On-line Measurement of Refractive Index Variations in Optical Foils

This paper addresses the problems arising in continuous film extrusion of thin foils that need to result in optical quality films but due to processes still not completely understood give suboptimal quality of the resultant foil. The foils show more or less severe chatter marks dependent on certain system parameters like feed rate of the product, temperature of the product casted, spinning rate and differential spinning rate of the rolls, cooling rate and so on. An in–line capable optical quality measurement system is devised that allows to continuously assess the product quality within seconds after the foil is extruded from the casting unit, thus allowing rapid corrective actions if quality declines below a set standard. It also allows for a continuous documentation of the amount of residual chatter marks for each and every inch of product. The system is based upon a schlieren-optical setup able to resolve schlieren-caused refraction of collimated light of less than 10 second of arc.