https://www.usgs.gov/publications/optical-properties-pseudovitrinite-implications-its-origin
Optical properties of pseudovitrinite; implications for its origin | U.S. Geological Survey
A set of Pennsylvanian coals from the North American coal basins, ranging in vitrinite reflectance from 0.65% to 1.75%, was examined, with special emphasis on...
optical properties