Modulation monitoring system for use with an imaging system that includes a high speed periodically modulated variable focal length lens

A method for operating a modulation monitoring system for use with an imaging system that images a workpiece surface and that includes a high speed periodically modulated variable focal length (VFL) lens, wherein:

• the imaging system comprises:
• an imaging light source that is not the workpiece surface and that is configured to provide source light along a source light path to illuminate the workpiece surface, wherein the source light travels from the imaging light source to the workpiece surface and the source light is at least one of reflected or transmitted by the workpiece surface as image light;
• an objective lens configured to input the image light arising from the workpiece surface, the VFL lens configured to receive image light transmitted by the objective lens along an imaging optical path, and a camera configured to receive light transmitted by the VFL lens along the imaging optical path;
• a VFL lens controller configured to control the VFL lens to periodically modulate its optical power at high speed and thereby periodically modulate a focus position of the imaging system over a plurality of imaging system focus Z heights along a Z-height direction, and
• the modulation monitoring system comprises:
• a VFL-traversing light source, comprising a light source configured to provide VFL-traversing light along a modulation monitoring light path through the VFL lens, wherein the VFL-traversing light source comprises a different light source than the imaging light source and the modulation monitoring light path does not include the workpiece surface;
• and
• a modulation signal determining portion comprising an optical detector;
• wherein the method comprises:
• operating the VFL-traversing light source to provide the VFL-traversing light along the modulation monitoring light path through the VFL lens and for which the optical detector receives the VFL-traversing light and the modulation monitoring light path does not include the workpiece surface, wherein the VFL-traversing light travels from the VFL-traversing light source through the VFL lens and to the optical detector but does not travel to or from the workpiece surface that is imaged by the imaging system;
• receiving from the optical detector at least one optical detector signal that corresponds to the modulated optical power of the VFL lens;
• and
• providing at least one modulation monitoring signal based on the at least one optical detector signal.