Dark-field optical microscopy techniques

Two techniques for overcoming the limitations of optical microscopy are of particular value in the study of colloidal systems. They are electron microscopy36-37, in which the limit of resolution is greatly extended, and dark-field microscopy, in which the minimum observable contrast is greatly reduced. 

The reflection technique is more limited in resolution and magnification than is transmission. Reflection optical microscopy of adhered surfaces is often revealing, but the very limited depth of field is a severe handicap. Nevertheless, examination of surfaces after testing or failure is a simple, but important, first step in determining locus of failure (see Stress distribution mode of failure). Surface reflectivity of specimens is often improved by deposition of a layer or metal. The reflection microscope is especially useful when operated in dark-field mode, where topographical differences are accentuated. 

The second step in the analysis involves identifying the extent of sample chemical heterogeneity. In order to do so, the unknown samples are imaged with conventional optical microscopy, using all available contrast-enhancement techniques (bright field, dark field, Die, polarized light, etc.). Regions of the sample that exhibit contrast are then sampled with the Raman microprobe in an attempt to correlate microscopic appearance... 

Advanced developments in optics and other components of OM have enabled more sophisticated variations in the technique, including bright-field, dark-field, polarization, interference and phase-contrast microscopies [5,6]. In the following... 

Another microscopy technique, which is actually based on light scattered by colloids, is dark-field or ultra-microscopy. In this technique, an ordinary optical microscope is used, but the sample is illuminated in such a way that light does not enter the objective unless scattered by the object under investigation. This technique does now allow a direct observation of, for example, a particle, but is particularly useful for detecting the presence of particles and investigating the Brownian motion of colloids. An important requirement is that the refractive index of the coUoids... 

Since soil micas are mostly present in the medium-to-coarse-sized clay fraction especially in soils of temperate regions, their particle size is close to or beyond the resolution boundary of the light microscope. Therefore, techniques such as phase contrast or dark-field illumination, which have been developed in order to improve the optical investigation of fine-grained minerals, are of special use. Phase-contrast microscopy permits the accurate determination of... 

---