A method has been developed to detect underdrawings in paintings more completely than is possible with ir photography. A modified Barnes ir camera has been used equipped with a lead sulfide detector. The results can be explained in terms of an increase in hiding thickness of paint layers with the wavelength. Practical aspects of reflectography of paintings are briefly described and an example is given.
J. R. J. van Asperen de Boer, "Infrared Reflectography: a Method for the Examination of Paintings," Appl. Opt. 7, 1711-1714 (1968)