How to choose cameras and lighting for an inspection station
Lighting and mounting decide more than the software does. Start from the smallest defect you need to catch and work back.
Most inspection problems are solved, or caused, before the software sees a single pixel. A good image makes almost any check easy; a poor one makes even the best AI unreliable. The way to a good image is to start from the defect and work back.
1. Describe the defect
Write down, for each defect you need to catch: how small the smallest one is, where on the part it can appear, and what makes it visible to a person (a change in brightness, shape, colour or texture). Use real sample parts, good and bad, not drawings.
2. Work out the resolution you need
A common rule of thumb is that the smallest defect should cover at least three to four pixels across. Measurements need more, because their accuracy depends on how finely edges are resolved.
Example: a field of view 100 mm wide on a camera 2448 pixels wide gives 100 ÷ 2448 ≈ 0.04 mm per pixel. A 0.2 mm scratch then covers about 5 pixels: enough to detect. A 0.05 mm one covers about 1 pixel: too small for this setup.
If the numbers do not work, you can use a narrower field of view, a higher-resolution camera, or several cameras each looking at part of the part.
3. Check motion blur
If the part moves while the image is taken, it smears by its speed multiplied by the exposure time. Keep the blur under about one pixel.
Example: at 500 mm/s with a 1 ms exposure, the part moves 0.5 mm, about 12 pixels at 0.04 mm per pixel: far too much. To stay under one pixel, the exposure must be about 80 microseconds or less, which needs a bright, strobed light. Stopping the part for the image avoids the problem altogether.
4. Choose the lighting
Lighting creates the contrast the check needs. Each type shows different things:
| Lighting | Shows well | Typical use |
|---|---|---|
| Backlight | Outlines and holes, as a sharp silhouette | Dimensions, presence of holes, part outline |
| Diffuse dome | Surfaces without glare, even on shiny parts | Machined or reflective parts, labels |
| Ring light (bright-field) | General features, print, components | Assemblies, PCBs, codes on matt surfaces |
| Dark-field (low angle) | Scratches, edges, embossing, dents | Surface defects on flat or slightly curved parts |
| Coaxial | Flat, mirror-like surfaces | Laser marks and codes on metal |
Keep the inspection light stronger than the room light, so the image does not change when a door opens or the sun moves. An enclosure, a shroud or a filter matched to the light colour all help.
5. Choose camera and lens
- Monochrome cameras suit most checks and give finer detail for the same price. Use colour only when colour is what you are checking.
- For measurement, a telecentric lens avoids perspective error, so a part slightly closer or further away still measures the same.
- Check depth of field: the whole area you inspect must be in focus, including on taller parts.
- Industrial GigE Vision (GenICam) cameras suit stations with longer cables to the PC; USB cameras suit short runs and simpler stations.
6. Present the part the same way every time
A fixture or a stop that places the part in the same position makes every check simpler and more reliable. Complex parts need several views, either from several cameras or from a robot presenting the part at several positions. Whatever the number of views, the station should give one verdict per part.
7. Test before you buy
Before committing to hardware, image real good and bad parts under the proposed lighting, at the real working distance and, if the part moves, at line speed. A short sample test answers most of these questions with evidence rather than opinion.