**4.2.6 Three-dimensional measurements**

3-D metrics have first been developed for simplification purposes but three-dimensional image assessments causes new challenges (Fig. 11.). There are measurement distortion

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Fig. 12. Interface of digital image analysis software Myriad v8.0

Fig. 13. Milimetric paper test of image processing software.

reflective plate.

Colour measurements were determined with Minolta Cr200 model colourmeter which is based on L\*a\*b\* measurement system (Fig. 14.). Avarage of three sample points are used as colour value (Fig. 15.). Minolta Cr200 model colourmeter was calibrated with a white

Fig. 11. 3D measurement system that it developed jointly with the National Institute of Advanced Industrial Science and Technology (Anonymous, 2011a.).

between an original 3-D surface and its deformed version. The other important challenge is analysis of 3-D views by using 2-D screens. Image measurements still require identifying the pixels that are connected to each other. In two dimensions, it is necessary to determine differences between touching pixels.

Existing 3D measurement techniques are classified into two major types—active and passive. In general, active measurement employs structure illumination (structure projection, phase shift, moire topography, etc.) or laser scanning, which is not necessarily desirablein many applications. On the other hand, passive 3D measurement techniques based on stereo vision have the advantages of simplicity and applicability, since such techniquesrequire simple instrumentation.
