Use of Cross-Polarized Lighting in Archeological Photography

James W. Henderson, RBP

Abstract

This paper describes the development of a multiple-step photographic procedure that can be used to make records of faded archaeological artifacts, such as pictographs (rock paintings), pottery, and emigrant inscriptions (along the Oregon Trail). Degradation of porous surfaces from natural abrasion and weathering has made photographing painted prehistoric and historic artifacts using natural lighting and traditional photographic techniques difficult. Natural lighting often produces harsh reflections on degraded surfaces and reduces the visibility of faint pigment colors. Portable strobes equipped with polarizing filters produce cross-polarized lighting that selectively eliminates surface reflections without affecting internal, backscattered reflections. Digital capture using cross-polarized lighting, along with subsequent digital enhancement, increases the brightness and saturation of backscattered colors in these artifacts. The completed procedure produces detailed color information not visible using other methods.

Historical Perspective

Since its invention in 1839, photographic documentation has played a significant role in the sciences, which depend heavily on visual cues for information. Medical diagnosis, botany, and archaeology are among the scientific disciplines that rely upon visual appearance for detailed descriptions and comparisons (Schaff, 1997).

Photography also has been used to make detailed records of natural phenomena. Technical photographs have been extensively employed in the natural sciences, since an effective technical photograph not only creates an accurate visual record of two- and three-dimensional objects, but also highlights the subject’s most significant details. A good technical photograph is, to a large extent, the result of the photographer’s knowledge of the subject, controlled use of lighting, and precise enhancement of the captured image data. The photographic medium itself is less critical than how the photographer controls the lighting and recording techniques in order to capture the most important details. In essence, the intent and approach are important here. Enhanced details in the image can reveal information heretofore unknown soon after observation, and even provide new comparisons long after the time the record was made. Photographs thus become visual data records of details that cannot be easily seen or that occur outside the realm of human perception. Along with written observations, these photographs extend the initial value of field observations (Blaker, 1980). Susan Sontag (1978) writes: “Photographs furnish evidence. Something we hear about, but doubt, seems proven when we're shown a photograph of it. Photographs are incontrovertible proof that something occurred or took place.” Photographs have increasingly assumed a greater role in the processes of gathering evidence and scientific proof.

Light and Reflection

Both human vision and photography depend heavily upon reflected light for visual clues about shape, texture, and color. Without these, we would have little understanding of the three-dimensional world.What one sees—and makes photographic records of—is not always what is actually present. 

This is especially true in the fields of prehistoric and historic archaeology. Many of the prehistoric artifacts encountered consist of porous material such as rock, pottery, and skin that have been buried for centuries beneath abrasive sand and detritus. Some artifacts are large, immoveable objects out in the open, and are subject to wind-blown sand and rain. Many have been painted and decorated with various colored pigments. Indigenous peoples have used red ochre extensively to decorate their material culture. Finely ground, and mixed with a binding agent such as animal fat or fish eggs, ochre adheres well to any porous surface and is clearly visible.

Figure 2. The visual appearance of a newly painted red pictograph.

Colored pigments fade over time due to prolonged exposure to light; furthermore, visualization is impeded by the interaction of light with the different types of surfaces it encounters on an object.

Objects reflect, absorb, or transmit light incident to their surfaces in predictable ways. More opaque objects allow less light to pass through their surfaces, while more translucent objects allow more light to pass through their surfaces. The relative smoothness or roughness of a surface itself influences the quality and behavior of light interacting with it. A very smooth, highly reflective surface reflects nearly all the light incident to it, and at the same angle (Stevens, 2005). Thus, a person standing in front of a highly reflective illuminated surface at an angle opposite that of the light source will see a perfect reflection of that light source. A good example of this is the glare off a metal car hood that oftentimes reflects directly into the driver’s eyes when the driver is headed into the sun. This occurs because the smooth, metal hood behaves as a single reflective body. The reflections off the surface of a pool of water are among common examples (in this case, the reflections of various objects behave as individual light sources).

Figure 3. The smooth surface of a small water puddle reflects the sky clearly when the angle of observation is identical to the angle of illumination.

On the other hand, when the subject is comprised of a porous surface, the light waves incident to that surface behave in a very different manner. Numerous small, irregular pits cover such a surface, and scatter each incident wave of light into many separate waves traveling in many different directions. The rougher the surface, the greater the likelihood it will scatter the incident light over its surface. Most objects consist of rough or matte surfaces to one degree or another. Skin, wood, rock, and paper are examples. This light-object interaction causes colored reflections at or below the surface to be obscured to one degree or another. High magnification photography of a porous surface illuminated by directional light reveals irregular, circular pits, each of which becomes its own 360-degree reflecting surface.

Figure 4. A photomicrograph (x25) made from a small section of red pictograph under conditions of natural daylight. The porous rock surface contains many irregular pits which diffuse and scatter the light rays illuminating it. This condition inhibits visualization of the colored details lying beneath the surface and degrades the visual appearance of the design. Compare with Figure 2.

When light strikes one of these pits, it reflects a miniature facsimile of the light source. These are sometimes called specular highlights, or collectively, surface glare. Specular highlights often dominate the sum total of all reflections from a very pitted and uneven surface. As the principal light incident to an object reflects off the surface, a small quantity of light passes through the surface into the interior where it produces secondary reflections called backscatter. These then pass back out through the surface and combine with primary surface reflections (Stevens, 2005).

Visual Noise

Color pigments are frequently the principal details responsible for recognition of an object's content; thus, any degradation of a pigment-bearing surface affects their visibility and legibility. Other sources of degradation, such as lichens, graffiti, scratches, and salt accretions also interfere with the visibility of the painted colors on or beneath the surface. Such interference could be referred to as "visual noise."

Figure 5. Close-up photograph of a red pictograph made with natural light from the open sky. Opaque, minerals have precipitated out of water solution and accumulated on the surface of the rock face, obscuring the underlying red pigment and making it difficult to see and to produce a detailed photograph.

This, in a visual sense, could be thought to be analogous to the low rumbling noise from an air conditioner that interferes with low frequency musical notes during a concert performance inside a large building. Many listeners cannot hear the low tones under such conditions (Henderson, 2002). Similarly, many natural lighting conditions negatively affect the appearance of an object's colors. Consequently, photographing weathered surfaces, such as pictographs, and capturing their very faint colors has been historically difficult (Stuart, 1978).

Polarized Light

An undergraduate student from Harvard University ultimately solved the problem. The student discovered that an artificial reflection-filtering device could be made by lining up iodoquinine sulphate crystals in the same direction and embedding them in transparent plastic to prevent the crystals from drifting apart. The student was Edward Land, founder of the Polaroid Company and inventor of the polarizing filter.

Land understood the nature of radiating waves of light and recognized that these waves could be passed through a special chemical polymer that would reorient all randomly radiating waves of light into one plane. Light so modulated was said to be linearly polarized.

Figure 6. Schematic of cross-polarized lighting. A polarizing filter placed over a light source causes all light rays to become oriented in the same linear plane of propagation. Polarized light strikes the rough surface of a pictograph panel, resulting in a polarized surface reflection. Light also passes through the surface and in doing so becomes depolarized. When interior elements reflect the light back, it again passes through the surface of the rock where it joins the polarized surface reflections. Together they pass through a second polarizing filter attached to the camera that has been rotated perpendicular, or crossed, to the orientation of the polarized light. Polarized reflections are absorbed, but the depolarized light containing the pigment reflections passes through the filter.

His real genius became apparent when a colleague took a piece of Land’s polarizing filter with him on a fishing trip and returned with an amazing story. By rotating the filter slowly in front of his eyes, he was able to see through the reflections of the sky on the surface of the pool and locate the position of a large trout (Campbell, 1994). Land immediately realized the implications of his friend’s experience. Some of the light reflecting off the smooth surface was already plane polarized in one direction. Looking through the filter while pointing it in the direction of the water and slowly rotating it caused some of the naturally occurring plane polarized reflections to be absorbed when the filter’s orientation was perpendicular to the polarized lake reflections. Land subsequently produced polarizing sunglasses and anti-reflective automotive glass. He later adapted a polarizing filter for photography to suppress reflections and glare (Polaroid, 1993).

Cross-Polarized Lighting

An important modification to Land's polarization technique furthered the use of polarized light in medical photography. Dermatologists had historically been unable to avoid and overcome distracting glare while examining the skin's surface. Thus, physicians could not observe important details that might have been present. Noting that human skin was relatively transparent and readily allowed light to pass through it, R. R. Anderson, M.D., discovered that the skin caused penetrating light to backscatter and lose its polarization when it passed through the surface. Reflected light from the skin surface, however, retained its polarization. Since the deadly skin disease malignant melanoma originates in the layer of cells beneath the skin, he reasoned that if he polarized the light source with one filter, and then placed a second filter over the camera lens and rotated it until it was perpendicular to the axis of the polarized light, the second filter would selectively absorb the polarized surface reflections of the skin but permit the internal, backscattered reflections from the melanoma to pass freely. He had discovered a unique method of selectively eliminating surface reflections and capturing faint internal reflections from the colored pigments (Anderson, 1991).

Since skin is simply another type of porous surface, applying the same technique to porous archaeological artifacts seemed quite feasible. I was puzzled when I discovered that cross-polarized lighting had never been attempted out in the field on archaeological artifacts. The reason for this was the unavailability of powerful, battery-powered portables strobes with modeling lights. In 1990, I obtained battery-operated strobes with sufficient power to make field photography possible and began examining a wide range of pigment-bearing objects using cross-polarized light (Henderson 1995, 1997, 2000). Subjects ranged in diversity from the rock paintings of Indigenous Peoples to skin tattoos belonging to Generation-X Peoples, and they included 1 BCE inscribed clay tablets from the Dead Sea, 400 AD Peruvian pottery jars, and 19th Century Oregon Trail emigrant names.

This application made production of clear photographs of historically significant artifacts possible. The goal of preserving this aspect of the past became a reality.

Summary

Most of the problems associated with photography of poorly preserved artifacts are a result of the approach taken to document them. The Cross-Polarization approach improves photographic recording of faded, colored artifacts, using either traditional film or digital capture. The procedure is an advanced photographic technique that requires cross-polarized illumination followed by digital enhancement to produce the highest level of legibility. If surface reflections have not been eliminated, subsequent attempts of digital or photographic enhancement will not eliminate visual noise that masks the colored pigments. This multi-step procedure is particularly effective in revealing pigments such as those found in severely weathered pictograph panels, faded pottery, and faint axle grease inscriptions. This would not be possible using conventional photographic techniques.

The Cross-Polarization Enhancement procedure, however, has two significant limitations. First, it requires more than just basic photographic knowledge along with more precise control of the lighting. Individuals with advanced theoretical and practical knowledge of photography will be able to translate the general descriptions in this paper into a specific step-by-step procedure to use in the field. Second, the procedure requires a substantial monetary investment in field equipment in order to attain a high level of light output to compensate for the large amounts of reflected light absorbed by the polarizing filters. Well-intentioned attempts to substitute point-and-shoot cameras and tungsten lights are not only misplaced, but their use may very well result in irreversible damage to the artifact.

Cross-polarization photography can be added to a growing list of valuable techniques to be used, when the goal is to document endangered cultural resources before vandals and natural processes obliterate them forever from our cultural milieu. Until there is a more universal acceptance by the public of the preservation of our ancestors’ material culture, and reliable methods are developed to physically preserve the sites, indirect preservation by documentation and photography, for the present, seems to be the best solution.

REFERENCES

Anderson, R. R. 1991, Polarized Light Examination and Photography of the Skin. Archives of Dermatology 127:1000-1005.

Blaker, A. A.1980. Handbook for Scientific Photography. San Francisco: W. H. Freeman and Company.

Campbell, F. W. 1994. Biographical Memoirs of Fellows of the Royal Society. 40: 195-219.

Henderson, James W. 1995. An Improved Procedure for the Photographic Enhancement of Rock Paintings. Rock Art Research 12:2: 75-85.

Henderson, James W. 1997. Cross-Polarized Enhancement: A New Way to Decipher Ancient Texts. American Society of Oriental Research Annual Meeting, Napa, CA.

Henderson, James W. 2000. Preservation of Pictographs and Petroglyphs of the Confederated Tribes at Warm Springs, Unpublished Final Report submitted to the Meyer Memorial Trust, Madras, OR.

Henderson, James W. 2002. Digitizing the Past: A New Procedure for Faded Rock Painting Photography. Canadian Journal of Archaeology 26:25-40.

Polaroid Corporation. 1993. Linear and Circular Polarizers. Norwood, Massachusetts: Publication TP186C-2.

Schaaf, L. J. 1997. Invention and Discovery: First Images, in Beauty of Another Order, ed. Thomas, A. New Haven: Yale University Press.

Sontag, S. 1978. On Photography. New York: Farrar, Straus and Giroux.

Stevens, Hallam. Sparknote on Optical Phenomena. February, 2005.
http://www.sparknotes.com/physics/optics/phenom.

Stuart, D. R. 1978. Recording Southwestern Rock Art Sites. The Kiva Vol. 43, Nos. 3-4: 189-193.

Acknowledgments

I am grateful for the opportunity and continued support of this research. Special thanks to the Confederated Tribes at Warm Springs; Long Distance Trails Office, National Park Service; Idaho State Parks; Archaeological Society of Alberta (Canada); and the Herrett Center for Arts and Science, College of Southern Idaho. Thanks also to the many unnamed volunteers who have selflessly assisted with the task of locating and recording remote places. Special thanks to colleagues Francoise Simoneau, Karen and Steve Buck, Judy Miles, Shara Anslow, and especially Alison Henderson, whose unswerving patience and hard work under adversity kept me well focused.

Author

James Henderson is a Registered Biological Photographer who specializes in difficult imaging problems in the sciences and and medicine. Henderson’s undergraduate major at Oberlin College was biology. Upon completion of military service, he majored in Applied Science and Photography at The Rochester Institute of Technology under the late Nile Root.

Correspondence: polarizedphoto@earthlink.net
Jim Hendersen, RBP
Applied Photographic Research
804 Center Street
Oregon, City, OR 97045