Machine Vision Lighting
Lighting is an often overlooked area of machine vision system design, but correctly designed lighting can make the difference between a system that reliably captures suitable images and one that does not. Cameras are not as tolerant as the human eye to inadequate illumination and consequently lighting generally needs to be optimised in order to operate dependably.
So, where do you start when selecting a light source for your application? Before reading the remainder of this document ask yourself the following questions:
- Is the subject shiny or matt? Note: just because an object is dark in colour does not mean it is not reflective.
- Do you need colour or monochrome analysis?
- Will the subject be moving or stationary?
- Is high or low frequency image acquisition required?
- How large is the object to be illuminated?
- What is the background?
- Are there any mechanical constraints, both operational and maintenance?
- What is the practical service life of the light source?
The following table summarises the recommended light source for a given application:
| Application Requirement |
Example Object |
Recommended Lighting |
| Low specular reflection |
Integrated Circuit on shiny PCB |
Diffuse front lighting e.g., ringlight with polarizing filters |
| Highlight surface contours, scribed or embossed features |
Stamped digits on a metal component |
Directional dark field front lighting. e.g., large ring light close to the measurement plane |
| Inspecting transparent objects |
Glass or plastic items |
Directional dark field front lighting. e.g., large ring light close to the measurement plane |
| Measure outside dimensions |
Bolt thread |
Back lighting with lateral protection |
| Even illumination with no shadows |
Printed solder paste inspection |
Combined on axis and front diffuse lighting |
| Presence verification with no grey scale contrast |
Plastic moulding |
Structured lighting. e.g., laser with band pass filter |
| 3D profiling |
Glue volume |
Structured lighting, e.g., laser with band pass filter |
For more detailed information see Light Sources and Lighting Techniques. |
