Light source holder for semiconductor based light sources, comprising: a one-piece body made of plastic material and comprising a mounting surface (31, 131) for receiving, and thermally connecting, the semiconductor based light source to the one-piece body; means (68, 69, 131) for mechanically fixing the semiconductor based light source on the body; contact means (72, 172) for electrically connecting the semiconductor based light source and lead wires (47, 48); heat dissipating fins being an integral part of the one-piece body and extending away therefrom, so that heat received on the mounting surface (31, 131) is conducted along a continuous thermal path from the mounting surface (31, 131) into the fins for dissipating heat to the environment; wherein the light source holder comprises a collar surrounding the mounting surface and being an integral part of the body, the collar being designed to position the light source holder in a luminaire; wherein the collar is connected to, or abuts against, at least one end of at least one of the fins; wherein the body comprises a centre portion with the fins (27, 29, 30) extending laterally or axially away therefrom; wherein the collar may cover the rim of the cutout of a fixture the light source holder is to be fitted in; wherein the collar comprises a ring shaped planar surface at the back side thereof, which surface forms a thermal interface thermally connecting the light source holder to the luminaire; characterized in that the light source holder comprises at least one internal chamber with the electrical contact elements disposed therein, forming an electrical interface for electrically connecting the semiconductor based light source to a power source; wherein openings (68, 69) are provided through the mounting surface (31, 131) for allowing electrical terminals (62, 62) of the light source to be introduced into the internal chamber and to come into abutment with the electrical contact elements; and wherein the plastic material of the body is a heat conductive and electrically insulating material. a one-piece body made of plastic material and comprising a mounting surface (31, 131) for receiving, and thermally connecting, the semiconductor based light source to the one-piece body; means (68, 69, 131) for mechanically fixing the semiconductor based light source on the body; contact means (72, 172) for electrically connecting the semiconductor based light source and lead wires (47, 48); heat dissipating fins being an integral part of the one-piece body and extending away therefrom, so that heat received on the mounting surface (31, 131) is conducted along a continuous thermal path from the mounting surface (31, 131) into the fins for dissipating heat to the environment; wherein the light source holder comprises a collar surrounding the mounting surface and being an integral part of the body, the collar being designed to position the light source holder in a luminaire; wherein the collar is connected to, or abuts against, at least one end of at least one of the fins; wherein the body comprises a centre portion with the fins (27, 29, 30) extending laterally or axially away therefrom; wherein the collar may cover the rim of the cutout of a fixture the light source holder is to be fitted in; wherein the collar comprises a ring shaped planar surface at the back side thereof, which surface forms a thermal interface thermally connecting the light source holder to the luminaire; characterized in that the light source holder comprises at least one internal chamber with the electrical contact elements disposed therein, forming an electrical interface for electrically connecting the semiconductor based light source to a power source; wherein openings (68, 69) are provided through the mounting surface (31, 131) for allowing electrical terminals (62, 62) of the light source to be introduced into the internal chamber and to come into abutment with the electrical contact elements; and wherein the plastic material of the body is a heat conductive and electrically insulating material.