Wavelength selection module, illumination system and metrology system

A wavelength selection module comprising:

• a pair of rotatably mounted filter elements disposed in series being operable to receive an input radiation beam comprising multiple wavelengths to provide selective control of a wavelength characteristic of a corresponding output radiation beam,
• wherein at least one of the filter elements comprises at least one overlapping band-pass pair of linear variable filters,
• wherein each of the pair of filter elements comprises a plurality of different regions having different transmission characteristics, such that the wavelength selection module is operable to select a desired narrow pass band with a variable center frequency by having a combination of at least two of any of:
• a respective band-pass pair of linear variable filters defining a respective pass band,
• a respective band-stop pair of linear variable filters defining a respective stop-band,
• a single short pass linear variable filter, and
• a single long pass linear variable filter,
• wherein each of the pair of filter elements is configured to rotate in-plane to allow the input radiation beam to be incident on a selected one of the plurality of different regions of each of the filter elements, and
• a controller configured to rotate one or both of the pair of filter elements to select the desired narrow pass band with variable center frequency.