Since the advent of photography, the craft has completely changed the world — from its profound effects on communication and documentation in practical applications, to being a powerful form of personal expression and a visual art. Now, photography is being used to look into the past and discover significant historical information thought to be lost forever.
Photography makes it possible to see things that are impossible to detect with the naked eye by way of utilizing a technique called multispectral imaging. This is done by illuminating an object with a series of specific bands of light and then using filters for some images, resulting in a final combined image that reveals all sorts of underlying information. Multispectral imaging is being used for astrophotography, satellite imagery, and even in medical fields such as dentistry and ophthalmology.
One emerging and fascinating use of multispectral imaging is uncovering hidden information found on medieval palimpsests. Because writing materials such as parchment were expensive to produce, ancient manuscripts would commonly be reused (or “palimpsested”) by washing or scraping off any existing text to create a page which could be rewritten on. This practice means that palimpsests feature an underwriting of hidden information that was washed away as far as the human eye could tell, and overwritten text which further obscures any previous writing’s faint remains.
Natural and spectral palimpsest images. Courtesy of the Owner of the Syriac Galen Palimpsest.
Prior to the development of multispectral imaging techniques, scholars and researchers would use potentially destructive chemicals in order to read any palimpsest underwriting. Thankfully, with the use of photography, these historical documents can be made more easily readable than ever before, and without harming them in the process.
Most recently, a high-end camera company that we all know, Phase One, has become formally involved with multispectral imaging development. In partnership with leading researchers Michael B. Toth of R.B. Toth Associates and William A. Christens-Barry of Equipoise Imaging, LLC, Phase One is looking to help cultural heritage institutions analyze and preserve these important heritage objects. “With Phase One, we look forward to developing solutions that capitalize on our already 15 years of imaging support for cultural heritage studies around the globe — from spectral imaging at the Walters Art Museum and in the Sinai Desert to ongoing research with the University College London and Museum of New Mexico,” said Mike Toth of the new alliance.
Interestingly, as a side note, both Toth and Christens-Barry have actually worked on the draft of the Declaration of Independence at the Library of Congress, the location where “National Treasure: Book of Secrets” was filmed.
How does this multispectral imaging system with the Phase One camera work?
First, let’s start with the equipment. The camera used for image capture is a Phase One IQ260 Achromatic system, which has a 60-megapixel, 16-bit monochrome digital back with 350 nm (ultraviolet) to 1,100 nm (infrared) sensitivity. For comparison, the human eye is sensible to light between about 400 nm (violet) to 700 nm (red). Without a color filter array on the sensor, this digital back is all about capturing fine details, perfect for black and white photography and also of course imaging priceless historical manuscripts. The digital back is attached to the recently released Phase One XF camera body (previously a 3x4 view camera was used) with an apochromatic lens.
The specialized lighting in multispectral imaging plays a vital role in seeing more than what is apparently there with the naked eye. The multiple light banks of narrowband LEDs developed by Equipoise Imaging, LLC emit light in 13 or more ultraviolet, visible, and infrared wavelength bands. A series of images of the palimpsest are captured in an “image cube,” or stack of registered images, at each of the narrowband wavelengths.
There is also a six-position motorized filter wheel containing two-inch by two-inch square optical glass filters. The filter wheel may be used selectively in combination with the narrowband illumination to further increase the range of captured information by including flourescence emission and UV reflectance images. This information can then be used to analyze and completely determine the characteristic spectra of substrates such as parchment, colorants such as iron gall inks, and contaminants such as mold.
Spectral XV operating software — which is based on the Capture One engine and created with the Capture One SDK — controls the integrated digital back, filter system, lighting, and stores the collection of metadata. The Paleo image processing software brings all the captured images together, integrates the data, and enhances or reveals text, drawings, or residues that were previously difficult or impossible to see by eye.
In a way, it is similar to what we already know in how to perform focus-stacking or HDR photography. We take a consecutive series of images of the same unmoved subject, but change a single variable in each frame for our desired outcome: focal distance for focus-stacking, exposure time for HDR photography, or spectral wavelength band for multispectral imaging. Then these multiple frames are combined in software for a single enhanced result.
There’s a compelling new project beginning in September where Professor Melissa Terras, who is the Director of the University College London Centre for Digital Humanities, will be leading a team to perform multispectral imaging on Egyptian mummy masks to potentially uncover lost writings of poems and literature in the reused papyrus. Recent imaging work on Egyptian papyri has yielded lost lines of poetry written by Sappho, for example. It is unpredictable what new information may be revealed with multispectral imaging, but past studies have already proven the potential of this non-destructive photographic method. R.B. Toth Associates’ website has a list of previous accomplishments that are worth checking out to see more applications of multispectral imaging.
Lead photo courtesy of R.B. Toth Associates. All images used with permission.