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On-line Measurement of Refractive Index Variations in Optical Foils

Author(s): Dominik Hofer, Bernhard Zagar
Affiliation(s): Inst. for Measurement Technology

Topic: 6. Applications for optical 1D and 2D sensors
Presentation: Oral

Periodic disturbances in sheets of transparent media may occur during its manufacturing. These inhomogeneities are either results of a varying refractive index in sheets of equal thickness or of a varying thickness in an otherwise homogeneous sheet. By means of a Schlieren--system it is possible to accurately determine the periodicity and magnitude of optical deviations with the aim to be able to intervene in the process of production to improve quality. In a typical Schlieren--system light from a point--like light source is collimated, passes through the assumed homogeneous object under test and is refocused exactly onto an edge to block it out. Some light deflected out of collimation due to the action of Schlieren within the object would pass by the edge and be focused by a projection lens onto a properly placed photo detector. In this proposed paper we present a system realizing a highly sensitive Schlieren--system able to measure precisely the optical quality of sheets with reaction times sufficiently small to allow for immediate corrective actions at the extruder and the subsequent rollers. It consists of the Schlieren--system, a highly sensitive optical detector with read--out electronics and associated PC--based visualization and measurement software.