Dispersion staining

The optical properties of all liquid and solid materials change as a function of the wavelength of light used to measure them.

The graph created by plotting the optical property of interest by the wavelength at which it is measured is called a dispersion curve.

Finally, the sample must be properly mounted under a coverslip to minimize any other optical effect that could complicate the interpretation of the color seen.

The Becke' Line method takes advantage of the fact that particles behave basically like lenses because they tend to be thinner at the edges than they are at the center.

Closing the sub-stage condenser iris decreases the resolution of the particle and increases the depth of field over which other objects may interfere with the effect seen.

The color of these two bands of light will vary depending on where the particle and liquid match in refractive index, the location of λo.

For large particles (greater than 25 micrometers in diameter) the colored Becke` Lines may be sufficiently distinct to determine the lo with the required accuracy.

The side farthest from the source of the light shows all the wavelengths for which the particle has the higher refractive index.

All of the features of objects in the field of view that don't match the refractive index of the mounting medium appear as bright white.

With “Positive Phase Contrast”, the particle appears colored from the contributing wavelengths for which the mounting medium has the higher refractive index.

Because of the physical size of the phase plate and its image onto the back focal plane of the objective where it is modified, a halo is formed around the particle.

[15] The difference in the optical properties of materials as a function of the “simple” or monochromatic colors of light is called dispersion.

[16] This paper discussed the occurrence of colored Becke` lines when a particle was in a liquid of matching refractive index.

For optimal effects the powder and the liquid had to be carefully selected so that the intersection of their dispersion curves created as large an angle as possible over the full range of visible wavelengths.

[17] He observed that the colored Becke` lines noted by Maschke could be used to distinguish between two materials with the same refractive index but different dispersion curves.

The colors could also indicate the region of the visible light spectrum for which a particle and liquid it was mounted in had a refractive index match.

Wright also noted that by using oblique transmitted illumination the particle would show these colors without having to inspect the Becke` line.

Dodge, and co-authors R. C. Emmons and R. N. Gates all wrote papers on the use of dispersion effects through the microscope to characterize particles.

[18][19][20] Crossmon seems to have coined the term “Dispersion Staining” as any optical technique that used the “Christiansen Effect” to produce color in the image of colorless particles.

[21] He demonstrated the use of Becke` Line, Oblique Illumination, Darkfield, and Phase Contrast Dispersion Staining methods.

S. C. Crossmon and W. C. McCrone have published numerous papers on the use of objective back focal plane stop dispersion staining techniques since that time.

For the relatively high energy bonds in most inorganic solids this means their refractive indices change very little over the visible range of frequencies.

The refractive indices of organic compounds on the other hand, with their lower bonding energies, change significantly over the visible range.

Crutcher, E. R., “The Role of Light Microscopy in Aerospace Analytical Laboratories”, PROCEEDINGS OF THE NINTH SPACE SIMULATION SYMPOSIUM, 1977.

Crutcher, E. R., “Optical Microscopy: An Important Tool for Particulate Receptor Source Apportionment”, PROCEEDINGS OF THE AIR POLLUTION CONTROL ASSOCIATION, pp.

and R. M. Gates, “The use of Becke line colors in refractive index determination”, AMERICAN MINERALOGIST, vol.

612–619, 1948 Hoidale, Glen B., “The color identification of transparent crystalline particles with an optical microscope: a literature survey of dispersion staining”, U.S. ARMY MICROFICHE, AD 603 019, 1964.

Bolleter, “Microscopic determination of the degree of nitration of nitrocellulose with dispersion staining”, JOURNAL OF APPLIED POLYMER SCIENCE, vol 12, no.1, pp.

Laskowski, Thomas E. and David M. Scotford, “Rapid determination of olivine compositions in thin section using dispersion staining methodology”, AMERICAN MINERALOGIST, vol.

Su, Shu-Chun, “Dispersion Staining - A versatile complement to Becke line method for refractive index determination”, GEOCHIMICA ET COSMOCHEMICA ACTA SUPPLEMENT, vol.