A Note on the Use of an Improved Infrared Vidicon for Reflectography of Paintings

In infrared reflectography of paintings infrared-to-visible image translation systems are used, operating around 2.0 microns; paint layers become most transparent in that spectral region to detect an underdrawing beneath. Earlier work was successfully carried out with a Barnes Infrared Camera equipped with a lead sulphide detector peaked at about 2.0 microns [1]. Later an infrared vidicon was used manufactured by Heimann PTW in a television system [2]. This vidicon is not ideal because it is not peaked at 2.0 microns-in fact its responsivity extends only to about 1.8 microns. In practice, however, reflectograms were often comparable with those obtained with the earlier Barnes system, although penetration of malachite-containing paint was inferior and interference of overlying paint layers more pronounced. Hori et al. have reported the development of an infrared vidicon with a multi-layer PbO-PbS-sensitive target showing a very short time-lag, a higher resolution and a wavelength response up to 2.0 microns [3]. This vidicon would have been preferable but it is not commercially available. Recently, however, an infrared vidicon manufactured by Hamamatsu Inc. [4] has been made commercially available with a wavelength response up to 2.4 microns. This vidicon has indeed the responsivity required for optimal infrared reflectography; in addition it can be estimated that its signal current when using a 0.8 micron cut-on 100