For determining a disease activity as a single value by means of optical measurements in order to facilitate an analysis of a current disease status and a future course of disease thus assisting a doctor s diagnosis or decision on a therapy a system and a method are provided wherein a blood perfusion is varied in an area of interest the area of interest is irradiated with light of at least two wavelengths an intensity of light reflected and/or transmitted by the area of interest is detected features are derived from detected intensity curves of at least two predetermined wavelengths under at least two different perfusion conditions and the disease activity is determined using these features.
