Abstract: An optical flow cell is disclosed that includes a flow cell body having an inlet and an outlet with a flow opening therebetween to allow a fluid to pass therethrough. A light entry fixture and a light imaging fixture are transversely carried by the flow cell body to allow viewing of the flow opening, wherein the light entry fixture is positioned at one side of the body and the light imaging fixture is positioned at an opposite side of the body. The fixtures are made from at least some materials different than then flow cell body but having a thermal rate of expansion that matches a thermal rate of the body so as to maintain a predetermined size of the flow opening during temperature fluctuations.