Optical measuring instruments that measure the refractive index of liquids to control concentration and quality. Used for concentration management of cutting oils and coolant fluids, quality control of plating and cleaning solutions, and component analysis in food and chemical industries. Available types include Abbe refractometers, digital refractometers, portable refractometers, and automatic refractometers. These are essential measuring instruments for liquid management and quality assurance in manufacturing sites. Unlike spectrometers and microscopes, they are positioned as specialized equipment dedicated to refractive index measurement.