Abstract: An illuminator for use in a hand-held refractometer used for measuring the composition of substances. The refractometer comprising a hand-held body housing a prism and measuring assembly with a prism surface for supporting the substance to be measured, wherein the illuminator illuminates the substance to be measured. The illuminator comprises a light source positioned on or about the prism surface supporting a substance to be measured, and a power source for operating the light source. An operating assembly is also provided for manually activating the light source by selectively bringing it into communication with the power source. The light source, power source, and operating assembly form an illuminator assembly which supports the light source in a position on or about a prism surface in order to illuminate a substance to be measured at an angle with respect to the prism surface.