Focus-tunable liquid lenses have become a widely adopted solution for applications that require rapid, precise focus changes without mechanical movement. From semiconductor inspection and metrology to medical imaging and automated optical systems, these lenses enable focus adjustments in milliseconds while reducing system complexity.
However, speed alone is not enough.
For many engineers, the more important question is:
Can a liquid lens return to exactly the same focal position every time, and can it hold focus reliably over long periods?
When depth of field is measured in tens of microns, even a small drift in focal power can affect measurement accuracy, image quality, and process consistency. This is especially critical in inspection systems, where repeatable results are often more important than focusing speed itself.
In this article, we’ll explore why stability and repeatability matter, how they are evaluated, and what recent testing reveals about the performance of Optotune’s electrically tunable liquid lenses.
Why stability matters
Many industrial imaging systems spend most of their operating life focused at a fixed working distance.
Examples include:
- Semiconductor inspection
- Metrology systems
- Die bonding inspection
- Precision manufacturing quality control
In these applications, autofocus may only occur occasionally. Once focus has been achieved, the system must maintain that focal position for hours or even days.
Any focus drift can become problematic when working with high-magnification optics. Telecentric inspection systems often operate with extremely shallow depth of field, meaning even a small change in focal power can move the image out of optimal focus.
As a result, long-term stability becomes a key performance metric for any dynamic optical component.
Why repeatability is equally important
Other applications have the opposite requirement.
Instead of maintaining a fixed focus position, they repeatedly jump between known working distances.
Examples include:
- Multi-height inspection
- Automated focusing systems
- Robotic machine vision
- Dynamic imaging processes
In these systems, the lens may change focus hundreds or thousands of times per day.
The important question becomes:
When the lens returns to a previously calibrated focal position, does it return to the exact same optical state?
If not, additional calibration routines may become necessary, reducing speed and increasing system complexity.
For manufacturers designing automated optical equipment, repeatability determines whether a predefined focus table remains accurate over time.
Testing liquid lens performance under realistic conditions
To evaluate these characteristics, Optotune conducted a series of experiments using an inspection-style optical setup based on the EL-16-40-TC electrically tunable lens integrated into a telecentric imaging system. The test configuration intentionally used a high-magnification optical system with a very shallow depth of field, creating demanding conditions similar to those found in semiconductor inspection and metrology applications.Â
Instead of examining focus subjectively, image contrast was continuously monitored using a precision target. Any variation in focal power would directly affect measured contrast, providing a reliable indication of optical performance.Â
Two key operating scenarios were investigated:
- Long-term focus holds where focal power remained unchanged for extended periods.
- Rapid focus jumps where the lens repeatedly moved away from and back to predefined focal positions.Â
These two scenarios represent many of the real-world operating conditions encountered in machine vision and industrial automation systems.
Long-Term focus stability
The first series of tests examined what happens when a liquid lens remains focused at the same working distance for several days.
For users of traditional mechanical focusing systems, long-term stability can be affected by thermal expansion, vibration, wear, or mechanical tolerances.
Liquid lenses introduce a different mechanism of operation, using electrical actuation rather than moving lens groups. This raises a common question among engineers:
Will focus drift over time?
The experiments showed that focus remained highly stable over extended periods, demonstrating focal power variations within a very narrow range even after several days of operation. Additional measurements using a Shack-Hartmann sensor confirmed similarly stable behavior when external influences were minimized.
Repeatability during rapid refocusing
The second test scenario focused on repeatability.
The lens was repeatedly moved to random focal positions and then returned to predefined focus points. These focus jumps occurred over a range of operating conditions and working distances representative of practical inspection systems.Â
For automated optical systems, this type of behavior is often more relevant than long-term holding stability.
A barcode scanner, autofocus system, or robotic inspection platform may constantly switch between focal positions. To maintain consistent performance, the lens must reliably return to the correct focal power every time.
The results demonstrated repeatability within a narrow tolerance band, supporting the use of liquid lenses for applications that rely on frequent focus changes and predetermined calibration values.Â
What this means for optical system designers
The practical takeaway is straightforward.
When focus-tunable lenses are used in precision optical systems, their performance must be evaluated not only in terms of speed, but also in terms of long-term consistency.
A lens that can focus in milliseconds is only valuable if it can also:
- Maintain focus reliably over time
- Repeat calibrated focal positions accurately
- Operate consistently across thousands of focus cycles
- Remain within the tolerance requirements of the overall optical system
The testing presented by Optotune showed both stability and repeatability within approximately ±10 mdpt under the evaluated conditions, demonstrating suitability for demanding inspection and metrology applications.Â