5. Lighting Geometries

The direction and angle of illumination play a critical role in the type of image formed. A dark area in an image often results from the reflection of light away from the camera. There are different illumination techniques available for machine vision applications to control the direction of incident light.

5.1 Bright field illumination

In bright field illumination, the light shines on the object and then reflects directly into the camera. Light from surface texture, scratches, and defects scatters or reflects away from the camera, appearing as dark features on a bright background. Bright field illumination with a diffuse light source creates a more uniform exposure on reflective surfaces.

5.2 Dark field illumination

In dark field illumination, the light shined on the object reflects outside the camera's field of view. The image appears dark, but surface features such as scratches and inclined edges will show up as bright features on a dark background because the scattered light reflects towards the camera.

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Imaging oil and scratches on glass with Computational Imaging. Bright field imaging shows the droplets and larger particles on a glass plate, while dark field imaging highlights all the surface details, readily showing the scratches, pits and microscopic particles. The composite image shows all of these features in a single image.

5.3. Coaxial illumination

Coaxial (on-axis) illumination is a uniform bright field lighting solution for flat, highly reflective surfaces. The light source shines down on the object's surface and then reflects directly back to the camera. Very shallow dents and defects are more easily detectable if using collimated coaxial light rays.

5.4 Backlighting

Backlight sources illuminate the object from the opposite side to the camera. Bright field backlight illumination is used to image an object's silhouette for inspecting shape integrity. Dark field backlight illumination is used with transparent objects since only light transmitted by the workpiece by scattering from non-flat features will enter the camera.

5.5 Structured Lighting

Structured lighting involves the projection of light in a particular pattern onto the surface of an object. This could be in the form of a line, grid, or dots and is used where the deformation of the pattern can reveal information about the object. One of the most common uses is 3D reconstruction applications, such as laser line triangulation or stere vision applications. Laser diodes are the standard light source, but LED structured lighting is also available. LED pattern projectors produce uniform patterns and sharp edges without the lifetime, speckle, and safety issues of laser sources. And unlike lasers, they come in different wavelengths, including white.

For rough surfaces, individual rays can hit a point on the surface with a different local orientation. The specular reflectance at each of these local microsurfaces reflects light in various directions, even for direct incident light. This diffuse reflection, or scattering, occurs to a greater extent when the illumination itself is diffuse.