Ruggedisation of Imaging Lenses
Published on : Friday 01-10-2021
There are many available options for ruggedising machine vision lenses and no single option will work for every application, says Adarsha Sarpangala.
Imaging lenses used in harsh environments have special requirements beyond those of standard imaging lenses. Some of the applications in aerospace, healthcare, factory automation, robotics, and other industrial inspections need to work in demanding environments with vibrations, shocks, temperature changes, and contaminants. Standard fixed focal length lenses may fail to operate normally or provide expected results in these challenging conditions, so a new category of ruggedised lens models has been introduced. Ruggedisation is the act of strengthening a lens to resist these harsh environments and nowadays it is growing in importance. Generally, we see three types of ruggedisation: Industrial Ruggedisation, Ingress Protected Ruggedisation, and Stability Ruggedisation.
In order to understand the unique qualities of ruggedised lenses, let us first look at the structure of standard imaging lenses. Standard imaging lenses will have a manual focus adjustment, typically consisting of a threaded barrel within another threaded barrel, to allow for smooth movement without rotation of the optics. The iris will be manually adjustable to specific f-stops with the help of ball detents. This iris is made of many thin moving leaves that overlap and move together when adjusting f-stop. These lenses also have a thumb screw to fix the focus and iris position. Due to the moving parts in the lens assembly, the lenses are susceptible to change in focus and aperture when a vision system undergoes shock and/or vibrations.
Most imaging lenses labelled as ruggedized, tough, or harsh environment utilise Industrial Ruggedisation to prevent changes in focus from shock and vibration. This is done by simplifying the mechanics of standard fixed focal length lenses. The iris diaphragm is removed and replaced by a fixed aperture stop, eliminating many moving parts. Thin iris leaves in standard imaging lens design can easily spring out of place and be damaged in a high shock or vibration environment; eliminating them is a simple change to greatly improve the survivability of the lens. The threaded barrel of the lens focus mechanism is also replaced by a simple single thread with rigid locking. These changes are ideal for applications where the system will be set up once and not change 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 significant part reduction and cost savings.
The disadvantage of an Industrial Ruggedised lens is the lack of flexibility. Changing the f-stop is not a simple task and in most cases would require a different lens to be used. The focus travel will not be smooth and will rotate the optics while it focuses, which can cause a change in pointing in the lens (how an object point maps to an image point) as the focus is changed. Additionally, the locking method will typically make it more difficult to adjust focus and will likely require special tools, such as small hex drivers or large wrenches. However, in most industrial applications these disadvantages will not cause problems since standard lenses will likely be used to determine the setting for a machine. Once the machine setting is determined, an Industrial Ruggedised lens can be substituted and set to the appropriate focus and f-stop without having to be changed again.
Ingress Protection Ruggedisation
The next type of ruggedisation is Ingress Protection Ruggedisation. In this type of ruggedised lens, the assembly is sealed with an O-ring and RTV silicone to prevent moisture or foreign debris from entering the lens. This retrofit is typically applied to a lens that already possesses features of an Industrial Ruggedised lens including a fixed iris and simplified focus, as the adjustments for f-stop and focus are often problematic for sealing. This type of ruggedisation is used primarily in applications where the environment has a great deal of humidity or moisture and space is not available to fully enclose the lens or camera for protection. Another compatible environment for this type of lens is when spatter, dust, or small particles are present in the air around the lens.
Ingress protection is specified with two digits, which make up the IP rating according to the IEC 60529 standard. The first digit describes the level of protection against solids and particles and ranges from zero to six. If a component has not been tested for protection against solid intrusion, the first digit is changed to an X. The second digit describes the protection from moisture and ranges from zero to nine.
Industrial Ruggedisation is not sufficient for the most demanding applications like high accuracy measurement and gauging, robotic sensing, 3D stereo vision, and object tracking where the calibration of an imaging system must be maintained. A more robust type of ruggedisation, called Stability Ruggedisation, is required to reduce pixel shift and truly protect lenses from shock and vibration. In Stability Ruggedisation, in addition to replacing the iris and a simplified focus mechanism, individual lens elements are glued in place to prevent movement within the housing. This ensures optical pointing and positioning is maintained during shock and vibration.
In an optical system, 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 miniscule (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 Ruggedised lens, if an object point is in the centre of the field of view and falls on the exact centre pixel, it will always fall there even if the lens has been heavily vibrated, thus stabilising the image.
There are many available options for ruggedising machine vision lenses and no single option will work for every application. In order to determine the best ruggedised lens option, the application and its environment must be taken into consideration. As each ruggedised lens type features different but similar advantages and disadvantages, each factor should be reviewed for each application type. Knowing the best sort of protection for your application will help extend the life of your system and provide the best performance and price.
Adarsha Sarpangala is Imaging Business Key Account Manager in Edmund Optics India Pvt Ltd, Bengaluru office. He manages the machine vision business of Edmund Optics for the Indian market. Adarsha has around 10 years of experience in the machine vision industry including selection of cameras, lens, lighting and machine vision solutions. For any queries on the articles or machine vision applications he can be contacted on email@example.com