458
S. Perrin et al.
17.3 Three-Dimensional Microsphere-Assisted Microscopy
Optical interference microscopy is a powerful imaging technique allowing a quantitative measurement and reconstruction of the surface topography [139], the inside
tomographic view [140] or the volume [74, 141]. Interferometry consists of superposing two light beams from a unique light source in which the first beam is reflected (or
transmitted) by the object and the second beam is reflected from a reference surface.
The detection of the interferogram by the detector, resulting from the combination
of the two beams, makes it possible to extract the wavefront deformation and, hence,
the height distribution. Several interferometric methods and optical interference configurations exist depending on the nature of the illumination light, the measurement
mode, i.e. transmission or reflection, and the signal processing.
17.3.1 Interference Methods
Mainly used for the characterization of material surfaces and micro-components,
coherence scanning interference microscopy (CSI), also called white light interference microscopy is based on the acquisition of the interference signal which is gated
by an envelope function when broadband illumination is employed. Optical coherence tomography for biological imaging [142] is based on the same principle, but
using a remote fibred interferometer and point detection. The interference signal
occurs only when the optical path difference between the two arms of the interferometer is less than the coherence length of the light source. The maximum intensity
of the gate function, thus, reveals the exact axial position of the object with respect
to the reference mirror at a given pixel. A variation in the optical path difference
between the two arms of the interferometer enables the probing of the surface or
volume of the object by the fringes from which a height distribution can be retrieved.
Different algorithms have been implemented to process the interferogram [143]. In
the case of digital holographic microscopy (DHM), spatially and temporally coherent illumination is required (e.g. a laser source), resulting in the need for only one
interference pattern acquisition to retrieve the surface topography or the volume
distribution of the sample. A numerical processing of the hologram uses 2D fast
Fourier transform operators to perform numerical wavefront propagation, allowing
the phase distribution of the sample to be retrieved [144]. Enhancements of DHM
have been reported through software developments [145] and optical setups [146].
Phase-shifting interference microscopy (PSM) requires the measurement of three
or more shifted interference patterns by introducing a known optical delay between
the reference and object arms [142]. Depending on the algorithm and the displacement technique, the phase difference distribution can then be recovered through
simple mathematical formulas, making possible the point-by-point reconstruction of
the wavefront deformation from the object. Coherent as well as low-coherent light
sources can be used in PSM. More easier to implement from a software development
S. Perrin et al.
17.3 Three-Dimensional Microsphere-Assisted Microscopy
Optical interference microscopy is a powerful imaging technique allowing a quantitative measurement and reconstruction of the surface topography [139], the inside
tomographic view [140] or the volume [74, 141]. Interferometry consists of superposing two light beams from a unique light source in which the first beam is reflected (or
transmitted) by the object and the second beam is reflected from a reference surface.
The detection of the interferogram by the detector, resulting from the combination
of the two beams, makes it possible to extract the wavefront deformation and, hence,
the height distribution. Several interferometric methods and optical interference configurations exist depending on the nature of the illumination light, the measurement
mode, i.e. transmission or reflection, and the signal processing.
17.3.1 Interference Methods
Mainly used for the characterization of material surfaces and micro-components,
coherence scanning interference microscopy (CSI), also called white light interference microscopy is based on the acquisition of the interference signal which is gated
by an envelope function when broadband illumination is employed. Optical coherence tomography for biological imaging [142] is based on the same principle, but
using a remote fibred interferometer and point detection. The interference signal
occurs only when the optical path difference between the two arms of the interferometer is less than the coherence length of the light source. The maximum intensity
of the gate function, thus, reveals the exact axial position of the object with respect
to the reference mirror at a given pixel. A variation in the optical path difference
between the two arms of the interferometer enables the probing of the surface or
volume of the object by the fringes from which a height distribution can be retrieved.
Different algorithms have been implemented to process the interferogram [143]. In
the case of digital holographic microscopy (DHM), spatially and temporally coherent illumination is required (e.g. a laser source), resulting in the need for only one
interference pattern acquisition to retrieve the surface topography or the volume
distribution of the sample. A numerical processing of the hologram uses 2D fast
Fourier transform operators to perform numerical wavefront propagation, allowing
the phase distribution of the sample to be retrieved [144]. Enhancements of DHM
have been reported through software developments [145] and optical setups [146].
Phase-shifting interference microscopy (PSM) requires the measurement of three
or more shifted interference patterns by introducing a known optical delay between
the reference and object arms [142]. Depending on the algorithm and the displacement technique, the phase difference distribution can then be recovered through
simple mathematical formulas, making possible the point-by-point reconstruction of
the wavefront deformation from the object. Coherent as well as low-coherent light
sources can be used in PSM. More easier to implement from a software development