The working distance between the object and the camera system usually does not change or only changes slightly in the mass production of products. This means that the optical design of an image processing system is relatively simple in these cases: the optical parameters of the system are calculated from the existing working distance and the desired resolution, and the system then does its job without any further adjustments once the appropriate components have been selected and installed in the system. Converting the system to products with a different geometry requires a one-off adjustment of the image processing system, but the system then runs again - ideally without any further intervention until the next changeover.

However, in many applications, image processing systems have to implement frequent changes to the working distance. In extreme cases, the distance between the inspection plane and the camera system changes from one object to the next - for example in a system for reading addresses on parcels, where the height of the parcels can vary constantly. In order to always be able to capture the desired information in this example, the working distance to the image processing system or the focus of the optics would have to be adjusted for each packet. This often cannot be done mechanically at the required speed.

Variable curvature of the lens

Although traditional optical systems allow focusing within certain limits depending on the required distance, it is necessary to move one or more lenses along the optical axis. This requires motors and mechanical guides, which not only limits the size and response time, but also the robustness and lifetime of a solution.

Variable-focus lenses provide an alternative for such applications. The technical basis is the variable curvature of a lens consisting of a membrane filled with a liquid. If the pressure within the lens is changed, the curvature can be adjusted. At the Swiss company Optotune, for example, a current-controlled actuator presses liquid through a ring from the outside into the inside of the lens, which changes the curvature of the lens and therefore the focal length. Changing the lens radius by just a few micrometers can achieve the same optical effect as mechanically shifting a lens by several centimetres. This makes it possible to design more compact optical systems, often with fewer lenses and without translational movement. Optotune has implemented current-controlled control of the electro-optical components via an electromagnetic actuator. The change in refractive power (measured in diopters) is linear to the applied current, is reproducible and also free of hysteresis.

© Stemmer Imaging

However, the ratio between refractive power and current varies from lens to lens due to production tolerances, and temperature fluctuations also influence the control. In order to enable accurate control of the refractive power, the variable-focus lenses contain a temperature sensor on which the calibration data of the respective lens is also stored. An absolute accuracy of typically 0.1 diopters can therefore be achieved with the current driver also offered. Communication with the driver takes place via a USB connection and a serial protocol that can be implemented in various programming languages. The source code for the control is available in C# and Labview. An alternative current driver with GigE, RS232 and analog interfaces is available from the British company Gardasoft. To simplify system integration for the user, Optotune works together with various camera manufacturers and software companies in the field of image processing to implement built-in autofocus functions.

Mechanical drives superfluous

Liquid lenses do not require any expensive mechanical drives and enable a robust design that can work with higher accuracies than mechanical-based optical solutions. While mechanical autofocus systems often reach their limits in terms of speed and reliability, variable-focus lenses enable focusing over large differences in working distance within milliseconds. They can also be completely sealed so that no dust can enter, which reduces the susceptibility to errors during image acquisition and subsequent evaluation.

Another feature of variable-focus lenses is that they can be used with different optical materials. A low dispersion liquid with a refractive index of 1300 and an Abbe number of 100 is particularly suitable for polychromatic imaging optics. Virtually no chromatic aberration (color deviation) occurs with such lenses. For this reason, variable-focus lenses can also be combined with commercially available lenses to create high-quality autofocus systems without the need for additional color correction measures.

View of the application

The Optotune model 'EL-16-40', installed between camera and lens.

© Stemmer Imaging

The importance of variable-focus lenses is also increasing against the backdrop of Industry 4.0: Anyone who wants to manufacture small batch sizes down to a quantity of 1 must equip their production with solutions that can react quickly and flexibly to changing geometries of the test parts.

There are already numerous examples of Optotune's electro-optical components doing their job. One obvious application for variable-focus lenses is the reading of 2D codes, for example on objects of different sizes in the logistics, pharmaceutical or automotive industries. While 1D codes can be scanned with a laser, reading 2D codes requires a camera as it opens up additional inspection and measurement functions. Variable-focus lenses enable a considerable extension of the working range, for example from infinity to a few millimetres. In a typical optical setup, the variable focus lens is mounted directly in front of a lens with a fixed focal length.

There are various principles for the control: If the distance to the object is known to the system, the working distance can be controlled directly in the form of an open control loop by setting the lens to the corresponding focal length. The distance information can either be provided by a suitable sensor or the system knows from the programming which object will be inspected next. In this mode, focus adjustment times of 5 to 15 ms are possible.

If the distance is not known, the lens can also be operated in oscillation mode. At low frequencies of 5 Hz, for example, several images can be taken, each with different working distances, until a code has been successfully read. Although this approach is not particularly fast, it is easy to implement and can be used without calibration.

At frequencies of up to several 100 Hz, the entire working range can be tuned during the shutter speed. This results in an image with extended depth of field, but with reduced contrast, as the individual 'images' with different focus overlap additively during the shutter speed. Codes with good contrast can still be easily recognized.

The inspection of optical components with multiple surfaces, such as camera lenses in cell phones or the counting of particles in a three-dimensional liquid volume, are also examples of applications in which optotune lenses have proven to be a suitable tool.

In applications that require high magnification, the variable-focus lens is usually placed between the objective and the tube lens. The achievable Z-range depends on the magnification factor. A typical system achieves a Z-range of 16 mm at 5x magnification. If the variable-focus lens is operated by a 12-bit current source that allows 4096 steps, an axial resolution of 4 µm can be achieved.

Applications with high magnification

One possible application example for such a system is the inspection of printed circuit boards. Most inspection machines use mechanical drives to move a camera along the X and Y axes. Due to distortion of the PCBs, alignment problems and play in the mechanics, it is difficult to stay focused over the entire inspection area. Even at 10x magnification, it is easy to refocus with a variable-focus lens thanks to a Z-range of 4 mm. A sensor can also be used here to determine the working distance, provided the PCB is flat across the field of view. Otherwise, individual image areas can be quickly focused using autofocus algorithms.

Stemmer Imaging has been selling Optotune's variable-focus lenses for some time now. In combination with components such as suitable illuminators, optics and cameras, the Gardasoft power driver and helpful services such as feasibility studies or support with system design, the company offers everything from a single source to simplify the use of this technology.

Author:
Peter Stiefenhöfer is Head of Marketing and Public Relations at Stemmer Imaging in Puchheim.