Abstract: An apparatus for receiving an analyte comprises two opposing housings that clamp onto a substrate. One of the housings includes passageways that deliver the analyte and optical signals to the substrate. Another one of the housings includes passageways that allow optical signals, which have passed through the substrate, to travel to photometric sensors which may be used to study the analyte or its effects. The apparatus may include a light guide that uniformly distributes light from a plurality of point emitters to multiple areas of the substrate. The apparatus may include an actuator assembly that opens and closes the two housing to allow for installation and removal of the substrate. The substrate may be carried in a cartridge that is removable from the two housings.