Patent ID: 7027138

Claim:
An improved differential refractometer to measure the refractive index difference between a sample and reference fluid comprised of a) A mask ( 18 ) restricting an incident light beam ( 1 ) produced by a light source ( 17 ) to fall onto a collimating lens ( 19 ) that produces a parallel beam of light falling on an aperture ( 7 ), said aperture restricting the cross section of said emerging beam before said beam is incident on the entrance face ( 8 ) of b) A measurement cell, said restricting aperture limiting said emerging parallel light beam to illuminate sample solution contained in said sample chamber ( 2 ) of said flow cell; c) A transparent partition ( 3 ) through which said parallel light beam passes, after having traversed said sample chamber ( 2 ), into d) A reference chamber ( 4 ) containing a reference fluid and bounded by said transparent partition ( 3 ) separating said sample chamber ( 2 ) from said reference chamber ( 4 ); e) A lens element ( 20 ) forming an image of said beam restricting mask ( 18 ) onto a f) A plurality of photo detector elements ( 23 ) each of width small compared to the diameter of the width of said mask ( 18 ) image at the plane of the photodetector elements such that several of said detector elements are illuminated by said mask ( 18 ) image; the measurement of the position of said mask image being improved by reference to the detected intensity variation across said plurality of illuminated detector elements.