Method and device for determining the heating state of an optical element in an optical system

A method of estimating an average temperature of an incidence surface of an optical element of an optical system, the optical element configured to have electromagnetic radiation incident thereon during use of the optical system, the method comprising:

• measuring a temperature using at least one temperature sensor arranged at a distance from the incidence surface;
• determining a calibration parameter based on an illumination setting in the optical system;
• and
• using the calibration parameter to estimate the average temperature of the incidence surface based on the temperature measurement,
• wherein the method further comprises determining the calibration parameter based on at least one measurement or simulation of a variable for the illumination setting currently set in the optical system, wherein the variable depends on a thermal state of the optical element, and
• wherein:
• the variable dependent on the thermal state of the optical element is an intensity distribution generated during the operation of the optical system in a plane located downstream of the optical element along a beam path of the electromagnetic radiation through the optical system;
• or
• the variable dependent on the thermal state of the optical element is a wavefront generated during the operation of the optical system in a plane located downstream of the optical element along a beam path of the electromagnetic radiation through the optical system.