One problem in the quantitative

One problem in the quantitative evaluation of holographic interferograms is to measure the absolute fringe order and not only the relative one. This relative fringe order can be measured with high accuracy by means of methods like heterodyning or phase shifting. To get the absolute value a method using two different wavelength to perform the interfero- grams. The mathematical treatment and an experimental application is reported. 1. INTROCUCTION Holographic Interferometry is a proven technology for nondestructive testing in certain areas of industrial use. An example for this application is the holographic tire tester. However, in any case the evaluation is done by a visual inspection of the interferometric fringe system; there is no example of a fully automated evaluation with the test result "yes" or "no" till today. One of the problems to reach this goal is an automated evalua­ tion of the absolut fringe order. Holographic Interferometry is a proven tool for quantitative deformation measurements in a laboratory environment. Examples for this application are the experimental stress ana­ lysis or the design optimization measurements. ' However, in all these applications the additional input of the absolute fringe order in at least one surface point in necessary to determine the correct deformation and by this the correct stress in a given point. The future of holographic interferometry will see more applications in measurement tech­ nologies additionally to the NOT. This application requires a fully automated evaluation of the fringe system with high accuracy, since the phase or the fringe order, respective­ ly, is related to the dislocation of a surface point by the simple multiplication with the sensitivity vector. Today,, several methods are developed to determine the phase relative to any point of the surface. ' ' However, the problem to determine the absolute value from the Rinterferogram is unsolved till today, although hints are given in former reports. /J° In this report a new method is discussed to measure the absolute phase by the use of two more different wavelengths: the interferograms are performed with different colors of a dye laser. The change of the wavelength can be measured and controlled very precisely. This change results a variation of the fringe ^order, which is proportional to the fringe order itself: The absolute order can be determined from the precise measurement of the phases in the different interferograms.