fig1 shows a schematic representation of coordinate measuring device 1 according to the present invention . the coordinate measuring device has measurement table 20 carrying mask 2 . measurement table 20 is positioned on air bearings 21 to be movable in the x - coordinate direction and in the y - coordinate direction . in the embodiment shown , granite block 25 forms plane 25 a on which measurement table 20 is movably positioned . the position of the measurement table is determined by means of interferometer system 24 emitting a measurement light beam 23 . granite block 25 is positioned on vibration dampers 26 . it is clear to one skilled in the art that any other material may be used for providing plane 25 a in which measurement table 20 may be moved . the use of granite block 25 shown here is therefore not to be considered as limiting the invention in any way . mask 2 carries a plurality of structures 3 to be measured . incident light illumination means 14 emitting light along incident light optical illumination path 5 is provided for the illumination of the mask . similarly , transmitted light illumination means 6 emitting light along transmitted light optical illumination path 4 is provided for the transmitted light illumination of mask 2 . the light of transmitted light illumination means 6 is directed to the substrate by means of condenser 8 . the light of incident light illumination means 14 reaches mask 2 via measurement objective 9 . measurement objective 9 is arranged to be translatable for focusing in the z - coordinate direction by means of displacing means 15 . in the incident light optical illumination path , there is arranged coupling - out means 12 which directs the light collected from mask 2 and objective 9 to camera 10 including detector 11 . detector 11 is connected to a computer generating a digital image from the acquired signals . the following paragraph refers to both fig1 and 2 . in order to determine the optical properties of mask 2 , measurement table 20 is moved such that first area 41 and / or second area 42 on mask 2 may be imaged by objective 9 . first area 41 or second area 42 is thus imaged on detector 11 of the coordinate measuring machine by objective 9 . detector 11 then determines the transmission and / or reflection properties of first or second area 41 , 42 based on the acquired data ( see fig2 a or 2 b ). in order to prevent fluctuations of incident light illumination means 14 and / or transmitted light illumination means 6 during the examination of first and / or second area 41 , 42 , part of the light in the respective optical illumination path 4 , 5 is directed to at least one monitor detector 30 , 31 . the signal generated by monitor detectors 30 , 31 is also supplied to computer 16 so that potential light fluctuations of transmitted light illumination means 6 and / or incident light illumination means 14 during the measurement of first and / or second area 41 , 42 may be compensated . a variety of means may be used to direct part of the light in incident light optical illumination path 5 or in transmitted light optical illumination path 4 to the respective monitor detectors 30 , 31 . in the embodiment shown , coupling - out mirrors 7 , 32 are used , which direct a fraction of the light in incident light optical illumination path 5 and in the transmitted light optical illumination path 4 , respectively , to the monitor detector or transmits it to the monitor detector . it is also contemplated that part of the light in incident light optical illumination path 5 or in transmitted light optical illumination path 4 is coupled out by a respective fiber and directed to the corresponding monitor detector . it is clear to one skilled in the art that the suggested manner of coupling out or directing the light to the monitor detectors is only one possibility that may not be considered as limiting the invention in any way . fig2 a shows a schematic view of mask 2 provided with first area and second area 41 , 42 , which may be used for determining the optical properties of mask 2 . first area 42 consists of a material differing from that of second area 42 . with the help of first area 41 and second area 42 on mask 2 , the optical properties of mask 2 , such as transmission and / or reflection , may be examined . fig2 b shows the arrangement of first area 41 and second area 42 not connected to each other . areas 41 , 42 may be arranged at any location on the mask . first area 41 and second area 42 differ in their transmission and reflection properties . the material of areas 41 , 42 may , for example , correspond to the material of mask 2 and commonly consist of sio 2 . it is also possible that the material of one of areas 41 , 42 is a cr layer or a phase - shifting material , such as mosi . areas 41 , 42 applied to mask 2 are preferably larger than the resolution of the imaging means or measurement objective 9 . in a preferred embodiment , first area 41 and second area 42 or areas 41 , 42 together have a size of about 10 μm . in order to determine the transmission and / or reflection properties of mask 2 , first and / or second areas 41 , 42 may be imaged on detector 11 of the coordinate measuring machine by measurement objective 9 , if areas 41 , 42 are arranged adjacent to each other . fig3 a shows a first embodiment of the arrangement of first area 41 and second area 42 on mask 2 . first and second areas 41 , 42 are arranged to be adjacent to each other , and they differ in material . in a preferred embodiment , the mask consists of sio 2 . fig3 b shows a second embodiment of the arrangement of first area 41 and the second area 42 on mask 2 . second area 42 is a surface of the mask , so that the material of second area 42 corresponds to the material of the mask . the mask preferably consists of sio 2 . with this arrangement , the transmittance and / or reflectance of mask 2 may be determined relative to material of first area 41 . fig3 c shows a third embodiment of the arrangement of first area 41 and second area 42 on mask 2 . second area 42 is applied to the surface of mask 2 as an intermediate layer . the mask preferably consists of sio 2 . with this arrangement , the transmittance and / or reflectance of the material of first area 41 may be determined relative to the material of second area 42 . the transmission / reflection properties are always determined relatively between areas 41 and 42 . for this purpose , one area is defined as a reference . the invention has been described with reference to particular embodiments . however , it is contemplated that modifications and changes may be made to the invention without departing from the scope of the following claims .