fig1 illustrates a mask blank 10 , which essentially comprises a carrier device 11 , e . g . a glass body having a thickness of about 6 mm with a rectangular base area , and a layer sequence 12 deposited thereon . the layer sequence 12 on the carrier device 11 has , by way of example , a molybdenum - silicon nitride layer , at the periphery a chromium layer and in the vertical direction a terminating photoresist layer . an illumination device 14 oriented toward a side area 13 of the mask blank 10 , preferably a laser , radiates a predetermined light 15 into the mask blank 10 essentially perpendicularly to the side wall 13 of the mask blank 10 . the light 15 , running essentially parallel to the layer sequence 12 , runs through the mask blank 10 and is if appropriate scattered at dislocations , strains , other deformations such as mechanical stresses or flexures or at defects or impurity particles in the mask blank 10 . light 16 which runs through the mask blank 10 or is scattered therein is received in a detection device 17 . the detection device 17 is diametrically opposite the illumination device 14 , the mask blank 10 being arranged such that it lies inbetween . the illumination device 14 , the detection device 17 and the mask blank 10 lie in one plane . in order now to be able to create a tomogram or a map of the blank interior the illumination device 14 coupled with the detection device 17 is moved at least by an angle α of at least 180 ° around the mask blank 10 , so that in an evaluation device 18 , e . g . a computer , scattering sources present in the blank material and the position of said scattering sources can be computationally deduced from the combination of the emitted and detected light beams 15 , 16 . the laser light 16 scattered in the mask blank 10 is stored in a location - dependent manner in the computer 18 . the blank map thus generated may then be used as a reference for any further tool ( pattern generator , metrology , etc .) or as a basis for corrections . a map generated in this way is likewise suitable for identifying the mask blank quality , in which case this determination of the predetermined properties such as e . g . mask blank flexures and included particles in the mask blank 10 may likewise be carried out during the machining process of a mask blank 10 in a blank machining tool . an evaluation of the data of the detection device 17 in the evaluation device 18 is preferably effected on the basis of a fourier transformation . the evaluation device 18 not only processes and analyzes the data of the detection device 17 but preferably likewise controls the illumination device 14 and thus the incident light 15 . in a second embodiment in accordance with fig2 , the mask blank 10 is identical to the mask blank described with reference to fig1 . in accordance with the second embodiment , an illumination device 14 comprises a plurality of light sources 19 , e . g . laser diodes . a detection device 15 likewise comprises a plurality of detectors 20 , preferably laser detectors , which , like the laser diodes 19 , are arranged in a manner distributed around the mask blank 10 in one plane with the mask blank 10 . the illumination device 14 and the detection device 17 are preferably arranged internally in a recess 21 for receiving the mask blank 10 , both the illumination device 14 and the detection device 17 being positioned essentially perpendicularly to the side areas 13 of the mask blank 10 . the laser diodes 19 and the laser detectors 20 are preferably arranged alternately next to one another . the laser diodes 19 are connected in series , and the detectors 20 simultaneously receive the laser scattering waves which have been scattered in the mask blank 10 . the data supplied by the detectors 20 are preferably detected or evaluated by utilizing the kerr effect . in this case , too , as described with reference to fig1 , a data analysis of the output data of the detectors 20 is carried out in an evaluation device 18 , preferably a computer , e . g . with the aid of the fourier transformation , a blank characteristic , tomogram or a map of the mask blank 10 being created from the items of information generated . such a tomogram may thereupon be used , in each step for producing the mask blank 10 , as a reference for measurement data or blank alterations in the individual process steps . if the mask blank 10 is fixed in a fixing device ( not illustrated ) with an adjustable clamping force , then a flexure of the mask blank 10 possibly brought about by the clamping may be identified and corrected if the evaluation device 18 is connected to the adjustable clamp or fixing device . fig3 illustrates a diagrammatic oblique view of a blank map 10 ′, distortions , mechanical stresses in the mask blank 10 or flexures 22 of the mask blank 10 being illustrated by the flexed grid arrangement . in order to ensure a high quality standard of the mask blank 10 , it is necessary to avoid distortions or flexures 22 of the mask blank 10 , which can be identified in a blank map 10 ′. fig4 illustrates a detail , namely a fixing device 23 in the form of adjustable clamping elements , preferably piezoelements such as tube piezoelements , for example . the mask blank 10 is preferably fixed by means of three piezo devices 23 , which can each be driven variably by means of a separate dc voltage 24 . if an apparatus according to the invention , for example according to the embodiment as shown in fig1 or 2 , has localized a mechanical strain 22 of the mask blank 10 with the aid of a blank map 10 ′ in accordance with fig3 , then the clamping elements 23 , which can be driven independently of one another , can act on the mask blank 10 in such a way that the mechanical strain or flexure 22 is corrected or reduced . application of a voltage 24 to the tube piezos used as clamping or fixing device 23 causes them to expand . a tube piezoelement has a tube structure in this case , the adjustable dc voltage 24 being connected to the outer wall , i . e . the tube exterior , by one pole and to the inner wall , i . e . the tube interior , by one pole . the regulation of the dc voltage 24 may be provided externally for example with the aid of a potentiometer ( not illustrated ). in this way , a variable clamping force and thus the mechanical stress in a mask blank 10 can be controlled actively and , consequently , detected flexures of the mask blank 10 can be reduced . although the present invention has been described above on the basis of preferred exemplary embodiments , it is not restricted thereto , but rather can be modified in diverse ways . thus , the illumination device is not restricted to laser - based devices , but rather may also be provided for example by an illumination device with long coherent wavetrains such as a mercury vapor lamp , for example . furthermore , geometries other than rectangular arrangements and thus also geometries of the illumination device or of the detection device such as a circular configuration , for example , are also conceivable , in principle .