The measurement accuracy in non contact profilometric techniques is generally limited by mechanical vibrations and by
geometrical defaults of the micro-scanning table. In order to free the measurement from these environnemental
perturbations, we describe a novel type of interferometric microscopy based on the well-known Spectroscopic Analysis
of White Light Interferograms (SAWLI). The originality of the presented set-up lies in the fixation of the reference plate
on the inspected object. As reference plate and sample are fixed together, the mechanical vibrations do not affect the
measurements. As a result the potential nanometric accuracy of interferometric microscopy is effective. This method
consists in measuring the air gap thickness between the reference plate and the sample. At the output of the spectral
interferometric microscope a channelled spectrum is observed. From this signal, the spectral phase is calculated using a
numerical seven points phase shifting algorithm allowing the measurement of the local height of the analyzed surface.
These preliminary results demonstrate the ability of this method as a point sensor. Then this technique will be
implemented in a high frequency scanning STIL technology named MPLS 180.
The idea which initiated this work is to adapt the principle of phase coding to a Confocal
Chromatic type sensor in order to enhance the dynamic range and the axial resolution of height
measuring systems. When using a classical Confocal chromatic type measurement, we obtain a
"coarse" value of the distance. Then by the means of a spectrally adjustable light source and a
chromatic objective, we choose the wave length in order to focus in the plane of the object. Next,
by measuring the phase coding, we can precisely define the profile of the object. So, in the end, we
obtain the subnanometric preciseness of the phase coding while clearing up the ambiguity that is
inherent to interferometric methods thanks to the confocal chromatic measurement.
The initial idea is to use a Linnik interferometer, with a dedicated chromatic objective in the
object arm, and an achromatic objective in the reference arm. A change of the wave length leads to
a variation of the focus plane in relation to the object. This equivalence between a variation in wave
length and a variation in distance, caused by the axial chromatism, leads us to imagine a system of
phase coding in which the usual movement of the reference mirror is replaced by a variation of the
wave length. We then obtain an interferometer without any moving part, which produces the
extensive depth of field that is peculiar to chromatic systems.
A novel optoelectronic set up based on a confocal extended field proprietary design has been developed for high resolution 3D surface sensing as well as for roughness and surface flaw characterization.
The classical optical sectioning property of the basic confocal imaging design assumes that a monochromatic light beam is propagating forth and back from the elementary point source to the spatial filtering pinhole. When using a polychromatic light source, the residual chromatic aberration of the optical system reduces the optical sectioning global performance by enlarging the axial resolution (optical sectioning) by a quantity almost equal to the length of the axial chromatic aberration.
When dealing with 3D surface sensing of the axial chromatic aberration can be considered as generating a highly accurate axial color coding, provided that an adequate color decoding of the backreflected light beam is realized.
Consequently, it appears that it is possible to design customized confocal extended field point sensors with depths of field ranging from a few tens of microns (with subnanometric axial resolution) up to tens of millimeters (with micrometric axial resolution).
Owing to the large Numerical Aperture of the confocal imaging set up perfectly specular optical surfaces can be easily captured with this type of instrument. Examples of Metrological 3D surface sensing of aspheric ophtalmic progressive lenses, small lens arrays and MOEMS will be presented and discussed.
A novel optoelectronic setup based on a quasi confocal, z- axis extended field, proprietary design has been developed for High Resolution Non Contact 3D Surface Metrology including roughness characterization and surface flaw detection.