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Introduction

Optical techniques are now becoming widely used on the production line for the automatic inspection of objects and their components, bringing significant advantages in the cost-effective improvement in the quality of products. By reasons of their sensitivity, accuracy and non-contact as well as non-destructive characteristics, methods such as holographic interferometry, speckle metrology, moiré and fringe projection techniques have found an increasing interest not only for laborious investigations but also for applications on the factory floor.
These applications cover such important fields as testing of lenses and mirrors with respect to optical surface forms, construction optimization of components by stress analysis under operational load, on site investigation of industrial products with diffusely reflecting surfaces for the purpose of material fault detection and last but not least optical shape recognition as an area of topical interest for the solution of problems in reverse engineering.

The basic principle of these methods consists either in a specific structuring of the illumination of the object by incoherent projection of fringe patterns onto the surface under test or by coherent superposition (interference) of light fields representing different states of the object. A common property of the methods is that they produce fringe patterns as output. In these intensity fluctuations the quantities of interest - coordinates, displacements, refractive index and others - are coded in the scale of the fringe period.
Using coherent methods this period is determined by the wavelength of the interfering light fields. In the case of incoherent fringe projection the spacing between two neighboring lines of the projected transparency controls the scale of the measurement. Consequently the task to be solved in fringe analysis can be defined as the conversion of the fringe pattern into a continuous phase map taking into account the quasi sinusoidal character of the intensity distribution.

Fringe pattern of a valve 3D model of a valve

Techniques for the analysis of fringe patterns are as old as interferometric methods themselves, but until into the late 1980s routine analysis of fringe patterns for optical shop testing, experimental stress analysis and non-destructive testing was mainly performed manually.
One example may illustrate this highly subjective and time consuming process: If only one minute is estimated for the manual evaluation per measuring point the total time necessary for the reconstruction of the three-dimensional displacement field of a 32×32 mesh would take almost 6 working days because at least 3 holographic interferograms with 1024 grid points each have to be evaluated. Such a procedure must be qualified as very ineffective with respect to the requirements of modern industrial inspection, to say nothing of the limited reliability of the derived data.
However, the step from a manual laborious technique done by skilled technicians to a fully automatic procedure was strongly linked to the availability of modern computer technology. Therefore the development of automatic fringe pattern analysis closely followed the exponential growth in the power of digital computers with image processing capabilities.

For several years various commercial image processing systems with different efficiency and price level have been developed and specialized systems dedicated to the solution of complex fringe analysis problems are commercially available now. Advanced hardware and software technologies enable the use of desktop systems with several image memories and special video processors nearby the optical set-up. This online connection between digital image processors and optical test equipment opens completely new approaches for optical metrology and non-destructive testing as real time techniques with high industrial relevance. In this sense automatic fringe analysis has developed mainly during the 1980s into a subject in its own right.