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False-Color Photography

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Anglo-Australian Observatory, RMIT University

The word “false” in this application means unreal or artificial; it does not necessarily mean fraudulent. False-color images can be made in many ways but in general they are the combined display of two or three monochrome images (information channels) taken of the same subject at different wavelengths or showing variations of some kind. These images may be made in radiation that we cannot see (UV or IR), or cannot photograph in any conventional way (acoustic, gamma ray, radio etc.).

Photographic or digital processes can then be used to apply a color to the monochrome information channels and combine them to reveal or emphasize their differences. If these separate information channels approximate to visible parts of the spectrum and they are combined using similar passbands, the result is a true-color image. Where any one or all of the channels is made of invisible radiation or the recombination is made with non-corresponding colors the result is a false-color picture. Because the human eye can discern a much greater variety of colors than gray tones, subtle differences and often unexpected relationships become obvious in the reconstructed color images that are not seen when comparing the black and white originals side by side.

Where two colors are involved a more natural-looking result is obtained if the channel colors are complementary (e.g., blue and orange), so that adding equal parts produces gray or white. The same is true with three channels (RGB), but in scientific false-color imagery the colors are often selected for optimum scientific return rather than a visually pleasing result. Because of the tri-color nature of the human visual system, using more than three channels can result in loss of information. Note that colorized monochrome (single channel) images are described as pseudocolor.

The most common false-color pictures and the false-color materials most accessible to the general photographer are those made with infrared film. This emulsion is a tri-pack color film, but with the uppermost of the three layers specially sensitized to infrared light. Like most color films the layers are also sensitive to UV and blue light, so it is used with a sharp-cut minus blue (i.e., yellow) filter, which does not allow blue light or UV to reach the film. The result is that infrared radiation appears as red and green reproduces as blue while red reproduces as green. Similar results but more versatility can be obtained with a digital camera and the appropriate software.

Applications for false-color imaging include detecting plant disease, camouflage, examining buildings for heat loss, masking, scientific recording including UV microscopy, scanning electron microscope imaging, differential autoradiography, all kinds of astronomical and satellite imaging and, of course, unusual pictorial effects.

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