An optical fiber with a section of its cladless core immersed in a fluid uses a refractive index (RI), n, matching method to measure n and (delta) n/(delta) (lambda) (dispersion) of the fluid. Current methods of measuring n and (delta) n/(delta) (lambda) often require light transmission through the fluid; thus turbidity, which diffuses and attenuates the light, restricts RI analysis. This device, which senses the minimum light remaining within the fiber core, is immune to such limitations. Temperature, a factor in any precise measurement of n, is the only accurate measurement needed; no critical system calibration is required. Tests were performed on clear and turbid edible oil samples, and the method matched the +/- .0001 RI unit accuracy of the Abbe refractometer. A tunable laser was used to measure the oil RI within the wavelength range of 543 to 633 nm, and the data fitted to Cauchy's dispersion equation. Calculations, using (lambda) equals 589.3 nm, agree with Abbe measurements of the oil samples.