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WAVELENGTH CALIBRATION FOR OPTICAL METROLOGY The present disclosure relates to calibration of a system for selection of wavelengths (or colors) from a broadband radiation source. Wavelength (or color) selection is employed in optical metrology systems (e.g. for applications in wafer metrology), and requires that the center wavelength (CWL) of the selected wavelength band be known within a minimal error (e.g. within an error of < 2 nm). In a known metrology system, color switching is achieved using one or more motorized filter wheels located in the path of a broadband (e.g. white) light beam. The filters in the filter wheel(s) may be linearly varying transmissive filters. In another example, fast steer mirrors may be used to direct the broadband light towards one or more continuous variable filters. In known examples, wavelength calibration may be performed using a spectrometer and a reference light source, internal to the metrology system, to tune the selected colors by comparing them to known atomic transitio...