  We derive a relationship between the optical conductivity scattering rate
1/\tau(\omega) and the electron-boson spectral function \alpha^2F(\Omega) valid
for the case when the electronic density of states, N(\epsilon), cannot be
taken as constant in the vicinity of the Fermi level. This relationship turned
out to be useful for analyzing the experimental data in the pseudogap state of
cuprate superconductors.
