An improved differential refractometer incorporating a photodetector array is disclosed. Using a multi-element photo array provides the basis for measurement of differential refractive index values with a heretofore unattainable combination of sensitivity of measurement and concurrent range of measurement. Within the large dynamic range attainable, the detector structure provides equal sensitivity irrespective of deflection within the range. The transmitted light beam is tailored to provide a spatial variation of the light intensity at the array improving thereby the precision of measurement of its displacement. This in turn results in improved precision in the reported differential refractive index and in the calculation of the differential refractive index increment dn/dc. Integrating the detector array into the flow cell structure of the parent case results in a detector of exceptional sensitivity and range for sample quantities far smaller than required by conventional differential refractometers.