Tunable acoustic gradient lens system utilizing amplitude adjustments for acquiring images focused at different z-heights

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:

during a first timeframe:

operating 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 Z1max+ and Z1 max−; and

operating the light source and camera to expose a first image using a first exposure increment having a first exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z1max+ or Z1 max−;

during a second timeframe:

adjusting the TAG lens controller to drive the periodic modulation of the TAG lens optical power at the resonant frequency of the TAG lens, using a second amplitude driving signal that provides a second amplitude of the periodic modulation at the resonant frequency that is different than the first amplitude of the periodic modulation, the second amplitude corresponding to a second focal Z range extending between peak focus distances Z2max+ and Z2max−, wherein the second focal Z range is different than the first focal Z range; and

operating the light source and camera to expose a second image using a second exposure increment having a second exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z2max+ or Z2max−; and

performing processing that includes determining focus metric values for the first and second images.

2. The system of claim 1 , wherein the processing includes comparing the focus metric values for the first and second images.

3. The system of claim 1 , wherein the processing includes determining that a focus metric value for the second image is higher than a focus metric value for the first image.

4. The system of claim 1 , wherein the focus distance during the first exposure increment moves over a first exposure Z range that is less than 10% of the first focal Z range.

5. The system of claim 1 , wherein the focus distance during the first exposure increment moves over a first exposure Z range that is less than 2 DOF (depth of field) of the system.

6. The system of claim 1 , wherein the focus distance during the second exposure increment moves over a second exposure Z range that is less than 10% of the first focal Z range.

7. The system of claim 1 , wherein the first exposure increment time interval is equal to the second exposure increment time interval.

8. The system of claim 1 , wherein the first exposure increment time interval is different than the second exposure increment time interval.

9. The system of claim 1 , wherein the first image and the second image are part of an image stack obtained by the system operating in a points from focus (PFF) mode, and the focus metric values are processed to determine a Z-height of the first workpiece surface.

10. The system of claim 1 , wherein the one or more processors are configured to execute the programmed instructions to perform operations further including:

during a third timeframe:

adjust the TAG lens controller to drive the periodic modulation of the TAG lens optical power at the resonant frequency of the TAG lens, using a third amplitude driving signal that provides a third amplitude of the periodic modulation at the resonant frequency that is different than the first amplitude and the second amplitude of the periodic modulation, the third amplitude corresponding to a third focal Z range extending between peak focus distances Z3max+ and Z3max−, wherein the third focal Z range is different than the first focal Z range and the second focal Z range; and

operate the light source and camera to expose a third image using a third exposure increment having a third exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z3max+ or Z3max−; and

perform processing that includes determining focus metric values for the first, second and third images.

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) during a first timeframe:

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 Z1max+ and Z1 max−; and

operating a light source and a camera to expose a first image using a first exposure increment having a first exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z1max+ or Z1 max−;

(c) during a second timeframe:

driving a periodic modulation of the TAG lens optical power, using a second amplitude driving signal that provides a second amplitude of the periodic modulation at the resonant frequency that is different than the first amplitude of the periodic modulation, the second amplitude corresponding to a second focal Z range extending between peak focus distances Z2max+ and Z2max−, wherein the second focal Z range is different than the first focal Z range; and

operating the light source and camera to expose a second image using a second exposure increment having a second exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z2max+ or Z2max−; and

(d) performing processing that includes determining focus metric values for the first and second images.

12. The method of claim 11 , wherein the processing includes comparing the focus metric values for the first and second images.

13. The method of claim 11 , wherein the processing includes determining that a focus metric value for the second image is higher than a focus metric value for the first image.

14. The method of claim 11 , wherein the focus distance during the first exposure increment moves over a first exposure Z range that is less than 10% of the first focal Z range.

15. The method of claim 11 , wherein the focus distance during the first exposure increment moves over a first exposure Z range that is less than 2 DOF (depth of field) of the VFL lens system.

16. The method of claim 11 , wherein the focus distance during the second exposure increment moves over a second exposure Z range that is less than 10% of the second focal Z range.

17. The method of claim 11 , wherein the first exposure increment time interval is equal to the second exposure increment time interval.

18. The method of claim 11 , wherein the first exposure increment time interval is different than the second exposure increment time interval.

19. The method of claim 11 , wherein the first image and the second image are part of an image stack obtained by the VFL lens system operating in a points from focus (PFF) mode, and the focus metric values are processed to determine a Z-height of the first workpiece surface.

20. The method of claim 11 , further comprising:

(e) during a third timeframe:

driving a periodic modulation of the TAG lens optical power, using a third amplitude driving signal that provides a third amplitude of the periodic modulation at the resonant frequency that is different than the first amplitude and the second amplitude of the periodic modulation, the third amplitude corresponding to a third focal Z range extending between peak focus distances Z3max+ and Z3max−, wherein the third focal Z range is different than the first focal Z range and the second focal Z range; and

operating the light source and camera to expose a third image using a third exposure increment having a third exposure increment time interval that is approximately centered at the timing of either the peak focus distance Z3max+ or Z3max−; wherein

the processing performed in (d) includes determining focus metric values for the first, second, and third images.