A method to correct signal light intensities measured by a detector of a detection unit in a laboratory instrument, wherein the laboratory instrument comprises the detection unit and a control device, wherein 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, wherein the light source is configured to emit light towards the sample plane, wherein the reference light sensor is mounted on or next to the sample plane so that the reference light sensor does not interrupt with the emitted light towards the sample holder or is comprised by the sample plane or sample holder, such that the reference light sensor and the sample holder are exposed to the substantially same light emitted from the light source towards the sample plane, wherein the reference light sensor is configured to measure an initial light intensity of emitted light towards the sample plane and at least one reference light intensity of emitted light towards the sample plane, wherein the detector is configured to measure a signal light intensity of emitted light from the sample plane, the method comprising the following steps (44, 46, 48, 50, 52, 54): a) the control device receives a basic light intensity of the light source b) the control device activates the light source in the detection unit c) the reference light sensor measures an initial light intensity of emitted light towards the sample plane and transmits the measured initial light intensity to the control device d) the control device calculates the sensitivity of the reference light sensor based on the measured initial light intensity and the basic light intensity of the light source e) after calculating the sensitivity of the reference light sensor, the detector measures at least one signal light intensity of emitted light from the sample plane and transmits the at least one measured signal light intensity to the control device and at the same time the reference light sensor measures at least one reference light intensity of emitted light towards the sample plane and transmits the at least one measured reference light intensity to the control device f) the control device corrects the at least one measured signal light intensity with the at least one reference light intensity and the calculated sensitivity of the reference light sensor, wherein the basic light intensity of the light source is measured by a calibrated light sensor of a light source test-system, wherein the light source test-system is separate from the laboratory instrument, wherein in step a) of the method the control device receives the basic light intensity of the light source when the light source is mounted in the detection unit for the first time. a) the control device receives a basic light intensity of the light source b) the control device activates the light source in the detection unit c) the reference light sensor measures an initial light intensity of emitted light towards the sample plane and transmits the measured initial light intensity to the control device d) the control device calculates the sensitivity of the reference light sensor based on the measured initial light intensity and the basic light intensity of the light source e) after calculating the sensitivity of the reference light sensor, the detector measures at least one signal light intensity of emitted light from the sample plane and transmits the at least one measured signal light intensity to the control device and at the same time the reference light sensor measures at least one reference light intensity of emitted light towards the sample plane and transmits the at least one measured reference light intensity to the control device f) the control device corrects the at least one measured signal light intensity with the at least one reference light intensity and the calculated sensitivity of the reference light sensor, wherein the basic light intensity of the light source is measured by a calibrated light sensor of a light source test-system, wherein the light source test-system is separate from the laboratory instrument, wherein in step a) of the method the control device receives the basic light intensity of the light source when the light source is mounted in the detection unit for the first time.