One of the major contributor to the reliability and robustness of the inspection system is the method of lighting used. The desired image quality can typically be met by improving the illumination scheme. Every component affects the amount of light incident on the sensor and, therefore, affects the image quality of the system.
The aperture (f/#) of the imaging lens has a direct impact on the amount of light incident on the camera. The higher the aperture number, the more is the required amount of illumination. High power lenses will usually require more illumination since the smaller areas viewed reflect less light back into the lens. The lens aperture, lens magnification, camera settings, filtering and other illumination parameters will all affect the light incident on the sensor. These factors need to be adjusted in order to accommodate objects with different characteristics (profile, reflectivity, etc.).
Selecting the right method of lighting is crucial to the success of an inspection system. The different methods of lighting are:
Front Illumination – The lighting and the camera are on the same side of the product. Variations of this lighting are Oblique, Diffused, and Coaxial Lighting. Oblique lighting positions the ray at an angle from the product. Diffused lighting covers the product from multiple angles. Coaxial lighting lights the product through a half mirror presenting the ray of light at the same axis as the camera lens.
Back Illumination – The lighting and the camera are on opposite sides of the product. Variations of this lighting are Directional Back, Diffuse Back, and Dark Field Lighting. Directional Back lighting positions the ray at an angle from the product back side. The Diffused Back lighting presents the ray from multiple angles. Dark Field lighting presents the ray at a very acute angle.
Unpolarized Illumination – The light waves are restricted to a single plane of oscillation or whose oscillations are controlled in some manner. It reduces some glinting and is good for illuminating specular or shiny parts.
Light Tent Illumination – This lighting method emulates cloudy day illumination to present lighting from all angles to create a uniform image of the part with uniform intensity.
Structured Illumination – This lighting method projects plane or grid of light onto the product to obtain 3D part information.
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