How to Choose Machine Vision Lenses
Important Considerations When Choosing Machine Vision Lens
Lenses are complex devices, fitted with multiple layers of optical glass and moving, metallic parts that are precisely layered for specific focal points and levels of optical zoom.
Because of this, purchasing these devices is a complicated affair and sometimes requires custom-made solutions to meet your needs properly. This is especially true in the case of machine vision, where the slightest error in the lens, sensor or even the lighting can hinder quality assurance pipelines.
In this informative guide, we’ll discuss some of the important factors to understand when selecting a lens for your machine vision operation. This includes the ruggedization of your lens for industrial environments, how to calculate focal length, and how your lighting solution might impact your lens.
What is Ruggedization For Machine Vision Lenses?
Ruggedization is the process of making something rugged or more sturdy. Here, ruggedization is important because machine vision operations are designed to operate within completely controlled conditions, meaning that there is little room for error.
Specifically, in regards to lenses, this means making them able to withstand conditions such as heat, moisture, and shock from moving components. Quality Magazine has an excellent guide on the three primary types of ruggedization that we will discuss here: Ingress protection, stability, and Industrial.
Ingress Protection Ruggedization
Ingress protection is protection against particulates that might be able to enter the lens, such as dust or liquids. Typical commercial leases will have multiple moving components and lenses that are threaded together, thus creating gaps where materials can enter the lens and obscure the sensor’s vision.
These dust particles can cause potential errors in the machine’s assessment of equipment, and thus, is a problem. These ingress-protected lenses forgo moving components and utilize O-rings, which seal these potential points of entry. This means that these lenses do not feature moving internals, and their lenses are very specifically arranged to focus on the necessary point.
These rings are placed between the front lens retainer, lens elements, and the housing to protect everything aside from the very surface of the lens.
The surface of the lens requires its own attention, as a buildup of water can also obscure the sensor’s vision. Here, ingress-protected lenses utilize a hydrophobic window that has an anti-reflection coating that prevents water from sticking to the substrate, forcing it to bead up and fall from the lens.
These lenses are required in environments that utilize water, such as food or pharmaceutical pursuits that require equipment to be frequently washed and sanitized. They are also crucial to lenses on outdoor vehicles and vehicle assembly lines, as vehicles are both washed and encounter natural water outdoors.
Stability ruggedization is the process of preparing a lens so that elements within it will not move when exposed to shock, such as on a moving piece of machinery.
In a standard lens, experiencing a 50g shock can cause displacement between 1 and 20 microns; in a ruggedized stability lens, this figure plummets to less than one micron. This effect is achieved by providing a permanent point of contact to the lens barrel in the form of a steel c-mount clamp that locks the focus position of the lens.
Elements within the lens are also glued into place to firmly secure them, as even minute motions can negatively affect machine vision applications.
These lenses are commonly seen in situations that require the lens to move, such as on robotics or a mechanical assembly line. Additionally, these are also helpful in environments where there are loud sounds or other vibration sources that can cause components to become misaligned.
Industrial Ruggedization is the process of overhauling the core lens mechanisms for industrial purposes, which includes the aforementioned strategies.
This overhauling configuration means the lens’s barrel is a single piece of metal, which benefits industrial environments and makes the lens assembly more manageable, as there are fewer parts involved.
Here, every component is locked into place, which is exactly what an industrial environment needs. However, this means that if for any reason these components would become misaligned, they can be exceedingly difficult to rearrange, and so lenses must be constructed with these potential shortcomings in mind.
Many high-quality machine vision lensmakers are introducing ruggedization into their offerings to prepare their equipment to be reliable for industrial environments. In working on selecting a lens manufacturer, be sure to discuss this topic with them to ensure that their offerings are suitable for your needs.
Here, some manufacturers might even offer customized solutions especially suited to you.
What Lighting Is Best For My Machine Vision?
Lighting is a complicated subject in the world of machine vision that includes both brightness levels and color presence, both of which are important for properly presenting a piece of equipment to be analyzed.
Not only that, but different materials react to light differently, leading to reflections and glares that can sometimes make them more difficult to read. Luckily, there are solutions to lighting issues, and all of them involve high-quality optical lenses to read the situation properly.
National Instruments has an in-depth guide on the subject of lighting for machine vision and an overview of the topic as a whole. They detail the effects of ambient light, some lighting samples, color analysis, and more, which we’ll discuss here.
Ambient light, such as sunlight and overhead factory lights, can all impact your vision setup, offering additional lighting that causes your equipment to be unevenly lit. Here, they suggest using a physical enclosure to physically block the excess light, drown it out with a strobe light, or utilize a pass filter that can block some of these wavelengths.
As we alluded to earlier, different materials react to light differently, requiring different solutions. Depending on a surface’s shape and texture, you might have to employ different tactics, such as different shapes of light or differing backgrounds.
Take a soda can as an example, showcasing how diffused light creates an even background on the aluminum surface instead of a ring light that lacks the contrast needed to read text printed on the can.
Ultimately these solutions require some experimentation and trial and error, but a helpful approach understands how the surface interacts with the light. If it is highly reflective, you will want to diffuse the light to minimize the reflection, but you may need a more concentrated solution if it is not.
They also note that IR or colored lighting can offer different effects in capturing images, such as UV lights exposing fluorescent dyes and IR lights neutralizing color contrast. These offer different benefits and detractions in different situations, as IR lights are better for penetrating polymer materials and can be used to see beyond surface layers.
Your solution must be tailored to your desired outcome, as inspecting a motherboard with an IR light will help you examine its circuitry, but looking at printed materials will be much more difficult due to the lack of color contrast.
How Do I Calculate Focal Distance?
While we can’t dedicate too much space in this guide to these complicated calculations, we can direct you to the Optical Wizard, a helpful online calculator. These calculations only require the size of your sensor, focal length, and the size of your object in millimeters to help properly position your lens.
They even have multiple different formulas depending on your preferred result, so you can input data like your sensor’s pixel count or start with your object’s size in the field of view and work backward from there.
If you have questions about your sensor, there is even a sensor database on the page with helpful information that you may need. All of these options can help you reach the perfect viewing distance for your machine vision lens.
We’ve discussed the importance of lens ruggedization and how different lighting configurations yield different results. Developing a machine vision assembly requires careful thought and planning to meet your goals, and no factor or part can be overlooked in an industrial setting.
We hope this information is helpful to you and gives you better insight into how these complicated pieces of equipment all interact with one another.
Quality Magazine - Ruggedization of Machine Vision Lenses | Qualitymag.com
National Instruments - A Practical Guide to Machine Vision Lighting | ni.com