, a coating layer index of refraction n2, wherein coating layer index of refraction n2 is at least 1.4, and a coating layer thickness, wherein the coating layer material has a composition different from the support material, wherein the coating layer material comprises a luminescent material configured to absorb one or more of UV radiation and visible light, and to convert it into luminescent material light having a wavelength at a spectral maximum of the luminescent material light in one or more of the visible and the infrared; - the support material is transmissive for the luminescent material light, and -0.2≤n1-n2≤0.2 and d1/P≤0.25 apply, - wherein the coating layer, or an outer layer configured over the coating layer, has a root mean square height Sq of at maximum 1/10 of a wavelength at a spectral maximum of the luminescent material light, and - wherein the elongated luminescent body comprises one or more side faces, wherein the elongated luminescent body comprises a radiation input face and a radiation exit window, wherein the radiation input face is configured in a light receiving relationship with the plurality of light sources, wherein the luminescent material configured to absorb at least part of the light source light and convert it into the luminescent material light, and wherein the radiation input face is configured perpendicular to the radiation exit window.