We model the optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) using a modified Lorentz oscillator model and compare our results to the conventional Lorentz oscillator model. It is found that an adjustable broadening function incorporated in the modified Lorentz oscillator model should be used instead of a Lorentzian broadening function in the conventional oscillator model for modeling successfully the optical constants of organic materials. We attribute this to the assumption that each absorption peak is composed of a large number of individual transitions closely spaced in energy.