Although the information brought out by image interference between lighting modes, colour channels, and so on is often significant and useful, the application of these techniques must be done with care. Visual artifacts may be created, not only because Adobe Photoshop’s “Difference” blending mode, as mentioned, renders all subtraction results of pixel values positive, but also because the signal intensity in each image results from interactions of a number of factors (colour, morphology, reflectance, spectrum and angle of incident light, etc.). When several complex signals are combined the results become more difficult to interpret.
Nonetheless, when the difference between the images is pronounced and due to only one or a few factors, image interference may yield spectacular results. Figure 12 shows an assemblage of fossils, two chancelloriids and one sponge from the Burgess Shale, photographed under water without (Figure 12A) and with (Figure 12B) crossed nicols. The chancelloriids have three distinct types of tissue preservation: sclerites preserved in pyrite (bright in both Figures 12A and 12B), sclerites preserved as a shiny film (semibright in Figure 12A, dark in Figure 12B), and integument preserved as a nonshiny film (same colour as matrix in Figure 12A, darker than the matrix in Figure 12B). When Figure 12B (with its darker fossil relative to the matrix) is subtracted from Figure 12A (with its lighter fossil), the brightness gap between the films (most sclerites and integument; the pyritized sclerites acquire a brightness intermediate between matrix and films) comes out in bright pixels, contrasting sharply with the dark matrix (Figure 12C; cf. also Figure 10A–B, E–F, in which the central part of the picture acquires higher pixel values after the subtraction).
Subtracting channel A from B, the inverse picture is obtained (Figure 12D; cf. Figure 10E–F vs. G–H). Such a negative image is most easily produced directly through the “Inverse” command in Adobe Photoshop and may turn up to be better for viewing details than the positive image (compare the frequent use of negative images in astronomy).
An Adobe Photoshop PSD file (1.8 MB), containing the original colour images (in reduced resolution) for Figure 12, is enclosed to enable the reader to experiment with layer and channel interference.
Another example of the same image interference technique is between colour channels of one colour picture. Figure 13A shows a specimen of Chancelloria from the Middle Cambrian Wheeler Shale of Utah. The sclerites are spectacularly preserved in limonite (presumably arising from the weathering of pyrite) and would seem to need no enhancement. Crossed nicols are often applied with advantage on this material, as in this picture, because the rock is a friable mudstone that often does not survive immersion in liquid. Nonetheless, the sclerites form an intricate meshwork, and many rays are indistinct because they are somewhat buried and lie under a very thin layer of matrix. By subtracting the green channel (Figure 13C) from the red one (Figure 13B), we achieve a highly enhanced contrast between sclerites and matrix, making the bright sclerites appear as if they were suspended over a black background (Figure 13D). The uneven colouring of the central versus peripheral sclerites in the original picture (Figure 13A) has disappeared in the final image, because the procedure singles out spectral colour differences rather than differences in intensity. As the sclerites lie in several layers, a three-dimensional effect obtains. The result could not have been achieved simply by enhancing the contrast of the red channel.
Whenever a colour difference exists in a picture, it may be enhanced by this procedure. The picture of Yunnanozoon (Figure 6), in addition to the typical reddish tint, has bluish areas marking out the tissues surrounding the gut. By subtracting the blue channel from the green one, the bluish areas were enhanced by darkening; the resulting channel was then blended with the original colour image (using Adobe Photoshop’s “Multiply” mode, which has the same effect as superimposing the images upon each other) to get back to a more natural-looking image (Figure 14).
In the case of the graptolite image in Figure 8, subtraction of the green channel of the image taken under crossed nicols (Figure 8B) from that of the unpolarized image (Figure 8A) enhances the carbonized structures of the graptolite rhabdosomes and brings out features that were obscure in the original image (Figure 15). Subtracting the green channel of the original picture of Burgessia (Figure 5A) from the red one in the image taken under crossed nicols (Figure 5B) simultaneously brings out structures in the darkest and the lighter parts of the original images (Figure 16).