Residual flux R of a saturated absorption line from Eq. (4) for various values of A_λ = k_λ E_B-V in a clumpy ISM geometry. When dust is present, the residual flux is higher than (1 − C_f). However, in a uniform dust screen geometry, the residual flux is always R = (1 − C_f). The covering fraction derived from the observed residual flux and a clumpy dust geometry is underestimated when dust is present.