The present invention relates to a method and device of obtaining a beam hardening correction coefficient for carrying out beam hardening correction on computed tomography data. The method includes the steps of: firstly, acquiring an original reconstructed image and an original sinogram of an object of a particular size; secondly, obtaining an error-reduced sinogram after processing the original reconstructed image by error reduction; thirdly, sampling and calculating an average value of the original sinogram and an average value of the error-reduced sinogram; fourthly, optimizing the original sinogram according to the error-reduced sinogram to determine a coefficient vector of optimization function for the object of the particular size; and finally fitting the coefficient vector of the optimization function of the original sinogram to obtain the beam hardening correction coefficient for the object of the particular size.

An optical system has a light source having an original etendue of less than 0.1 mm.sup.2 for an illumination system for projection lithography. An optical assembly serves for simultaneously increasing the etendue of a used emission of the light source. The optical assembly is embodied such that an increase in the etendue by at least a factor of 10 results. A component of the optical assembly that is impinged on is displaced relative to the light source such that an impingement region of the emission of the light source on the optical component of the optical assembly varies temporally.

An optical system has a light source having an original etendue of less than 0.1 mm.sup.2 for an illumination system for projection lithography. An optical assembly serves for simultaneously increasing the etendue of a used emission of the light source. The optical assembly is embodied such that an increase in the etendue by at least a factor of 10 results. A component of the optical assembly that is impinged on is displaced relative to the light source such that an impingement region of the emission of the light source on the optical component of the optical assembly varies temporally.