Method of variably attenuating a beam of projection light in an optical system of a microlithographi

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A pattern of light absorbing regions is created on or within a diffractive or reflective optical element 44 mounted in the optical system of a microlithographic apparatus. Projec­tion light is directed onto the absorption pattern then either the light absorbing regions (Fig. 6 : 120) disappear by themselves or they are actively removed. These steps are carried out at least at two consecutive portions (Fig. 4 : 56a, 56b) of the optical element, so that at a given time the projection light is directed either to the first or to the second portion. As an example a rotatable disk shaped optical element 44 may be placed at one of the pupil planes 36, 38 of the projection objective 20 as a time variable apodization filter. A print head 50 may be used to form absorbing regions 58 which are removed using an erasing head 52, or a laser printer may provide writing light (Fig. 4 : 72) causing illuminated areas on a rotating photosensitive drum (Fig. 4 : 78) to attract toner from a toner drum (Fig. 4 : 80). Writing light from a laser printer may be directed onto regions of photochromic materials including CaF2 or BaF2 doped with rare earth elements, to cause excited state absorption (ESA). Light absorbing regions may disappear as a result of recombination processes, or using an erasing light, and a rotating filter plate (Fig. 10 : 344) including regions of the material may be placed very close to the mask plane. Excited state absorption may alternatively be induced in a replaceable polymer film (Fig. 12a : 445) formed on a substrate.