Understanding Reflection Geometry

Understanding reflection geometry is understanding where light is going. The objective is to understand how light reflects into the camera and how to do it in a uniform manner. The key is to control the unwanted reflections so that we can eliminate hot spots.

Reflection Geometry - The Angle of Incidence equals the Angle of Reflection

If the part is flat it looks like this, if parts are curved, bumpy, or shiny it will make a difference. At that point we will need to analyze what angle or angles of reflection will be required for even illumination.

Doing an Analysis of the Part forces you to look at the features of the surface

  1. Specular - (Example: Silicone Wafer) - Light will come in at one angle, bounce back at the other angle and unless it bounces back at the certain right angle it is never going to get where the camera is.
  2. Diffused Rough Surfaces (Example: Piece of Paper) - Part that is going to reflect at multiple angles
  3. Spread - (Example: Character recognition on a shiny label) - Part where some light bounces off at an angle, but because a label is not a completely mirrored surface, some light is going to scatter back.

Light at the wrong angle will never reach the CCD.

When illuminating a mirrored surface, even the brightest light will not reach the CCD array if it is at the wrong angle.

The Most Critical Step in understanding Reflection Geometry is the "W"

The W will teach you about where you need to place light and why you have seen hotspots in the past.

The W will define the true boundaries between bright field and dark field. In this example we are looking at a very shiny surface. If you have the brightest light in the world placed at an angle in the dark area (dark field) it will always bounce off at an angle and never reach the camera. Any light placed in the bright field ( inside the W), you will see everything. If that is the case, you must have an evenly uniform light or you will get reflection.  If you have a curved surface, the W will get bigger and bigger and the darkfield (W) will be expanded.

How do I wrap lighting around a Cylinder or Sphere

The best approach to lighting a cylindrical part is Continuous Diffused Illumination (CDI). In this case you want the light source to be as close as possible to the part to generate a solid angle which will wrap around the part. In this case the maximum degree of a solid angle is +/- 80 degrees. This is the maximum angle because you cannot get light at that angle to reflect back. The farther away you get, the less wrap around lighting you will have.

The Solid angle of Illumination effects Uniformity

When evaluating an application, one must decide how much uniformity is needed and what is good enough.
In this example from left to right you will see:

  1. Doal - (Diffused On Axis Luminator) - Good for flat. Will see a dark spot and bright spot in image
  2. SCDI - (Square Continous Diffused Illuminator) - Good some contour - See some non-unifromity
  3. CDI - (Continuous Diffused Illuminator) - Great for contour - Perfect for uniformity. Must be close to the part

Key Points for Machine Vision Reflection Geometry

  • Light always reflects at the Angle Of Incidence
  • Darkness is Either Surface Absorption or Absence of Reflected light to the camera
  • Surface Angularity determines the Light Position and Width Size

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