A reference flat surface is positioned sufficiently closely to the wafer that an interference fringe pattern is formed that is a measure of the air gap between the reference flat surface and the surface of the wafer being measured.

The working surface defines a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane.

A reference face of a measurement instrument is placed against the fastener reference face (12) and the instrument determines the deviation of the observation face (24) from the plane of the instrument reference face.

The second reference plane is a function of the calculated deviations between the inspection surface for each site and the first reference plane.

The reference plane is the plane formed by the underside of the bevelled lead-in flange of the cap.

The reference plane is the plane formed by the underside of the bevelled lead-in flange of the cap.

The output connector (4) comprises a surface forming a reference plane (5).

The jacket including at least one reference surface oriented at a predetermined angle to the reference plane.

The reference plane is the plane formed by the underside of the three radiused tabs of the cap.

The reference plane is defined by the three ramp inside surface.

The reference plane is defined by the three ramp inside surface.

The reference plane is defined by three ramps inside surface.

The reference plane is defined by three ramps inside surface.

The shroud includes a capping surface, with a surface profile that deviates from a reference plane by more than a target deviation.

The angular orientation of the measuring surface with respect to a reference plane is measurable.