Measuring polarisation via a gating frequency

An apparatus for measuring polarisation of electromagnetic illumination, comprising:

• an infinity corrected optical element disposed such that illumination passes through the infinity corrected optical element before passing through the modulator;
• a modulator operable to modulate a polarisation state of illumination received from a target object and to generate modulated intensity illumination;
• a detector and a gating unit, said gating unit operable to periodically gate an exposure of an imaging device of the detector to the modulated intensity illumination at a first gating frequency, thereby enabling the detector to selectively measure an intensity of the modulated intensity illumination, and
• a processor operable to:
• determine calibration data associated with polarising effects of different optical paths taken by the received illumination after being received and before being selectively measured;
• and
• determine, from the selective measurement of the intensity of the modulated intensity illumination in accordance with the determined calibration data polarisation parameters of the received illumination and to generate image data corresponding to the target object with a plurality of the polarisation parameters,
• wherein the modulator modulates the received illumination in accordance with a first frequency and the first gating frequency is associated with and synchronised with at least the first frequency, and
• wherein to determine the calibration data the processor is operable to pass illumination from a patterned target object of known optical properties through a calibration optical element of known optical properties after being passed through the infinity corrected lens and before being modulated.

A module for use in a polarisation measuring device, comprising:

• an infinity corrected optical element disposed such that illumination passes through the infinity corrected optical element before passing through the modulator;
• a modulator operable to modulate a polarisation state of illumination received from a target object and to generate modulated intensity illumination;
• a detector and a gating unit, said gating unit operable to periodically gate an exposure of an imaging device of the detector to the modulated intensity illumination at a first gating frequency, thereby enabling the detector to selectively measure an intensity of the modulated intensity illumination, and
• a processor operable to:
• determine calibration data associated with polarising effects of different optical paths taken by the received illumination after being received and before being selectively measured;
• and
• determine, from the selective measurement of the intensity of the modulated intensity illumination in accordance with the determined calibration data polarisation parameters of the received illumination and to generate image data corresponding to the target object with a plurality of the polarisation parameters,
• wherein the modulator modulates the received illumination in accordance with a first frequency and the first gating frequency is associated with and synchronised with at least the first frequency, and
• wherein to determine the calibration data the processor is operable to pass illumination from a patterned target object of known optical properties through a calibration optical element of known optical properties after being passed through the infinity corrected lens and before being modulated.