Light mixing chamber and a luminaire

A light mixing chamber comprising:

• a first light mixing chamber part comprising a light exit window and a first side wall having a first inner wall portion and a first outer wall portion, spaced apart and together forming a first groove,
• a second light mixing chamber part comprising a bottom wall and a second side wall having a second inner wall portion and a second outer wall portion, spaced apart and together forming a second groove,
• an inner cavity, and
• an LED substrate having a front surface facing the inner cavity and a back surface facing away from the inner cavity, the front surface supporting a plurality of LEDs,
• wherein the LED substrate is arranged into the first and second grooves, interconnecting the first and the second light mixing chamber parts such that the light mixing chamber is formed,
• wherein the first inner wall portion of the first side wall is extending further away from the light exit window than the first outer wall portion of the first side wall and/or the second inner wall portion of the second side wall is extending further away from the bottom wall than the second outer wall portion of the second side wall, and
• wherein between 5% to 50% of the back surface of the LED substrate is covered by the first and second grooves.