Extending the vertical range of wavelength scanning interferometry

The mass production of high-precision components
has created a demand for in-line inspection
tools that are able to perform in-situ surface
topography measurement as part of a quality assurance
tool and as feedback to control the manufacturing
process. Wavelength scanning interferometry
(WSI) is a candidate for in-line inspection
of manufactured parts with nanometre height features
[1] [2] [3]. WSI has advantages over other
widely known optical interferometric techniques,
such as coherence scanning interferometry and
phase shifting interferometry, as it involves no mechanical
scanning, allowing for higher measurement
speed, and does not suffer from 2π phase
ambiguity. In this paper, a number of different
improvements to the standard WSI signal processing
technique are proposed that can extend
the measurement range while increasing accuracy
and resolution, with little to no impact on
the measurement speed. In next section the WSI
technique is described and an optimisation of the
Fourier Transform Method (FTM) for phase slope
estimation reported. The concept of the quadrature
WSI (Q-WSI) is then described, and a simulation
of the FTM algorithm and Q-WSI is compared
and discussed