Ruggedization of Imaging Lenses
This is Section 7.2 of the Imaging Resource Guide
Imaging lenses used in many industrial machine vision applications have special requirements beyond those of standard imaging lenses. The lenses used in factory automation, robotics, and industrial inspection have to be able to work in specific and demanding environments, which could involve vibrations, shocks, temperature changes, and contaminants. Because of these environmental requirements, new classes of ruggedized lenses are being designed specifically to work in a multitude of different scenarios and therefore create different types of ruggedization. There are three distinct types of ruggedization available:
Industrial Ruggedized lenses are designed to survive vibration and shock without damage to the lens or change in focus and f/#. To achieve this, flexibility is sacrificed by eliminating moving parts and making them easier to lock down.
A standard fixed focal length lens utilizes a complicated iris and focus mechanism. A typical iris is comprised of thin leaves and ball detents for adjusting f/#, which can spring out of place during shock and vibration. For Industrial Ruggedization, the iris is removed and replaced with a fixed aperture stop. Additionally, the typical focusing mechanism of a threaded barrel within another threaded barrel is replaced by a single thread and rigid locking mechanism.
Figure 1: Standard Lens with complex mechanics and an adjustable iris vs. Industrial Ruggedized Lens with simplified mechanics.
Industrial Ruggedization is ideal for applications where the system will be set up once and not changed afterwards. An added cost advantage is also present in this type of lens due to the removal of the complex movements and adjustments, which results in a significant part reduction and cost savings. There are many applications for Industrial Ruggedization, such as high vibration factory environments, situations where the camera is rapidly accelerated and decelerated, inspection systems where many similar camera setups are repeated, and robotic vision.
Ingress Protection Ruggedization
Ingress Protection Ruggedization ensures that a lens assembly is sealed to prevent moisture and foreign debris from entering the lens through the use of O-rings and RTV silicone. This protection is typically added to an Industrial Ruggedized lens, since an adjustable focus and iris would be problematic for sealing. These lenses are used in environments of high humidity/moisture, sputter, dust, or small particles, and also where space is not available to fully enclose the lens and camera.
Figure 2: Ingress Protection Ruggedized Lens with an O-ring to seal out contaminants.
Similar to Industrial Ruggedized lenses, Stability Ruggedization protects the lens from damage, but also ensures optical pointing and positioning is maintained after shock and vibration. In addition to replacing the iris and a simplified focus mechanism, individual lens elements are glued in place to prevent them from moving within the housing. Figure 3 shows a Stability Ruggedized lens in which the lens elements are glued in place and a clamping lock is used to simplify the focus.
Figure 3: Stability Ruggedized Lens with all lens elements glued in place.
In an imaging assembly, lens elements sit within the inner bore of the barrel. The space between the outer diameter of the lens and inner diameter of the barrel is small (typically less than 50 microns). Despite the minimal amount of space, decenters of tens of microns are enough to significantly affect the pointing of the lens. When using a Stability Ruggedized lens, if an object point is in the center of the field of view and falls on the exact center pixel, it will always fall there even if the lens has been heavily vibrated (Figure 4). Stability Ruggedization is important in applications where the field of view has to be calibrated, such as measurement equipment, 3D stereo vision, lenses used for sensing for robotics, and lenses used for tracking object locations. These applications often require the optical pointing to be stabilized to values much smaller than a single pixel.