To "null" the fringes is to adjust the pitch and roll of your sample so it is parallel to the plan of the interference fringes, which is perpendicular to the Z-scan axis.
On a Spherical Part, adjust the stage and the focus to center the circular fringe pattern. This can be done by moving in X/Y or in P/R, depending on what you are trying to measure. Depending on the curvature of the sample, nulling a spherical sample can significantly reduce collection time.
Below is a video showing the actual field of view (right) and a cartoon (let) depicting what is happening on a curved part as the instrument is moved up in Z with the joystick. Notice how the fringes travel across the field of view from right to left. The fringes indicate that not only is the sample offset, the direction of travel describes how it's offset. The fringe travel indicates, in this example, that the back sample curvature continues to rise to the left. Either X/Y or P/R can be adjusted to center the circular pattern depending on the users specific needs.
Nulling the stage for a curved part minimizes the total sample height within the field of view. As the instrument is moved up in Z now, the fringes go through a circular, bullseye pattern centered in the field of view.