A new method for noncontact, high-resolution measurement of gas density is described. The method uses a two-frequency Zeeman-split He–Ne laser and cumulative phase-measuring electronics. The measurement is resolved in two dimensions and provides density that is averaged only along the length of the laser beam that passes through the test section. The technique is based on highly accurate measurement of the optical path-length change of the laser beam as it passes through a test cell (in principle, to within 0.001λ, where λ is the wavelength of the laser). The technique also provides a very large dynamic range (again, in principle, up to 1010), which makes the method additionally attractive. Although the optical path length through the test section is directly related to the index of refraction, and hence to the density of the gas, the method can also be used to measure temperature (if the gas pressure is known) or pressure (if the temperature is known).
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