Observation of Newton black film (NBF) in foam film is possible only with a certain probability W which depends on the concentration C of surfactant in the solution and on the time t(a) during which adsorption of surfactant at the solution/air interface has taken place. In the paper, the W(C, t(a)) dependence is derived and used to analyze the effect of ta on the critical surfactant concentration C-c below which NBF in foam film practically cannot be observed. An expression for the C-c(t(a)) function is obtained which reveals that C-c decreases substantially with increasing t(a). This expression is found to describe well experimental C-c(t(a)) data for foam films obtained from aqueous solution of the therapeutic surfactant INFASURF. (c) 2008 Elsevier Inc. All rights reserved.