Difficulties_with_Digitally_Obtained_Interferogram_Quality

3.6.3 Difficulties with Digitally Obtained Interferogram Quality

The results of interferometric measurements can be distorted in consequence of poor quality optical elements, poor quality model, poor quality measurement systems or if we use analysing relations for ideal interferometry (not considering the beam curvature at the transmission through the field of inhomogenities, or boundary effects in the case of two-dimensional objects).

Besides these distortions the results can be also influenced by the used method of recording, the quality of interference records and the used algorithm of computer interferogram analysis. Poor quality records complicate mainly the process of automatic interference fringe detection, which directly influences the precision of interferometric measurements or can totally disable the process of interference fringe detection.

We can influence the quality of recording mainly by desirable adjustment of the contrast and brightness in the CCD camera image recording software. The contrast and brightness of interference fringes are also influenced by adjustment of suitable intensity ratio of interfering beam intensities and suitable laser performance. The trouble sometimes is in non-uniform illumination of interferometer field of view that is manifested either in non-uniform brightness of the recorded image or by non-uniform contrast of the fringes in the field of view, caused by undesirable intensity ratio of the object and reference branches of the interferometer.

An important inconvenience is also the graininess that is caused mainly by laser radiation that after incidence on any diffusive surface causes mutual interference of the individual beams, which is manifested as a "speckle".

Since the resolution of the CCD camera and mainly of the monitor is lower than that of holographic plate, the image obtained by this "dry" method is grainier than the image formed by so called "wet method" (Vejbor, Zapletálek, 1996).

The quality of interferometric records depends also on stability of the whole equipment. Even the smallest vibrations of interferometer cause significant vibrations of interference fringes which are then blurred or they vanish. The quality of the interference fringe record is conditional by minimally six points per a fringe width (Fig. 3–74 b). If the value of the fringe width falls under two points the obtained interferogram is devalued or decreased to the level of the shadow method (Fig. 3–74 a) (Pavelek et al, 2001).

On the interferogram records also other defects can be registered that are caused by dust on the optical components of the interferometer. Removal of defects caused by dust or other inhomogenities in the field of view of objectives can be executed by various program products, for example such as Photo Shop, Photo Finish, Corel Draw, etc. High quality records are possible to obtain by storage medium with higher resolution. That is why work with high-resolution CCD camera and adequate graphic card is more preferable.