Photon-counting integrating nephelometer

Abstract

An improved integrating nephelometer having superior sensitivity and long-term stability to previous designs includes a container defining a measurement chamber. A continuously energized light source, such as a tungsten filament incandescent bulb, illuminates the measurement chamber through an opal glass diffuser. The intensity of the light source is regulated in one embodiment by a simple optical-electric feedback system, and in another embodiment by a voltage regulated supply. A cone of observation of the chamber is defined by a plurality of spaced, apertured plates located in the chamber, at right angles to the light source. A measuring photomultiplier tube views the cone of observation and provides an output signal including components resulting from detected photoelectrons, noise, and thermally emitted 'dark current' electrons. A photon-detecting apparatus removes the noise component and provides an output pulse for each photoelectron and 'dark current' electron. The output pulses are counted, and converted into an analog signal representing a desired extinction coefficient b.sub.scat by any one of a number of apparatus, including an averaging circuit, or a second counter and a D-to-A converter. Dark current compensation, if necessary, is effected by either subtracting a fixed value, or a value dependent on some environmental variable, from the measured value of b.sub.scat, or by utilizing a reversible counter apparatus and light source apparatus. Variations of this nephelometer are also described, including an instrument for automatically compensating for the component of b.sub.scat due to Rayleigh scatter by gases, an instrument which is sensitive to a plurality of wavelengths, and a self-calibrating instrument which is adapted for the measurement of total light scatter and back scatter at a plurality of wavelengths.