A method to correct signal light intensities measured by a detector of a detection unit in a laboratory instrument is presented. The detection unit comprises a light source, a sample plane comprising a sample holder configured to hold at least one sample vessel comprising a test sample to be illuminated, a reference light sensor, and the detector. Based on a basic light intensity of a newly manufactured light source and an initial light intensity measured by the reference light sensor the sensitivity of the reference light sensor can be determined. And signal light intensities measured by the detector can be corrected based on the determined sensitivity and subsequently measured reference light intensities of the reference light sensor in order to generate comparable test results.