Project introduction  (Page 2/3)

Unfortunately, conventional NIR cameras that cover the spectral range required for a clear image of underdrawings typically costfrom $30,000 to $50,000. By reducing the cost of the NIR camera, underdrawings could be examined by a much larger set ofuniversities, museums, and collectors. This opens numerous opportunities to gain deeper insight into both the creativeprocess and the history of artwork.

Problem requirements and specifications

The spectral range of a camera depends on the type of detectors it employs. Thus, whatever the overall structure of our design,it will have to incorporate a sensor capable of imaging through paint. All pigments except black are somewhat transparent in the900 - 1700 nm range, which can easily be covered by any NIR detector . Even though our problem probably constrains us to using an NIR detector of some sort, this stillleaves us a variety of options. We will want to choose a detector that has minimal cost, in order to keep the final camera pricelow. We will also want a detector that is appropriately sized so that it can easily be physically integrated into our overalloptical system. Additionally, we will need to ensure that any detector we choose is capable of providing the dynamic rangerequired for underdrawing imaging. That is, 256 grey levels should be distinguishable in the image .

As Boer found in his original experiments with reflectography, for high-quality underimaging a specialized lightsource is necessary, to provide coverage of all relevant wavelengths in the NIR range . Our system would also need such a source of radiation. For this source, or sources, 800 W of poweris a high enough value to give a good signal-to-noise ratio (SNR) in standard room temperature conditions . Using significantly higher power is not advisable, as most sources willemit a certain portion of their light in the UV and visibleranges, and it will be difficult to filter out all of this. Intense light from these high-energy ranges could damage apainting during imaging.

The image acquisition time when imaging underdrawings is quite flexible. While experience with conventional charge-coupleddevice (CCD) or complementary metal-oxide-semiconductor (CMOS) digital camera technology in the visible range would lead one toexpect image acquisition time on the order of µsec, such a short acquisition time is not critical for this application. Arelatively small number of images are typically taken with a static target. The resulting images are typically subjected toextensive analysis and comparison with similar images of underdrawings from the same artist. When all of these factorsare taken together, a small image acquisition time becomes a nice extra feature when possible, but is not crucial for success inthis application domain. Typical systems targeted towards underdrawing imaging take about 15 minutes to acquire andprocess an image due to the mosaicing system used. For the underdrawing imaging application, the relaxed image acquisitiontime requirement makes it possible to consider sacrificing low acquisition time for improved image quality, improved imageresolution, or reduced camera cost.