Key Takeaways
Machine vision optics include unique and distinct features, such as the following:
- Field of view determines the visible area and magnification
- Working distance impacts lens placement flexibility
- Resolution makes sure images with small features are seen clearly
- Depth of field maintains focus across object depths
- Sensor size and magnification influence image clarity and lens selection
Machine vision technology enables machines to "see" and “understand” their surroundings using cameras, sensors, and software. Machines can inspect and evaluate things as the human eye does – only faster and with far greater precision. This technology is used to quickly and accurately automate inspections, measurements, and decisions in manufacturing. Choosing the right optics can make all the difference in ensuring high-quality results.
If you're working with machine vision, you might wonder: How do different lenses impact inspection and defect detection? Do you need to invest in high-end lenses for high resolution, or can a budget-friendly option do the job? Optimizing your system starts with understanding six key factors that directly affect image quality.
1. Field of View (FOV): How much can be seen by a camera at one time
Field of view (FOV) is measured by the relationship between the camera’s sensor and chosen lens. The focal length of a lens, measured in millimeters, determines its angular FOV, measured in degrees. A lens with a longer focal length gives a narrower view but more zoom (higher magnification), while a shorter focal length gives a wider view but less zoom.
For example:
- Long focal length lens: zooms in to inspect tiny details on a printed circuit board (PCB). The FOV is narrower, but you can see small components, such as solder joints, in detail.
- Short focal length lens: captures an entire assembly line at once. The FOV is broader, helping to see multiple parts or products moving through production.
2. Working Distance: The space between the front of the lens and the object
Working distance is largely influenced by your application needs: Are you working in a confined space where the vision system must be mounted close to the part? Or do you have more flexibility, like inspecting large automotive components on a production line? These factors can guide your lens selection to optimize the inspection process.
Working distance between the front of the lens and the part that is to be inspected.
3. Resolution: The imaging system’s ability to capture fine features
Image sensors are made up of pixels, with each pixel representing a specific size in micrometers. A higher pixel count results in greater resolution, allowing for more precise imaging. For smaller objects, higher resolution is necessary to capture intricate details during inspection. When choosing a sensor, make sure its resolution matches the size and complexity of the parts you're inspecting.
4. Depth of Field: The range of distance where objects remain sharp and in focus.
Depth of field (DOF) makes sure objects at different distances from the camera remain visible – playing an important role in many applications. For example, in a logistics environment with barcode readers, a large DOF is required. This is because objects within the FOV can be different sizes and scales, putting barcodes at varying working distances relative to the lens’ focal point.
DOF is influenced by the lens aperture (f-number or f-stop), the opening that lets light in. A smaller aperture (larger f-number) allows less light but creates a larger DOF, keeping more of the scene in focus. A larger aperture (smaller f-number) lets in more light but creates a shallower DOF – only objects within specific distances will be sharp.
How it works:
- Large DOF: When scanning products on shelves in a warehouse, both foreground and background items remain in focus, allowing the system to read barcodes at varying distances.
- Shallow DOF: When inspecting smaller parts on a conveyor belt, the camera focuses on the part itself while the background machinery is blurred.
Working distance between the front of the lens and the part that is to be inspected.
Working distance between the front of the lens and the part that is to be inspected.
5. Sensor Size: The size of a camera sensor’s active area, specified in the horizontal dimension
Image sensors come in many sizes, ranging from 1 inch to as small as ¼ inch. A sensor’s size impacts the amount of light it can capture, influencing image quality and low-light performance.
In action:
- Larger sensor: Captures more light, enhances image clarity and reduces noise, especially in dim conditions.
- Smaller sensor: Compact design and higher focus on finer details, despite capturing less light.
The horizontal dimension of an image sensor can be used, along with your desired FOV in millimeters, to determine the necessary primary magnification (PMAG) for your application.
The image sensor inside a machine vision camera is what turns reflected light into a digital image.
6. Primary Magnification (PMAG): The ratio between sensor size and FOV
PMAG is another key factor when choosing lenses for machine vision. It’s determined by the lens's focal length, sensor size, and desired FOV, all of which affect the level of magnification needed for the inspected object.
If you're inspecting a PCB and need to capture a tiny resistor, the right PMAG can zoom in to see it clearly. A larger sensor typically provides better clarity, while the FOV determines how much of the board is visible at once.
Use this equation to calculate PMAG:
Sensor Size / FOV = PMAG
For instance, with a 10mm sensor and a 50mm FOV, PMAG would be 0.2. This means the object is magnified 0.2 times its actual size. It’s good to be aware of PMAG in order to choose the best-fitting lens for your operation. PMAG also affects depth of field and resolution, including the level of detail and focus of a final image, determining how much of the scene stays in focus and the level of detail in the final image.
This image explains how to calculate PMAG (Primary Magnification) in a machine vision or imaging system. PMAG is the ratio of the horizontal sensor size of a camera to the desired horizontal field of view (FOV).