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Lighting Techniques

The most important overall guideline of lighting is creating contrast. This is usually achieved by feature separation or segmentation, by creating maximum contrast for features of interest, by creating minimum contrast for features of no interest, and by setting minimum sensitivity to normal variations.  Changing contrast can be realized by utilizing the following lighting cornerstones:

• Changing Light / Sample / Camera Geometry
• Changing Light Pattern (Structure)
• Changing Spectrum (Wavelength)
• Changing Light Character (Filtering)

Back lighting

Back lighting is a lighting method to provide separation between subject (dark silhouettes) and its bright background thus creating instant contrast. In inspection applications, this method is perfect for detecting presence or absence of holes and gaps or measuring targets. Furthermore, using monochrome light in back lighting, especially with light control polarization, will be very helpful to detect more subtle details.

Diffuse (Full Bright Field) Lighting

Diffuse lighting is intended to provide an intense uniform and multi-directional light for the inspected objects which usually have very reflective, shiny, or uneven surface, by removing shadows and big reflections. The primary most common implementations of diffuse lighting are dome and tube lights.

Partial Bright Field or Directional Lighting

Directional lighting, also called partial bright field lighting, is a lighting technique providing shadows, lots of contrast and topographic details. During inspections, this is useful because it can help locate surface flaws such as burrs or pits. Often a pair of lights is set up at certain degrees to the subject for close up to get as many details as possible at directional lighting. However, it is not suitable for every lighting scene, such as on-axis with mirror-like surfaces.

Dark Field Lighting

By positioning lighting in a place where the imaging objective cannot see, dark field light is generated. By this method, a perfect even surface will come out dark and other high-angle features appear bright due to scattering, so this lighting method is useful for detecting scratches, edges, textures, or other details.

Lighting direction, height, and angle should be carefully designed to provide proper dark field lighting.

Specific Lighting Techniques - Oblique Lighting

Its elegant feature of capturing diffuse reflection, oblique lighting is much more of a stable lighting technique used in inspections. The following illustrates several types of oblique lighting.

Diagonally Positioning an Area Light

If an object is illuminated by putting the area light to manipulated positions with carefully design, e.g. by two lights with opposite 45-degree angles, it can get an even illumination to achieve an image without any shadowing.

Position a Ring Light Closer

By putting a ring light close to the object, the object will be illuminated at a very low angle, so the light reflected from the sleek area will disperse. This technique is efficient for distinguishing areas with various levels of reflectivity but similar colors.

Indirect Lighting

Indirect lighting refers to the lighting method utilizing reflected or diffused light instead of direct light, so the object can be illuminated from multiple directions, to provide an even illumination without shadows or glare.

Additional Methods

Besides these lighting techniques, other methods can be used to create contrast, such as using colored light and Filters, according to the lighting cornerstones. Choosing which lighting techniques to use involves determining the features of interest, analyzing part access, analysis surface characteristics, and determining cornerstone issues.

Related

• Depth of Field
• Imaging System Resolution