Very thin layer of oil (less than 0.0002 in. or 0.005 mm) floating on the water surface; this is the most common form of oil seen in the later stages of a spill.
NOTE According to their thickness, sheens vary in color from rainbows (for the thicker layers) to silver/gray (for thinner layers) to almost transparent (for the thinnest layers).
Source: API RP 98, Personal Protective Equipment Selection for Oil Spill Responders, First Edition, August 2013. Global Standards
Sheen—An iridescent appearance on the surface of the water.
Source: Oil and Gas Wells, Pennsylvania Code, Title 25, Chapter 78, December 2012. Regulations