Focus Monitoring Arrangement and Inspection Apparatus Including Such an Arrangement

摘要: A focus monitoring arrangement ( 1000 ) is provided for a scatterometer or other optical system. first sensor 510 provides signal (S 1 -S 2 indicating relative to reference distance (z ). second 1510 providing (C -C processor 1530 calculates third by combining the and signal. By varying proportions of signals in calculating signal, an effective offset can be varied electronically, without moving elements.