Fig. 5a-c. luminosity ratio as a function of the mean hydrogen column density of the cloud. The luminosity ratio is based on the PDR models by Mochizuki & Nakagawa (2000) with the following modifications: (1) the hydrogen number density is constant in the model cloud; (2) the flux of the incident stellar radiation is times the solar neighborhood value (Mathis et al. 1983) at , and times at . The solid, dotted, and dashed curves indicate , , and 10 models, respectively. a  The assumed density is . b  . c  .