Autofocus system for a high speed periodically modulated variable focal length lens

A system for providing an automatically focused image using an imaging system that includes a high speed periodically modulated variable focal length (VFL) lens, the system comprising:

• an imaging system including at least an objective lens configured to input image light arising from a workpiece surface, a VFL lens configured to receive image light transmitted by the objective lens, and a camera configured to receive light transmitted by the VFL lens;
• a VFL lens controller configured to control the VFL lens to periodically modulate its optical power 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,
• a VFL-projected light source, comprising a light source configured to provide VFL-projected light along a focus monitoring light path to a back side of the VFL lens and through the VFL lens and the objective lens to the workpiece surface;
• a focus determining portion comprising an optical detector configured to input reflected VFL-projected light that has been reflected from a workpiece surface region and back through the objective lens and back through the VFL lens and back along the focus monitoring light path, and to provide at least one optical detector signal that is responsive to a difference between a focus Z height of the VFL-projected light and a Z height of the workpiece surface region, wherein the VFL-projected light focus Z-height is indicative of the imaging system focus Z-height, and the focus determining portion outputs at least one focus deviation signal based on the at least one optical detector signal;
• an exposure timing adjustment circuit, which inputs the focus deviation signal and determines an exposure timing adjustment signal related to a time when the imaging system focus Z-height approximately coincides with the workpiece surface region Z height, based on the focus deviation signal;
• and
• an exposure strobe time controller which controls an image exposure time of the imaging system, wherein the exposure strobe time controller is configured to input the exposure timing adjustment signal and automatically provide an adjusted image exposure time based on the exposure timing adjustment signal, wherein the imaging system focus Z-height approximately coincides with the workpiece surface region Z height at the adjusted image exposure time, and
• the system is configured to provide a focused image comprising at least one exposure increment at the adjusted image exposure time.