How do I estimate the smallest detail a camera can resolve for detecting 50 µm defects?
Asked 12/8/2017
2 views
2 answers
0
I want to estimate the smallest detail a camera system can resolve from specs like sensor resolution/size, pixel size, focal length, field of view, and working distance.
My application is machine vision: plastic beads about 3 mm in diameter are moving through a pipe, and I want to capture an image of each bead and detect surface defects mainly by color contrast. Typical defects are around 100 µm, but I’d like to reliably detect defects down to about 50 µm.
Ideally, a 50 µm defect would cover several pixels, preferably tens of pixels. How can I work out whether a given camera/lens setup can do this, and what magnification or field of view would I need?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
9
If you want tens to hundreds of pixels per 50 micron area, you need a microscope. Based on your object size, with that resolution, you want at least 10px/50um * 3000um/50um a 600px wide view of your bead, and up to a 6000px wide view of your bead.
Common sensor sizes are 24x36mm, 16x24mm, and smaller in industrial cameras.
You're going to need, roughly, 3-5x magnification for the bigger sensor sizes (36mm and 24mm), and lower for smaller sensor sizes (closer to 1x).
Shop for microscope objectives with this sort of field of view. They will mostly all be diffraction limited, ish, at pretty big apertures (f/2 and faster). You can use 2.44*wavelength * fno to get the spot size. In vis, that's about 2.44*.55*2 = 2.684 microns.
A 16mm wide sensor with 2.64 micron pixels is 5500px wide.
So you have not violated any resolution limits, but you are operating close to them for your 100px/50um target.
I do not expect you will find a system that can get images out of a camera at this speed, and processed, in anything resembling real time. Your best possible speed is probably 1-3fps.
The working distance of microscopes is usually/always short. If that is not suitable for your needs, you need a custom system (be prepared to spend upwards of $100,000/system) that would be near the limits of what is possible re: element sizes and resolution.
Originally by user40937. Source · Licensed CC BY-SA 4.0
user40937
8y ago
0
Generated from our catalog & community — verify before relying on it.
Start from required sampling on the object, not just camera specs.
If you want a 50 µm defect to span about 10 pixels, that means 5 µm per pixel on the object. For a 3 mm bead, the full field across the bead would then need about 3000/5 = 600 pixels. If you want much more detail—tens to hundreds of pixels on a 50 µm defect—you quickly get into microscope territory and may need several thousand pixels across the 3 mm bead.
So the key relationship is: object sampling = field of view / image pixels
Then compare that to your target defect size.
To achieve that sampling on a 3 mm object, you’ll likely need roughly 1× to 5× magnification depending on sensor size and pixel pitch. Larger sensors generally require higher magnification for the same field of view.
Also remember resolution is limited by both pixel size and optics. Even with enough pixels, the lens must resolve that detail. Microscope objectives or close-up machine-vision optics are often the right choice here, since they are designed for small fields of view and high resolving power.
Recommended products
UniqueBot
AI8y ago
Your Answer
Related Questions
How do I calculate the smallest detail a camera can resolve at a given distance?
How can I estimate the minimum magnification needed to resolve a feature of a given size?
How can I estimate the minimum printable QR code size a smartphone camera can reliably resolve?
How do you choose circle of confusion for depth of field on a 16K line-scan camera?
How can I estimate an object's real size from pixel size, focal length, and subject distance?