Optical Microscopy

By Clifford K. Schoff

Optical (light) microscopy is a powerful aid in defect and problem identification and root cause analysis. The author considers it the best single tool in problem solving. This article describes the main types of magnifying devices used in the coatings industry, in­cluding low magnification lenses, stereomicroscopes, and compound microscopes.

It dis­cusses reflected and transmitted light microscopy and the use of different kinds of illumi­nation, particularly bright field and dark field. It reviews specific applications, including the examination of surfaces, fibers and other particles, cross sections, and liquids. Ancillary equipment such as cameras, hot and cold stages, and reticules (reticles) are mentioned. The final word is that as useful as the microscope is, users should be careful in drawing conclusions strictly from what they see. You can be fooled!

Coatings chemists, technologists, and service people face many problems in their day-to-day work-defects and dirt on freshly applied paint, color that is not the correct shade, field failures, and unexpected corrosion. Although I am convinced that the most important problem-solving tools still are eyes and minds, there are limitations to what we can see. We need to be able to extend our vision. That is where optical (light) microscopy comes into play.

Microscopes and related devices en­able us to magnify and resolve what we are seeing. Is that speck of color on the part or panel rust, dirt, or a pigment particle? Is that defect a crater or a pop? Does it originate in the substrate, primer, or topcoat? Has the part been painted more than once? Is the pigment in the off-color paint flocculated or only partially dispersed? What is the particle size of a pigment dispersion or latex?
These and other questions can be answered using optical microscopy. Light microscopy does not usually pro­vide chemical information (exceptions being the identification of fibers and minerals), but it can be used to select areas for analytical techniques such as x-ray, FTIR, and SEM. I need to point out that I am not a microscopist, but a paint chemist who has used micro­scopes in the definition and solving of problems for 30 years.

I consider a light microscope the most valuable single tool for solving coatings problems. In addition, I firmly believe that all paint chemists and technologists should be­come familiar with microscopes and learn how to use them. More informa­tion on microscopes and microscopy can be found in the literature1·6 and in web sites on the Internet.