Machine Vision Lighting
one key to successful machine vision results. Machine vision systems create
images by analyzing the reflected light from an object, not by analyzing the
object itself. A lighting technique involves a light source and its placement
with respect to the part and the camera. A particular lighting technique can
enhance an image such that it negates some features and enhances others, by
silhouetting a part which obscures surface details to allow measurement of its
edges. Most common lighting approaches
enhances an object’s outline for applications that need only external or edge
measurements. Back lighting helps detect shapes and makes dimensional
measurements more reliable.
Axial diffuse lighting
Axial diffuse lighting couples light into the optical path from the side (coaxially). A semi-transparent mirror illuminated from the side, casts light downwards on the part. The part reflects the light back to the camera through the semi-
Structured light is the projection of a light pattern (plane, grid, or more complex shape) at a known angle onto an object. It can be very useful for providing contrast-independent surface inspections, acquiring dimensional information and calculating volume.
Directional lighting more easily reveals surface defects and includes dark-field and bright-field illumination. Dark-field illumination generally preferred for low-contrast applications. In dark-field illumination, specular light is reflected away from the camera, and diffused light from surface texture and elevation changes are reflected into the camera.
Bright-field illumination is ideal for high-contrast applications. However, highly directional light sources such as high-pressure sodium and quartz halogen may produce sharp shadows and generally do not provide consistent illumination throughout the entire field of view. Consequently, hot-spots and specular reflections on shiny or reflective surfaces may require a more diffused light source to provide even illumination in the brightfield.
Diffused dome lighting
Diffused dome lighting gives the most uniform illumination of features of interest, and can mask irregularities that are not of interest and may be confusing to the scene.
Strobe lighting is used in high-speed applications to freeze moving objects for examination. Using a strobe light also helps to prevent blurring.