METHOD AND APPARATUS FOR PHOTOMETRIC DETECTION IN FLUIDS

摘要

One difficulty encountered in photometric detection apparatus is the effective noise figure of the equipment. This limits the precision with which the equipment can operate. To solve this problem, a photometric detection method and apparatus employ a variable beam modulation device, such as a planar rotating disk (17) whose reflectance and transmittance with respect to an incident beam of electromagnetic radiation vary with disk angular position, to form, from an incident beam of radiation (14), two amplitude modulated correlated component beams (19, 20). Preferably the amplitudes of the component beams vary sinusoidally, 180o out of phase. In one embodiment, one of the component beams passes through a sample fluid, and the second component beam passes through a reference path and provides a reference relative to which changes in the sample fluid are measured. The component beams are combined and are directed onto a detector (35) which produces an output proportional to the intensity of the combined beam. By properly selecting the parameters of the system, the AC component of the combined signal, if any, provides a measure of presence generally and if calibrated the relative density, of the selected sample fluid. In a second embodiment, a beam (50) of polychromatic radiation passes through, and a portion thereof is absorbed by, the sample fluid before the beam impinges on a beam modulation device (17) for forming the time-varying component beams. The thus formed component beams are filtered so that one component beam has energy substantially only at the sample fluid characteristic wavelength, and the other component beam has energy in energy bands which are unaffected by changes in the component of the sample fluid being measured. The component beams are combined and the AC component, if any, is detected and/or measured. In a third embodiment, a second movable element (59) is supported in the path of the incident polychromatic radiation beam and serves as a variable, substantially monochromatic, filter for sequentially transmitting different energy bands to permit time sequential detection and/ or analysis of a fluid medium at a plurality of wavelengths of interest.