Variable focal length lens system with focus monitoring and control

Patent metadata

Field	Value
Publication number	US-10151962-B2
Application number	US-201615280501-A
Country	US
Kind code	B2
Filing date	Sep 29, 2016
Priority date	Sep 29, 2016
Publication date	Dec 11, 2018
Grant date	Dec 11, 2018

Abstract

A variable focal length (VFL) lens system is provided including a tunable acoustic gradient (TAG) lens and an optical focus monitoring configuration for providing a focus monitoring signal that reflects a focus state with high accuracy and without significant latency. An input illumination pattern is transmitted through the TAG lens to provide a corresponding output illumination pattern that has a size and intensity that depends on the optical power of the TAG lens. An optical focus signal detector portion includes a filtering configuration and a focus photodetector that provides a focus output signal that varies in relation to the total light energy that the focus photodetector receives, wherein the filtering configuration receives the output illumination pattern and limits the amount of included focus detection light that reaches the focus photodetector. A focus monitoring signal is provided based on the focus output signal provided by the focus photodetector.

First claim

What is claimed is:

1. A variable focal length (VFL) lens system comprising:
a tunable acoustic gradient (TAG) lens operated to periodically modulate its optical power over a range of optical powers at an operating frequency; and
an optical focus monitoring configuration for monitoring a focus state of the VFL lens system, the optical focus monitoring configuration comprising:
a monitoring light source configured to input focus detection light into the TAG lens during the periodic modulation, wherein:
the input focus detection light is configured to provide an input amount of light energy distributed in an input illumination pattern having an approximately constant size; and
at least a central portion of the input illumination pattern is transmitted through the TAG lens during the periodic modulation to provide a corresponding output illumination pattern from the TAG lens, the output illumination pattern having a size and intensity that depends on the optical power of the TAG lens; and

an optical focus signal detector portion positioned at an approximately constant distance from the TAG lens to receive focus detection light included in the output illumination pattern output from the TAG lens, the optical focus signal detector portion comprising a filtering configuration and a focus photodetector that provides a focus output signal that varies in relation to a total light energy that the focus photodetector receives, wherein the filtering configuration receives the output illumination pattern and limits an amount of the included focus detection light that reaches the focus photodetector and a focus monitoring signal is provided based on the focus output signal provided by the focus photodetector.

2. The VFL lens system of claim 1 , wherein the focus detection light is at least approximately collimated in the input illumination pattern.

3. The VFL lens system of claim 1 , wherein the input illumination pattern is configured such that the corresponding output illumination pattern comprises a solid pattern, and the filtering configuration comprises a spatial filtering aperture that is defined by limits of the focus photodetector, and the solid pattern overfills the focus photodetector at all times during the periodic modulation.

4. The VFL lens system of claim 1 , wherein:
the filtering configuration includes a mask comprising a spatially filtering aperture;
the mask blocks a blocked portion of the output illumination pattern at all times during the periodic modulation;
the spatially filtering aperture transmits a transmitted portion of the output illumination pattern at all times; and
the spatially filtering aperture is shaped such that a ratio of the transmitted portion to the blocked portion varies depending on the size of the output illumination pattern.

5. The VFL lens system of claim 4 , wherein the input illumination pattern is configured such that the corresponding output illumination pattern comprises an annular pattern.

6. The VFL lens system of claim 1 , wherein the filtering configuration includes a mask comprising a density filter having a non-uniform density pattern configured to attenuate the transmission of the output illumination pattern depending on the size of the output illumination pattern.

7. The VFL lens system of claim 6 , wherein the density filter is configured to receive and transmit the entire output illumination pattern to the focus photodetector at all times during the periodic modulation.

8. The VFL lens system of claim 6 , wherein the input illumination pattern is configured such that the corresponding output illumination pattern comprises an annular pattern, and the non-uniform density pattern is axisymmetric and a density of the non-uniform density pattern varies as a function of radius within the output illumination pattern.

9. The VFL lens system of claim 1 , wherein the periodic modulation corresponds to a frequency of at least 50 kHz, the focus monitoring signal comprises a time varying signal that is indicative of the focus state of the TAG lens throughout the modulation period, and the time varying signal is provided with a latency compared to the focus state of not more than 100 nanoseconds.

10. The VFL lens system of claim 1 , comprising a controller that operates to drive the TAG lens at a resonant frequency in order to periodically modulate the TAG lens optical power over the range of optical powers at the operating frequency, wherein the focus monitoring signal is input to the controller and is used to adjust at least one of amplitude, frequency, or phase of the periodic modulation of the TAG lens.

11. The VFL lens system of claim 1 , further comprising an imaging configuration comprising the TAG lens, an objective lens, and a camera portion, wherein:
the objective lens inputs workpiece light from an imaged surface region of a workpiece in a field of view (FOV) of the imaging configuration and transmits the workpiece light through the TAG lens, and the camera portion receives the workpiece light from the TAG lens and provides an image exposure such that it is focused at a corresponding imaging system focal plane having at least one of a focus distance or Z-height relative to the imaging configuration;
at least one of the focus distance or Z-height of the imaging system focal plane is controlled by the TAG lens optical power; and
the focus monitoring signal is indicative of at least one of the focus distance or Z-height of the imaging system focal plane.

12. The VFL lens system of claim 11 , wherein the VFL lens system includes calibration data that relates respective focus distances or Z-heights to respective focus monitoring signal values.

13. The VFL lens system of claim 11 , wherein:
the VFL lens system is configured to control the image exposure using an image exposure timing that determines the corresponding imaging system focal plane; and
the VFL lens system is configured to control at least one of a timing of a controllable strobe light source that is included in the VFL lens system or a timing of a controllable image integration period of the camera portion, to provide the image exposure timing.

14. The VFL lens system of claim 13 , further comprising a latching circuit configured to latch a focus monitoring signal value at a time corresponding to the image exposure timing, wherein the latched focus monitoring signal value is indicative of the focus distance or Z-height for the corresponding image exposure.

15. The VFL lens system of claim 13 , further comprising a comparator circuit configured to input the focus monitoring signal and input a reference signal related to a desired imaging focus distance or Z-height, and output a trigger signal that controls the image exposure timing to occur when the focus monitoring signal corresponds to the reference signal.

16. The VFL lens system of claim 11 , wherein the monitoring light source is configured to provide focus detection light consisting of a first set of wavelengths in the input illumination pattern, and the optical focus monitoring configuration further comprises:
a first beamsplitter that is located between the objective lens and the TAG lens and receives focus detection light from the monitoring light source and directs the input illumination pattern to pass through the TAG lens along with the workpiece light; and
a second beamsplitter that is located between the TAG lens and the camera portion wherein the second beamsplitter is configured to receive the output illumination pattern from the TAG lens along with the workpiece light and reflect the first set of wavelengths included in th

Assignees

• Mitutoyo Corp

Inventors

• Gladnick Paul Gerard
• Atherton Kim

Classifications

• H04N23/56

  provided with illuminating means · CPC title

• H04N23/67

  Focus control based on electronic image sensor signals · CPC title

• H04N23/73

  by influencing the exposure time · CPC title

• H04N23/60

  Control of cameras or camera modules · CPC title

• H04N23/74

  by influencing the scene brightness using illuminating means · CPC title