An instrument that measures the concentration of a compound that has been dissolved in a solvent (such as water, alcohol, etc.). The instrument shines a light through the solution, measures the fraction of the light that is absorbed by the solution, and calculates the concentration from that absorbance value.
An instrument (a spectrometer) for measuring the intensity of the wavelengths of the components of light (visible or ultraviolet).
An instrument that measures the transmission of a specified light wavelength through a material (e.g., a material in solution). Contrasting the light transmission through a standard solution and the test solution, one can determine a variety of characteristics of the material, especially its concentration.