Interference effects occur in the optical spectroscopy of thin film samples, and they can obliterate absorption spectra, especially if these are weak. A solution to this problem, originally proposed in 1976, is to place the sample inside an integrating sphere in the Edwards configuration so as to collect both reflected and transmitted light simultaneously. Surprisingly, the subsequent published literature on this technique is virtually nonexistent, despite its simplicity and effectiveness. This paper describes its use to measure absorption dips as weak as2×10−4in thin film samples. ©1997 American Institute of Physics.