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Abstract

An exposure system and method comprising a radiation generator (15'); and a modulator (4) which comprises a device for controlling the transmission of radiation passing through the modulator in accordance with an applied control function, the response characteristic of the modulator to the control function exhibiting a maximum or minimum and onto which radiation from the generator is incident. A microprocessor (19) controls the transmission condition of the modulator (4), and is responsive to predetermined control signals during an exposing stage and, during a compensation stage prior to the exposing stage, generates a test control function having a DC component and a cyclically varying component for application to the modulator. A photodiode (17) monitors radiation transmitted through the modulator, and a phase detector (18) detects the phase relationship between the cyclical component of the transmitted radiation and the cyclically varying component of the test control function and determines the position of the DC component of the test control function along the response characteristic of the modulator. The microprocessor (19) determines a correction function for application to the modulator (4) to compensate for any change in the response characteristic.

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First Claim