A body module of an optical measurement instrument comprises: a reception de- vice (201) for receiving samples, a first plate (202), a second plate (203) substantially parallel with the first plate and movably supported relative to the first plate in a direction perpendicular to the first and second plates, and walls extending from outer edges of the first plate to outer edges of the second plate. The reception de- vice is located in a measurement chamber constituted by the walls and the first and second plates. At least the second plate comprises a fastening interface provided with an aperture. The fastening interface is suitable for an optical module to be mounted to the second plate. The measuring chamber provides protection against undesired stray light from surroundings. The movability of the second plate allows adjustment of a distance between a sample and an optical module mounted to the second plate.