Non-turning-point monitoring improves narrow bandpass filters.

Historic practice has been to optically monitor narrow bandpass filters by the termination of each layer at a turning point. The problem is that turning points are the least sensitive points to the change of the optical signal with thickness and, thereby, those points are most prone to errors. It is shown that better performance in the production results can be achieved by designing and monitoring in order to terminate layers at non-turning points. A further advantage has been discovered wherein nonoptical monitoring of some layers is used to achieve even better stability in the production result. Simulation programs have been applied to such designs and demonstrate the advantages as compared to the historical approach.