a housing forming a circumferentially enclosed and elongated fluid channel arranged along a first axis;

an inlet arranged to connect a first outer surface area of the housing to a first end of the fluid channel and an outlet arranged to connect a second outer surface area to a second end of the fluid channel; and

a first light guide and a second light guide concentrically arranged along a second axis and on opposite side walls of the fluid channel,

wherein the housing comprises at least a first part, a second part, and a seal located between the first and the second part, wherein an interface between the first part and the second part is contoured,

wherein the first part and the second part together form the circumferentially enclosed and elongated fluid channel, the first part comprising a portion of the fluid channel with an open side positioned along the elongated fluid channel in a lengthwise direction of the first part, the open side of the first part oriented so that the portion of the fluid channel is not circumferentially enclosed and the second part configured to close the open side and thereby circumferentially enclose the fluid channel,

wherein the seal comprises a seal body extending along an interface between the first part and the second part, and

wherein the seal body has a continuous contoured surface conformed to the interface between the first part and the second part.