We calculate the optical absorption spectra associated with transitions from the n = 1 valence subband to the donor level in a dielectric quantum well. It is found that the impurity binding energies evaluated from photoluminescence experiments are lower than the practical binding energies, because of the effect of the image potential on the exciton binding energy in the dielectric quantum well. The broadening of absorption spectra due to the impurity distribution and the fluctuation in well width is also discussed.