Extended range interferometric refractometer

Claim 1

A method for measuring the refractive index difference, Δn, 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 of a) producing a plane polarized beam of monochromatic light incident normally on a Wollaston prism means and whose plane of polarization lies at 45° 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° 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 max and I min , respectively, corresponding to a phase difference π, 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 π/2; h) applying sequentially, and at a frequency sufficient to insure that said plane of polarization does not rotate more than π/2 while said retarder switches between said two retardance states, said two voltages and measuring with said detector means the intensities I 1 and I 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 0 is defined as I.sub.0 =(I.sub.max +I.sub.min)/2; and j) calculating said refractive index difference, Δn, of said two solutions from θ.sub.1 =πΔnL/λ, where L is The length of each of said cells and λ is the wavelength in vacuum of said incident plane polarized beam of light.

Claim 2

A method for measuring the refractive index difference, Δn, 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 of a) producing a plane polarized beam of monochromatic light incident normally on a Wollaston prism means and whose plane of polarization lies at 45° 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° 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 max and I min , respectively, corresponding to a phase difference π, 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 π/2; h) applying sequentially, and at a frequency sufficient to insure that said plane of polarization does not rotate more than π/2 while said retarder switches between said two retardance states, said two voltages and measuring with said detector means the intensities I 1 and I 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 0 is defined as I.sub.0 =(I.sub.max +I.sub.min)/2; and j) calculating said refractive index difference, Δn, of said two solutions from θ.sub.1 =πΔnL/λ, where L is The length of each of said cells and λ is the wavelength in vacuum of said incident plane polarized beam of light.