Abstract: Method for operating a mask-metrology measurement apparatus, wherein an image of a section of a photomask is recorded with a first image sensor and wherein an aerial image is generated by virtue of the image raw data obtained with the image sensor being subjected to a clear normalization. The aerial image is subjected to a non-linearity adaptation, which comprises the following steps. In step a., the aerial image is mathematically combined with a clear image (CT2T). In step b., linearity correction (Plin1) is applied to the image data generated in step a. to correct a linearity error of the first image sensor. In step c., a non-linearity adaptation (P?1lin2) is applied to the linearity-corrected image data obtained in step b. to imprint a linearity signature of a second image sensor not arranged in the beam path of the measurement apparatus on the image data. In step d., a clear normalization is applied to the linearity-adapted image data generated in step c.