Machine Vision Lighting Techniques
Lighting techniques for machine vision systems include:-
Diffuse Front Lighting
Diffuse front lighting, as its name suggests, consists of a light source positioned on the same side of the subject as the camera. The light has no discernable directionality, consequently shadows are eliminated and the effects of specular reflection are reduced.
Diffuse lighting is achieved by placing the light source behind a semi translucent surface or by using indirect light reflected from white screens.
Directional Front Lighting
Directional front lighting generally comes in two flavours, bright field and dark field illumination. Referring to the left hand diagram, the Bright field is the region in which any reflected light is within the field of view (FOV) of the camera. Conversely the Dark field is the region in which light reflected from a mirrored surface would be outside the FOV.
The diagram actually depicts a ring light operating just inside the Bright field. Dark field illumination is often used to highlight indentations or imperfections on otherwise flat surfaces.
Polarizing filters can be used to reduce unwanted reflections. If an object is illuminated using polarized light any specular reflections, which result from light dispersing perpendicular to the direction of travel, will be removed. This is achieved by placing a polarizing filter in front of the camera lens at right angles to these rays they will be removed from the image.
On-Axis (Coaxial) Lighting
The idea behind on-axis lighting is to introduce light along the optical axis of the camera. Probably the most popular way of doing so is to use a beam splitter. More advanced on-axis lighting systems incorporate a reflection chamber and provide only indirect lighting paths to the object under investigation.
Such devices are ideal for lighting textured surfaces such as PCBs with soldered components or wrinkled paper.
Structured lighting denotes the projection of crisp lines, grids or circles and is often accomplished using lasers.
In the left hand diagram, a laser is used to project a thin line onto a three dimensional object. When used with bandpass filters the resulting image gives an indication of the objects height.
Diffuse back lighting is achieved using a large light emitting area such as a fluorescent tube or halogen bulb covered with a semi transparent plastic or glass diffuser. The object to inspect is placed between the light source and the camera. The camera will see the object as a silhouette.
Problems can occur with this type of backlighting when light from the edges of the diffuser provide lateral illumination of the object thereby reducing the sharpness of the silhouette. This can be cured either by protecting the object from lateral illumination or using a lens before the diffuser to provide parallel rays of light. The later is sometimes known as telecentric back lighting and has the advantage of making the image size independent of the distance between the light source and measuring plane.