A light generating system comprising a plurality of light sources, an elongated luminescent body, and a body holder structure, wherein: - the plurality of light sources are configured to provide light source light, wherein the light sources are solid state light sources, wherein the plurality of light sources are configured in a light source array; - the elongated luminescent body has a length L and a width W, wherein the elongated luminescent body comprises luminescent material configured to convert at least part of light source light into luminescent material light; - the body holder structure comprises an elongated slit for hosting the elongated luminescent body, wherein the elongated slit has a cavity wall defining the elongated slit and a slit opening, wherein the slit opening has a slit opening width WS1, wherein WS1≥1.05*W, wherein the cavity wall and the elongated luminescent body have first shortest distances d11 that vary over the cavity wall, wherein at least part of the cavity wall is reflective for light source light; - the light sources are configured at second shortest distances d21 from the elongated luminescent body, and wherein one or more of the plurality of light sources are configured to irradiate with the light source light the elongated luminescent body both (i) directly and (ii) indirectly via the cavity wall, characterized in that: - the elongated slit has a second slit width WS2 at a slit end most remote from the slit opening, wherein the slit opening and the slit end are bridged by cavity wall parts (2131,2132), wherein the second slit width WS2 at the slit end is smaller than the slit opening width WS1, wherein WS1/WS2 is at least 1.1, - the cavity wall parts (2131, 2132) comprise first parts that are configured conformal to part of the elongated luminescent body at first shortest distances selected from the range of ≤ 100 μm, wherein the first parts are configured closer to the slit end than to the slit opening, and - the slit end is in thermal contact with the elongated luminescent body. - the plurality of light sources are configured to provide light source light, wherein the light sources are solid state light sources, wherein the plurality of light sources are configured in a light source array; - the elongated luminescent body has a length L and a width W, wherein the elongated luminescent body comprises luminescent material configured to convert at least part of light source light into luminescent material light; - the body holder structure comprises an elongated slit for hosting the elongated luminescent body, wherein the elongated slit has a cavity wall defining the elongated slit and a slit opening, wherein the slit opening has a slit opening width WS1, wherein WS1≥1.05*W, wherein the cavity wall and the elongated luminescent body have first shortest distances d11 that vary over the cavity wall, wherein at least part of the cavity wall is reflective for light source light; - the light sources are configured at second shortest distances d21 from the elongated luminescent body, and wherein one or more of the plurality of light sources are configured to irradiate with the light source light the elongated luminescent body both (i) directly and (ii) indirectly via the cavity wall, characterized in that: - the elongated slit has a second slit width WS2 at a slit end most remote from the slit opening, wherein the slit opening and the slit end are bridged by cavity wall parts (2131,2132), wherein the second slit width WS2 at the slit end is smaller than the slit opening width WS1, wherein WS1/WS2 is at least 1.1, - the cavity wall parts (2131, 2132) comprise first parts that are configured conformal to part of the elongated luminescent body at first shortest distances selected from the range of ≤ 100 μm, wherein the first parts are configured closer to the slit end than to the slit opening, and - the slit end is in thermal contact with the elongated luminescent body. - the elongated slit has a second slit width WS2 at a slit end most remote from the slit opening, wherein the slit opening and the slit end are bridged by cavity wall parts (2131,2132), wherein the second slit width WS2 at the slit end is smaller than the slit opening width WS1, wherein WS1/WS2 is at least 1.1, - the cavity wall parts (2131, 2132) comprise first parts that are configured conformal to part of the elongated luminescent body at first shortest distances selected from the range of ≤ 100 μm, wherein the first parts are configured closer to the slit end than to the slit opening, and - the slit end is in thermal contact with the elongated luminescent body.