Light reflected from a surface is differently polarized depending on the angle of incidence, the optical properties of the material, and the topography of the surface. If polarized light is used for illumination, changes in the polarization of the returned light can be analyzed using an additional polarization filter in front of the detecting device. This is the principle of epi-polarizing microscopes and other similar instruments. The ability of such devices to separate directly reflected light from backscattered light is used, for example, in ophthalmology (Fariza et al. 1989) and dermatology (Philip et al. 1988; Anderson 1991; Phillips et al. 1997).
The principle is eminently useful for fossil photography as well. Rayner (1992) used it to obtain high-contrast images of coalified fossils in dark shales, and Boyle (1992) applied it to Burgess Shale fossils. The technique is simple. In the setup used here, the camera lens is fitted with a regular polarizing filter, and spot lamps used for the illumination are also provided with polarizing filters that can be rotated in front of the lamps (filters of the appropriate size can be cut from commercially available gelatin filters). The filter at each of the light sources is then rotated individually so as to obtain maximum extinction of reflections from the object (or a reflecting object temporarily inserted in front of the camera lens); this is most easily done if the other light sources are covered or put out when the filter of one source is adjusted. The procedure is analogous to crossing the nicols in a petrographic microscope and will consequently be referred to here as crossed nicols (the term nicol in current usage refers not only to a Nicol prism, but to any filter that polarizes light). Further practical considerations are discussed by Rayner (1992) and Boyle (1992).
With this setup, dramatic contrasts may be obtained from otherwise very low-contrasting material, depending on whether the light at reflection keeps its original polarization or becomes more or less strongly repolarized. Also, because direct reflections are repressed, the effect is similar to that obtained when a specimen is immersed in water or some other clear fluid. Both these effects are very useful when photographing fossils from two of the classic Cambrian preservation lagerstätten, the Burgess Shale and the Maotianshan mudstone (with the Chengjiang fauna), as well as other fossils, such as graptolites and plants, preserved in shales or mudstones.
The Middle Cambrian Burgess Shale in British Columbia is not only famous for its exquisitely preserved fossils, but also infamous for the difficulties it presents to the photographer. The fossils are generally preserved in a shiny film that differs only slightly in colour from the surrounding rock. Commonly interpreted as an aluminosilicate film (Conway Morris 1977; Whittington 1985; Conway Morris 1990; Towe 1996; Orr et al. 1998), its reflectant matter appears to consist mainly of thermally altered organic carbon (Butterfield 1996). The reflectance of this film makes it possible to photograph the fossils by tilting them so that the directly reflected light (ultraviolet light is commonly used for increased contrast) falls into the camera lens (Conway Morris 1985). The same property, however, also allows us to make use of polarized light to increase the contrast between fossils and shale (Boyle 1992).
In
Figure 2, a specimen of Waptia
(cf. Briggs et al. 1994, pp. 157–158)
from the Burgess Shale has been immersed in water and photographed without (Figure
2A) and with (Figure 2B) crossed
nicols. The image taken without crossed nicols shows the low contrast between
the dark shale and the films representing the fossil soft parts. When crossed
nicols are applied (Figure 2B), the
improvement is dramatic: the outlines of the soft parts are now clearly visible
against the shale surface.
The
same procedures were applied to the images in Figure
3, showing the sponge Vauxia (cf. Rigby
1986) from the Burgess Shale. The details of the organic skeleton are
considerably enhanced under crossed nicols (Figure
3B).
Figure 4 and Figure
5 show grey-scale images of two more Burgess Shale fossils, Marrella
(cf. Whittington 1971) and Burgessia
(cf. Hughes 1975).
In Figure 4A and Figure
5A, the specimens have been immersed in water and photographed without
crossed nicols. (The dark irregular patch in Figure
4 is squeezed-out internal fluids and/or decomposed body tissues, a common
occurrence with Marrella.) In Figures 4B
and 5B, the specimens are photographed
with crossed nicols. All four pictures represent a single colour channel.
A
number of high-quality photographs of Burgess Shale fossils have been produced
throughout the years; see, for example, the photograph of Thaumaptilon by
B.K. Harvey in Conway Morris (1993),
figures 1–2, the suite of photographs by C. Clark in Briggs
et al. (1994), the ctenophore images by several photographers (including B.
Boyle) in Conway Morris and Collins
(1996), or the figures of Alalcomenaeus in Briggs
and Collins (1999). These have been taken using various methods, including
ultraviolet radiation, direct reflections, low-angle lighting, water immersion,
and crossed nicols.
The early Cambrian Maotianshan mudstone in south China, containing the exquisitely preserved Chengjiang fauna (e.g., Hou and Bergström 1997), represents a different problem for photography than the Burgess Shale. Although flattened, the fossils are preserved in considerably higher relief than those of the Burgess Shale. Consequently, low-angle light can bring out good details. Also, there is commonly a colour difference between fossils and matrix, brought out by iron-rich red films representing part of the soft bodies. The main problem in photographing them is, instead, that the mudstone is very friable and cannot be immersed in a fluid without breaking apart. Thus the common technique of photographing fossils immersed in water, glycerin or some other suitable liquid to remove reflections and increase contrast cannot be used. This is where the technique of polarizing the light comes in useful. (The red colour of the Chengjiang fossils also makes them suitable for photography with orthochromatic films, which are insensitive to red, as shown in the photographs by U. Samuelsson in Hou and Bergström 1997.)
Figure
6 shows a specimen of Yunnanozoon from the Chengjiang mudstone (cf. Hou
et al. 1991; Chen et al. 1995; Dzik
1995; Shu et al. 1996). Figure
6A is taken without, and Figure 6B
with, crossed nicols. The difference in result is less dramatic than in the case
of the Burgess Shale fossils; however, the use of crossed nicols has an effect
similar to that of immersing the specimen in liquid, namely to reduce
reflections and enhance contrasts.
The effects of using polarized light for the photography thus will range from good to spectacular, depending on the differences in reflectance of the objects. Only experimentation will tell how useful the method is in any particular case, but carbonized fossils seem consistently to yield fine results, as noted by Rayner (1992).
Two
further examples of such fossils are given here. Figure
7 shows a Tertiary leaf from Spitsbergen. In this case, the surface
topography of the leaf comes out best in unpolarized light (Figure
7A), whereas the use of crossed nicols brings out the contrast with the
matrix as well as the colour differences within the leaf (Figure
7B). Figure 8 shows graptolites
from Ordovician grey shales of Scania, Sweden. Although there is a colour
difference between fossils and matrix that comes out without crossed nicols (Figure
8A), a clear contrast is not obtained until crossed nicols are applied (Figure
8B).
