The quantity measured is the electrical response of the circuit, coupled with the spectrograph obtained by varying the incident light energy by a monochromator. The circuit and optics are coupled by use of a lock-in amplifier. The measurements give information related to the band gap of the semiconductor, allowing for identification of various charge transitions like exciton and trion energies. This is highly relevant for studying semiconductor nanostructures like quantum wells, and other nanomaterials like transition metal dichalcogenide monolayers.