Tunable acoustic gradient lens system with amplitude adjustment corresponding to z-height as indicated by calibration data

1. A variable focal length (VFL) lens system, comprising:

a tunable acoustic gradient (TAG) lens;

a TAG lens controller that controls the TAG lens to periodically modulate the optical power of the TAG lens over a range of optical powers at an operating frequency;

a light source;

an objective lens that inputs workpiece light arising from a first workpiece surface, which is illuminated by the light source, and transmits the workpiece light along an imaging optical path that passes through the TAG lens;

a camera that receives the workpiece light transmitted by the TAG lens along the imaging optical path and provides a corresponding workpiece image exposure;

a memory for storing programmed instructions; and

one or more processors to execute the programmed instructions to perform operations including:

adjusting the TAG lens controller to drive a periodic modulation of the TAG lens optical power at a resonant frequency of the TAG lens, using a first amplitude driving signal that provides a first amplitude of the periodic modulation at the resonant frequency, the first amplitude corresponding to a first focal Z range extending between peak focus distances Z 1 max+ and Z 1 max−, wherein the first amplitude driving signal is selected based on a first target focus distance and stored calibration data, wherein the calibration data relates different amplitude driving signals to different target focus distances and indicates that the first amplitude driving signal corresponds to the first target focus distance; and

operating the light source and the camera to expose an image using at least one exposure increment having an exposure increment time interval wherein the focus distance during the exposure increment moves over a first exposure Z range that includes the target focus distance.

2. The system of claim 1 , wherein the calibration data comprises a plurality of target focus distances and indicates that a different amplitude driving signal corresponds to each target focus distance.

3. The system of claim 1 , wherein the exposure increment time interval is approximately centered at the timing of either the peak focus distance Z 1 max+ or Z 1 max−.

4. The system of claim 1 , wherein the first exposure Z range is less than 10% of the first focal Z range.

5. The system of claim 1 , wherein the first exposure Z range is less than 2 DOF (depth of field) of the system.

6. The system of claim 1 , wherein the first target focus distance is proximate to either the peak focus distance Z 1 max+ or Z 1 max− in that the difference between the first target focus distance and the proximate peak focus distance is less than 10% of the first focal Z range.

7. The system of claim 1 , wherein the first target focus distance is proximate to either the peak focus distance Z 1 max+ or Z 1 max− in that the difference between the first target focus distance and the proximate peak focus distance is less than 2 DOF (depth of field) of the system.

8. The system of claim 1 , wherein the operating of the light source comprises operating the light source to provide pulse light with a first phase pulse timing that corresponds to the timing of the peak focus distance Z 1 max+ or Z 1 max−.

9. The system of claim 8 , wherein the operating of the camera comprises operating the camera to acquire an image of a first workpiece feature at the first target focus distance as illuminated by the pulse light at the first phase pulse timing.

10. The system of claim 1 , wherein the first workpiece surface is at a first Z height and the first target focus distance corresponds to the first Z-height.

11. A method for operating a variable focal length (VFL) lens system comprising a tunable acoustic gradient (TAG) lens and with a workpiece surface in a measurement/imaging volume of the VFL lens system, the method comprising:

(a) periodically modulating the optical power of the TAG lens over a range of optical powers at a resonant frequency of the TAG lens;

(b) driving a periodic modulation of the TAG lens optical power, using a first amplitude driving signal that provides a first amplitude of the periodic modulation at the resonant frequency, the first amplitude corresponding to a first focal Z range extending between peak focus distances Z 1 max+ and Z 1 max−, wherein the first amplitude driving signal is selected based on a first target focus distance and stored calibration data, wherein the calibration data relates different amplitude driving signals to different target focus distances and indicates that the first amplitude driving signal corresponds to the first target focus distance; and

(c) operating a light source and a camera to expose an image using at least one exposure increment having an exposure increment time interval wherein the focus distance during the exposure increment moves over a first exposure Z range that includes the target focus distance.

12. The method of claim 11 , wherein the first target focus distance is proximate to a middle point of the first exposure Z range along the Z direction.

13. The method of claim 11 , wherein the exposure increment time interval is approximately centered at the timing of either the peak focus distance Z 1 max+ or Z 1 max−.

14. The method of claim 11 , wherein the first exposure Z range is less than 10% of the first focal Z range.

15. The method of claim 11 , wherein the first exposure Z range is less than 2 DOF (depth of field) of the VFL lens system.

16. The method of claim 11 , wherein the first target focus distance is proximate to either the peak focus distance Z 1 max+ or Z 1 max− in that the difference between the first target focus distance and the proximate peak focus distance is less than 10% of the first focal Z range.

17. The method of claim 11 , wherein the first target focus distance is proximate to either the peak focus distance Z 1 max+ or Z 1 max− in that the difference between the first target focus distance and the proximate peak focus distance is less than 2 DOF (depth of field) of the VFL lens system.

18. The method of claim 11 , wherein the operating of the light source comprises operating the light source to provide pulse light with a first phase pulse timing that corresponds to the timing of the peak focus distance Z 1 max+ or Z 1 max−.

19. The method of claim 18 , wherein the operating of the camera comprises operating the camera to acquire an image of a first workpiece feature at the first target focus distance as illuminated by the pulse light at the first phase pulse timing.

20. The method of claim 11 , wherein the at least one exposure time increment comprises a plurality of exposure time increments that are repeated cyclically, each using the same exposure increment time interval.