The invention relates to a spectrophotometer for the automated optical characterization of solar collector tubes and to a method for the operation thereof, that measures the coefficient of transmission of the glass tube and of reflection of the metal tube. The device includes all the necessary components for taking said measurement, such as the optical bench supporting the tube, the standard or framework, and the optical modules for the device, a main module that generates the light bundles, a measuring module that measures the coefficient of transmission of the outer glass tube and the coefficient of reflection of the inner metal tube, a mechanical system of tube rotation, an electronic system of data acquisition and processing, an external computer for controlling the device and processing the data measured, and a system of communication between the device and the computer.