Patent ID: 6128080
Filing Date: 2000-10-03
Classification: G01N

Abstract:
A method for measuring the refractive index difference, .DELTA.n, of two solutions contained, respectively, in two separate optical cells of equal length L, one of which contains a reference solution and the other a sample solution comprising the steps ofa) producing a plane polarized beam of monochromatic light incident normally on a Wollaston prism means and whose plane of polarization lies at 45.degree. to a principal optical axis thereof;b) passing both of the two beams leaving said Wollaston prism through a lens means so as to direct one said beam whose polarization is vertical through said sample cell and the other said beam whose polarization is horizontal through said reference cell;c) superimposing said two beams upon their leaving said cells by lens means at a second Wollaston prism means; said superimposed beam emerging therefrom;d) passing said emerging beam of (c) through a liquid crystal retarder means whose fast axis is aligned with said vertically polarized beam that had passed through said sample cell;e) inserting after said liquid crystal retarder means and normal to the beam emerging therefrom sequentially a quarter waveplate means, a plane polarizing analyzer means, and a light detector means, whereby said superimposed beam emerging from said quarter waveplate is a superposition of two counter rotating circularly polarized beams producing an essentially plane polarized resultant beam whose plane of polarization has been rotated with respect to the 45.degree. plane of polarization of said incident beam of step (a);f) changing the voltage applied to said liquid crystal retarder to measure thereby at said detector means the maximum ad minimum intensity, I.sub.max and I.sub.min, respectively, corresponding to a phase difference .pi., of said beam emerging therefrom;g) calibrating said liquid crystal retarder by selecting two applied voltages producing two corresponding retardance states whose difference therebetween is exactly .pi./2;h) applying sequentially, and at a frequency sufficient to insure that said plane of polarization does not rotate more than .pi./2 while said retarder switches between said two retardance states, said two voltages and measuring with said detector means the intensities I.sub.1 and I.sub.2, respectively, after each corresponding voltage has been applied sequentially thereto;i) determining said rotation angle of said plane polarized resultant beam of step (e) by performing the four quadrant arctangent ##EQU16## where the intensity I.sub.0 is defined asI.sub.0 =(I.sub.max +I.sub.min)/2;andj) calculating said refractive index difference, .DELTA.n, of said two solutions from.theta..sub.1 =.pi..DELTA.nL/.lambda.,where L is The length of each of said cells and .lambda. is the wavelength in vacuum of said incident plane polarized beam of light.