The invention provides a lighting device (1) comprising: —one or more light sources (10) configured to provide light source light (11); —a luminescent element (5) comprising an elongated light transmissive body (100); the elongated light transmissive body (100) comprising a luminescent material (120) configured to convert at least part of light source light (11) received at one or more radiation input faces (111) into luminescent material light (8), and the luminescent element (5) configured to couple at least part of the luminescent material light (8) out at the first radiation exit window (112) as converter light (101); wherein the light transmissive body (100) has a first index of refraction n1; —a beam shaping optical element (224) optically coupled with the first radiation exit window (112), the beam shaping optical element comprising a radiation entrance window (211) configured to receive at least part of the converter light (101); wherein the beam shaping optical element (224) has a second index of refraction n2; wherein: −0.75≤n1/n2≤1.25; 15 —the beam shaping optical element (224) or an optical connector (300) configured between the elongated light transmissive body (100) and the beam shaping optical element (224) comprise a glass material (310), wherein the glass material (310) is based on at least one or more of bismuth oxide, boron oxide, potassium oxide, lithium oxide, phosphorus oxide, lead oxide, tin oxide, antimony oxide, tellurium oxide, silicon dioxide, and vanadium oxide.