Method and system for detecting localized birefringence defects in a

Abstract

A method and system are provided for reliably detecting localized birefringence defects of an object such as an optical media substrate in a dynamic local birefringence field created during cooling of the optical media substrate at an inspection station. A birefringence digital filter filters out high frequency defect data while eliminating background birefringence data in a digital image created by a camera adapted to respond to the birefringence of the optical media substrate under control of a computer. Resulting filtered high frequency defect data is then processed to determine the localized birefringence defects in real-time. In one embodiment, the birefringence digital filter includes an addressable storage device such as a PROM which stores a lookup table which is able to produce filtered data in real-time at a data rate in excess of 8 million pixels per second. In another embodiment, a finite impulse response (FIR) filter, including registered multiplier-adders may be used to calculate or obtain filtered data at a higher rate (i.e. 10 million pixels per second). The filtered data may be post processed to accomplish gain and offset correction as well as to impose nonlinear characteristics on the output data if desired to implement a data squelch. The data squelch can reduce undesirable noise generated as an artifact of the filtering process.