In soap films, two types of equilibrium film are often observed, sometimes successively in the same system: one characterized by thicknesses of the order of 7 nm or more which varies significantly with minor changes in composition such as ionic strength, and the other having a lesser thickness relatively independent of such changes. When a distinction is needed, the former are designated as common black films, and the latter as Newton black films. The current use of first or secondary for the common black film and of second, primary or Perrin's for the Newton black film is discouraged.
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 614 (https://doi.org/10.1351/pac197231040577)