Explanation:
Derive an expression for the equivalent width in a saturated line. Assume a Voigt profile, with the difference in optical depth between the center of the line and the wings being ~104. The wings of the line can be ignored. Define a frequency x1 = (v1 − v0)/ΔvD, where the optical depth τv = 1. Inside of x1 the line is fully saturated, and outside x1 the line is optically thin. Show that the equivalent width is

Note that the equivalent width is practically insensitive to the number density of absorbing material.