Lithographic apparatus and humidity measurement system

摘要

A humidity measurement system (10) for use in, for example, a lithographic apparatus. The humidity measurement system (10) includes a tunable laser diode (11) configured to emit a measurement radiation beam (5) having a wavelength in a wavelength range. The wavelength range includes a first wavelength associated with an absorption peak of water molecules. A signal processing unit (14) is connected to a radiation detector (12). The signal processing unit (14) is configured to measure an intensity of the measurement radiation beam (5) of the tunable laser diode (11) subjected to absorption. The signal processing unit (14) is also connected to the tunable laser diode (11) for obtaining wavelength information. The signal processing unit (14) is arranged to detect an extreme value in a measured intensity as function of the wavelength and to calculate a humidity value from the detected extreme value.