Liquid refractometer based on interferometric fringe projection technique

Measurement of the refractive index of liquids is of great importance in applications such as the control of adulteration of liquids and pollution monitoring. We present and discuss a fringe projection technique for measuring the index of refraction of transparent liquid materials. In the proposed technique a fringe pattern produced by a reflective grating interferometer is imaged by a lens before passing through a cell containing the liquid sample under test. The interference pattern consists of a family of parallel straight fringes that are digitized by a CCD array. The focal shift, which is induced by the liquid sample, produces changes in the spatial frequency of the projected pattern. Spatial frequency of the fringe pattern is analysed by means of a one-dimensional Fourier transform and phase demodulation procedure. The principle of the proposed fringe projection refractometer relies on a comparative measurements of the refractive index of the sample under test with respect to a reference liquid. The method is simple and can be easily interpreted geometrically.