Machine Vision Lighting techniques - MSTAR Machine Vision System

Illumination techniques

How to determine the best illumination for a specific machine vision task?

There are in fact several aspects that must be taken into account to help you choose the right illumination for your vision system with a certain degree of confidence.

Application purpose

This is by far the first point that must be clear.

If we want to inspect the surface of an object to look for defects or features such as printed text, then front illumination is needed -

i.e. light coming from the camera side. Selecting the proper light direction or angle of incidence on the target surface as well as other

optical properties such as diffuse or direct light depends on the specific surface features that must be highlighted.

If, on the other side, we plan to measure the diameter or the length of an object or we want to locate a through-hole, the best choice

to maximize contrast at the edges is back illumination - i.e. light is blocked by the object on its way to the camera.

The choice is not so obvious when dealing with more complex situations such as transparent materials and sometimes mixed solutions must be taken into account.

Illumination Structures

Illumination	Structure
Bright field light Light is aimed directly at an object, often creating distinct shadows. This type of lighting is effective when used on objects requiring high degrees of contrast, but creates specular reflections when used with shiny or reflective materials.
Dark field light Light is projected at an angle to the surface, causing any variations to deflect light up into the camera, creating bright spots on a dark background or field. Nothing is seen by the vision system if there are no aberrations on the surface.
Back Lighting An even field of illumination is projected from behind an object, which is seen as a silhouette by the camera. Backlighting is most commonly used for taking measurements or determining part orientation.
Diffuse Light: (Dome/Tunnel) Reflected light, providing a non-directional, soft illumination free of harsh shadows that is well suited for highly specular objects. This illumination effect is similar to the type of light found on an overcast day.
Co-Axial Illumination: A variation of diffuse light in which light is aimed at an angled beam splitter that reflects the light down. The object is viewed from above through the beam splitter. This light type is particularly helpful on highly reflective objects or in situations where the area of inspection is obscured by shadows from its surroundings.
Each light type has a specific purpose, but is also adaptable for a range of applications not immediately related to its function. For example, a spotlight, which provides bright field illumination, can be placed at an angle to create a dark field effect. For some applications, the best results are achieved by combining multiple light types.