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A Deeper Dive into the Illumination “W”

You probably recall the illumination “W” that is a foundational concept for machine vision illumination design (see Figure 1). You also probably recall that the illumination “W” is based on a flat, perpendicular, specular or mirror-like surface. Further examination of the “W” reveals characteristics that can further help with your design of lighting in implementing bright field or dark field lighting.

Diagram of illumination "W" that is a foundational concept for machine vision illumination design
Figure 1 — Illumination “W”

What happens, though, when the surface being imaged is tilted? And remember that tilt, even if very small, is always a real possibility. With tilt, the legs of the illumination “W” also tilt, but at twice the angle of the surface tilt (see Figure 2). That means that your lighting design, the separation of bright field and dark field lighting changes. There is a zone of confusion where, depending on the tilt of the part, the lighting might be bright field for one part and dark field for another. Light must be kept out of this zone of confusion for dark field illumination. For bright field illumination, the light must span across the entire zone of confusion as well as the interior of the “W”.

Diagram showing zones of confusion when the surface being imaged is tilted
Figure 2 — Illumination “W” with Tilted Part
Diagram: Legs of the illumination "W" move outward when imaging a rounded part
Figure 3 — Illumination “W” on Curved Surface

Another interesting situation is when imaging a rounded part. If the background is shiny and the part is not, then there is no change in the illumination “W”; it is defined by the background. However, if the part is shiny and the background is not, then the legs of the “W” move outward depending on the size and curvature of the part (see Figure 3).

Diagram: Outer legs of the Illumination "W" become blurry when illuminating a diffuse object
Figure 4 — Illumination “W” for Diffuse Surface

The lighting design must take into account not only the part’s curvature but also the location of the part in the image. If the part is cylindrical, then it is possible for the illumination “W” to disappear and become a “V” (see Figure 4). This is a very difficult situation to illuminate well for machine vision since it requires the lighting to extend behind the object where we would normally place back lighting. There is very little opportunity to support the part being viewed.

Diagram: Illumination "W" disappears and becomes a "V" when illuminating a cylinder
Figure 5 — Illumination “W” Becomes a “V” on Cylinder

Now if we change our object or scene to no longer be specular but to be diffuse, then the outer legs of the illumination “W” become extremely blurry (see Figure 5). This is very useful. It tells us that diffuse objects are very tolerant of the direction of illumination for bright field illumination and very difficult to light for dark field illumination. Finally, the illumination “W” is not two dimensional, it is three dimensional (see Figure 6). Understanding this can help improve your machine vision lighting design.

Three-dimensional diagram of Illumination "W"
Figure 6 — Illumination “W” in Three Dimensions

The illumination “W” introduced as an aid to machine vision lighting has proven extremely useful even if a simplification. This post expands on the “W” concept to aid in even better and easier machine vision lighting design.