Eccentricity

For all wafer types you can specify the expected degree of eccentricity of the wafer. While all wafers are round, if your camera is not perpendicular to the wafer, then foreshortening will cause the wafer to appear elliptical, as shown in the figure below. Ordinarily, this would cause problems in attempting to align the wafer.

Foreshortening causes elliptical-appearing wafers

You can specify the maximum expected degree of eccentricity for the Wafer Pre-Align tool. The eccentricity e is computed using the following formula:

where min is the length of the minor principal axis and max is the length of the major principal axis. A value of 0.0 indicates a perfect circle.