Calibration of scanning interferometers

A method of calibrating a scanning interferometry imaging system, comprising:

• configuring the scanning interferometry imaging system for operation with a low NA interference objective using light having a narrowband wavelength spectrum;
• using the scanning interferometry imaging system to direct measurement light and reference light along different paths and to overlap the measurement and reference light on a detector, the measurement and reference light having the narrowband wavelength spectrum;
• scanning an optical path length difference between the measurement light and the reference light at the detector while acquiring intensity data using the detector, the detector acquiring the intensity data at a frame rate and the scanning being performed at a scan speed;
• determining information about the scan speed based on the acquired intensity data, geometric information about the scanning interferometry imaging system, and the narrowband wavelength spectrum;
• calibrating the scanning interferometry imaging system based on the information about the scan speed;
• and
• using the calibrated scanning interferometry imaging system to determine information about a test object, wherein the calibrated scanning interferometry imaging system is used with an objective different from the low NA interference objective used for calibrating the scanning interferometry imaging system.

A scanning interferometry imaging system, comprising:

• a light source module for providing light having a narrowband wavelength spectrum;
• an aperture stop;
• an interference objective;
• a detector for acquiring intensity information at a frame rate;
• one or more optical elements arranged to derive measurement light and reference light from the light source, to direct the measurement and reference light along different paths, and to overlap the measurement and reference light at the detector module;
• an actuator for scanning an optical path length difference between the measurement light and the reference light at a scan speed;
• and
• a data processing apparatus in communication with the detector and the actuator, the electronic processing module being programmed acquire intensity data using the detector while scanning the optical path length difference, and determine information about the scan speed based on the acquired intensity data, geometric information about the scanning interferometry imaging system, and the narrowband wavelength spectrum, and calibrating the scanning interferometry imaging system based on the information about the scan speed
• wherein, during the scanning, the measurement light reflects from an object and the scanning interferometry imaging system is arranged to image the object to the detector module.

A non-transitory computer readable medium encoded with instructions that when executed by a data processing apparatus cause the data processing apparatus to perform operations comprising:

• scanning an optical path length difference between measurement light and reference light at a detector, where the measurement and reference light is light derived from a common source and directed along different paths before being overlapped on the detector using a scanning interferometry imaging system, the measurement and reference light having a narrowband wavelength spectrum;
• acquiring intensity data using the detector while scanning the optical path length difference, the intensity data being acquired at a frame rate and the scanning being performed at a scan speed;
• determining information about the scan speed based on the acquired intensity data, geometric information about the scanning interferometry imaging system, and the narrowband wavelength spectrum;
• and
• calibrating the scanning interferometry imaging system based on the information about the scan speed,
• wherein, during the scanning, the measurement light reflects from an object and the scanning interferometry imaging system is arranged to image the object to the detector.