an apparatus for determining physical properties of a mask blank . the apparatus includes , for example , an illumination device for radiating a predetermined light laterally into the mask blank , a detection device opposite the illumination device for detecting the light which has been scattered and / or runs through the mask blank , and an evaluation device for determining predetermined properties of the mask blank from the light which has been scattered and / or has run through the mask blank and has been detected in the detection device . the present invention likewise provides a method for determining physical properties of a mask blank .