An illumination system of a microlithographic apparatus, comprising a) a light entrance configured to receive projection light that is produced by a light source, b) an irradiance adjustment unit arranged to receive the projection light from the light entrance and configured to adjust an irradiance distribution of the projection light on a surface, wherein the irradiance adjustment unit comprises - a spatial light modulator (54a, 54b; 154) having an array of light deflection elements that are configured to be individually controlled, and - a light splitter (56a, 56b; 56; 156a) configured to split the projection light into modulated light (58a, 58b; 158) that propagates along a modulation light path (59a, 59b; 159) and non-modulated light (60a, 60b; 160a, 160c, 160d) that propagates along a non-modulation light path (63; 163a, 163c, 163d) which is distinct from the modulation light path, wherein - the spatial light modulator (54a, 54b; 154) is only arranged in the modulation light path (59a, 59b; 159), - no spatial light modulator is arranged in the non-modulation light path (63; 163a, 163c, 163d), and - the modulated light (58a, 58b; 158) and the non-modulated light (60a, 60b; 160a, 160c, 160d) superimpose on the surface. a) a light entrance configured to receive projection light that is produced by a light source, b) an irradiance adjustment unit arranged to receive the projection light from the light entrance and configured to adjust an irradiance distribution of the projection light on a surface, wherein the irradiance adjustment unit comprises - a spatial light modulator (54a, 54b; 154) having an array of light deflection elements that are configured to be individually controlled, and - a light splitter (56a, 56b; 56; 156a) configured to split the projection light into modulated light (58a, 58b; 158) that propagates along a modulation light path (59a, 59b; 159) and non-modulated light (60a, 60b; 160a, 160c, 160d) that propagates along a non-modulation light path (63; 163a, 163c, 163d) which is distinct from the modulation light path, wherein - the spatial light modulator (54a, 54b; 154) is only arranged in the modulation light path (59a, 59b; 159), - no spatial light modulator is arranged in the non-modulation light path (63; 163a, 163c, 163d), and - the modulated light (58a, 58b; 158) and the non-modulated light (60a, 60b; 160a, 160c, 160d) superimpose on the surface. - a spatial light modulator (54a, 54b; 154) having an array of light deflection elements that are configured to be individually controlled, and - a light splitter (56a, 56b; 56; 156a) configured to split the projection light into modulated light (58a, 58b; 158) that propagates along a modulation light path (59a, 59b; 159) and non-modulated light (60a, 60b; 160a, 160c, 160d) that propagates along a non-modulation light path (63; 163a, 163c, 163d) which is distinct from the modulation light path, - the spatial light modulator (54a, 54b; 154) is only arranged in the modulation light path (59a, 59b; 159), - no spatial light modulator is arranged in the non-modulation light path (63; 163a, 163c, 163d), and - the modulated light (58a, 58b; 158) and the non-modulated light (60a, 60b; 160a, 160c, 160d) superimpose on the surface.