Optical instrument and parts thereof for optimally defining light pathways

An optical assembly is disclosed that includes an illumination source, a detection sensor, a monitor sensor, and an optical piece having a first side adapted to face a sample. The optical piece defines an illumination channel extending from the illumination source toward the first side, a detection channel extending from the first side toward the detection sensor, and a monitor channel extending from the illumination channel toward the monitor sensor. A spectrophotometer is also disclosed that includes a circuit board, illumination source and one or more sensors. The circuit board includes an optically transparent region, wherein the illumination source is mounted and situated relative to a first surface of the circuit board, so as to direct light through the optically transparent region. Each sensor is mounted and situated relative to a second surface of the circuit board opposite the first surface.