Project introduction

Problem statement

The ability to image sketches hidden by layers of paint is a valuable asset to art purchasers in their attempts toexhaustively examine the works they collect. Art conservators value a painting's underdrawings for two main reasons. The firstis that underdrawings can be exploited as an aid in determining whether the painting is an original or a forgery, by comparingthe style of the underdrawings to those of an artist's other works. The second is that comparisons between a painting and itsown underdrawing can give new insights into an artist's creative process for that work of art. Underdrawing images are bestrevealed by near infrared (NIR) cameras because all paint pigments, except black, are somewhat transparent in the 900 - 1700 nm range . However, current NIR cameras that are sensitive in this wavelength range are exorbitantlyexpensive, typically costing around $50,000 . This leaves the market of small art museums and individual artcollectors unaddressed. We intend to deliver an NIR camera that meets or exceeds key imaging system parameters in this market,such as spectral range, image resolution, capture time, and portability, while also reducing cost below the $5,000 level.

Background

Before the 1960s, there was only one way to examine a painting's underdrawing: the layers of paint in front of the underdrawing had tobe removed, thereby destroying the painting in the process. This was unacceptable for multiple reasons. Since underdrawings are typicallyof less interest than the finished painting, removing the layers of paint on top is not justified. Also, many interesting results comefrom a detailed comparison between the original painting and its underdrawing. This comparison is difficult if the paint layers mustbe destroyed to access the underdrawing.

J. R. J. van Asperen de Boer was the first to image underdrawings using NIR reflectography in 1968 . Previous attempts had used an NIR camera to passively capture light inthat range. However, with passive capture it was difficult to see through pigments such as green, since most of the lightcollected was in the range of 750 - 900 nm, which is below the range where those colors are transparent. Boer solved thisproblem by capturing the reflected light from a tungsten lamp that produced radiation of wavelengths up to 2500 nm, giving amuch clearer image of the underdrawing. While Boer's design has been improved in numerous ways during the past four decades, itremains fundamental to most techniques for imaging underdrawings nondestructively.

and below are examples of comparisons between original paintings and their underdrawings ascaptured with a typical modern NIR camera. In , the underdrawing shows that the man in the top hat, the artist'saccountant, was initially looking towards the viewer, but later this changed so that he looked away, purportedly because ofdisagreements between the two men . In , a large arrow can be seen on the chest of a horse on the underdrawing of a painting by Laib. This arrow is absentfrom the overlaid painted image.