Middle Cambrian fossils from the Burgess Shale have traditionally been photographed in UV light (Conway Morris 1985). The use of polarizing filters to dampen directly reflected light has proven to be even more effective in bringing out details not otherwise visible (Boyle 1992; Bengtson 2000). The application of the reflectance transformation method on a Marella specimen, preserved mainly as a dark film on dark shale, was highly successful (Fig. 2). The image brightness drops to almost zero when using the low-angle lights, testifying to the very low relief of fossil features. The method of specular enhancement leads to a dramatic enhancement of relief, showing detailed anatomical and taphonomic structure in three dimensions (Fig. 2.3). However, the reduction of color contrast obscures the structures preserved as dark surface films. The best results were therefore obtained by combining (adding) the image obtained from specular enhancement with an inverted version of an unprocessed image made using low-angled virtual light.
Lower Cambrian fossils from the Chengjiang deposit in the Maotianshan Shale of China (Chen et al. 1997), preserved with low relief on a light-colored shale, might be expected to be a good target for PTM imaging. Figure 3 shows a specimen of the sponge Paraleptomitella dictyodroma with different enhancement techniques. Whereas oblique lighting enhances low-relief spicules, specular enhancement does not seem to offer any improvement in this case.
Low-relief
fern impressions in claystone from the Carboniferous Coal Measures of the Forest
of Dean, Gloucestershire, England, are devoid of color contrast, and can be
difficult to photograph and study with conventional techniques. Figure
4 shows some results from applying PTM techniques. The 3D reconstruction
from surface normals (Fig. 11) does not
add significant information.
Late Neoproterozoic Ediacaran fossils are commonly preserved in low relief on the soles or tops of sandstone beds. Imaging them usually employs whitening in combination with very low-angle lighting. The two examples illustrated here (Fig. 9, Fig. 10) from the Rawnsley Quartzite of South Australia show that the physically impossibly low position of the virtual light source through extrapolation of PTM space may help to bring out faint structures (Fig. 9.3; Fig. 10.3, Fig. 10.4); also that specular enhancement may bring out details that are otherwise obscured by the granularity of the sandstone (Fig. 9.5–9.9).
PTM imaging of a slab of Cambrian shelly fragments (hyolithids and trilobites) in a dark shale of the Alum Shale Formation of Krekling, Norway, illustrates the use of specular enhancement for amplifying relief and substituting for physical whitening of the specimen (Fig. 5.3).
Fossils of high relief are generally easier to photograph than those of low relief. Here, traditional photographic methods usually produce good results using a proper choice of lighting in combination with surface treatment. The Cambrian trilobite in Fig. 8, from the Alum Shale Formation of the Närke, Sweden, is a classic object for palaeontological photography. The different orientations of the virtual light in Fig. 8.2 and Fig. 8.3 serve to illustrate the importance of light directions for the understanding of the morphology; the full effect is best seen in the interactive models. As with the specimen in Fig. 5, specular enhancement may substitute for physical whitening in producing a more even surface color (Fig. 8.4).
A promising application of PTM imaging is in trace fossils, where the structures are often faint and oriented in many different directions so that a single lighting mode cannot capture the full morphology. Figure 6 and Figure 7 show two slabs of Lower Cambrian trace fossils in "neutral" virtual lighting (1), and with two directions of the virtual light to demonstrate how differently oriented structures are picked up in other ways (2, 3). Interestingly, specular enhancement serves to outline all structures independently of their orientation because of the strong differences in tone between surfaces nearly parallel to the main plane and those nearly normal to it (4).
These experiments indicate that reflectance transformation techniques offer significant improvement over non-transformed images in some cases, whereas in others the improvement, if any, is marginal compared with traditional techniques. However, in all cases the possibility of interactive manipulation of virtual light in PTM images adds a new dimension to the electronic illustration of fossil specimens.
