There are literally hundreds of ways to enhance contrast at the edge of a particle. Simply using crossed polarized light for birefringent partices is one example. Phase contrast and any of the other hundreds of interference methods are examples. Rhineberg filters and darkfield illumination are two more examples. Selecting a contrasting refractive index mounting medium is an important consideration in desiding whether the surface features or interial structuctures of the particles are of interest. A few of those methods are shown here.

Nomarski, Jamin-Lebedeff, Mach-Zehnder, Hoffman, and many other systems that alter the diffraction beam path to bring it into interference with the image beam path create contrast. Many of these systems can be used quanitatively to measure a few nanometers difference in pathlengths.