Nephelometer having means semiautomatically canceling components from scattering by particles smaller or larger than those of interest

A nephelometer for assaying immunochemical complex of a sample and antigen mixture held in a conventional test tube having a right circular cylinder construction, comprising:

• means for holding the tube such that its central axis is substantially vertical;
• a laser light source for producing a beam of essentially monochromatic light, said beam being narrow with respect to the diameter of the tube holding the mixture, and said source being positioned to project the laser beam through the side wall of the tube such that it passes through liquid contained therein at or near the central axis of the tube, and such that the laser beam is at a substantial right angle to the central axis of the tube;
• light collecting means for receiving forward scattered light, said light collecting means including a passageway and light shielding means around said passageway for shielding all but light from and about a path through the passageway and through the tube, said light shielding means comprising a pair of field stops with each of said field stops defining an opening directed to provide a scatter volume entirely within the test tube, said passageway and light shielding means being operative to substantially exclude shattered light at the tube side wall interface so as to collect the scattered light only substantially from the volume within the test tube, and said path intersecting the laser light beam at a point within the liquid in the tube and away from the tube walls and lying in a plane which is approximately coplanar with a plane containing the laser beam and the tube axis;
• said path intersecting the laser beam at an angle of approximately 30.degree. with respect to the laser beam;
• said holding means including a cover, and said nephelometer further including shutter means for blocking the laser beam and preventing the laser beam from reaching the tube when said cover is open, said shutter means being operable to automatically move said shutter means to a position blocking the laser beam from reaching the tube when said cover is opened;
• and
• means for sensing the light scattered along the path defined by said light collecting means and for providing an assay output based on the sensed light.