Comparison Type Nephelometer

A comparison type nephelometer has been designed and constructed with which turbidities are measured by both absorption and scattering of light. The especially designed cells permit accurate measurement of small changes in intensity of either transmitted or scattered light over a wide range of intensities. A single light source with a sector scanner alternately illuminates two separate optical paths for simultaneous comparison of two cells, one of which may be a standard. The intensities of the two optical paths can be varied independently for comparison measurements. A light integrating sphere with phototube assembly furnishes a single photoreceiver for the two optical paths. One such sphere compares the transmitted beams, another, the scattered beams. The reproducibility of resetting in the normal operating range is two parts in ten thousand. The apparatus will operate over a range of more than five density units. When making series of measurements, it was found possible to make and record readings for different cells every 15 sec. The apparatus was used successfully for two years in bacteriological, silica gel, and colorimetry studies. A slight modification permits measurement of the depolarization of the scattered light as well as the usual Tyndall scattering. This depolarization is measured for both polarized and unpolarized incident light, thus characterizing the size and optical anisotropy of the particles or scattering centers.