This work isolated the cause of the observed discrepancy between the electron temperature (T) measurements before and after the JET Core LIDAR Thomson Scattering (TS) diagnostic was upgraded. In the upgrade process, stray light filters positioned just before the detectors were removed from the system. Modelling showed that the shift imposed on the stray light filters transmission functions due to the variations in the incidence angles of the collected photons impacted plasma measurements. The shift in the stray light filter transmission function was examined for all channels of the JET Core LIDAR spectrometer. Due to proximity to the ruby laser wavelength and filter configuration, two of the diagnostics high wavelength channels were affected. To correct for this identified source of error, correction factors were developed using ray tracing models for the calibration and operational states of the diagnostic. The application of these correction factors resulted in an increase in the observed T, resulting in the partial if not complete removal of the observed discrepancy in the measured T between the JET core LIDAR TS diagnostic, High Resolution Thomson Scattering (HRTS), and the Electron Cyclotron Emission (ECE) diagnostics.