The present invention relates to a determination of a physical state of an optical device by measuring a response signal. At least two subsequent measurements are executed. Therefore, a first optical signal is transmitted to the optical device, wherein a first optical property of said first signal is varied according to a first function of the time. A second optical property of a first response signal returning from the optical device is measured over the time and a first result function of the second optical property of the first response signal over the first optical property of first optical signal is established. Further, a second optical signal is transmitted to the optical device, wherein the first optical property of said signal is varied according to a second function of the time that is different from the first function of time. The second optical property of the corresponding second response signal is measured and a second result function of the second optical property of the response signal over the first optical property of the transmitted optical signal is established. The physical state of the optical device is determined on the base of a combination of the first and second result functions.