One of the most important properties of optical glass is the excellent spatial homogeneity of the refractive index of the material. Nevertheless, sometimes spatially short-range inhomogeneities are formed during the production process. These striae are strongly anisotropic due to the process of glass melting. In optical systems, they cause degradation of the performance with a complicated behavior. The quality specification of the glass homogeneity usually is given by simple values of phase differences along the main propagation direction of the light in an area of a certain size. For the measurement of these effects, interferometry can be used, which is a quite expensive method in reality. The observation of striae shadowgraph pictures is a faster and more frequently used method. The evaluation and quantitative reconstruction of the inhomogeneities in glass based on the striae technique are the main goal of this work. We revise the experimental setup and develop models to simulate the measurements for thin and thick samples. The results of the shadowgraph method are compared with interferometric measurements. A more refined evaluation which is not only based on the image contrast allows a unique and accurate reconstruction of the size and the phase height of striae with negligible axial extension. A simple procedure shows how one can estimate the effect in thick samples in practice approximately.

Date Published: 17 June 2009
PDF: 9 pages
Proc. SPIE 7389, Optical Measurement Systems for Industrial Inspection VI, 73891C (17 June 2009); doi: 10.1117/12.827677

Published in SPIE Proceedings Vol. 7389:
Optical Measurement Systems for Industrial Inspection VI
Peter H. Lehmann, Editor(s)