The solar cells used today have a performance rate of about 30% in theory, but most solar cells on the market only utilize about 20% of the energy provided by sunrays. A prominent reason that the performance rate is far from 100% is the large variety of energies and corresponding wavelengths in white light. Tandem solar cells utilize two different solar cells, where the light not absorbed by the top cell travels through the top cell and onto the bottom cell. This can lead to an efficiency upward of 40%.    The purpose of this thesis was to evaluate how to use the machine Bentham PVE300 optimally for measurements of transmittance, reflectance and EQE (external quantum efficiency) with the aim to calculate the IQE (Internal quantum efficiency).   To optimize the efficiency of the tandem cells, the reflectance, transmittance and EQE needed to be measured. To do this Bentham PVE300 was used. The properties of Bentham PVE300 were explored beforehand to get a better understanding of the equipment. By reading the instrument manual and simultaneously working on the instrument, methods for the measurement of EQE, reflectance and transmittance were compiled into a manual.   The results of measurements performed by Bentham PVE300 were compared to results from other equipment to determine if the measurements were viable. Agilent Cary 7000 was used to validate the measurements of reflectance and transmittance. Bentham PVE300 was ultimately determined to be reliable and in most cases more reliable than the currently used instruments.