With the extinctions measured in the PNe of the Galactic Bulge we had verified that the classical small clouds model for extinction in the solar neighbourhood can easily be generalized to the entire Galaxy. I use this model to explore the behaviour of the individual extinctions of PNe observed in an external galaxy. With a resonable number of objects one can determine the opacity of the extinction disk. With a larger number would be required to determine the ratio of the scale heights of PNe and extinction disk. Thus the measurement of the Halpha/Hbeta intensity ratio of PNs in disk galaxies provides a useful probe to learn about the interstellar medium in external galaxies.