Backside failure analysis techniques rely heavily on transmission of near infrared (IR) radiation through the silicon substrate. This statement applies both to emission techniques and active laser probing. Heavy doping of substrates causes them to become highly absorptive in the near IR due to band gap shifts, which effects phonon-assisted absorption, and to free-carrier absorption. Substrate thinning is often required to allow adequate optical transmission. This paper describes an empirical approach to determining the absorption coefficient in a heavily doped substrate and use of the coefficient in determining the amount of substrate thinning required.