Abstract: A nephelometer wherein a laser beam is split to define reference and sample beams. The reference beam is directed to a photodetector and the sample beam is passed through a test sample, the emerging beam being directed to a separate photodetector. The output from the respective photodetectors are compared via a comparator, and the comparator output is divided by the reference beam photodetector output to provide a recordable signal. Preferably, the photodetectors are dispersed within Ulbrecht type spheres whereby they are indirectly irradiated by the respective beam. The invention is particularly effective for measuring turbidities of very minute degree.