It is shown that the sensitivity of an interferometer for optical thickness measurement can be reduced by passing both the beams through the sample but at different angles. A method of compensation is described, to make the effective widths of the two beams equal at the sample surface. This technique allows the sensitivity of an interferometer to be varied over a wide range and is suitable for use with samples of thickness less than 1 mm. Two interferometers with reduced sensitivity are described; one of these is a Twyman-Green type and the other is a Mach-Zehnder type. At reduced sensitivity the interferometers do not give correctly the thickness variation in a sample which has an appreciable curvature. The precautions to be taken for testing such samples are discussed.