Page 51 - Handbook of Properties of Textile and Technical Fibres

32 Handbook of Properties of Textile and Technical Fibres

(Weisenburger and Sandoghdar, 2015). These recent imaging methods are competing
with more traditional ones such as the SEM, which nevertheless remains very useful,
thanks to efforts to reduce costs and improved ease of use. The Section 2.3.1 will
describe the current developments of the SEM technique. Nevertheless, regarding
three dimensional imaging, very new “light imaging” techniques have emerged using
the visible-IR range of light.
As mentioned in the previous paragraph, 3D imaging is often required and is even
essential to study and determine the dimensions of irregular fibers. Plant fibers present
generally intricate structures and morphologies that make the determination of their di-
mensions and thus mechanical properties more complex than that for more regular and
homogeneous synthetic fibers. Furthermore, for some plant fibers, the apparent diam-
eter may vary from a minimum value to three times that value along the length (Malek
et al., 2016). For years, scientists have thus resorted to the use of X-ray microtomog-
raphy techniques. Although excellent in results (Abbey et al., 2010), this technique is
very cumbersome to implement, requires an access to a high-resolution synchrotron
system and remains difficult to be associated with an in-situ mechanical
characterization.
Optical coherence tomography (OCT) (Huang et al., 1991) is one of the novel
optical techniques which constitute a promising method for nondestructive, accurate,
and real-time 3D characterization of fibers (Placet et al., 2014). Over the last two de-
cades, this technique has been increasingly used in the field of medical imaging and
finds today more and more applications such as in fiber analysis. Depending on the
light source properties, OCT has achieved submicrometer resolutions (Povazay
et al., 2002) required to study fibers. Placet et al. (2014) have investigated the internal
structure of hemp fibers with success using this technique and they have concluded that
“OCT is also a promising technique able to deliver the 3D morphology of the fiber.”
Other promising optical imaging techniques have emerged in recent years and are
based on digital holography (Schnars and J€ uptner, 1994). With respect to material
science, techniques derived from digital holography are among the most interesting
recent routes. For example, the digital holographic microscopy, a patented technology
by Lyncée Tec, can operate both in reflection and transmission modes and allows
optical topography information to be retrieved without any scanning. These digital
holographic techniques have also opened the field to optical tomography. Using this
technique, Nanolive provides a tomographic microscope able to provide real-time sub-
micron 3D images of living cells without requiring any fluorescence markers (Cotte
et al., 2013). As illustrated in Fig. 2.6, by building their own digital holographic
tomography (DHT) equipment, Malek et al. (2016) have showed very interesting
results on flax fibers. By overcoming some limitations coming from diffraction tomog-
raphy, DHT has several advantages such as a broader potential to get the 3D shape of a
fiber (Malek et al., 2016).
Recent improvements of optical imaging techniques are particularly promising for
obtaining the true dimensions of the fibers. Nevertheless, optical methods also make it
possible to access information relating to the fiber internal structure. More generally,
3D imaging techniques based on tomography are further described in the dedicated
Section 2.3.6.