Crystal defects such as grain boundaries affect the overall performance of a solar cell. The light beam induced current method allows for the localized quantification of the impact on the internal quantum efficiency of such defects. This work presents a method to estimate the separate impact of grain boundaries on the internal quantum efficiency (IQE) of multicrystalline silicon solar cells by correlating LBIC topographs with optical images of etched samples. Segmenting the impact of the grain boundaries on the IQE against those of other defects in our samples showed that the grain boundaries remain the most detrimental. The average IQE at 826 nm was reduced by up to 1.29 % (vs 0.25 % for other defects) absolute for standard multicrystalline and up to 1.15 % (vs 0.28 % for other defects) absolute for high performance multicrystalline silicon.