Ruggedization of Imaging Lenses
This is Section 5.3 of the Imaging Resource Guide.
Imaging lenses used in industrial machine vision applications have special requirements beyond those of standard imaging lenses. The lenses used in factory automation, robotics, and industrial inspection must work in specific, demanding environments involving vibration, shock, temperature change, and contaminants. As such, new classes of ruggedized lenses have been designed to work specifically in a multitude of different scenarios. There are three distinct types of ruggedization available:
Industrial ruggedized lenses are designed to survive vibration and shock without taking damage or changing focus or f/#. Flexibility is sacrificed by eliminating moving parts to achieve this. A standard fixed focal length lens uses a focus mechanism and an iris that is comprised of thin leaves and ball detents to adjust f/# which can spring out of place during shock and vibration. The iris is removed and replaced with a fixed aperture stop in industrial ruggedized lenses and the focusing mechanism, typically consisting of a focusing mechanism of a threaded barrel within another threaded barrel, is replaced by a single thread and rigid lock mechanism.
Figure 1: A standard lens with complex mechanics and an adjustable iris vs. an industrial ruggedized lens with simplified mechanics.
Industrial ruggedization is ideal for applications where the system will be set up once and not changed. An added cost advantage is also present in this type of lens due to the removal of the complex movements and adjustments, resulting in significant part reduction and cost savings. There are more applications for industrial ruggedization, such as high vibration factory environments, situations where the camera is rapidly accelerated, inspection systems where many similar camera setups are repeated, and robotic vision.
Ingress Protection Ruggedization
Ingress protection ruggedization ensures a lens assembly is sealed using O-rings and RTV silicone to prevent moisture and debris from entering the lens. This protection is typically added to an industrial ruggedized lens as sealing an adjustable focus and iris would be problematic. These lenses are used in environments of high humidity/moisture, sputter, dust, or small particles, and where space to fully enclose the lens and camera is not available.
Figure 2: An ingress protection ruggedized lens with an O-ring to seal out contaminant.
Like 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.
Lens elements sit within the inner bore of the barrel of an imaging assembly. The space between the outer diameter of the lens and inner diameter of the barrel is typically less than 50 microns. Despite the
minimal amount of space, decenters, on the order 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 FOV 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 FOV must be calibrated, such as measurement equipment, 3D stereo vision, lenses used for sensing in 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.