Abstract: A non-reciprocal nephelometer is disclosed herein that uses an integrating sphere with attached truncation-reduction tubes to contain the sample volume and to integrate the scattered light. The disclosed nephelometer improves on the imperfect angular response by using an integrating sphere design with forward (backward) truncation angles of ?1° (?179°), it reduces sampling losses by employing a substantially straight vertical flow path. In one disclosed embodiment, an illumination assembly consisting of one or multiple diffuse light sources is provided for homogenously illuminating the integrating sphere. An illumination aperture admits light from the light sources, a sensing aperture admits scattered light to an optical detector, and a dark aperture provides a dark background viewing area for the optical detector.