Illumination optical unit, and method for measuring a dependence of an intensity of illumination light, incident on an object field, on at least one detection object field coordinate using such an illumination optical unit

An illumination optical unit, comprising:

• a first facet mirror comprising first individual mirrors configured to reflectively guide illumination light from an EUV light source, the first individual mirror configured to be arranged in a used region of an illumination light far field;
• a second facet mirror configured to reflectively guide the illumination light reflected from the first facet mirror to an object field;
• and
• an intensity sensor,
• wherein:
• the second facet mirror comprises second individual mirrors defining second facets which, via partial beams of the illumination light, image individual-mirror groups of the first facet mirror defining first facets at least into partial fields of the object field;
• the intensity sensor device is configured to measure a dependence of an intensity of the illumination light incident on the object field on at least one detection object field coordinate;
• and
• the intensity sensor device comprises:
• a plurality of the first individual mirrors constituting sensor individual mirrors of the first facet mirror;
• and
• a plurality of intensity sensors;
• the sensor individual mirrors are assigned to a plurality of different object field positions along the detection object field coordinate;
• and
• the sensor individual mirrors are configured to reflectively guide the illumination light to the intensity sensors.